The distribution of global tropical SSTs remains similar to about a week ago. This includes general warmth from the Indian Ocean toward the central and South Pacific while cooling continues along the equatorial cold tongue east of 150W. However, recent tropical cyclone activity has led to a negative SST tendency across the South Indian Ocean while positive tendencies are present across the central and South Pacific.
After the recent burst of intense tropical cyclone activity across the South Indian Ocean, a renewed organization of the tropical forcing is occurring centered ~10S/120-140E. General enhancement is present in a west-northwest to east-southeast oriented band roughly 10 deg wide from ~5S/90E to ~15S/160E which does include the SPCZ. I continue to maintain that this forcing is tied to a decent MJO signal. Monitoring tools such as the Wheeler index, velocity potential and animations of 150mb, 250mb and surface daily mean vector wind anomalies support my contention. Care must be taken to remember that it is not only the convective signal which defines a MJO, but also the winds (the multivariate EOF Wheeler technique considers this). The atmosphere is a dynamical system that consists of forcing, response and feedback processes all the time, and the MJO is just one of those components. About 10-20% of the tropical variability can be explained by the MJO, which only adds importance to diagnosing that signal when it is present.Care must also be taken to understand the details of tools utilized to interpret various modes of subseasonal variability. For instance, the Wheeler technique can be unrepresentative when there are multiple regions of tropical forcing.
I think the dynamical signal with the MJO is centered ~10S/100E, which does not have to be at the same location of the enhanced tropical convection. The global circulation has strongly responded with features such as twin upper tropospheric subtropical anticyclones across Africa and the Indian Ocean and downstream cyclones just east of the date line. Circulation anomalies are starting to reverse in the lower troposphere; for instance, there are westerly wind anomalies just north of Australia. This is a signal of a tropical baroclinic mode. Finally, zonal mean easterly wind anomalies remain ~good 5m/s throughout the tropical and subtropical atmospheres and global relative AAM is at least 2 standard deviations below normal per reanalysis data/climatology. The circulation has been in GSDM Stage 4 for roughly the past week, and GSDM Stage 1 appears probable during the next 1-3 weeks.
A serious question for weeks 1-3+ predictions is how far east is this MJO signal going to propagate? Our thought has been for this particular event to be truncated (linked to a possible transition to La-Nina) meaning it may not get by roughly Indonesia. In the presence of periodic flare-ups across the central/South Pacific, I am going to continue that feeling. This means the East Asian Jet (EAJ) will remain retracted and more troughs are probable to impact the west coast into the Plains. Should the convection come out to the date line, then the EAJ would be expected to expand east.
Most models have now captured the recent EAJ retraction and I do agree with the notion of trough development from the Alaska into the East Pacific during week 1. That would favor a less active pattern for much of the USA after the upcoming Plains storm development, consistent with the “lull” I have discussed in past postings. I think that will change during week 2 and continue week 3. Initially, heavy precipitation should spread southward along the USA west coast, including the ARB region (for the Hydrometeorological Testbed Project). Troughs would then be expected to move through the Rockies leading to baroclinic storm development on the Plains with an Arctic cold air source. The latter has been somewhat lacking with our past active situation.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I may not be able to post another discussion until early next week (~Tuesday) due to travel and shift work obligations. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Tuesday, February 27, 2007
Friday, February 23, 2007
MJO and Atmospheric Angular Momentum (AAM) - Reprisal
There is not much “new” information I can add to my posting from 2/20. SSTs generally continue the trend of cooling along the equatorial cold tongue while above average warmth remains from the South Indian Ocean into Indonesia. Above average SSTs also linger from the equatorial date line into the South Pacific. A transition from El-Nino to La-Nina must be monitored during the upcoming months. Such a transition could have implications for the upcoming Atlantic hurricane season.
Tropical convective forcing continues to get better organized across the Eastern Hemisphere. Remembering that we have a slowly evolving situation, many monitoring and diagnostic tools suggest that at least a weak-moderate signal of the MJO is present centered ~10S/80-90E. The development of at least 3 tropical cyclones across the South Indian Ocean during the past week is a response. I think this MJO signal has “stalled”, and that may be part of the evolutionary process of transitioning from El-Nino to La-Nina.
