I want to use this posting as a “precedent” to shortening (still need to work on that!) these discussions given time issues. Interested readers should know where to look on the web to examine details such as, for example, SSTs. There may also be “obvious features” and issues that will be skipped over.
Basin wide (within 5 deg of the equator) moderate La-Nina SST anomalies remain having magnitudes colder than minus 2C near the equator at ~125W per TAO buoy data. The west central Pacific Ocean still has totals in excess of 30C along the equator at 140E slowly shifting south with the annual cycle. Weak warm anomalies remain across the Indian Ocean while the Atlantic basin is still warmer than climatology.
Even though it is probable for this moderate cold event to persist at least through December 2007, I am unclear afterwards. Understanding the seasonal cycle, there is actually weak subsurface warming occurring to at least 150W with anomalies ~plus 1C/150m. This may be a response to equatorial westerly wind events in the region of Indonesia to the date line late September and again about 10 days ago (anybody want to call this an oceanic Kelvin wave?). Trades are again intensifying around the date line in response to tropical convective forcing increasing back to the west. Stay tuned.
Satellite imagery and other diagnostic monitoring tools do give a signal of tropical convective forcing intensifying and organizing in the region the equatorial central Indian Ocean into northern Indonesia. Per animations of upper tropospheric velocity potential fields, the Western Hemisphere dynamical signal is propagating into Africa and the Indian Ocean, enhancing tropical forcing across those regions. The latter was forced by interactions with the extratropics.
Animations of upper tropospheric daily mean vector wind anomalies present an improving signal of twin tropical/subtropical anticyclones ~60-100E with cyclones near 170E. In fact, equatorial westerly wind anomalies having magnitudes ~25m/s have responded over the Western Hemisphere Pacific Ocean. Finally, there are lower level twin cyclonic circulation anomalies ~60-90E which may develop into tropical cyclones. In any case, at least a weak MJO may be in the early stages of formation given the baroclinic structure. The WH (2004) RMM phase space plots present ~1 standard deviation MJO signal across the Eastern Hemisphere with no clear sense of what it will do. My own feeling is we will see at least a truncated weak MJO during the next few weeks, having eastward propagation to ~140-150E where tropical SSTs are the warmest. This may be similar to what happened late September, except that situation not a “true” MJO.
Through 24 October, global relative AAM has dipped to ~minus 2.5-3 standard deviations (approximately AMUs) below the R1 data climatology, after our positive global mountain torque (mostly East Asian; ~25 Hadleys) roughly 10-14 days ago. The Global Wind Oscillation (GWO) which takes into consideration non-oscillatory extratropical dynamics linked to the surface torques and AAM transports, has orbited to ~minus 1.5-2 standard deviations into the phase plane of GSDM Stage 4-1 through 22 October. Zonal mean easterly wind flow anomalies (~5-10m/s at 200mb) have re-intensified throughout the global subtropics. In summary, ocean-atmosphere coupling to La-Nina appears to be once again strengthening, including a poleward AAM transport signal, with perhaps a MJO perturbation.
For the PNA sector, predictability has been VERY LOW for both myself and the numerical models during the last couple weeks. In retrospect, speculation suggests a mountain torque enhanced North Pacific jet (which we had) collapsing in a base state favoring anticyclonic wave breaking (AWB) may not be very predictable even at "short" lead times in regard to synoptic details, particularly during boreal fall. In fact, the above sequence of events greatly contributed to the severity of the wild fires across California. I do think a “classic” GSDM Stage 1 cold season response meaning tilting the odds toward an anomalous western USA trough (with synoptic variations) is in the near future. In the meantime, seasonal strengthening of the westerlies has finally led to a relatively tranquil weather pattern across much of the USA.
Going into week 2, I do have a concern the trough that most models suggest to come into the western USA will be slower and deeper, with even another one of those __________ AWB closed lows. In fact, these kinds of synoptic situations may be probable this cold season, as the southwest flow storm track presumably becomes established across the central states.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. There is evidence at this time a tropical depression may be forming across the southeast Caribbean. Locations including the Arabian Sea, Bay of Bengal and even the South Indian Ocean may need to deal with at least 1 tropical cyclone week 1. The Northwest Pacific Ocean around the Philippines may be a concern for tropical cyclone development through at least week 2. While dryness returns to tropical South America for at least week 1, portions of equatorial Africa may remain quite wet. Will there be another “Gabrielle” across the North Atlantic Ocean sometime soon?
