There is little change in the spatial distribution of global tropical SSTs from a week ago. SST anomalies along the equatorial cold tongue from ~165E to the west coast of South America vary from ~minus 1-3C with the coldest around 120W, extending to depths of at least 200m. SST totals of 29C and warmer are well entrenched across the Tropical Northwest Pacific (TNWP) into portions of the South Pacific Ocean loosely centered ~10N/140E. Recall that we use SST totals of 29C as a threshold for supporting persistent deep tropical convective forcing. Recent enhanced convection has slightly reduced the magnitudes of the warm anomalies from a week ago while this anomalously expanded warm pool slowly shifts south with the seasonal cycle. The equatorial Indian Ocean SSTs remain roughly .5-1.5C above normal as do portions of the Caribbean into the deep tropical Atlantic Ocean.
Per time-longitude section plots derived from the TAO buoy data, the evolution of this La-Nina appears not only ahead of the 2005-06 event, but slightly stronger. For instance, SST anomalies of minus 2C and lower have more persistently covered a larger region of the equatorial Pacific Ocean cold tongue, especially east of 140W. Based on the SSTs, the 2005-06 event peaked around February 2006, while this may be in the process of maturing as I type. However, there are complex coupled subseasonal dynamical forcing-response-feedbacks, etc., working with the seasonal cycle. Hence it is still unclear to me when this cold event will mature.
From this point I am not going to repeat what I wrote nearly a week ago since it is still valid, particularly in the interest of striving for some brevity (which failed!). Subseasonal variation #4 of the tropical convective forcing has progressed east-northeast into the west central and TNWP. Full disk satellite imagery and other monitoring tools suggest this forcing is centered ~10-15N/140E, with negative OLR extending from this region into portions of the very warm South Pacific Ocean. An almost separate region of enhanced rainfall is ~20N/60-90E. Finally, linked with a Southern Hemisphere Rossby wave energy dispersion (RWD), a separate intense tropical convective flare-up is developing ~60-70E just south of the equator. The latter may be part of a dynamical process to initiate what could become subseasonal event #5.
However, the most intense tropical forcing currently is across the west central Pacific Ocean, and is impacting the global circulation such as adding weak zonal mean westerly wind flow anomalies to the northern subtropical atmosphere (more said below). Surface westerly wind anomalies ~5m/s extend from the equator into the northwest Pacific Ocean, and it is probable 1 or more tropical cyclones will get spun off from the intense tropical thunderstorm clusters. At this point I do not think an oceanic Kelvin wave will get generated by the weak equatorial surface westerly anomalies.
Satellite animations and OLR/A Hovmoller plots do suggest the west Pacific convection to be drifting generally west, and may consolidate with the forcing loosely centered on the Bay of Bengal. Adding a bit more detail, during ~weeks 1-2 I would expect a portion of the west central Pacific convection to shift back to the northwest possibly in the form of tropical cyclones while the South Pacific Convergence Zone (SPCZ) becomes enhanced. Tropical convection may once again become quite intense from 60-90E while consolidating with the forcing farther east and north. I would speculate that during the next few weeks, tropical convective forcing should be nicely consolidated centered in the region of 100-120E along and just north of the equator. The latter is a consistent tropical convective signal with both La-Nina and the seasonal cycle. Again, make no mistake about the valid concerns I have of our nemesis. Whatever the case may be, the problem of 2 regions of Eastern Hemisphere tropical convective forcing will be a global weather-climate issue through at least DJF 2007-08.
Global relative AAM remains well below the R1 data climatology (updated through 24 September) by roughly 2 atmospheric momentum units (1 AMU = 1*10**24 Kg m/s**2) which is also approximately 2 standard deviations. A week ago I thought given the fairly strong poleward transport AAM signal across the Northern Hemisphere midlatitudes we would see a global response from the surface torques. However, that has not occurred. Updated through 21 September the Global Wind Oscillation (GWO) was still stuck at GSDM Stage 1 at roughly 2 standard deviations, and I do not think there has been much change since. From a zonal mean perspective there has been a recent weakly positive East Asian mountain torque, with a positive frictional torque of roughly 10 Hadleys (1 Hadley = 1*10**18 Kg m**2/s**2 = 1*10**18 J) at 30N compensated by dissipation at 50N. The latter are linked to an increase of storm track activity across the Northern Hemisphere midlatitudes.
The most significant point in regard to global relative AAM and zonal mean variations is that zonal mean anomalous easterly wind flow throughout the troposphere has been propagating off the equator into the subtropical atmospheres of both hemispheres since around 10 August (~10m/s at 200mb). This propagation has been to about 35N and 20S, and was tied to subseasonal tropical events #3 and # 4. In fact, the tropical convective flare-up across the TWNP discussed above has actually added weak zonal westerly wind flow anomalies (coming from the Western Hemisphere Pacific Ocean) from the equator to ~20N. As a La-Nina matures, it is typical to observe this kind of atmospheric behavior.
On the poleward flanks of these anomalous zonal mean easterly winds, westerly wind flow anomalies have also been increasing, especially across the higher latitudes of the Northern Hemisphere. The latter is not only due to seasonal transition into boreal autumn, but also a direct response to poleward shifted anomalous ridges dynamically forced by the meridional propapagtion of the zonal mean easterly wind flow anomalies discussed above. In fact, the zonal mean earth AAM is ~plus .2-.4 AMUs around 40-50N meaning an increase of atmospheric mass suggestive of anomalously high surface pressures (see the references in WB2007). I believe this is why extratropical storm track activity across the Northern Hemisphere midlatitudes has been increasing. This whole diagnosis is yet another example how our evolving La-Nina is already impacting the global synoptic variability (and has been since boreal winter). I disagree with any statements to the contrary from anywhere, particularly from the viewpoint of subseasonal atmospheric variability.
The global circulation is coupled to the developing cold event. RWDs from the west central and TNWP tropical convective forcing have also been strengthening westerly flow across the North Pacific Ocean. In fact, a fairly well defined residual of RWD activity linking both the Indian Ocean and west central Pacific forcing can be observed from weekly mean anomalies of 150mb vector winds. Wind speed anomalies greater than 15m/s have been common with these RWDs. Animations of upper tropospheric daily mean vector wind anomalies show twin tropical/subtropical anticyclones across both the Indian and west central Pacific Ocean basins, with the usual downstream cyclones generally around the date line, typical of GSDM Stage 1. So far the equatorial westerly wind anomalies tied to these twin lows have been mostly propagating into the Southern Hemisphere via South America (through RWDs), increasing their extratropical westerly flow.
Remembering the AAM considerations discussed above, these twin anticyclones are anchoring RWDs which are in the process of generating an extremely strong poleward shifted (~50-60N) north Pacific Ocean jet. This past JJA tropical forcing remained generally in the region of India, and in the presence of the boreal summer base state, troughs stayed offshore of North America favoring “Devil Ridges”. Now, anomalously intense troughs are probable to pound the western half of the USA for at least the next couple of weeks. This situation is stronger than I would have expected a week ago. The first of these will be an anticyclonically wave breaking event typical of GSDM Stage 1 this weekend. However, that character may locally change across North America given the west central Pacific Ocean signal. I do think there will be a mountain-frictional torque index cycle like variation during the next few weeks meaning a brief GWO orbit toward GSDM Stage 2.
The global numerical ensemble prediction systems (as part of a complete forecast process including the GSDM) continue to struggle with the synoptic details. However, there is good agreement for a western USA trough and eastern states ridge. There are some ensemble members suggesting a Stage 2 like pattern for North America starting late week 2. That response suggests a ridge west of North America well into Alaska and a deep central USA trough.