As of February 19th, ESRL/PSD reanalysis data plots of global AAM tendency were still ~minus 50 Hadleys. Contributions were from large negative global mountain and frictional torques as well as deep zonal mean anomalous easterly flow (~5-10m/s) throughout the tropical and subtropical atmospheres. A trade wind surge from the South Pacific into the South Indian Ocean is currently lessening the negative frictional torque and I suspect this may become positive during the next week or so. Finally, about a week ago the general pattern, observed since ~December 1st 2006, of westerly flow being transported out of the subtropics into the northern extratropics resumed. The latter is a characteristic of La-Nina situations and is described by Stage 1 of the GSDM.
I think we are still in GSDM Stage 4 (due to the very large negative global AAM tendency) and it is probable the global circulation will evolve into Stage 1 during the next 1-3 weeks. The East Asian Jet (EAJ) retracted significantly during the past week and it should stay that way. In fact, as some ensemble members and week-2 means show, there may be a surge of cold air into the east Pacific during that period due to this retraction. However, to me this would only be one of those “lulls” for much of the USA (except, of course, the west coast) in an otherwise stormy regime with an Arctic cold air source. What may persist this situation would be a “stationary MJO signal” ~100-120E while flare-ups of convection occur from the west central into the South Pacific. Rossby wave energy dispersions tied to the MJO signal would favor western USA troughs that would subsequently interact with subtropical jets forced by the central/South Pacific signal. My outlook from now through March 2007 posted on February 20th remains unchanged.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post another update on about next Tuesday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Tropical convective forcing continues to get better organized across the Eastern Hemisphere. Remembering that we have a slowly evolving situation, many monitoring and diagnostic tools suggest that at least a weak-moderate signal of the MJO is present centered ~10S/80-90E. The development of at least 3 tropical cyclones across the South Indian Ocean during the past week is a response. I think this MJO signal has “stalled”, and that may be part of the evolutionary process of transitioning from El-Nino to La-Nina.
As of February 19th, ESRL/PSD reanalysis data plots of global AAM tendency were still ~minus 50 Hadleys. Contributions were from large negative global mountain and frictional torques as well as deep zonal mean anomalous easterly flow (~5-10m/s) throughout the tropical and subtropical atmospheres. A trade wind surge from the South Pacific into the South Indian Ocean is currently lessening the negative frictional torque and I suspect this may become positive during the next week or so. Finally, about a week ago the general pattern, observed since ~December 1st 2006, of westerly flow being transported out of the subtropics into the northern extratropics resumed. The latter is a characteristic of La-Nina situations and is described by Stage 1 of the GSDM.
I think we are still in GSDM Stage 4 (due to the very large negative global AAM tendency) and it is probable the global circulation will evolve into Stage 1 during the next 1-3 weeks. The East Asian Jet (EAJ) retracted significantly during the past week and it should stay that way. In fact, as some ensemble members and week-2 means show, there may be a surge of cold air into the east Pacific during that period due to this retraction. However, to me this would only be one of those “lulls” for much of the USA (except, of course, the west coast) in an otherwise stormy regime with an Arctic cold air source. What may persist this situation would be a “stationary MJO signal” ~100-120E while flare-ups of convection occur from the west central into the South Pacific. Rossby wave energy dispersions tied to the MJO signal would favor western USA troughs that would subsequently interact with subtropical jets forced by the central/South Pacific signal. My outlook from now through March 2007 posted on February 20th remains unchanged.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post another update on about next Tuesday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Tuesday, February 20, 2007
MJO and Atmospheric Angular Momentum
Tropical SSTs remain roughly 0.5-1.0C above average over much of the South Indian Ocean as well as around the equatorial date line into the South Pacific. Actual SSTs are generally in the 29-30C range across these regions. Significant cool anomalies are now being observed by the TAO buoy array around 0/120W with magnitudes of about minus 1C. Values as low as minus 6C at ~50m deep have been recently observed across this region. The latter is the result of an upwelling oceanic Kelvin wave from the December 2006 MJO and may be contributing to a transition from El-Nino to La-Nina.