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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 late next week ~ 2-4 November before travel the following week.
Ed Berry
Saturday, October 27, 2007
Thursday, October 18, 2007
Major Delay
Travel and other obligations preclude me from comfortably writing a complete posting useful to others. In fact, much of the latter is strongly linked to an effort to implement “operationally” at some time (hopefully reasonably soon) the objective and quantification work discussed in the Appendix. I will try to do a complete discussion during the period of 28 October-3 November.
Global relative AAM remains very low, ~2.5 AMUs per R1 data through 15 October, and a coupled GSDM Stage 1 base state to La-Nina persists. In the zonal mean anomalous easterly wind flow continues throughout the subtropical atmospheres of both hemispheres, having weekly mean magnitudes of ~3-6m/s. I think it is likely for this GSDM Stage 1 regime to continue (at least on average) through the rest of 2007, possibly longer. Hence our circulation state tilts the odds toward life cycle-1 anticyclonic wave breaking baroclinic development, including the next trough to impact the western USA (addressed below). As discussed in past postings, we need to monitor tropical convective forcing across the west central Pacific Ocean.
Regionally for the PNA sector, the positive East Asian mountain torque (~20 Hadleys) enhanced North Pacific Ocean jet is collapsing as I type. One major synoptic cyclonic baroclinic development event is currently in progress across the Upper Mississippi Valley, and I definitely favor the notion of a second development across the Southern Rockies and Plains by the end of this upcoming weekend. Models have been struggling with this second storm, and the slower and deeper solution is most probable. After a period of respite ~later week 1 into week 2 from active weather across the USA, I think this type of situation is again more likely than climatology by ~ week 3 (1-8 November).
I will be monitoring to see if a more robust western USA trough-southeast states ridge pattern becomes established by later November into December (with synoptic variations). Ramifications for particularly the central USA should be straight forward (to meteorologists) especially if there exists an Arctic cold air source and decent tropical moisture transport from the Gulf of Mexico. Translation is a stormy pattern favoring possibly significant/extreme winter weather from the Rockies into the Northern/Central Plains with heavy rainfall and thunderstorms focusing on the Ohio Valley.
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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 October-3 November.
Ed Berry
Saturday, October 13, 2007
Sea of Noise
There has not much been much change to the spatial distribution of global tropical SSTs since my 8 October posting. A basin wide cold event appears to be in the process of maturing across the equatorial Pacific Ocean basin (165E -South America) with anomalies ~minus 3C from 120-140W extending to around 200m deep. The warm Indian Ocean -cool Indonesia -warm west central Pacific Ocean pattern of Eastern Hemisphere SSTAs persists with totals in excess of 30C ~0-10N/150-160E (merging with the warm Pacific Ocean horseshoe). Other SST details can be found from the links below in the Appendix and my previous posting.
The gist is that the global circulation remains strongly coupled to La-Nina in terms of circulation response. Given the complex dynamics of forcing-response-feedback global circulation-tropical convection evolutions since around 1 December 2006, perhaps it is the SSTs that have coupled to the atmosphere particularly starting around 1 September 2007. Reiterating, I did not think we would see a basin wide cold event a couple of months ago. In any case, this coupling has been and continues to modulate synoptic variability globally including the USA. From the GSDM (and more generally subseasonal) framework the latter is scientifically defensible; I strongly disagree with statements to the contrary from anywhere. The latter are from a perspective of, for example, 3-month composites of temperature and precipitation anomalies which will average out important subseasonal weather events.
Per ESRL/PSD plots, global relative atmospheric angular momentum (AAM) has decreased during the past week to roughly 3 AMUs below the R1 data climatology. In addition to the well established anomalously strong zonal mean easterly wind flow throughout the subtropical atmospheres (~25N and 25S with magnitudes ~10m/s at 200mb), anomalous easterly wind flow is developing both in the tropics and higher latitudes. The former is tied to the tropical convective forcing and the latter is a response to extremely intense midlatitude anticyclones (anomalies in excess of 40m/s at times) amplifying into the global Polar Regions. In fact, a rare sudden stratospheric warming of the austral (spring) stratosphere may be in progress as a response (also discussed 8 October). Both AAM tendency and the surface torques have become slightly positive; however, the Global Wind Oscillation (GWO) remains solidly in GSDM Stage 1.