The bottom line is much of the USA is in for an unusually active early autumn regime for at least the next couple of weeks. Most of Alaska should have colder than normal temperatures with periods of rain and snow on the north side of the strong Pacific jet. In addition to significant mountain snowfall particularly west of the Continental Divide (for the CONUS), early season severe downslope wind storms are probable at times across the central and Northern Rockies. Periods of heavy rainfall along with episodes of severe local storms including tornadoes are probable for much of the Plains, starting this weekend. Much of the east will continue with summertime warmth. During week-2 colder polar air (still need the Arctic Ocean to “freeze” to have a good cold air source) should dump into much of the Rockies and even portions of the Northern Plains. It would not surprise me to see an early season significant snowstorm for locations such as eastern Wyoming-Montana into the western Dakotas.
I would expect a respite from this active pattern during the latter part of October into November. However, I will restate that as boreal winter approaches, western USA troughs should become stronger and more persistent, leading to an active southwest flow storm track across the Plains. This type of pattern can be favorable for Arctic outbreaks combined with intense precipitation. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008.
Careful daily monitoring of tropical forcing across the very warm west central Pacific Ocean is a must starting now. For instance, my assessment for the Eastern Hemisphere tropical forcing to again consolidate may get "blown out of the water (pun?)" if the current west central Pacific Ocean convection takes over now given the very warm SSTs. Ramifications could then be completely different much sooner, which is simply a testament to growing uncertainty.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Tropical cyclone activity did become more robust that I thought a week ago, with much of that coming from “Gabrielles”. Right now Tropical Storm Karen is fighting wind shear and stability per concerns discussed in past postings. Nevertheless, attention still needs to be paid to the tropical cyclone hazard not only from the Atlantic Ocean but also the Caribbean. The USA is by no means “out of the woods”.
The west central into the northwest Pacific Ocean is probable to continue with a tropical cyclone hazard through week 2. Impact areas include the Philippines possibly north into Japan. Another area of concern is the Bay of Bengal including coastal sections. In general, locations from India into Southeast Asia are still probable to continue with rounds of intense/severe thunderstorms through at least week 2. Much of the North Atlantic into Europe is probable to continue with active weather also for at least the next couple of weeks. On the latter, there are some ensemble members suggesting a period for troughs to remain west Europe especially week-2, meaning warmer and dryer weather.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will be on travel and my next discussion will not be until ~ 4-5 October at the earliest. Difficulty will increase keeping these discussions current.
Ed Berry
Thursday, September 27, 2007
Friday, September 21, 2007
La-Nina in Command, Nemesis Lurks
The spatial distribution of global tropical SSTs has become much more consistent with what may be evolving into a basin-wide cold event. Per TAO buoy data and CPC analyses, negative equatorial SST anomalies currently extend from about 160-165E to South America. Cold anomalies are greater than minus 2C ~130W extending to at least a depth of 200m near 160W. Given the seasonal cycle, these anomalies are not trivial.
The Indian and particularly the west central Pacific Ocean basins remain warmer than climatology. Anomalies are greater than 2C with totals in excess of 30C for the latter, with a warm horseshoe spatial pattern extending into the extratropics. The warmth across the west central Pacific has shifted slightly to the south during the past week. Slightly warmer than average SSTs remain from the Caribbean into the north tropical Atlantic Ocean.
My own feeling is during the past 4-6 weeks we have been observing the equatorial SSTs respond to the atmosphere (discussed below), going back to subseasonal variations late boreal fall 2006. There is now observational evidence that ocean-atmosphere coupling has been starting to occur during the last few weeks. However, the problem of 2 regions of tropical convective forcing: west central Pacific and Indian Ocean basins, remains. I also remain concerned this La-Nina will mature before the heart of boreal winter.
Tropical convective forcing continues strong across the Eastern Hemisphere, centered ~10-15N/120E while extending from the central Indian into the northwest and west central Pacific Ocean basins. This represents a loose consolidation between the Indian and west central Pacific Oceans tropical forcing since early this year. As a response the easterly trades along the equator have strengthened during the past several weeks, with wind speed anomalies in excess of 5m/s at times, leading to the recent SST cooling discussed above.
Four subseasonal variations consisting of northeastward propagation of tropical convection within this stationary signal have occurred since ~1 June 2007. These have had periods of ~30 days, and I do not consider these to be MJOs. These events have had strong interactions with the extratropics including ~15 day variations of the global mountain torque. Event number 3 occurred prior to 1 September and number 4 is in progress as I type, currently enhancing convection around the Philippines. In fact, from the full disk satellite imagery and OLR/A Hovmollers, I can still easily identify 2 regions of enhanced tropical rainfall, the north central Indian Ocean and from the west central into the northwest Pacific. The point is these subseasonal variations may lead us back to “nemesis” by boreal winter.
The last subseasonal event had a decent zonal mean anomalous poleward AAM transport signal in the 30-60N band around mid August, followed the ~30 Hadley positive global mountain torque just before the start of this month. As of 19 September per ESRL/PSD R1 AAM plots, once again there is a strong poleward AAM transport signal in the 30-60N band (~0.5-1 Hadleys), and another decent positive global mountain torque event may be probable. These types of behaviors have been contributing to the poleward propagation zonal mean easterly wind flow anomalies from the equatorial into the subtropical atmospheres of both hemispheres. In fact, at 200mb while zonal mean easterly wind anomalies of ~5-10m/s are present around 25-30N, there are weak zonal mean westerly wind flow anomalies along the equator. The latter are tied to twin upper tropospheric tropical cyclones ~150W. The Global Wind Oscillation (GWO) quasi-phase space plot updated through 18 September shows at least 2 circuits around GSDM Stage 1 since mid August due to the subseasonal events and meridional propagation of the anomalous zonal mean equatorial easterlies.
Global relative AAM remains low, ~ 2 AMUs below the R1 data climatology, while the GWO is still minus 2 standard deviations at GSDM Stage 1 (typical of La-Nina). The global frictional torque is only slightly positive but increasing. I think the global mountain torque will increase forcing the AAM tendency upward during the next 1-2 weeks. Anomalous zonal mean westerly flow, roughly 10m/s at 200mb, continues poleward of the easterly wind anomalies, ~50N and 40S. This translates to anomalously strong and poleward shifted ridges for both hemispheres.
Animations of upper tropospheric daily mean vector wind anomalies give a relatively simple and stationary picture (at least in a weekly mean) of twin tropical/subtropical anticyclones ~120E with downstream cyclones ~150W. Rossby wave energy disperions arcing from this tropical forced response into the extratropics link up nicely to central Pacific Ocean ridges and western continental troughs across the Americas. This whole pattern is roughly a 30 degree westward shift from about 10 days ago. In fact, poleward amplification of midlatitude ridges across the Southern Hemisphere into the high latitudes may lead to a rare sudden stratospheric warming (SSW) in the Antarctica region (currently austral spring). Per Tokyo Climate Center E-P fluxes have been strongly directed upward at 60S and 10hPa temperatures have risen at least 35C since early this month.
I think we are now observing a coupled response involving the tropical SSTs, enhanced Eastern Hemisphere tropical convection and the global circulation. To me, this whole evolution started late boreal fall 2006 first with the Indian Ocean tropical forcing dominating, then La-Nina like atmospheric base state commencing ~ 1 December 2006, and finally the SSTs catching up. Given that the anomalous zonal mean easterlies are propagating off the equator; I remain concerned this La-Nina may mature before 2008. In any case, the evolution toward a coupled La-Nina (meaning not just the atmosphere) is already impacting the synoptic weather variability not only across North America, but globally.