Full disk satellite imagery and other monitoring tools depict intense tropical convection across several regions of the globe. These include South Africa, the South Indian Ocean, the north coast of Australia into the South Pacific and much of Brasil. This is a rather complicated situation; however, a slowly evolving dynamical signal projecting onto the MJO is emerging currently centered ~10S/80E. Statistical tools such as the multivariate EOF Wheeler technique and empirical methods such as the time-filtered coherent modes Hovmollers support this notion. Furthermore, a rough phase speed computation of the convective envelope gives me about 4 m/s (3 deg long/day), consistent with a MJO.
There is also strong diagnostic support that a MJO signal is developing across the Eastern Hemisphere. Recall there was an intense flare-up of tropical convection across the warm South Pacific SSTs during about the middle of January. Through interactions with the Southern Hemisphere extratropics, a dynamical signal moved east to intensify convection across South Africa. It is this forcing that has now shifted into the South Indian Ocean. The global circulation has responded strongly to this redistribution of tropical forcing, with, for example, the tendency of global relative atmospheric angular momentum (AAM) down to about minus 50 Hadleys as of February 17th per re-analysis data! There are also other contributions to this tendency such as the current “negative phase” of a mountain-frictional torque index cycle. This is largest magnitude of a negative AAM tendency signal in at least a year and in itself represents an extreme weather-climate event. What is even more interesting is this negative AAM tendency is after the positive maximum of ~30 Hadleys around January 10th (roughly 50 days ago).
The point is westerly flow is being removed from the atmosphere by the earth and the spatial rearrangement of the tropical forcing has contributed to it. Furthermore, if one more carefully understands the time and space scales of what was first adding westerly flow to the atmosphere, early January, then the current removal of it, there is consistency with the MJO time scale. Currently zonal mean anomalous easterly flow prevails throughout the tropical and subtropical atmospheres with magnitudes of ~5-10m/s. Additionally, there are anomalous twin upper tropospheric subtropical anticyclones developing across the Indian Ocean with downstream cyclones and surface wind anomalies that are generally opposite. This means we already have a tropical baroclinic mode as a response to the developing MJO convection.
To me it is not a matter of whether there is a MJO, it is simply how strong will it be during the next several weeks. I do think the eastward movement of this MJO is going to be truncated (not get past roughly Indonesia) given recent SST trends. I also think our west central-South Pacific signal will continue with roughly 10-15 day flare-ups and assist with enhancing the climatologically strong southern USA subtropical jet.
We are currently in GSDM Stage 4 and it is probable to go into Stage 1 during the next 2-3 weeks (with at times strong subtropical jets). More models and their ensembles have become supportive of my past feelings about particularly the western and central portions of the USA going into an active regime. The ensemble means do struggle with differing solutions (along with a lot of spread) after about day 10. This is not at all a surprise to me given the abrupt shifts is circulation anomalies as measured by AAM tendency. With variations in amplitude including “lulls”, I think this regime may mature during March, especially if the tropical forcing becomes more persistent around 120E in the presence of the central and South Pacific signal.
As stated in my February 16th posting, we should several troughs first impact the west coast and then move northeast across the Plains (loosely – already starting). The western states will cool while the east warms. Arctic air may initially build up across Alaska (due to blocking west of the state) and then come into the western/central states during early March. March 2007 may be exceptionally active (above climatology) with heavy rain/severe storms across the Deep South into the east while late winter/early spring blizzards pound much of the Rockies into the Plains/Upper Mississippi Valley.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post at least a short update this Friday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Full disk satellite imagery and other monitoring tools depict intense tropical convection across several regions of the globe. These include South Africa, the South Indian Ocean, the north coast of Australia into the South Pacific and much of Brasil. This is a rather complicated situation; however, a slowly evolving dynamical signal projecting onto the MJO is emerging currently centered ~10S/80E. Statistical tools such as the multivariate EOF Wheeler technique and empirical methods such as the time-filtered coherent modes Hovmollers support this notion. Furthermore, a rough phase speed computation of the convective envelope gives me about 4 m/s (3 deg long/day), consistent with a MJO.