There has been a significant subseasonal perturbation onto our GSDM Stage 1 La-Nina basic state during the past week. As seen from animations of daily mean upper tropospheric vector wind anomalies and time-longitude sections of velocity potential (divergent part of the total velocity field), persistent Rossby wave energy dispersions arcing from the twin subtropical anticyclones ~120E through both hemispheres led to an increase of divergence centered around the Americas. Strong convection (OLRA ~ minus 50-90 W/m**2) developed from the East Pacific ITCZ into the Caribbean and the Amazons of South America. Weak anomalous twin subtropical anticyclones have formed as a response ~60-90W leading to Western Hemisphere equatorial easterlies.
Current full disk satellite imagery shows tropical convection enhanced around the Americas. There is also renewed tropical forcing around 0/80E, with strong suppression across Indonesia into the west Pacific. A week ago I thought there was some observational evidence that a MJO may be in the early stages of development. This notion is still not unreasonable. However, I did not think a MJO might develop as the result of an extratropical feedback forcing a Western Hemisphere tropical convective dynamical signal, currently ~60-70W. These dynamical signals typically propagate ~15m/s (roughly 12 deg long/day) when they are in the Western Hemisphere. Simple extrapolation would put this signal solidly across the Indian Ocean in roughly 8-10 days.
Even though we are in a stationary GSDM Stage 1 base state, my confidence for any week 1-3 forecast across the PNA sector is very low. In addition to getting “surprised” (RMM WH2004 phase space plots did suggest this possibility; these kind of chain of events have occurred in the past such as December 2005), seasonal transition into boreal cold season is also adding difficulty. It also seems like any predictive information I have offered lately has been poor. This only confirms once again we are dealing with a dynamical system characterized by a sea of noise that does not like to be predicted. Tools such as the GSDM, GWO quasi-phase space plot and risk assessment plots of subseasonal variability, along with rigorous daily monitoring, help to facilitate “forecasts of opportunity (strong coherent signals above the background noise)” especially when numerical and other statistical tools are also struggling. Like it or not, that is the way it is (the truth)!!
My feeling is to expect a reinvigoration of the tropical convective forcing across the equatorial Eastern Hemisphere from ~60-120E by the end of week 2. The La-Nina enhanced westerlies ~40N have been coming south, regionally recently slamming the USA west coast with several strong troughs. If the Indian Ocean convection becomes quite robust, and that is probable given our base state (which may further strengthen La-Nina), the North Pacific jet would be expected to collapse. In fact, a full-latitude ridge into Alaska may develop initially along the west coast then retrograde to ~140-150W by week 3.
For the USA, this suggests about 1 more week of an unseasonably active weather pattern followed by quieter conditions week 2 (models suggest this). If this were about a month or so later I would be expressing a concern for a cold/stormy regime (with Arctic air) across the western half of the country starting week 3. Should a decent MJO develop across the Indian Ocean along with the GWO, a “classic” GSDM Stage 1 cold season regime for North America may start to evolve by that time (~ 28 October-4 November) in spite of the seasonal cycle. My speculation for this upcoming boreal winter season stated in past discussions remains the same.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking reigns supreme across the North Atlantic. The spirit of “Gabrielle” may still live.
Heavy rainfall may become a concern for portions of equatorial Africa week 1 then into the Indian Ocean weeks 2-3 per above. Tropical cyclone hazards may actually become more probable for the South Indian Ocean (one already did try) than Bay of Bengal weeks 2-3. The west central Pacific will always be the wild card. One would expect general suppression from Indonesia into the west Pacific Ocean weeks 1-2, with activity increasing especially along the equator by week 3. However, who really knows the timing at this point?
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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 ~19-20 October.