Most week 1-2 ensemble means from the global operational centers show the general west coast-Rockies USA trough and eastern states ridge. Model agreement should be better now given possible coupling. This means an active flow regime for much of the western 2/3rds of the country, and temperature and precipitation anomalies should be understood by now. Particular attention will need to be paid to the Pacific Northwest as strong baroclinic storms containing high winds and heavy rain periodically move ashore. I have a thought for a brief GSDM Stage 2 response after an intense baroclinic storm moves across the Plains next weekend. I tie this to the possibility of a mountain-frictional torque variation linked to subseasonal #4 discussed above.
As boreal winter approaches, western USA troughs should become stronger and more persistent, leading to an active southwest flow storm track across the Plains. This type of pattern can be favorable for Arctic outbreaks combined with intense precipitation. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008. On the latter, careful daily monitoring of tropical forcing across the very warm west central Pacific is a must starting now.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Per reasons discussed in my 14 September posting, I think AWB has put the kibosh on the Atlantic tropical cyclone season, at least for now. Yes, there is another “Gabrielle” festering as TD#10.
The west central into the northwest Pacific Ocean is probable to continue with a tropical cyclone hazard through week 2. Impact areas include the Philippines possibly north into Japan. Another area of concern is the Bay of Bengal including coastal sections. In general, locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms. Much of Europe is probable to have active weather for at least the next couple of weeks.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~ 28 September.
Ed Berry
The Indian and particularly the west central Pacific Ocean basins remain warmer than climatology. Anomalies are greater than 2C with totals in excess of 30C for the latter, with a warm horseshoe spatial pattern extending into the extratropics. The warmth across the west central Pacific has shifted slightly to the south during the past week. Slightly warmer than average SSTs remain from the Caribbean into the north tropical Atlantic Ocean.
My own feeling is during the past 4-6 weeks we have been observing the equatorial SSTs respond to the atmosphere (discussed below), going back to subseasonal variations late boreal fall 2006. There is now observational evidence that ocean-atmosphere coupling has been starting to occur during the last few weeks. However, the problem of 2 regions of tropical convective forcing: west central Pacific and Indian Ocean basins, remains. I also remain concerned this La-Nina will mature before the heart of boreal winter.
Tropical convective forcing continues strong across the Eastern Hemisphere, centered ~10-15N/120E while extending from the central Indian into the northwest and west central Pacific Ocean basins. This represents a loose consolidation between the Indian and west central Pacific Oceans tropical forcing since early this year. As a response the easterly trades along the equator have strengthened during the past several weeks, with wind speed anomalies in excess of 5m/s at times, leading to the recent SST cooling discussed above.
Four subseasonal variations consisting of northeastward propagation of tropical convection within this stationary signal have occurred since ~1 June 2007. These have had periods of ~30 days, and I do not consider these to be MJOs. These events have had strong interactions with the extratropics including ~15 day variations of the global mountain torque. Event number 3 occurred prior to 1 September and number 4 is in progress as I type, currently enhancing convection around the Philippines. In fact, from the full disk satellite imagery and OLR/A Hovmollers, I can still easily identify 2 regions of enhanced tropical rainfall, the north central Indian Ocean and from the west central into the northwest Pacific. The point is these subseasonal variations may lead us back to “nemesis” by boreal winter.
The last subseasonal event had a decent zonal mean anomalous poleward AAM transport signal in the 30-60N band around mid August, followed the ~30 Hadley positive global mountain torque just before the start of this month. As of 19 September per ESRL/PSD R1 AAM plots, once again there is a strong poleward AAM transport signal in the 30-60N band (~0.5-1 Hadleys), and another decent positive global mountain torque event may be probable. These types of behaviors have been contributing to the poleward propagation zonal mean easterly wind flow anomalies from the equatorial into the subtropical atmospheres of both hemispheres. In fact, at 200mb while zonal mean easterly wind anomalies of ~5-10m/s are present around 25-30N, there are weak zonal mean westerly wind flow anomalies along the equator. The latter are tied to twin upper tropospheric tropical cyclones ~150W. The Global Wind Oscillation (GWO) quasi-phase space plot updated through 18 September shows at least 2 circuits around GSDM Stage 1 since mid August due to the subseasonal events and meridional propagation of the anomalous zonal mean equatorial easterlies.
Global relative AAM remains low, ~ 2 AMUs below the R1 data climatology, while the GWO is still minus 2 standard deviations at GSDM Stage 1 (typical of La-Nina). The global frictional torque is only slightly positive but increasing. I think the global mountain torque will increase forcing the AAM tendency upward during the next 1-2 weeks. Anomalous zonal mean westerly flow, roughly 10m/s at 200mb, continues poleward of the easterly wind anomalies, ~50N and 40S. This translates to anomalously strong and poleward shifted ridges for both hemispheres.
Animations of upper tropospheric daily mean vector wind anomalies give a relatively simple and stationary picture (at least in a weekly mean) of twin tropical/subtropical anticyclones ~120E with downstream cyclones ~150W. Rossby wave energy disperions arcing from this tropical forced response into the extratropics link up nicely to central Pacific Ocean ridges and western continental troughs across the Americas. This whole pattern is roughly a 30 degree westward shift from about 10 days ago. In fact, poleward amplification of midlatitude ridges across the Southern Hemisphere into the high latitudes may lead to a rare sudden stratospheric warming (SSW) in the Antarctica region (currently austral spring). Per Tokyo Climate Center E-P fluxes have been strongly directed upward at 60S and 10hPa temperatures have risen at least 35C since early this month.
I think we are now observing a coupled response involving the tropical SSTs, enhanced Eastern Hemisphere tropical convection and the global circulation. To me, this whole evolution started late boreal fall 2006 first with the Indian Ocean tropical forcing dominating, then La-Nina like atmospheric base state commencing ~ 1 December 2006, and finally the SSTs catching up. Given that the anomalous zonal mean easterlies are propagating off the equator; I remain concerned this La-Nina may mature before 2008. In any case, the evolution toward a coupled La-Nina (meaning not just the atmosphere) is already impacting the synoptic weather variability not only across North America, but globally.
Most week 1-2 ensemble means from the global operational centers show the general west coast-Rockies USA trough and eastern states ridge. Model agreement should be better now given possible coupling. This means an active flow regime for much of the western 2/3rds of the country, and temperature and precipitation anomalies should be understood by now. Particular attention will need to be paid to the Pacific Northwest as strong baroclinic storms containing high winds and heavy rain periodically move ashore. I have a thought for a brief GSDM Stage 2 response after an intense baroclinic storm moves across the Plains next weekend. I tie this to the possibility of a mountain-frictional torque variation linked to subseasonal #4 discussed above.
As boreal winter approaches, western USA troughs should become stronger and more persistent, leading to an active southwest flow storm track across the Plains. This type of pattern can be favorable for Arctic outbreaks combined with intense precipitation. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008. On the latter, careful daily monitoring of tropical forcing across the very warm west central Pacific is a must starting now.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Per reasons discussed in my 14 September posting, I think AWB has put the kibosh on the Atlantic tropical cyclone season, at least for now. Yes, there is another “Gabrielle” festering as TD#10.
The west central into the northwest Pacific Ocean is probable to continue with a tropical cyclone hazard through week 2. Impact areas include the Philippines possibly north into Japan. Another area of concern is the Bay of Bengal including coastal sections. In general, locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms. Much of Europe is probable to have active weather for at least the next couple of weeks.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~ 28 September.
Ed Berry
Saturday, September 15, 2007
Nemesis
The spatial distribution of global tropical SSTs has not undergone any huge changes since a week ago. The La-Nina pattern is in place along the equator with ~minus 1-2C anomalies from about the date line to South America with the coolest around 125W (including Nino 3.4). These anomalously cool ocean waters extend to roughly 200m (TAO data) with magnitudes less than minus 3C near 100m/140W. Increased trades during the last week or so have intensified the negative SST anomalies along the equatorial Pacific Ocean cold tongue.