There is also strong diagnostic support that a MJO signal is developing across the Eastern Hemisphere. Recall there was an intense flare-up of tropical convection across the warm South Pacific SSTs during about the middle of January. Through interactions with the Southern Hemisphere extratropics, a dynamical signal moved east to intensify convection across South Africa. It is this forcing that has now shifted into the South Indian Ocean. The global circulation has responded strongly to this redistribution of tropical forcing, with, for example, the tendency of global relative atmospheric angular momentum (AAM) down to about minus 50 Hadleys as of February 17th per re-analysis data! There are also other contributions to this tendency such as the current “negative phase” of a mountain-frictional torque index cycle. This is largest magnitude of a negative AAM tendency signal in at least a year and in itself represents an extreme weather-climate event. What is even more interesting is this negative AAM tendency is after the positive maximum of ~30 Hadleys around January 10th (roughly 50 days ago).
The point is westerly flow is being removed from the atmosphere by the earth and the spatial rearrangement of the tropical forcing has contributed to it. Furthermore, if one more carefully understands the time and space scales of what was first adding westerly flow to the atmosphere, early January, then the current removal of it, there is consistency with the MJO time scale. Currently zonal mean anomalous easterly flow prevails throughout the tropical and subtropical atmospheres with magnitudes of ~5-10m/s. Additionally, there are anomalous twin upper tropospheric subtropical anticyclones developing across the Indian Ocean with downstream cyclones and surface wind anomalies that are generally opposite. This means we already have a tropical baroclinic mode as a response to the developing MJO convection.
To me it is not a matter of whether there is a MJO, it is simply how strong will it be during the next several weeks. I do think the eastward movement of this MJO is going to be truncated (not get past roughly Indonesia) given recent SST trends. I also think our west central-South Pacific signal will continue with roughly 10-15 day flare-ups and assist with enhancing the climatologically strong southern USA subtropical jet.
We are currently in GSDM Stage 4 and it is probable to go into Stage 1 during the next 2-3 weeks (with at times strong subtropical jets). More models and their ensembles have become supportive of my past feelings about particularly the western and central portions of the USA going into an active regime. The ensemble means do struggle with differing solutions (along with a lot of spread) after about day 10. This is not at all a surprise to me given the abrupt shifts is circulation anomalies as measured by AAM tendency. With variations in amplitude including “lulls”, I think this regime may mature during March, especially if the tropical forcing becomes more persistent around 120E in the presence of the central and South Pacific signal.
As stated in my February 16th posting, we should several troughs first impact the west coast and then move northeast across the Plains (loosely – already starting). The western states will cool while the east warms. Arctic air may initially build up across Alaska (due to blocking west of the state) and then come into the western/central states during early March. March 2007 may be exceptionally active (above climatology) with heavy rain/severe storms across the Deep South into the east while late winter/early spring blizzards pound much of the Rockies into the Plains/Upper Mississippi Valley.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post at least a short update this Friday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Friday, February 16, 2007
Resurrection Update
I have been on travel this past week meaning this posting will be not as complete. Hopefully I will be able to post a better discussion around Tuesday next week.
Per full disk satellite imagery and monitoring tools, I think a renewed moist phase of the MJO is evolving from South Africa into the South Indian Ocean. The possibility of this development was offered in my January 30th posting based on the scientific principles utilized to derive our GSDM, including responses from the rapid weakening of warm ENSO.
The centroid of this convection was at ~10S/60E and extended east to about western Indonesia. Other sporadic tropical convection continued across the South Pacific and South America. SSTs are about plus 1C (at least 29C) over much of the South Indian Ocean and these warm waters will contribute to a lot of CAPE to our developing MJO. I suspect this MJO will become more robust ~90-110E by late week 2/early week 3.
