GSDM Stage 1 Perturbed

The west Pacific SST horseshoe of warmth remains; however, anomalies are weakening due to recent days of intense tropical rainfall. Magnitudes are generally 0.5-1C with SSTs ~29-30C, with the warmest waters now around 10S/160W as of March 29th. Conversely, the equatorial Indian Ocean continues to slowly warm due to suppression (with the exception of a tropical cyclone in the South Indian Ocean) with SST totals and anomalies comparable to the west Pacific warm pool. Cooling along the equatorial east Pacific cold tongue has again intensified this past week with negative anomalies ~minus 1-3C east of 140W and at least minus 4C in the subsurface. These colder than normal waters extend down to roughly 200m at the date line per latest TAO buoy data. Whether or not La-Nina is evolving is an open question (recall May 2003); however, it is not unusual to see “pulses” of cooling if this is the case.

Hard times continue trying to understand the tropical convective forcing, and there is a seasonal cycle consideration to remember. There is no coherent MJO dynamical signal. Most significant behavior during the last 10 days has been an eastward shift of intense (OLRA ~ minus 50-70 W/m**2) forcing from ~120E to 160E just south of the equator. I maintain this is a transient event linked to very complicated tropical-extratropical interactions. These can only be reasonably understood observationally from disciplined detailed daily monitoring of fields such as animations of upper and lower tropospheric wind fields utilizing the GSDM framework. Some of these interactions include last week’s extension of the North Pacific jet shifting this forcing to the east, possibly reinforced by feedbacks from the extratropics including a strong positive East Asian mountain torque.

Full disk satellite imagery already shows the west Pacific tropical forcing drifting west and northwest, and the coherent modes Hovmollers suggest a weak projection on the convectively coupled Rossby mode. In fact, a tropical cyclone may develop out of this region north of the equator during the next few days per JTWC. Weaker forcing (diurnal cycle understood) also persists across northwest South America, west central Africa and ~0/120E.

Our best weather-climate signal remains global relative AAM including the zonal mean contributions. This quantity is a good diagnostic measure of the character and evolution of the global and zonal mean circulation anomalies. Relative AAM continues at around 1.5 standard deviations below the 1968-1997 reanalysis data climatology, and its tendency has been weak for the last couple of weeks. Furthermore, zonal mean upper tropospheric easterly wind anomalies remain at around 5m/s throughout much of the tropical and subtropical atmospheres. These easterlies are contributing to a zonal mean sink of AAM ~20N while flux convergence of its transports occur ~35N. All of these observations are consistent with a GSDM Stage 1 base state (most probable during a La-Nina) that has been around since at least late February.

Given the warm Indian Ocean SSTs, feedbacks from our low AAM regime and the slow westward shift of tropical forcing back to 120E discussed on 3/27, we need to monitor for the development of intense tropical convection across the Indian Ocean into Indonesia sometime weeks 2-3. In the meantime, the transient SPCZ convection discussed above has generated enough anomalous (~20-30m/s at 150mb on 3/29) upper tropospheric equatorial westerly wind flow over the east Pacific to perturb our GSDM Stage 1 state toward Stage 2 for week 1. In fact, animations of daily mean upper tropospheric vector wind anomalies show well defined twin tropical anticyclones at 160E as of 3/29. These anticyclones are already interacting with Rossby wave energy dispersion (RWD) processes from Asia to North America, including the ridge building into Alaska and the developing downstream trough across western Canada. It is possible the models may be underestimating the impacts of this trough onto the Rockies and Plains early-mid next week given the large circulation anomalies tied to this RWD and the complex dynamics. Overall, as shown by most ensemble numerical models, a colder regime is probable for the eastern two-thirds of the USA for at least week 1.

My thoughts for weeks 2-3 remain the same. I speculate whatever “renegade” North Pacific jet that does come out may lead to a similar evolution of synoptic events that was observed during the last 7-10 days. If the tropical convective scenario discussed above does occur, a shift back to a GSDM Stage 4-1 would be most probable by sometime week 3 (April 14-20). Although models do not predict the evolution of tropical convection very well after week 1, the NCEP week 2 ensemble mean total precipitation forecast lends loose support to renewed Indian Ocean convection. The latter is likely a SST response. This would lead to a renewed active pattern for much of the western and central USA, including severe local storms across the Plains. Careful monitoring will be needed to see if Eastern Hemisphere tropical forcing eventually develops into a MJO.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I am unclear when I will be able to another discussion next week due to travel. We are also working on a weather-climate discussion for the ESRL/PSD MJO web site, which will hopefully be posted during the next couple of weeks. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