Across the Eastern Hemisphere I am getting a bit concerned about an emerging recent familiar spatial SST pattern, in this case featuring a warm Indian Ocean-cool Indonesia and very warm west central/northwest Pacific Ocean. SST totals across the equatorial Indian Ocean are ~29C and in excess of 30C over the Tropical Northwest Pacific (TNWP). This has some similarities to what was observed about a year ago, and may signal the return to 2 (at least episodic) regions of Eastern Hemisphere tropical convective forcing; the Indian and west central Pacific Oceans. As I have previously stated, this is a nemesis going back to the 2001-02 boreal cold season (global warming SST signal?).
During boreal autumn 2006 the Indian Ocean tropical convective forcing dominated and contributed to putting the kibosh on El-Nino ~ 1 December. Will the warm west central Pacific Ocean tropical forcing start to dominate going into 2008? My own speculation is to have that concern (read between the lines!). In any case, La-Nina SSTs and a coupled circulation response (~GSDM Stage 1.5) are probable the rest of this year.
Weekly averaged anomalies of outgoing longwave radiation (OLRA) present a nice signal of enhancement ~10N/60E and ~10N/140E, with suppression in between. The stationary component is the former while the latter has been forced by dynamical processes linked to the extratropics. Specifically, the Global Wind Oscillation (GWO) has done a reasonable job of capturing recent circulation impacts linked to a strong positive global mountain torque (~ plus 30 Hadleys; mostly Andes and East Asia) just prior to 1 September. Updated through 7 September the GWO was ~ GSDM Stage 1.5. Any MJO signal is weak and incoherent.
These complex interactions involved a chain of events leading to a strong inter-hemispheric symmetry of Rossby wave energy dispersions from the west central Pacific. Shown in weekly mean anomalies of 250mb vector winds (for example), strong ridges developed across eastern Alaska and west of South America. Wind flow anomalies were ~20-30m/s. This evolution led to cold regimes across the central USA and central South America, and the numerical models did not predict any of these behaviors very well for week 2.
Full disk satellite imagery and other monitoring tools have shown the TWNP tropical forcing to generally shift northwest with at least a couple of tropical cyclones during the last week. The Indian Ocean forcing has drifted to ~10N/80E and may consolidate with the convection to the east, typical of La-Nina. Global relative AAM remains ~minus 2 Atmospheric Momentum Units (AMUs, using the mks system) per R1 data, having decreased a bit during the past few days. I think the GWO is drifting back toward GSDM Stage 1, particularly since the global mountain torque and AAM tendency was weakly negative ~minus 10 Hadleys through 11 September, and the AAM transport signal is again directed poleward.
There is some evidence of the strong anomalies of deep zonal mean easterly wind flow to have propagated poleward from the equator into the subtropical atmospheres of both hemispheres since ~10 August. The recent TNWP convection did contribute, with, in fact, ~5m/s at 200mb westerly wind flow anomalies developing along the equator. A key circulation response to La-Nina is for these zonal mean easterly wind anomalies to propagate off the equator. Perhaps this process may occur with a series of subseasonal events. However, already starting to observe this response expresses a concern for this cold event to peak during boreal autumn.
Animations of upper tropospheric daily mean vector wind anomalies present a reasonable signal of twin tropical/subtropical anticyclones near 90E with twin cyclones near 150E. These circulation gyres are directly linked to both the Eastern Hemisphere tropical convective forcing and PNA/PSA RWDs discussed above. There has been a westward shift of all the above for the past few days, meaning the next trough to impact North America will maximize in the region of the west coast. This is consistent with the GWO, and all numerical ensembles forecast this general idea for weeks 1-2.
I think readers should be familiar with USA weather impacts due to a western states trough and eastern USA/Deep South ridge (understanding the seasonal cycle). More strong/severe baroclinic storms developing off of East Asia are probable to impact Alaska and the Gulf of Alaska periodically for at least the next couple of weeks.
Particularly with the recent meridional propagation of zonal mean easterly wind flow anomalies, our base state is very much susceptible (probabilistically) to lower and midlatitude anticyclonically wave breaking (AWB) lows. One of these has been limiting the development of Tropical Cyclone Ingrid across the western Atlantic (more said below). I think the models have the right idea of an AWB low along the USA west coast later week 1 only to eject into the Plains week 2 (linking with a subtropical jet emanating from the twin 150E tropical cyclones). I think seasonal transition to the boreal cold season will limit western USA troughs until the November-December period. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. A week ago I tried to be favorable for the notion of Atlantic tropical cyclogenesis. Given what I have been monitoring and writing in these postings all summer, what was I thinking??? AWB, which has been a limiting factor for Atlantic tropical cyclogenesis all summer, has only intensified during the past week. The ~20-30m/s upper tropospheric equatorial westerly wind flow anomalies across the Pacific downstream from the twin tropical cyclones appear to heading into central South America, apparently having limited local impact across the tropical Atlantic Ocean. This situation of AWB needs to change if additional true tropical cyclones are to develop in the Atlantic basin (above climatology). For the USA, the concern may have to focus on hybids like Gabrielle and Humberto.
The TNWP is probable to continue with a tropical cyclone hazard week 1 hopefully relaxing week 2. Impact areas include the Philippines north into at least Japan. There is evidence that the Eastern Hemisphere convection is starting to drift south, meaning locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms through week 2. Finally, I can also see some concern for 1-2 strong/severe extratropical cyclones to slam at least northern Europe weeks 1-2.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~ 21 September.
Ed Berry
Across the Eastern Hemisphere I am getting a bit concerned about an emerging recent familiar spatial SST pattern, in this case featuring a warm Indian Ocean-cool Indonesia and very warm west central/northwest Pacific Ocean. SST totals across the equatorial Indian Ocean are ~29C and in excess of 30C over the Tropical Northwest Pacific (TNWP). This has some similarities to what was observed about a year ago, and may signal the return to 2 (at least episodic) regions of Eastern Hemisphere tropical convective forcing; the Indian and west central Pacific Oceans. As I have previously stated, this is a nemesis going back to the 2001-02 boreal cold season (global warming SST signal?).
During boreal autumn 2006 the Indian Ocean tropical convective forcing dominated and contributed to putting the kibosh on El-Nino ~ 1 December. Will the warm west central Pacific Ocean tropical forcing start to dominate going into 2008? My own speculation is to have that concern (read between the lines!). In any case, La-Nina SSTs and a coupled circulation response (~GSDM Stage 1.5) are probable the rest of this year.
Weekly averaged anomalies of outgoing longwave radiation (OLRA) present a nice signal of enhancement ~10N/60E and ~10N/140E, with suppression in between. The stationary component is the former while the latter has been forced by dynamical processes linked to the extratropics. Specifically, the Global Wind Oscillation (GWO) has done a reasonable job of capturing recent circulation impacts linked to a strong positive global mountain torque (~ plus 30 Hadleys; mostly Andes and East Asia) just prior to 1 September. Updated through 7 September the GWO was ~ GSDM Stage 1.5. Any MJO signal is weak and incoherent.
These complex interactions involved a chain of events leading to a strong inter-hemispheric symmetry of Rossby wave energy dispersions from the west central Pacific. Shown in weekly mean anomalies of 250mb vector winds (for example), strong ridges developed across eastern Alaska and west of South America. Wind flow anomalies were ~20-30m/s. This evolution led to cold regimes across the central USA and central South America, and the numerical models did not predict any of these behaviors very well for week 2.
Full disk satellite imagery and other monitoring tools have shown the TWNP tropical forcing to generally shift northwest with at least a couple of tropical cyclones during the last week. The Indian Ocean forcing has drifted to ~10N/80E and may consolidate with the convection to the east, typical of La-Nina. Global relative AAM remains ~minus 2 Atmospheric Momentum Units (AMUs, using the mks system) per R1 data, having decreased a bit during the past few days. I think the GWO is drifting back toward GSDM Stage 1, particularly since the global mountain torque and AAM tendency was weakly negative ~minus 10 Hadleys through 11 September, and the AAM transport signal is again directed poleward.