Ed Berry
The gist is that the global circulation remains strongly coupled to La-Nina in terms of circulation response. Given the complex dynamics of forcing-response-feedback global circulation-tropical convection evolutions since around 1 December 2006, perhaps it is the SSTs that have coupled to the atmosphere particularly starting around 1 September 2007. Reiterating, I did not think we would see a basin wide cold event a couple of months ago. In any case, this coupling has been and continues to modulate synoptic variability globally including the USA. From the GSDM (and more generally subseasonal) framework the latter is scientifically defensible; I strongly disagree with statements to the contrary from anywhere. The latter are from a perspective of, for example, 3-month composites of temperature and precipitation anomalies which will average out important subseasonal weather events.
Per ESRL/PSD plots, global relative atmospheric angular momentum (AAM) has decreased during the past week to roughly 3 AMUs below the R1 data climatology. In addition to the well established anomalously strong zonal mean easterly wind flow throughout the subtropical atmospheres (~25N and 25S with magnitudes ~10m/s at 200mb), anomalous easterly wind flow is developing both in the tropics and higher latitudes. The former is tied to the tropical convective forcing and the latter is a response to extremely intense midlatitude anticyclones (anomalies in excess of 40m/s at times) amplifying into the global Polar Regions. In fact, a rare sudden stratospheric warming of the austral (spring) stratosphere may be in progress as a response (also discussed 8 October). Both AAM tendency and the surface torques have become slightly positive; however, the Global Wind Oscillation (GWO) remains solidly in GSDM Stage 1.
There has been a significant subseasonal perturbation onto our GSDM Stage 1 La-Nina basic state during the past week. As seen from animations of daily mean upper tropospheric vector wind anomalies and time-longitude sections of velocity potential (divergent part of the total velocity field), persistent Rossby wave energy dispersions arcing from the twin subtropical anticyclones ~120E through both hemispheres led to an increase of divergence centered around the Americas. Strong convection (OLRA ~ minus 50-90 W/m**2) developed from the East Pacific ITCZ into the Caribbean and the Amazons of South America. Weak anomalous twin subtropical anticyclones have formed as a response ~60-90W leading to Western Hemisphere equatorial easterlies.
Current full disk satellite imagery shows tropical convection enhanced around the Americas. There is also renewed tropical forcing around 0/80E, with strong suppression across Indonesia into the west Pacific. A week ago I thought there was some observational evidence that a MJO may be in the early stages of development. This notion is still not unreasonable. However, I did not think a MJO might develop as the result of an extratropical feedback forcing a Western Hemisphere tropical convective dynamical signal, currently ~60-70W. These dynamical signals typically propagate ~15m/s (roughly 12 deg long/day) when they are in the Western Hemisphere. Simple extrapolation would put this signal solidly across the Indian Ocean in roughly 8-10 days.
Even though we are in a stationary GSDM Stage 1 base state, my confidence for any week 1-3 forecast across the PNA sector is very low. In addition to getting “surprised” (RMM WH2004 phase space plots did suggest this possibility; these kind of chain of events have occurred in the past such as December 2005), seasonal transition into boreal cold season is also adding difficulty. It also seems like any predictive information I have offered lately has been poor. This only confirms once again we are dealing with a dynamical system characterized by a sea of noise that does not like to be predicted. Tools such as the GSDM, GWO quasi-phase space plot and risk assessment plots of subseasonal variability, along with rigorous daily monitoring, help to facilitate “forecasts of opportunity (strong coherent signals above the background noise)” especially when numerical and other statistical tools are also struggling. Like it or not, that is the way it is (the truth)!!
My feeling is to expect a reinvigoration of the tropical convective forcing across the equatorial Eastern Hemisphere from ~60-120E by the end of week 2. The La-Nina enhanced westerlies ~40N have been coming south, regionally recently slamming the USA west coast with several strong troughs. If the Indian Ocean convection becomes quite robust, and that is probable given our base state (which may further strengthen La-Nina), the North Pacific jet would be expected to collapse. In fact, a full-latitude ridge into Alaska may develop initially along the west coast then retrograde to ~140-150W by week 3.
For the USA, this suggests about 1 more week of an unseasonably active weather pattern followed by quieter conditions week 2 (models suggest this). If this were about a month or so later I would be expressing a concern for a cold/stormy regime (with Arctic air) across the western half of the country starting week 3. Should a decent MJO develop across the Indian Ocean along with the GWO, a “classic” GSDM Stage 1 cold season regime for North America may start to evolve by that time (~ 28 October-4 November) in spite of the seasonal cycle. My speculation for this upcoming boreal winter season stated in past discussions remains the same.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking reigns supreme across the North Atlantic. The spirit of “Gabrielle” may still live.