The global circulation is already responding with features such as twin upper tropospheric subtropical anticyclones ~60E and downstream twin cyclones. In fact, the cyclone just north of India tied to this MJO contributed to a recent extended period of extreme cold/wet weather in that region. Upper tropospheric easterly wind anomalies are increasing throughout the tropical and subtropical atmospheres and the tendency of global relative atmospheric angular momentum is about minus 15 Hadleys. The latter is still decreasing with a contribution coming from a strong negative frictional torque (~15 Hadleys) that began about 2 weeks ago. I think we are in Stage 4 of the GSDM and it is probable we will go into Stage 1 during the next 2-3 weeks. There will also remain periodic forcing from the South Pacific that may enhance a climatologically strong subtropical jet.
My feelings about particularly the western and central portions of the USA going into an active pattern for the next few weeks remain unchanged. In fact, I think this regime may mature going into March. We should several troughs first impact the west coast and then move east-northeast across the Plains (loosely). The western states will cool while the east warms. Arctic air may initially build up across Alaska (due to blocking west of the state) and then come into the western/central states during early March. March 2007 may be exceptionally active (above climatology) with (for example) heavy rain/severe storms across the Deep South into the east while late winter/early spring blizzards pound much of the Rockies into the Plains/Upper Mississippi Valley.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Per full disk satellite imagery and monitoring tools, I think a renewed moist phase of the MJO is evolving from South Africa into the South Indian Ocean. The possibility of this development was offered in my January 30th posting based on the scientific principles utilized to derive our GSDM, including responses from the rapid weakening of warm ENSO.
The centroid of this convection was at ~10S/60E and extended east to about western Indonesia. Other sporadic tropical convection continued across the South Pacific and South America. SSTs are about plus 1C (at least 29C) over much of the South Indian Ocean and these warm waters will contribute to a lot of CAPE to our developing MJO. I suspect this MJO will become more robust ~90-110E by late week 2/early week 3.
The global circulation is already responding with features such as twin upper tropospheric subtropical anticyclones ~60E and downstream twin cyclones. In fact, the cyclone just north of India tied to this MJO contributed to a recent extended period of extreme cold/wet weather in that region. Upper tropospheric easterly wind anomalies are increasing throughout the tropical and subtropical atmospheres and the tendency of global relative atmospheric angular momentum is about minus 15 Hadleys. The latter is still decreasing with a contribution coming from a strong negative frictional torque (~15 Hadleys) that began about 2 weeks ago. I think we are in Stage 4 of the GSDM and it is probable we will go into Stage 1 during the next 2-3 weeks. There will also remain periodic forcing from the South Pacific that may enhance a climatologically strong subtropical jet.
My feelings about particularly the western and central portions of the USA going into an active pattern for the next few weeks remain unchanged. In fact, I think this regime may mature going into March. We should several troughs first impact the west coast and then move east-northeast across the Plains (loosely). The western states will cool while the east warms. Arctic air may initially build up across Alaska (due to blocking west of the state) and then come into the western/central states during early March. March 2007 may be exceptionally active (above climatology) with (for example) heavy rain/severe storms across the Deep South into the east while late winter/early spring blizzards pound much of the Rockies into the Plains/Upper Mississippi Valley.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.
Ed Berry
Friday, February 09, 2007
Slowing atmosphere means more active weather for the lower 48 states?
Global tropical SSTs remain above average across most of the Atlantic into the east Pacific, as well as the South Pacific and much of the South Indian Ocean. Magnitudes are roughly plus 1-2C. Cooler than normal waters are present around much of Indonesia while weak pockets of negative anomalies are starting to appear along the central equatorial cold tongue. The former is due to recent persistent intense tropical thunderstorm activity while the latter is a response to our rapidly collapsing warm event. After the December 2006 to early January 2007 MJO, SSTs have been cooling particularly from about the equatorial date line region to ~140W. This MJO initiated a strong trade wind surge which led to upwelling of cooler subsurface waters as well as generating an upwelling oceanic Kelvin wave. In fact, latest TAO buoy subsurface data indicates negative anomalies ~minus 5C at roughly 50m around 120W. Anomalous cross-equatorial northerly flow has continued this cooling process. We are at the point to be concerned about a La-Nina evolving as we head into boreal summer.