There is some evidence of the strong anomalies of deep zonal mean easterly wind flow to have propagated poleward from the equator into the subtropical atmospheres of both hemispheres since ~10 August. The recent TNWP convection did contribute, with, in fact, ~5m/s at 200mb westerly wind flow anomalies developing along the equator. A key circulation response to La-Nina is for these zonal mean easterly wind anomalies to propagate off the equator. Perhaps this process may occur with a series of subseasonal events. However, already starting to observe this response expresses a concern for this cold event to peak during boreal autumn.
Animations of upper tropospheric daily mean vector wind anomalies present a reasonable signal of twin tropical/subtropical anticyclones near 90E with twin cyclones near 150E. These circulation gyres are directly linked to both the Eastern Hemisphere tropical convective forcing and PNA/PSA RWDs discussed above. There has been a westward shift of all the above for the past few days, meaning the next trough to impact North America will maximize in the region of the west coast. This is consistent with the GWO, and all numerical ensembles forecast this general idea for weeks 1-2.
I think readers should be familiar with USA weather impacts due to a western states trough and eastern USA/Deep South ridge (understanding the seasonal cycle). More strong/severe baroclinic storms developing off of East Asia are probable to impact Alaska and the Gulf of Alaska periodically for at least the next couple of weeks.
Particularly with the recent meridional propagation of zonal mean easterly wind flow anomalies, our base state is very much susceptible (probabilistically) to lower and midlatitude anticyclonically wave breaking (AWB) lows. One of these has been limiting the development of Tropical Cyclone Ingrid across the western Atlantic (more said below). I think the models have the right idea of an AWB low along the USA west coast later week 1 only to eject into the Plains week 2 (linking with a subtropical jet emanating from the twin 150E tropical cyclones). I think seasonal transition to the boreal cold season will limit western USA troughs until the November-December period. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. A week ago I tried to be favorable for the notion of Atlantic tropical cyclogenesis. Given what I have been monitoring and writing in these postings all summer, what was I thinking??? AWB, which has been a limiting factor for Atlantic tropical cyclogenesis all summer, has only intensified during the past week. The ~20-30m/s upper tropospheric equatorial westerly wind flow anomalies across the Pacific downstream from the twin tropical cyclones appear to heading into central South America, apparently having limited local impact across the tropical Atlantic Ocean. This situation of AWB needs to change if additional true tropical cyclones are to develop in the Atlantic basin (above climatology). For the USA, the concern may have to focus on hybids like Gabrielle and Humberto.
The TNWP is probable to continue with a tropical cyclone hazard week 1 hopefully relaxing week 2. Impact areas include the Philippines north into at least Japan. There is evidence that the Eastern Hemisphere convection is starting to drift south, meaning locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms through week 2. Finally, I can also see some concern for 1-2 strong/severe extratropical cyclones to slam at least northern Europe weeks 1-2.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~ 21 September.
Ed Berry
Saturday, September 08, 2007
Are we Sure???
The spatial distribution of global tropical SSTs has not changed too much since a week ago. There is a slightly anomalously warm (cool) Indian Ocean (Indonesia region) to significantly above average Tropical Northwest Pacific Ocean (TNWP) pattern across the Eastern Hemisphere. A horseshoe shape of anomalous warmth still extends into the extratropics from the latter. Starting around the date line all Nino regions appear to be cooler than normal per CPC 7 September 2007 daily mean SST analysis, with a tilt toward warmth from the Caribbean into the deep tropical Atlantic Ocean.
SST totals and anomalies from the west central into the northwest Pacific Ocean are in excess of 30C and plus 1 C over a large area. Negative SST anomalies of 1C and lower extend from ~160W to less than minus 2C ~110W per recent 5-day averaged TAO buoy data. All these anomalies are quite deep to at least 200-400 meters meaning a steeper than normal equatorial oceanic thermocline. So, at least for now our La-Nina has become an essentially basin wide event, having a pronounced westward shift since ~ 1 August forced by enhanced trades. In my past postings, I suggested the cool SSTs would be confined to the equatorial east Pacific Ocean. So far that appears to have been a poor assessment.
However, I do have what I think are fair concerns in regard to the future our developing cold event given ongoing subseasonal behaviors (discussed below). It is unfortunate that the resources are not (yet) presented to this effort which would allow weather-climate linkage risk assessment and probabilistic information (etc.) that may provide additional and useful predictive information. From speculation, I am concerned this La-Nina will peak during boreal fall; in a sense similar to El-Nino about a year ago (there has been a biennial character since ~2001). I will leave this path by again typing how interesting the weather-climate situation may become should the 30C and warmer waters of the west central Pacific Ocean become strongly convectively active by the start of 2008.
From full disk satellite imagery and other tools, there are 2 regions of tropical convective forcing across the Eastern Hemisphere. North equatorial Africa remains broadly active with periodic enhancement around the Americas. For the Eastern Hemisphere, one area is the stationary tropical signal centered ~10N/60-80E linked to the evolution of La-Nina discussed in my Labor Day Weekend postings. The TNWP forcing I think is another manifesting of the 20-30 day variability observed since around early June involving east and northward shifts. Even though recently updated RMM WH2004 phase space plots indicate some MJO projection with predicted eastward movement, I do not think the current TNWP flare-up is a result of MJO variability.
Since early June, the first couple were very weak MJOs while the last 2 I think have had an extratropical component of forcing. In fact, the most recent has involved a relatively strong positive global mountain torque of ~plus 30 Hadleys leading to a relative AAM tendency of similar magnitude just before 1 September. A circulation response has been for an increase of anomalous zonal mean westerly wind flow loosely from the equator to about 30N with magnitudes of 2-5m/s. There has even been a weak poleward propagation of anomalous zonal mean easterly wind flow from the equator to ~40N since about 10 August.
The most significant contribution to the most recent positive global mountain torque may have come from East Asia, but having nice dipole symmetry with the Southern Hemisphere due to the Andes Mountains. From daily monitoring of fields such animations of upper tropospheric daily mean vector wind anomalies, I can link extratropical Rossby wave energy dispersions (RWDs) tied to the Indian Ocean forcing to the positive global mountain torques. Subsequent dispersions into the TNWP enhancing upper tropospheric divergence led to the current observed flare-up in that region. These same animations suggest RWDs from the TWNP enhanced convection impacting the PNA sector as I type. A response has been for the GSDM Stage 2 response leading to a digging Rocky Mountain trough.
From a somewhat broader perspective, around 20 August we saw a poleward AAM transport signal maximized ~45N only to recently become directed equatorward ~35N. The interactions discussed above were involved in this reversal. When updated, the ESRL/PSD Global Wind Oscillation (GWO) quasi-phase space diagram should show an even larger circuit toward GSDM Stage 2, but still on the left hand side of the plot. Global relative AAM still remains low, ~ minus 2 Atmospheric Momentum Units (AMUs) below the R1 data climatology. Much of that contribution is still coming from the Southern Hemisphere equatorial and subtropical atmospheres. I think this will be the largest GWO circuit since the 2 large events during March and May 2007, having a contribution from non-MJO tropical convective forcing.
Finally, other roughly 20-30 day GWO circuits leading to at times 2 regions of tropical forcing across the Eastern Hemisphere may be probable going into boreal fall. These would likely involve both the Indian and west central Pacific Ocean basins. This is not a new concern on my end for numerous reasons, and any longer-term impacts on our La-Nina situation are unclear. However, monitoring is critical since numerical coupled ensemble prediction schemes are unlikely catch complicated subseasonal behaviors like GWOs until their impacts have already occurred.