Hard Times

Above average SSTs remain centered around 0/150E and continue to have a horseshoe distribution pattern into both hemispheres. Anomalies are ~0.5-1C having total SSTs of 29-30C with the warmest centered at roughly 5-10S/170E. These SSTs have cooled a bit during the last week as a response to enhanced rainfall in that region. Weak-moderate warm anomalies continue to exist across much of the equatorial Indian and north tropical Atlantic Oceans. In fact, suppression of tropical convection has allowed SSTs across the Indian Ocean to warm by about 0.5C over the past week. The equatorial east Pacific remains moderately cool east of 120W with negative anomalies as low as -4C down to roughly 200m.

Understanding the current tropical convective forcing is about as difficult as it can get. There is no coherent MJO dynamical signal, and these are the situations where additional understanding beyond linear MJO (and linear dynamics) thinking is needed. The strongest tropical convective forcing globally is centered ~5-10S/165E, in the region of warm SSTs. In fact, three-day averaged OLRA are at least minus 50-70 W/m**2 (including Hurricane Becky) and the Wheeler multivariate MJO index even projects this convection as a MJO as of March 26th. The latter is a response to the baroclinic vertical wind structure with this convection. For example, there are weak anomalous twin upper tropospheric tropical anticylones with cross-equatorial southerly flow (with lower level opposite wind flows) around the date line.

As discussed on my last Friday posting (3/23), the enhancement of the North Pacific jet helped to shift tropical convection into the southwest Pacific. I believe that additional dynamical forcing tied to a strong East Asian mountain torque around March 20th (~20 Hadleys with a global torque of 30 Hadleys) contributed to maintaining this jet and the SPCZ convection. Other weaker tropical forcing persists over northwest South America with respectable thunderstorm activity across west central Africa.

However, another region of now relatively weak tropical convection remains centered at roughly 0/120E. This is actually a westward shift out of the South Pacific from mid-January, and continues to be the most important tropical forcing to impact the global circulation. A response is that global relative AAM remains about 1.5 standard deviations below the 1968-1997 reanalysis data climatology, reflective of loosely deep zonal mean easterly flow throughout much of the tropical and subtropical atmospheres (~5m/s anomalies). I think this is a slowly evolving component and may be part of a coupled process involving a transition to La-Nina. The role of the seasonal cycle needs to be remembered, particularly around May. GSDM Stage 1 continues to define the current weather-climate/global circulation, which is consistent with La-Nina. Summing up, our strongest weather-climate signal remains global relative AAM while the subseasonal evolution of the tropical forcing is unclear. I can only offer the following speculation.

Discussion in past postings has addressed the fairly rapid subtropical variations (~10-15 days as seen from the global AAM tendency time series) within this GSDM Stage 1 base state. There are fairly symmetric anomalous wave trains across the subtropical atmospheres of both hemispheres, including twin upper tropospheric cyclones across the east Pacific and Indian Oceans. These are actually generating anomalous equatorial westerly flow that I think will locally interact with the subtropics. I also suspect tropical convection will increase across the warm Indian Ocean SSTs during the next 2-3 weeks once the twin cyclones in that region move east. A weak eastward propagating dyamical signal from the southwest Pacific may also arrive there. By weeks 3-4 tropical forcing may become very intense around 90-120E. The gist is there may be a shift toward GSDM Stage 1-2 by week 2 and then perhaps a GSDM Stage 4-1 response afterwards.

Needless to say, forecast confidence after week 1 is very low. I like the increasing number of solutions being displayed by most ensembles through around days 5-7. Blocking developing across Alaska is highly probable since I can already see this process, linked to the 120E tropical forcing, from animations of daily mean 150mb and 250mb vector wind anomalies. This will allow wave breaking troughs to develop across the central Pacific hence supporting a ridge off the USA west coast and a downstream central North American trough. This suggests a colder and wetter regime for much of the northern and central USA in the wake of the present western states trough. Daily monitoring will be needed in regard to the amplitude of these features, particularly for upwelling concerns off the California coast and southward penetration of Arctic air into the Northern Rockies and Plains. Afterwards, my thought would be for this regime to shift west and north, going back into an active (above climatology) spring pattern for especially the Plains by week 3. Careful monitoring will be needed to see if the Eastern Hemisphere tropical forcing eventually develops into a MJO.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another (hopefully shorter) discussion this Friday. We are also working on a weather-climate discussion for the ESRL/PSD MJO web site, which will hopefully be posted during the next couple of weeks. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

Renegade North Pacific Jet

Above average SSTs still persist across the tropical west central Pacific with the warmest centered ~ 5S/165E having magnitudes of 1-2C and actual SSTs ~30C. Weak-moderate warm anomalies are also present across much of the equatorial Indian and north tropical Atlantic Oceans. The equatorial east Pacific remains moderately cool east of 120W with negative anomalies as low as -4C down to roughly 200m.