As discussed in past postings, the global circulation has been generally La-Nina like since about late November. Tropical convective forcing from the Indian Ocean dominated during much of the fall and helped to add anomalous zonal mean easterly flow to the subtropics. This contributed to circulation regimes across the PNA sector not consistent with an El-Nino composite (episodes of GSDM Stage 1 and 2 situations). When the tropical forcing shifted into the South Pacific (in a very complicated manner) by late January, a weak ENSO response did occur (GSDM Stage 3). However, it was only a perturbation on an already established base state (one must always pay attention to the initial conditions). Global relative atmospheric angular momentum increased to about 2 sigma above the 1968-1997 reanalysis data climatology, with much of that coming from the Southern Hemisphere. In fact, the anomalous tropical forcing along the South Pacific Convergence Zone actually played a role in establishing the intense ridge into the Arctic observed off the USA west coast late January. This was an unusual situation where there was a cold regime across the USA (lower 48 states) with GSDM Stage 3.
Our warm event peaked in November-December 2006, which may be a trend that started with the 2002-03 warm ENSO. We also see a situation where there may be an alteration of El-Nino-La Nina having a quasi-biennial time scale, suggesting it is now “time” for a cold event. In my own mind I also see complex forcing-response-feedbacks meaning our recent trend of multiple regions of tropical convective forcing that started during 2001-02 has impacted ENSO (which may be a global warming signal). In our recent case, the Indian Ocean forcing during fall, unusually intense for an El-Nino, sent the circulation into La-Nina first. The tropical Pacific SSTs may now be responding afterwards (think about this).
Tropical convective forcing has been shifting back west into Indonesia since late January (weakly projecting onto a convectively coupled equatorial Rossby mode), helping to maintain the cold regime across much of the lower 48 states. Tropical forcing has also been weakening across the South Pacific. We have also been monitoring South Africa and the South Indian Ocean for a re-emergence of a moist MJO. Since the mid January eruption convection along the SPCZ, there has been a weak dynamical signal moving through the Southern Hemisphere. Interacting with the extratropics, tropical forcing has been increasing across South Africa into the Indian Ocean for about the past week (with a couple of tropical cyclones). I think this notion looks reasonable, and there are even a few empirical and statistical tools supporting our thinking. Tropical forcing also remains across the South Pacific and South America (mainly Brasil and the South Atlantic Convergence Zone).
Zonal mean easterly flow has been increasing throughout the tropical and subtropical atmospheres for the last week or so. In fact, the global tendency of relative atmospheric angular momentum dipped to about minus 20 Hadleys ~ February 5th assisted by both negative global mountain and frictional torques. GSDM Stage 4 best describes the current global weather-climate situation, and I think we will transition to GSDM Stage 1 (or a combination of 4-1) during weeks 2-3. Other processes contributing but not discussed in this already much too lengthy posting are the roles of high latitude blocking and a possible late season warming of the stratosphere. The latter will help to keep sea level pressures above normal across the Arctic maintaining a cold air source for the USA (even though temperatures are slightly above average across most of the Arctic, keeping in mind climatology).
At this time most of the CONUS has below normal temperatures with an active storm track from California into the eastern states. During weeks 2-3 I would expect all of this to slowly shift northwest meaning by late this month an active southwest flow storm track across from the Rockies into the Plains would be probable. Cold and moist energetic troughs would be expected to dig into the western and central Rockies and then lift northeast into the Mid/Upper Mississippi Valley. Given the South Pacific signal (and we need to monitor the east Pacific where there are still warm SSTs – at least for now remembering the cold subsurface), a moist subtropical jet with closed lows may interact with this storm track (GSDM 4-1 instead of Stage 1). While late winter storms would occur in the cold sectors, heavy rain and severe storms would be probable for much of the Deep South into the eastern states. In general, this would be an active pattern for much of the lower 48 states. There would be cold to the northwest and warmth for the far southeast. Alaska may stay under the ridge while closed lows may develop near/west of Hawaii. Finally, with variations in amplitude, I could see this type of pattern persisting well into spring.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion next week. The WB(2007) paper on the GSDM is scheduled to appear this month's publication of MWR.