Heavy rainfall may become a concern for portions of equatorial Africa week 1 then into the Indian Ocean weeks 2-3 per above. Tropical cyclone hazards may actually become more probable for the South Indian Ocean (one already did try) than Bay of Bengal weeks 2-3. The west central Pacific will always be the wild card. One would expect general suppression from Indonesia into the west Pacific Ocean weeks 1-2, with activity increasing especially along the equator by week 3. However, who really knows the timing at this point?
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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 ~19-20 October.
Ed Berry
Monday, October 08, 2007
Coupled
Across the tropical Indo-Pacific region the spatial distribution of SSTs clearly represents a basin wide cold event meaning negative anomalies in all Nino regions. TAO buoy recent five-day averaged anomaly magnitudes are at least minus 2.5C ~0/120W with totals colder than 20C off the west coast of South America. These negative anomalies (~minus 4C at 120W/100m) extend to roughly 200m depth along the cold tongue east of the date line. Please read the latest CPC ENSO diagnostics discussion and the MEI discussion from ESRL/PSD for further details (links given in Appendix).
The west central Pacific Ocean remains warmer than average with positive anomalies ~plus 1-2C and totals in excess of 30C. The warmest waters were from about 0/150E extending southeast into the region of 18S/180. Overall the warm spatial horseshoe pattern of SSTs extending into the extratropics remains well pronounced. The latter is also a characteristic of a La-Nina. However, I think the magnitudes and spatial coverage of the positive anomalies particularly across the Southern Hemisphere (greater than 1C) are impressive. These anomalously warm waters of the west central Pacific Ocean are slowly drifting south with the seasonal cycle.
Similar to a year ago, the spatial pattern of an anomalously warm equatorial Indian Ocean, cool around Indonesia then very warm west central Pacific has also become very well defined during the past 4-6 weeks. I think some of the coolness around Indonesia is a response to enhanced southeasterly surface winds from the Southern Hemisphere responding to Indian Ocean convection, along with cold air surges from the southern extratropics. Impacts onto La-Nina from this “Indian Ocean dipole” are unclear. Around 1 December 2006 dominate Indian Ocean tropical forcing “stopped El-Nino in its tracks”. Stay tuned. The tropical Atlantic Ocean extending into the Caribbean remains warmer than normal. In fact, anomaly magnitudes across the central and eastern tropical Atlantic Ocean have increased to ~plus 1C with totals of at least 27-28C over the past few weeks.
Obviously I went into greater length in regard to global tropical SSTs than I intended. However, other ocean basins also impact the global circulation besides Nino 3.4 (as supported by several modeling studies). For instance, is the recent warming across the Atlantic the start of an “Atlantic El-Nino” for those waters???
The global circulation has been strongly coupled to La-Nina since about 1 September in terms of SST forcing, tropical convection and circulation response, stuck in GSDM Stage 1. Around that time there was poleward propagation of zonal mean anomalous easterly wind flow from the equatorial into the subtropical atmospheres of both hemispheres. Anomaly magnitudes have been ~5-10m/s at 200mb located zonally ~30N and 20S. These easterlies have supported poleward shifted and at times very intense subtropical anticyclones (having enhanced westerly wind flow on their poleward sides). Global relative atmospheric angular momentum (AAM) has persisted around 2 atmospheric momentum units (AMUs) below the R1 data climatology, and there has been little variation of the Global Wind Oscillation (GWO). Broadly there has been poleward AAM transport ~40N and 30S with weak forcing from the surface torques.
Full disk satellite imagery and other diagnostic monitoring tools present a strong signal of tropical convective forcing developing back to the west across the equatorial Indian Ocean. Three-day averaged OLR/A suggest this convection to be centered ~5N/75E, with anomaly magnitudes ~minus 50-90W/m**2. Strong suppression continues across Indonesia and the west central/northwest Pacific convection has weakened considerably. Per animations of daily mean upper tropospheric vector wind anomalies the persistent Eastern Hemisphere twin subtropical anticyclones have shifted west as a response to ~110E. Rossby wave energy dispersions from these anticyclones are starting a slight retrogression of circulation anomalies across the PNA sector as I type.