For the USA week 1, the models have generally caught the GSDM Stage 2 response of the Rockies and Plains trough. Much of the country will have the first surge of autumn cool air, with the largest negative temperature anomalies generally focused on the Northern Plains and Upper Mississippi Valley. Heavy precipitation is likely to occur across locations such as from the Southern Plains into the eastern states. During weeks 2-3, the east Pacific Ridge-central states trough pattern should loose amplitude, with more troughs from the east Pacific into the western states becoming probable. The GWO should quickly orbit into the GSDM Stage 4-1 phase plane. Another “strong trough” situation may be probable for the Rockies and Plains ~ week 3. Again, predictability can be very low during transition seasons. Stay tuned if much of the western and central USA has a “real winter (assuming the Arctic Ocean freezes back over!)” with perhaps many GSDM Stage 4-1 situations per above.
Internationally, generally from the Indian Ocean into the TNWP periods of intense rainfall are probable week 1, including tropical cyclone activity across the latter. The focus should then shift back into the region of India into Southeast Asia and Southern China at least week 2, with still a tropical cyclone concern for the region around the Philippines. Northern equatorial Africa should stay convectively active for at least the next couple of weeks.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. As discussed a week ago, Gabrielle developed as a hybrid system. I can see a scenario favoring tropical cyclone development perhaps even above climatology across the deep tropics of the Atlantic Ocean week 1 and possibly continuing week 2.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will be on travel during the week of 10-14 September, and I am unclear when my next update will be.
Ed Berry
SST totals and anomalies from the west central into the northwest Pacific Ocean are in excess of 30C and plus 1 C over a large area. Negative SST anomalies of 1C and lower extend from ~160W to less than minus 2C ~110W per recent 5-day averaged TAO buoy data. All these anomalies are quite deep to at least 200-400 meters meaning a steeper than normal equatorial oceanic thermocline. So, at least for now our La-Nina has become an essentially basin wide event, having a pronounced westward shift since ~ 1 August forced by enhanced trades. In my past postings, I suggested the cool SSTs would be confined to the equatorial east Pacific Ocean. So far that appears to have been a poor assessment.
However, I do have what I think are fair concerns in regard to the future our developing cold event given ongoing subseasonal behaviors (discussed below). It is unfortunate that the resources are not (yet) presented to this effort which would allow weather-climate linkage risk assessment and probabilistic information (etc.) that may provide additional and useful predictive information. From speculation, I am concerned this La-Nina will peak during boreal fall; in a sense similar to El-Nino about a year ago (there has been a biennial character since ~2001). I will leave this path by again typing how interesting the weather-climate situation may become should the 30C and warmer waters of the west central Pacific Ocean become strongly convectively active by the start of 2008.
From full disk satellite imagery and other tools, there are 2 regions of tropical convective forcing across the Eastern Hemisphere. North equatorial Africa remains broadly active with periodic enhancement around the Americas. For the Eastern Hemisphere, one area is the stationary tropical signal centered ~10N/60-80E linked to the evolution of La-Nina discussed in my Labor Day Weekend postings. The TNWP forcing I think is another manifesting of the 20-30 day variability observed since around early June involving east and northward shifts. Even though recently updated RMM WH2004 phase space plots indicate some MJO projection with predicted eastward movement, I do not think the current TNWP flare-up is a result of MJO variability.
Since early June, the first couple were very weak MJOs while the last 2 I think have had an extratropical component of forcing. In fact, the most recent has involved a relatively strong positive global mountain torque of ~plus 30 Hadleys leading to a relative AAM tendency of similar magnitude just before 1 September. A circulation response has been for an increase of anomalous zonal mean westerly wind flow loosely from the equator to about 30N with magnitudes of 2-5m/s. There has even been a weak poleward propagation of anomalous zonal mean easterly wind flow from the equator to ~40N since about 10 August.
The most significant contribution to the most recent positive global mountain torque may have come from East Asia, but having nice dipole symmetry with the Southern Hemisphere due to the Andes Mountains. From daily monitoring of fields such animations of upper tropospheric daily mean vector wind anomalies, I can link extratropical Rossby wave energy dispersions (RWDs) tied to the Indian Ocean forcing to the positive global mountain torques. Subsequent dispersions into the TNWP enhancing upper tropospheric divergence led to the current observed flare-up in that region. These same animations suggest RWDs from the TWNP enhanced convection impacting the PNA sector as I type. A response has been for the GSDM Stage 2 response leading to a digging Rocky Mountain trough.
From a somewhat broader perspective, around 20 August we saw a poleward AAM transport signal maximized ~45N only to recently become directed equatorward ~35N. The interactions discussed above were involved in this reversal. When updated, the ESRL/PSD Global Wind Oscillation (GWO) quasi-phase space diagram should show an even larger circuit toward GSDM Stage 2, but still on the left hand side of the plot. Global relative AAM still remains low, ~ minus 2 Atmospheric Momentum Units (AMUs) below the R1 data climatology. Much of that contribution is still coming from the Southern Hemisphere equatorial and subtropical atmospheres. I think this will be the largest GWO circuit since the 2 large events during March and May 2007, having a contribution from non-MJO tropical convective forcing.
Finally, other roughly 20-30 day GWO circuits leading to at times 2 regions of tropical forcing across the Eastern Hemisphere may be probable going into boreal fall. These would likely involve both the Indian and west central Pacific Ocean basins. This is not a new concern on my end for numerous reasons, and any longer-term impacts on our La-Nina situation are unclear. However, monitoring is critical since numerical coupled ensemble prediction schemes are unlikely catch complicated subseasonal behaviors like GWOs until their impacts have already occurred.
For the USA week 1, the models have generally caught the GSDM Stage 2 response of the Rockies and Plains trough. Much of the country will have the first surge of autumn cool air, with the largest negative temperature anomalies generally focused on the Northern Plains and Upper Mississippi Valley. Heavy precipitation is likely to occur across locations such as from the Southern Plains into the eastern states. During weeks 2-3, the east Pacific Ridge-central states trough pattern should loose amplitude, with more troughs from the east Pacific into the western states becoming probable. The GWO should quickly orbit into the GSDM Stage 4-1 phase plane. Another “strong trough” situation may be probable for the Rockies and Plains ~ week 3. Again, predictability can be very low during transition seasons. Stay tuned if much of the western and central USA has a “real winter (assuming the Arctic Ocean freezes back over!)” with perhaps many GSDM Stage 4-1 situations per above.
Internationally, generally from the Indian Ocean into the TNWP periods of intense rainfall are probable week 1, including tropical cyclone activity across the latter. The focus should then shift back into the region of India into Southeast Asia and Southern China at least week 2, with still a tropical cyclone concern for the region around the Philippines. Northern equatorial Africa should stay convectively active for at least the next couple of weeks.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. As discussed a week ago, Gabrielle developed as a hybrid system. I can see a scenario favoring tropical cyclone development perhaps even above climatology across the deep tropics of the Atlantic Ocean week 1 and possibly continuing week 2.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will be on travel during the week of 10-14 September, and I am unclear when my next update will be.
Ed Berry
Monday, September 03, 2007
Short
Since it unlikely I will not be able to write another posting until (hopefully) next weekend (8-9 September), I thought to do a brief update. There is no change to my reasoning I discussed on 1 September. The spatial pattern of global tropical SSTs supports the possibility of a coupled ocean-atmosphere La-Nina response through at least boreal fall into early winter. Hence the continuation of a low AAM GSDM Stage 1 circulation is most probable.
The dominate global tropical convective forcing has loosely consolidated ~10N/80-90E, per monitoring tools such as full disk satellite imagery and various Hovmoller plots. There is some evidence of another east-northeast shift of the tropical forcing toward western Indonesia and Southeast Asia. I think this movement is a bit farther south than the past 3. However, another burst of strong convection is developing back to the west ~0/60E, illustrating the stationary character of our atmospheric base state.