Ed Berry
As discussed in past postings, the global circulation has been generally La-Nina like since about late November. Tropical convective forcing from the Indian Ocean dominated during much of the fall and helped to add anomalous zonal mean easterly flow to the subtropics. This contributed to circulation regimes across the PNA sector not consistent with an El-Nino composite (episodes of GSDM Stage 1 and 2 situations). When the tropical forcing shifted into the South Pacific (in a very complicated manner) by late January, a weak ENSO response did occur (GSDM Stage 3). However, it was only a perturbation on an already established base state (one must always pay attention to the initial conditions). Global relative atmospheric angular momentum increased to about 2 sigma above the 1968-1997 reanalysis data climatology, with much of that coming from the Southern Hemisphere. In fact, the anomalous tropical forcing along the South Pacific Convergence Zone actually played a role in establishing the intense ridge into the Arctic observed off the USA west coast late January. This was an unusual situation where there was a cold regime across the USA (lower 48 states) with GSDM Stage 3.
Our warm event peaked in November-December 2006, which may be a trend that started with the 2002-03 warm ENSO. We also see a situation where there may be an alteration of El-Nino-La Nina having a quasi-biennial time scale, suggesting it is now “time” for a cold event. In my own mind I also see complex forcing-response-feedbacks meaning our recent trend of multiple regions of tropical convective forcing that started during 2001-02 has impacted ENSO (which may be a global warming signal). In our recent case, the Indian Ocean forcing during fall, unusually intense for an El-Nino, sent the circulation into La-Nina first. The tropical Pacific SSTs may now be responding afterwards (think about this).
Tropical convective forcing has been shifting back west into Indonesia since late January (weakly projecting onto a convectively coupled equatorial Rossby mode), helping to maintain the cold regime across much of the lower 48 states. Tropical forcing has also been weakening across the South Pacific. We have also been monitoring South Africa and the South Indian Ocean for a re-emergence of a moist MJO. Since the mid January eruption convection along the SPCZ, there has been a weak dynamical signal moving through the Southern Hemisphere. Interacting with the extratropics, tropical forcing has been increasing across South Africa into the Indian Ocean for about the past week (with a couple of tropical cyclones). I think this notion looks reasonable, and there are even a few empirical and statistical tools supporting our thinking. Tropical forcing also remains across the South Pacific and South America (mainly Brasil and the South Atlantic Convergence Zone).
Zonal mean easterly flow has been increasing throughout the tropical and subtropical atmospheres for the last week or so. In fact, the global tendency of relative atmospheric angular momentum dipped to about minus 20 Hadleys ~ February 5th assisted by both negative global mountain and frictional torques. GSDM Stage 4 best describes the current global weather-climate situation, and I think we will transition to GSDM Stage 1 (or a combination of 4-1) during weeks 2-3. Other processes contributing but not discussed in this already much too lengthy posting are the roles of high latitude blocking and a possible late season warming of the stratosphere. The latter will help to keep sea level pressures above normal across the Arctic maintaining a cold air source for the USA (even though temperatures are slightly above average across most of the Arctic, keeping in mind climatology).