Now comes the punch line to all this, at least for today. In past postings I have mentioned there have been ~4 subseasonal variations of the Eastern Hemisphere tropical convective forcing since early June 2007. I think the Indian Ocean forcing is the initiation of another event, perhaps even a weak MJO (yes, you read it here first!). I base this speculation on seasonal cycle considerations (closer to the equator) and the very warm waters of the west central Pacific Ocean. However, if a MJO develops it is highly probable to be truncated to the Eastern Hemisphere and any enhanced rainfall may stay north of Indonesia. Finally, I will again repeat my past concerns of 2 regions of Eastern Hemisphere tropical forcing (Indian Ocean and west central Pacific) and the possibility for the west Pacific Ocean convection becoming the dominate area.
Finally, unlike the past several weeks a GWO signal may also be appearing as a response to the development of Indian Ocean forcing. Per ESRL/PSD R1 AAM plots updated through 6 October global tendency has dipped ~minus 30 Hadleys having contributions from the surface torques and possible eddy feedbacks from the extratropics (large anticyclones). In fact, per Tokyo Climate Center a rare sudden stratospheric warming (SSW) of the austral stratosphere may be in progress given very strong upward EP fluxes and 10-hPa temperatures recently soaring to minus 10C. I discussed this notion in my 21 September posting. The point is events like a SSW are linked to the dynamics involving atmosphere-ocean coupling and subseasonal variability.
At this time anomalous zonal mean westerly wind flow is present ~45-50N (~5-10m/s at 200mb) due to seasonal transition and La-Nina feedbacks. Focusing on the PNA sector, as the Eastern Hemisphere tropical forcing moves into the warm west central Pacific over the next few weeks (sooner??), it is probable for the western USA to get more strong full latitude troughs starting sometime week 2. Yes, many models are suggesting one or two weak wave breaking residual lows to move into the Rockies and Plains before than. Most week-2 ensemble means from global centers suggest a trough off the USA west coast-central states ridge with a trough around the east coast. I think these forecasts will not perform well. In fact, during ~week 3 full-latitude ridging around 140W into Alaska with strong anticylonically wave breaking lows across the southwest states may be probable should a brief GWO circuit toward Stage 2 occur. However, timing of circulation variations remains unclear particularly this time of year.
Much of the USA west coast is probable to be impacted with several troughs for at least the next couple of weeks. Yes, strong North Pacific jets leading to baroclinically amplifying west coast troughs can also happen during La-Nina years. Alaska should remain north of the storm track. The Rockies and Plains should become active with perhaps a couple of strong (unlike the earlier weak systems week 1) baroclinic synoptic-scale storms during week 2 possibly continuing into week 3. During week 3 some Arctic air (for this time of year) may become involved leading to snow events across the northern High Plains. Heavy precipitation and even autumn severe local storms may again focus on the central and northern Mississippi Valley into the Ohio Valley.
Western USA troughs may become stronger and more persistent, leading to an active southwest flow storm track across the Plains, as we progress through the colder part of boreal seasonal cycle. Ramifications should be understood. Given the recently observed pronounced tendency for wave breaking and hence slower moving lows across the southern Rockies, perhaps this particular La-Nina base will support some wetness across the Desert Southwest and southern High Plains going into winter. Even though there are some indications for a moderate La-Nina, I stress the importance of monitoring the west central equatorial and eventually the Tropical Southwest Pacific Ocean for persistent anomalously strong convection particularly starting later December into early 2008.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking still rules across the North Atlantic.
The west central into the northwest Pacific Ocean is probable to have a respite from typhoons and heavy rainfall week 1, with activity increasing weeks 2-3. Impact areas still include the Philippines and Southeast Asia. Another area of concern for tropical cyclones is the Bay of Bengal including coastal sections, especially if the convection moves east. 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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 for subseasonal variability). 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 ~13-14 October.
Ed Berry
The west central Pacific Ocean remains warmer than average with positive anomalies ~plus 1-2C and totals in excess of 30C. The warmest waters were from about 0/150E extending southeast into the region of 18S/180. Overall the warm spatial horseshoe pattern of SSTs extending into the extratropics remains well pronounced. The latter is also a characteristic of a La-Nina. However, I think the magnitudes and spatial coverage of the positive anomalies particularly across the Southern Hemisphere (greater than 1C) are impressive. These anomalously warm waters of the west central Pacific Ocean are slowly drifting south with the seasonal cycle.