Global relative AAM tendency has spiked to ~plus 30 Hadleys per R1 data plots through 31 August, and much of that appears to be driven by a decent global mountain torque (also ~plus 30 Hadleys). There also appears to be an equatorial contribution to this positive AAM tendency, perhaps linked with a weak west central Pacific tropical convective flare-up. Bottom line is I think we are having another perturbation to our GSDM Stage 1 regime forced by the global mountain torque and the weak eastward movement of the tropical convection discussed above. When updated, this will be observed as another circuit in GWO quasi-phase space well on the left hand side of the ESRL/PSD plot, similar to the event roughly a couple of weeks ago.
Trough development from the Rockies into the Plains for week 1 is a good bet. A closed low may anticyclonically wave break (AWB; probable to occur during a GSDM Stage 1 base state) across the central Rockies later this week before shifting northeast toward the Great Lakes. I still like the notion of a stronger and colder trough for the Rockies and Plains week 2. However, throughout this upcoming fall the polar jet stream westerlies may remain farther north on average than climatology across North America given our GSDM Stage 1 circulation. Western/central USA troughs may be a function of brief upward GWO orbits. By around mid-late November into December the seasonal cycle contribution may allow a more persistent and at times highly amplified negative phase of the Pacific-North American teleconnection (favoring anomalously strong western USA troughs). WB (2007) has already showed that the latter is probable during GSDM Stage 1 situations for the period of November-March. I hope we can formally illustrate these kinds of concepts much more objectively with signal to noise ratio plots and risk assessment maps for a period such as December-February (DJF).
The outlook internationally is unchanged from 1 September. A modification may be for a somewhat more active Tropical Northwest Pacific Ocean in the region of the South China Sea per above later this week into week 2. Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone information. The inability of Hurricane Dean and thus far Hurricane Felix to exhibit much of a northward movement from the deep tropical Atlantic Ocean is a result of the persistent anomalous zonal mean easterly wind flow throughout the tropical atmosphere. I still see quite a bit of AWB activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. In fact, Gabrielle may be the result of a hybrid frontal wave development off the USA east coast per some models.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post an update during the weekend of 8-9 September.
Ed Berry
The dominate global tropical convective forcing has loosely consolidated ~10N/80-90E, per monitoring tools such as full disk satellite imagery and various Hovmoller plots. There is some evidence of another east-northeast shift of the tropical forcing toward western Indonesia and Southeast Asia. I think this movement is a bit farther south than the past 3. However, another burst of strong convection is developing back to the west ~0/60E, illustrating the stationary character of our atmospheric base state.
Global relative AAM tendency has spiked to ~plus 30 Hadleys per R1 data plots through 31 August, and much of that appears to be driven by a decent global mountain torque (also ~plus 30 Hadleys). There also appears to be an equatorial contribution to this positive AAM tendency, perhaps linked with a weak west central Pacific tropical convective flare-up. Bottom line is I think we are having another perturbation to our GSDM Stage 1 regime forced by the global mountain torque and the weak eastward movement of the tropical convection discussed above. When updated, this will be observed as another circuit in GWO quasi-phase space well on the left hand side of the ESRL/PSD plot, similar to the event roughly a couple of weeks ago.
Trough development from the Rockies into the Plains for week 1 is a good bet. A closed low may anticyclonically wave break (AWB; probable to occur during a GSDM Stage 1 base state) across the central Rockies later this week before shifting northeast toward the Great Lakes. I still like the notion of a stronger and colder trough for the Rockies and Plains week 2. However, throughout this upcoming fall the polar jet stream westerlies may remain farther north on average than climatology across North America given our GSDM Stage 1 circulation. Western/central USA troughs may be a function of brief upward GWO orbits. By around mid-late November into December the seasonal cycle contribution may allow a more persistent and at times highly amplified negative phase of the Pacific-North American teleconnection (favoring anomalously strong western USA troughs). WB (2007) has already showed that the latter is probable during GSDM Stage 1 situations for the period of November-March. I hope we can formally illustrate these kinds of concepts much more objectively with signal to noise ratio plots and risk assessment maps for a period such as December-February (DJF).
The outlook internationally is unchanged from 1 September. A modification may be for a somewhat more active Tropical Northwest Pacific Ocean in the region of the South China Sea per above later this week into week 2. Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone information. The inability of Hurricane Dean and thus far Hurricane Felix to exhibit much of a northward movement from the deep tropical Atlantic Ocean is a result of the persistent anomalous zonal mean easterly wind flow throughout the tropical atmosphere. I still see quite a bit of AWB activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. In fact, Gabrielle may be the result of a hybrid frontal wave development off the USA east coast per some models.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post an update during the weekend of 8-9 September.
Ed Berry
Saturday, September 01, 2007
To be, or not to be La-Nina
Per NOAA/CPC SST plots and NOAA/PMEL/TAO buoy data; global tropical SSTs are slightly warmer than average across the Indian Ocean while significantly above normal across much of the west Pacific Ocean warm pool. Anomalies and totals are around plus 0.5-1C and 28-29C for the former and ~plus 1-2C and greater than 30C in region of 5-10N and 150E. Cooler than normal SSTs have become more coherent spatially across the equatorial central and east Pacific east of the date line, with anomalies as low as less than minus 2C ~100-120W extending to depths ~200m. Given that the seasonal cycle favors cooling, these anomalies are not trivial. The Caribbean into the tropical Atlantic Ocean remains near to slightly warmer than climatology.
So, is La-Nina becoming better defined? As stated previously, the global circulation is strongly La-Nina like with global relative AAM currently ~3-4 Atmospheric Momentum Units (AMUs ) below the R1 data climatology while the SSTs have only weakly supported that response. Since about January 2007 Hovmoller plots of OLR/A show a nice signal of tropical convective forcing consolidating across the Eastern Hemisphere ~90-120E. There were 3 weak northeastward propagating subseasonal (~20 to 30-day) variations (weaker in succession) of this tropical forcing since about 1 June. I even speculate that this whole situation started ~ 1 December 2006 when dynamical feedbacks from the atmosphere put the “kibosh” on what was supposed to be a basin-wide warm event. The atmosphere has since been trending toward La-Nina, seemingly followed by the tropical convective response and maybe finally the SSTs. There has been a surge of above normal trades during the last couple of weeks as a response to the Eastern Hemisphere tropical convection helping to cool the SSTs across the Nino regions.
At this point I think La-Nina circulation conditions (GSDM Stage 1) are highly probable through boreal fall into at least early winter (DJF). However, suppression across the west central Pacific Ocean is only going to favor expanding warmth west of the date line. These warm anomalies are also probable to remain very deep, to at least 400m. It will be interesting monitoring the tropical convective forcing this winter as the seasonal cycle makes its contribution. In the meantime, going into the boreal cool season the odds may slightly tilt toward a significant cold regime at times, focusing on the western 2/3rds of North America along with an active southwest flow storm track across the USA Plains.
GSDM Stage 1 continues to reign supreme. Full disk satellite imagery present a nice signal of tropical convective forcing centered ~10N/100E, with some enhancement across north equatorial Africa and flare-ups around the Americas and Tropical Northwest Pacific (on-going tropical cyclones understood). Animations of daily mean upper tropospheric vector wind anomalies show an emerging response of twin tropical/subtropical anticyclones ~90E with compensating cyclones just east of the date line. In fact, the combined distribution of anomalous tropical easterlies from the Indian Ocean into the Africa, westerly wind flow anomalies just east of the date line and OLRA have led to a decent MJO projection per Wheeler and Hendon (2004) phase space plots. A coherent Rossby wave energy dispersion from this tropical forcing and circulation response nicely hooks up with yet another sickening wicked Devil ridge across the western and central USA (even in a weekly mean).