At this time most of the CONUS has below normal temperatures with an active storm track from California into the eastern states. During weeks 2-3 I would expect all of this to slowly shift northwest meaning by late this month an active southwest flow storm track across from the Rockies into the Plains would be probable. Cold and moist energetic troughs would be expected to dig into the western and central Rockies and then lift northeast into the Mid/Upper Mississippi Valley. Given the South Pacific signal (and we need to monitor the east Pacific where there are still warm SSTs – at least for now remembering the cold subsurface), a moist subtropical jet with closed lows may interact with this storm track (GSDM 4-1 instead of Stage 1). While late winter storms would occur in the cold sectors, heavy rain and severe storms would be probable for much of the Deep South into the eastern states. In general, this would be an active pattern for much of the lower 48 states. There would be cold to the northwest and warmth for the far southeast. Alaska may stay under the ridge while closed lows may develop near/west of Hawaii. Finally, with variations in amplitude, I could see this type of pattern persisting well into spring.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion next week. The WB(2007) paper on the GSDM is scheduled to appear this month's publication of MWR.
Ed Berry
Friday, February 02, 2007
Break through update, then what?
The following is a posting from the HMT Blog at
http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/
We have posted some annotated figures on that site which you may find useful. This discussion focused on the American River Basin (ARB) in California for this project. I will adapt the information for this Blog.
The reasoning from our last posting on January 30th remains unchanged. Some models are continuing to advertise the break through of westerlies on the west coast with the PSD ensemble showing a distinct shift toward probability of above normal precipitation in the California region in week 2 (Feb 10-16). That notion appears reasonable given the increase of westerly flow throughout the subtropical atmospheres during the past 1-2 weeks. Tropical convection is now established over the west Pacific and continues to be active over the Indian Ocean. The tendency for convective forcing to be further west should continue to effect the North Pacific circulation with more retrogression of features. This could eventually (beyond week 2) lead to weakened westerlies in midlatitudes and a return to a trough along the US west coast, especially if a new MJO develops over the Indian Ocean (while a signal continues from the South Pacific).
Below normal temperatures are likely to remain entrenched from the Upper Mississippi Valley into the Northeast states for the next 1-2 weeks (with moderation). The rest of the country would be expected to have near to above normal temperatures. As troughs impact the USA west coast and shift inland, Arctic surges may become probable west of the Continental Divide into the Rockies and Northern Plains later week 2 into week 3 (~February 13-23). One would also expect an active southwest flow storm track across the central part of the country with warmth across the Deep South. High impact weather concerns may vary from severe winter weather from the Rockies into the northern and central Plains to heavy rainfall and severe local storms across the southeast states. Moisture sources may include an active subtropical jet (South Pacific signal) along with transport from the deep tropics through the Gulf of Mexico.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion by the end of next week.
Ed Berry and Klaus Weickmann
http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/
We have posted some annotated figures on that site which you may find useful. This discussion focused on the American River Basin (ARB) in California for this project. I will adapt the information for this Blog.
The reasoning from our last posting on January 30th remains unchanged. Some models are continuing to advertise the break through of westerlies on the west coast with the PSD ensemble showing a distinct shift toward probability of above normal precipitation in the California region in week 2 (Feb 10-16). That notion appears reasonable given the increase of westerly flow throughout the subtropical atmospheres during the past 1-2 weeks. Tropical convection is now established over the west Pacific and continues to be active over the Indian Ocean. The tendency for convective forcing to be further west should continue to effect the North Pacific circulation with more retrogression of features. This could eventually (beyond week 2) lead to weakened westerlies in midlatitudes and a return to a trough along the US west coast, especially if a new MJO develops over the Indian Ocean (while a signal continues from the South Pacific).
Below normal temperatures are likely to remain entrenched from the Upper Mississippi Valley into the Northeast states for the next 1-2 weeks (with moderation). The rest of the country would be expected to have near to above normal temperatures. As troughs impact the USA west coast and shift inland, Arctic surges may become probable west of the Continental Divide into the Rockies and Northern Plains later week 2 into week 3 (~February 13-23). One would also expect an active southwest flow storm track across the central part of the country with warmth across the Deep South. High impact weather concerns may vary from severe winter weather from the Rockies into the northern and central Plains to heavy rainfall and severe local storms across the southeast states. Moisture sources may include an active subtropical jet (South Pacific signal) along with transport from the deep tropics through the Gulf of Mexico.
Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion by the end of next week.
Ed Berry and Klaus Weickmann
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