Similar to a year ago, the spatial pattern of an anomalously warm equatorial Indian Ocean, cool around Indonesia then very warm west central Pacific has also become very well defined during the past 4-6 weeks. I think some of the coolness around Indonesia is a response to enhanced southeasterly surface winds from the Southern Hemisphere responding to Indian Ocean convection, along with cold air surges from the southern extratropics. Impacts onto La-Nina from this “Indian Ocean dipole” are unclear. Around 1 December 2006 dominate Indian Ocean tropical forcing “stopped El-Nino in its tracks”. Stay tuned. The tropical Atlantic Ocean extending into the Caribbean remains warmer than normal. In fact, anomaly magnitudes across the central and eastern tropical Atlantic Ocean have increased to ~plus 1C with totals of at least 27-28C over the past few weeks.
Obviously I went into greater length in regard to global tropical SSTs than I intended. However, other ocean basins also impact the global circulation besides Nino 3.4 (as supported by several modeling studies). For instance, is the recent warming across the Atlantic the start of an “Atlantic El-Nino” for those waters???
The global circulation has been strongly coupled to La-Nina since about 1 September in terms of SST forcing, tropical convection and circulation response, stuck in GSDM Stage 1. Around that time there was poleward propagation of zonal mean anomalous easterly wind flow from the equatorial into the subtropical atmospheres of both hemispheres. Anomaly magnitudes have been ~5-10m/s at 200mb located zonally ~30N and 20S. These easterlies have supported poleward shifted and at times very intense subtropical anticyclones (having enhanced westerly wind flow on their poleward sides). Global relative atmospheric angular momentum (AAM) has persisted around 2 atmospheric momentum units (AMUs) below the R1 data climatology, and there has been little variation of the Global Wind Oscillation (GWO). Broadly there has been poleward AAM transport ~40N and 30S with weak forcing from the surface torques.
Full disk satellite imagery and other diagnostic monitoring tools present a strong signal of tropical convective forcing developing back to the west across the equatorial Indian Ocean. Three-day averaged OLR/A suggest this convection to be centered ~5N/75E, with anomaly magnitudes ~minus 50-90W/m**2. Strong suppression continues across Indonesia and the west central/northwest Pacific convection has weakened considerably. Per animations of daily mean upper tropospheric vector wind anomalies the persistent Eastern Hemisphere twin subtropical anticyclones have shifted west as a response to ~110E. Rossby wave energy dispersions from these anticyclones are starting a slight retrogression of circulation anomalies across the PNA sector as I type.
Now comes the punch line to all this, at least for today. In past postings I have mentioned there have been ~4 subseasonal variations of the Eastern Hemisphere tropical convective forcing since early June 2007. I think the Indian Ocean forcing is the initiation of another event, perhaps even a weak MJO (yes, you read it here first!). I base this speculation on seasonal cycle considerations (closer to the equator) and the very warm waters of the west central Pacific Ocean. However, if a MJO develops it is highly probable to be truncated to the Eastern Hemisphere and any enhanced rainfall may stay north of Indonesia. Finally, I will again repeat my past concerns of 2 regions of Eastern Hemisphere tropical forcing (Indian Ocean and west central Pacific) and the possibility for the west Pacific Ocean convection becoming the dominate area.
Finally, unlike the past several weeks a GWO signal may also be appearing as a response to the development of Indian Ocean forcing. Per ESRL/PSD R1 AAM plots updated through 6 October global tendency has dipped ~minus 30 Hadleys having contributions from the surface torques and possible eddy feedbacks from the extratropics (large anticyclones). In fact, per Tokyo Climate Center a rare sudden stratospheric warming (SSW) of the austral stratosphere may be in progress given very strong upward EP fluxes and 10-hPa temperatures recently soaring to minus 10C. I discussed this notion in my 21 September posting. The point is events like a SSW are linked to the dynamics involving atmosphere-ocean coupling and subseasonal variability.