As discussed above, global relative AAM is ~3-4 AMUs below the R1 data climatology. Much of that signal is still coming from strongly anomalous deep zonal mean easterly wind flow across the Southern Hemisphere tropical atmosphere with, for instance, ~5-10m/s magnitudes at 200mb. The last subseasonal event of tropical forcing did allow some of the anomalous zonal mean easterly wind flow to propagate off the equator into the northern subtropics to ~30N. In fact, even weak zonal mean westerly wind anomalies have appeared ~15N. I think what the RMM plots are showing as a MJO is in reality another 20-30-day subseasonal variation similar to the past 3. As of 30 August the global mountain torque has spiked to ~plus 30 Hadleys contributing to the plus 20 Hadley AAM tendency. I think there will be another orbit in GWO quasi-phase space just like that seen about 10-15 days ago. In other words, another perturbation onto the GSDM Stage 1 base state is probable during the next couple weeks.
For the Pacific-North American sector the above reasoning suggests an eastward shift of the coherent Rossby wave train already discussed. My own feeling would be for at least a couple of troughs to dig into the western USA, with the strongest during week 2. This pattern should then shift east and decay in amplitude, particularly in the presence of seasonal transition to fall. Hence a trend toward cooler and wetter is probable for the west and central USA, perhaps targeting the northern and central Plains week 2. Numerical ensemble prediction schemes from international weather centers are coming into better agreement on this notion, which was first suggested in my posting about a week ago for this forecast period. It is likely deep tropical moisture transport from both the east Pacific and Caribbean will be involved with these precipitation events. Locations around Alaska are going to be “close” to the amplifying ridge ~140-160W. However, storms coming off of East Asia are probable to impact that state weeks 2-3.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. I still see quite a bit of anticyclonic wave breaking activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. However, easterly wave activity is quite robust over Africa tied to an anomalously strong Tropical Easterly Jet, and there may be a couple of opportunities for hybrid systems to develop close to the USA during the next 2-3 weeks. In fact, what could become Gabrielle (Xena’s sidekick?) during the next few days may be currently festering midway between Africa and the Lesser Antilles.
Locations from the central and eastern equatorial Indian Ocean and India into China and Southeast Asia still appear likely to continue with the poundings of intense monsoon/frontal band rainfall this week into week 2. Perhaps the upcoming subseasonal event will be the last northward shift for this boreal summer. With the exception of Typhoon Fitow, much of the west central and northwest Pacific should have generally suppressed conditions for at least week 1. Equatorial North Africa should remain active for at least week 1 and perhaps a few more.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post a short update during roughly the middle of next week.
Ed Berry
So, is La-Nina becoming better defined? As stated previously, the global circulation is strongly La-Nina like with global relative AAM currently ~3-4 Atmospheric Momentum Units (AMUs ) below the R1 data climatology while the SSTs have only weakly supported that response. Since about January 2007 Hovmoller plots of OLR/A show a nice signal of tropical convective forcing consolidating across the Eastern Hemisphere ~90-120E. There were 3 weak northeastward propagating subseasonal (~20 to 30-day) variations (weaker in succession) of this tropical forcing since about 1 June. I even speculate that this whole situation started ~ 1 December 2006 when dynamical feedbacks from the atmosphere put the “kibosh” on what was supposed to be a basin-wide warm event. The atmosphere has since been trending toward La-Nina, seemingly followed by the tropical convective response and maybe finally the SSTs. There has been a surge of above normal trades during the last couple of weeks as a response to the Eastern Hemisphere tropical convection helping to cool the SSTs across the Nino regions.
At this point I think La-Nina circulation conditions (GSDM Stage 1) are highly probable through boreal fall into at least early winter (DJF). However, suppression across the west central Pacific Ocean is only going to favor expanding warmth west of the date line. These warm anomalies are also probable to remain very deep, to at least 400m. It will be interesting monitoring the tropical convective forcing this winter as the seasonal cycle makes its contribution. In the meantime, going into the boreal cool season the odds may slightly tilt toward a significant cold regime at times, focusing on the western 2/3rds of North America along with an active southwest flow storm track across the USA Plains.
GSDM Stage 1 continues to reign supreme. Full disk satellite imagery present a nice signal of tropical convective forcing centered ~10N/100E, with some enhancement across north equatorial Africa and flare-ups around the Americas and Tropical Northwest Pacific (on-going tropical cyclones understood). Animations of daily mean upper tropospheric vector wind anomalies show an emerging response of twin tropical/subtropical anticyclones ~90E with compensating cyclones just east of the date line. In fact, the combined distribution of anomalous tropical easterlies from the Indian Ocean into the Africa, westerly wind flow anomalies just east of the date line and OLRA have led to a decent MJO projection per Wheeler and Hendon (2004) phase space plots. A coherent Rossby wave energy dispersion from this tropical forcing and circulation response nicely hooks up with yet another sickening wicked Devil ridge across the western and central USA (even in a weekly mean).
As discussed above, global relative AAM is ~3-4 AMUs below the R1 data climatology. Much of that signal is still coming from strongly anomalous deep zonal mean easterly wind flow across the Southern Hemisphere tropical atmosphere with, for instance, ~5-10m/s magnitudes at 200mb. The last subseasonal event of tropical forcing did allow some of the anomalous zonal mean easterly wind flow to propagate off the equator into the northern subtropics to ~30N. In fact, even weak zonal mean westerly wind anomalies have appeared ~15N. I think what the RMM plots are showing as a MJO is in reality another 20-30-day subseasonal variation similar to the past 3. As of 30 August the global mountain torque has spiked to ~plus 30 Hadleys contributing to the plus 20 Hadley AAM tendency. I think there will be another orbit in GWO quasi-phase space just like that seen about 10-15 days ago. In other words, another perturbation onto the GSDM Stage 1 base state is probable during the next couple weeks.
For the Pacific-North American sector the above reasoning suggests an eastward shift of the coherent Rossby wave train already discussed. My own feeling would be for at least a couple of troughs to dig into the western USA, with the strongest during week 2. This pattern should then shift east and decay in amplitude, particularly in the presence of seasonal transition to fall. Hence a trend toward cooler and wetter is probable for the west and central USA, perhaps targeting the northern and central Plains week 2. Numerical ensemble prediction schemes from international weather centers are coming into better agreement on this notion, which was first suggested in my posting about a week ago for this forecast period. It is likely deep tropical moisture transport from both the east Pacific and Caribbean will be involved with these precipitation events. Locations around Alaska are going to be “close” to the amplifying ridge ~140-160W. However, storms coming off of East Asia are probable to impact that state weeks 2-3.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. I still see quite a bit of anticyclonic wave breaking activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. However, easterly wave activity is quite robust over Africa tied to an anomalously strong Tropical Easterly Jet, and there may be a couple of opportunities for hybrid systems to develop close to the USA during the next 2-3 weeks. In fact, what could become Gabrielle (Xena’s sidekick?) during the next few days may be currently festering midway between Africa and the Lesser Antilles.
Locations from the central and eastern equatorial Indian Ocean and India into China and Southeast Asia still appear likely to continue with the poundings of intense monsoon/frontal band rainfall this week into week 2. Perhaps the upcoming subseasonal event will be the last northward shift for this boreal summer. With the exception of Typhoon Fitow, much of the west central and northwest Pacific should have generally suppressed conditions for at least week 1. Equatorial North Africa should remain active for at least week 1 and perhaps a few more.
Appendix
An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at
http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg
http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg
http://www.cdc.noaa.gov/map/clim/gsdm.shtml
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post a short update during roughly the middle of next week.
Ed Berry
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