The tropical forcing discussed on my March 20th posting has loosely separated into 2 regions. One area remains quasi-stationary ~0/120E while another region has moved into the warm waters of the southwest Pacific centered at roughly 5-10S/170E. The former region continues to have the strongest interactions with the extratropics while the latter may be a response to the transient strong Pacific jet stream embedded within the low relative AAM, GSDM Stage 1.

We speculate that a Rossby wave energy dispersion from west Pacific forcing last week contributed to the snow storm for the USA northeast. Animations of 150mb and 250mb daily mean vector wind anomalies suggest that both tropical forcing present across Indonesia and a strong east Asian mountain torque of roughly 20 Hadleys (30 Hadleys globally) combined to add more westerly flow to the Pacific subtropics. Now there is an intense jet across the North Pacific having anomalies of at least 30-40 m/s. This feature is “out running” the tropical forcing back at 120E, and perhaps shifting the convection east across the southwest Pacific. In fact, according to the coherent modes Hovmollers, the convection across the southwest Pacific projects onto a Kelvin wave.

The future evolution of the tropical forcing still remains unclear. However, for at least the next 1-3 weeks, the most important convection to impact the extratropics should remain quasi-stationary centered in the region of 120E (consistent with a low AAM regime) while the southwest Pacific has episodic flare-ups. The strong North Pacific jet is transient. As most models show, this jet should amplify a strong trough across the western USA by early next week. For weeks 2-3, given persistence of GSDM Stage 1, ridge amplification across the central and east Pacific into Alaska with more troughs impacting the western and central states is probable. This is an active pattern for the Rockies and Plains favorable for late season snowstorms and outbreaks of severe thunderstorms with individual synoptic baroclinic cyclones. Careful monitoring will be needed to see if the Eastern Hemisphere tropical forcing develops into a MJO, and if the current cooling of the tropical east Pacific develops into a La-Nina.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion next Tuesday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

GSDM Stage 1

This discussion will be “short and sweet”. The distribution of global tropical SSTs remains similar to that discussed on my posting last Friday (3/16/07). The horseshoe pattern of warmth persists across the west central Pacific with anomalies of ~1-2C and 29-31C actual SSTs while similar cool SST anomalies exist along the equator from 150W to South America. The latter have actually cooled a bit during the last few days while colder anomalies ~2-4C persist in the subsurface down to roughly 200m.

Tropical convective forcing is getting better organized across the Eastern Hemisphere centered at about 0/120E. Latest 3-day averaged OLRA are ~minus 50-70W/m**2. Other pockets of enhancement still remain across the South Pacific, Brasil and equatorial Africa. The dynamical MJO signal discussed in my last posting has lost coherence. The future evolution of the tropical convection is unclear. Based on its current position around 120-130E and the existing low AAM circulation anomalies, we speculate that convection will remain quasi-stationary or shift slightly west over the next one to two weeks. Kelvin waves excited by the convection should also help excite transient convective episodes over the west Pacific. This complicated picture may all be part of a slow process of the atmosphere transitioning to a more pronounced La-Nina base state.

GSDM Stage 1 (most probable during La-Nina) still best describes the current weather-climate situation. It is probable this situation will persist for at least the next 2-3 weeks. Relative atmospheric angular momentum (AAM) remains well below the 1968-1997 reanalysis data climatology at ~1.5 standard deviations. For the Pacific-North American sector, more strong troughs are expected for the western USA with a southwest flow storm track across the Plains probable at least into early April. This pattern is favorable for late season snowstorms across portions of the Rockies and northern/central High Plains with a greater than climatology risk for severe local storms across the Plains and Mississippi Valley.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion this Friday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

Recap on the Complexity

Tropical west Pacific SSTs still remain above average in a horseshoe pattern centered on 155E with anomalies ~0.5-1.5C and actual temperatures from 29-31C. There has also been some warming across portions of the equatorial Indian Ocean with positive anomalies ~0.5-1C and SSTs near 30C. Below normal SSTs remain from the north coast of Australia to the equator, with anomalies ~1.0C. With the exception of the equatorial Pacific ~120W, eastern hemisphere tropical SSTs are weakly positive. While the negative anomalies along the central equatorial Pacific cold tongue are currently minimal, colder subsurface anomalies remain down to roughly 200m. The latter are important for possible La-Nina development.