At this time anomalous zonal mean westerly wind flow is present ~45-50N (~5-10m/s at 200mb) due to seasonal transition and La-Nina feedbacks. Focusing on the PNA sector, as the Eastern Hemisphere tropical forcing moves into the warm west central Pacific over the next few weeks (sooner??), it is probable for the western USA to get more strong full latitude troughs starting sometime week 2. Yes, many models are suggesting one or two weak wave breaking residual lows to move into the Rockies and Plains before than. Most week-2 ensemble means from global centers suggest a trough off the USA west coast-central states ridge with a trough around the east coast. I think these forecasts will not perform well. In fact, during ~week 3 full-latitude ridging around 140W into Alaska with strong anticylonically wave breaking lows across the southwest states may be probable should a brief GWO circuit toward Stage 2 occur. However, timing of circulation variations remains unclear particularly this time of year.
Much of the USA west coast is probable to be impacted with several troughs for at least the next couple of weeks. Yes, strong North Pacific jets leading to baroclinically amplifying west coast troughs can also happen during La-Nina years. Alaska should remain north of the storm track. The Rockies and Plains should become active with perhaps a couple of strong (unlike the earlier weak systems week 1) baroclinic synoptic-scale storms during week 2 possibly continuing into week 3. During week 3 some Arctic air (for this time of year) may become involved leading to snow events across the northern High Plains. Heavy precipitation and even autumn severe local storms may again focus on the central and northern Mississippi Valley into the Ohio Valley.
Western USA troughs may become stronger and more persistent, leading to an active southwest flow storm track across the Plains, as we progress through the colder part of boreal seasonal cycle. Ramifications should be understood. Given the recently observed pronounced tendency for wave breaking and hence slower moving lows across the southern Rockies, perhaps this particular La-Nina base will support some wetness across the Desert Southwest and southern High Plains going into winter. Even though there are some indications for a moderate La-Nina, I stress the importance of monitoring the west central equatorial and eventually the Tropical Southwest Pacific Ocean for persistent anomalously strong convection particularly starting later December into early 2008.
Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking still rules across the North Atlantic.
The west central into the northwest Pacific Ocean is probable to have a respite from typhoons and heavy rainfall week 1, with activity increasing weeks 2-3. Impact areas still include the Philippines and Southeast Asia. Another area of concern for tropical cyclones is the Bay of Bengal including coastal sections, especially if the convection moves east. 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.
Links to CPC and PSD ENSO discussions:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
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 for subseasonal variability). 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 ~13-14 October.
Ed Berry
Thursday, October 04, 2007
Delayed
Given travel and other obligations, it is not likely I will write another posting until hopefully Monday, 8 October. In the interim, there is no real news I could offer in regard to the current ocean-atmosphere coupling to La-Nina. Weak evidence suggests a recent mountain-frictional torque variation along with another northward propagation event of Eastern Hemisphere tropical convective forcing. In any case, diagnostic monitoring tools support the notions I discussed a week ago of tropical convective forcing redeveloping across the Indian Ocean closer to the equator with northwest Pacific tropical cyclones.
Hovmoller plots indicate a slow retrogression of extratropical circulation anomalies since ~ 1 September. I think there is a seasonal cycle component contributing to this in addition to the other processess persisting our GSDM Stage 1 base state. Again, this also supports my notion of a break from the barrage of western USA troughs. As it typical of October (in this situation enhanced), starting later week 1-week 2 eastern Pacific troughs-western USA ridges and troughs amplifying across the eastern states appear probable. Loosely this translates to warmer and dryer than climatology for much of the USA focusing on the Plains. Even with the westward shift the Pacific Northwest is probable to stay wet while Alaska remains north of the storm track.
Ed Berry
Hovmoller plots indicate a slow retrogression of extratropical circulation anomalies since ~ 1 September. I think there is a seasonal cycle component contributing to this in addition to the other processess persisting our GSDM Stage 1 base state. Again, this also supports my notion of a break from the barrage of western USA troughs. As it typical of October (in this situation enhanced), starting later week 1-week 2 eastern Pacific troughs-western USA ridges and troughs amplifying across the eastern states appear probable. Loosely this translates to warmer and dryer than climatology for much of the USA focusing on the Plains. Even with the westward shift the Pacific Northwest is probable to stay wet while Alaska remains north of the storm track.
Ed Berry
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