Largely forced by the warm SSTs, favorable upper tropospheric winds and Rossby wave energy propagation from the extratropics, tropical convective forcing remains loosely organized centered ~0/140E. This area extends from the central Indian Ocean into the South Pacific. However, the convection in this region is not a large departure from climatology. There is still evidence from various fields such as animations of daily mean 150mb vector wind anomalies and velocity potential Hovmollers that a weak dynamical signal is moving through the Western Hemisphere. I think it is centered at roughly 0/60W and moving east at about 10 deg long/day. We expect this signal to come back into the Eastern Hemisphere sometime during week 2, and allow the tropical convective forcing to increase from Africa to the warming SSTs of the Indian Ocean. We may see the third MJO since late last fall evolve in that region by late week 2 or 3.

GSDM Stage 1 best describes the current weather-climate situation. Relative atmospheric angular momentum (AAM) remains well below the 1968-1997 reanalysis data climatology at ~1.5 standard deviations. The global tropical and subtropical atmospheres are dominated by zonal mean easterly wind anomalies with magnitudes of 5-10m/s. The fast dynamical signal discussed above should reinforce what is already a low AAM regime during the week 2-3 time frame. Synoptically, the most probable response across the Pacific North American sector would be for a discontinuous retrogression of existing circulation anomalous by at least 20-30 degrees of longitude. This means the ridge currently over the western North America should redevelop ~150W.

Even with the above reasoning, forecast uncertainty for weeks 2-3 remains very high. If a coherent MJO develops across the Indian Ocean by week 3, forecast confidence in subseasonal outlooks may improve. Most models forecast a fairly strong trough to impact the west coast early next week and then shift into the Rockies and Plains afterwards. It is possible this first trough may “split” with northern and southern stream developments during the course of its evolution. In fact, recent signals of AAM transports would support this possibility. Afterwards, a more “classic” GSDM Stage 1 springtime active pattern for the western and central USA may emerge for weeks 2-3. This would argue for late season snowstorms across portions of the Rockies and northern/central High Plains with a greater than climatology risk for severe local storms across the Plains and Mississippi Valley.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion next Tuesday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

Regrouping or What?

SSTs remain warm in a horseshoe pattern centered on 0/150E and extend to just southwest of Hawaii as well as into the South Pacific. Anomalies are ~plus 0.5-1.0C with actual SSTs from 29-30C. Other weak-moderate positive tropical SST anomalies are also present across portions of the Indian Ocean, far eastern Pacific and the North Tropical Atlantic. Negative anomalies of roughly 0.5-1.0C are present from Indonesia into Northern Australia as the result of recent tropical cyclone and heavy precipitation activity. Cool anomalies of ~0.5C also remain on the equator from about 120-150W with colder subsurface values down to 200m at the date line per latest TAO data. The latter may be attributable to a developing La-Nina. If one does develop, we expect the cooling to occur in “steps” related to forcing from subseasonal events.

Resolving the situation with the tropical convective forcing is difficult and about as tough as it gets in terms of uncertainty. A very weak dynamical signal with the recent weak-moderate MJO remains around 90W and appears to be stalling just west of South America. Tropical convection has also tried to get better organized ~0/140-150E for the past several days, largely forced by the warm SSTs in that region and favorable upper level winds. A Rossby wave energy dispersion from the Southern Hemisphere extratropics has led to an increase in upper tropospheric easterly wind anomalies across the west central Pacific. The locally strengthened upper level divergence is providing a source of lift for additional thunderstorm activity.

Animations of 150mb daily mean vector wind anomalies suggest twin subtropical anticyclones redeveloping ~150E. However, whether or not this convection will then shift east toward the warm SSTs as a MJO-like signal is unclear. Currently, a component actually shifting northwest from the west Pacific toward the Philippines is only complicating matters. Furthermore, should a strong eastward shift occur, the bulk of the forcing will stay south of the equator given the cool east Pacific SSTs and the time of year.

The uncertainty in the near term tropical convective forcing is not present in all circulation fields. Global relative atmospheric angular momentum (AAM) remains low at about -1.5 sigma anomaly. A large contribution is coming from the northern subtropical atmosphere centered ~25N. Zonal mean easterly wind anomalies at 200mb are from 5-10m/s in that region, having propagated off the equator starting back in mid February tied to the recent MJO. There have been a couple of episodes of weak positive global AAM tendency since then (~10 Hadleys), partly linked to variations involving mountain and frictional torques in addition to the MJO dynamical signal moving into the Western Hemisphere. For the latter, weak upper tropospheric westerly wind anomalies developed along the Western Hemisphere equator and are interacting with a subtropical wave train currently moving through South Asia.

A very strong storm system is currently pounding Tibet and northern India, part of the wave energy dispersion along the South Asian wave train. As this system interacts with the west central Pacific convection, an extension of the East Asia Jet (EAJ) is probable. Synoptically, this translates to the current trough just east of Japan moving east toward western North America while the upstream system replaces it. This was a possibility discussed in my March 9th posting, and most models predict this situation starting around next Tuesday. However, details remain very unclear. Nevertheless, there is hope for at least one IOP for the American River Basin HMT project with this system. This trough should then move into the Rockies and Plains during week 2 increasing the probability of high impact weather.

Afterwards, for weeks 2-3, uncertainty remains very high. I would think a GSDM Stage 1 base state (consistent with La-Nina) is most probable to persist, with subseasonal variations. Considering the seasonal cycle more strong troughs for particularly the Rockies and Plains while ridge amplification occurs across the central/east Pacific would be expected. However, if the tropical forcing shifts east along the equator to the date line (and/or a strong central Pacific flare-up occurs), the circulation could quickly transition to Stage 3.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion this Friday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

Details are Important

Not much change has occurred with the tropical sea surface temperatures (SSTs) since my posting on Tuesday (3/6). While equatorial Pacific SSTs remain cool east of 150W (~minus 1-2C), warm anomalies are still present across the west central into the South Pacific. The latter are roughly 0.5-1C extending to depths of about 100m with actual SSTs varying from 29-31C. Only weak anomalies exist to the west.

There are at least 2 important regions of tropical convective forcing. One is nearly stationary centered around 10S/120E, and may be part of a slower process linked to a transition from El-Nino to La-Nina. The other is the dynamical signal with the MJO ~15-20S/160W. About 10 days ago these regions consolidated, and there were at least 2-3 tropical cyclones in the area of Australia left in the wake.

An important point is that a MJO signal remains. In fact, this signal can be traced back to late January 2007 in the near equatorial velocity potential Hovmollers courtesy of CPC. The signal is weakening especially in the convective field while it continues to evolve as expected in the upper level wind field (see below). The convective field now has multiple regions of tropical forcing with well defined anomalous twin upper tropospheric subtropical anticyclones near 120E and also near 160W (but mainly south of the equator).

At this time upper tropospheric zonal mean anomalous easterly flow continues to propagate poleward through the northern subtropical atmosphere, with magnitudes of ~10m/s around 25N (as a response to the MJO). This has contributed to a -2 sigma global relative angular momentum anomaly. Locally, there are weak upper tropospheric equatorial westerly wind anomalies starting to interact with the subtropics.

The MJO dynamical signal is moving east at about 10 deg long/day, and it may re-emerge into the Eastern Hemisphere by late week 2 or week 3. During this time zonal mean westerly flow should increase throughout the northern subtropics possibly allowing the East Asian Jet to undercut the recent blocking across the North Pacific. Convective flare-ups from the west central-South Pacific are also probable while a stationary convective signal may remain near 120E. Forecast uncertainty remains tremendously high after week 1 but I think we will see persistence of a GSDM Stage 1 anomaly pattern.

I think the models have a reasonable handle on week1 across the lower 48 states with ridge amplification just off the west coast and a central and eventually eastern states trough. This will keep the western states relatively warm and dry with cooler and wetter weather for the central and east USA. However, while I did underestimate in recent posts the impacts from the subtropical easterlies discussed above, I do think there will be more strong troughs to impact the western USA by late week 2 into week 3. The latter is most probable during GSDM Stage 1, and also during seasonal transition into spring. In fact, should the tropical convective forcing become quite intense across the Eastern Hemisphere during week 3 (leading to MJO #3 since December 2006?) with central Pacific flare-ups, an active regime for the USA may persist well into April.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion next Tuesday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

Not What I Expected

Tropical sea surface temperatures continue to cool across the equatorial Pacific Ocean while weak-moderate positive anomalies remain from the west central to the South Pacific. In fact, negative temperature anomalies as low as -2C have recently been observed at around 120W per TAO buoy data, extending to depths of at least 200m. An evolution to La-Nina is looking more probable.

The dynamical signal with the MJO continues to propagate east at roughly 5m/s (4 deg long/day) toward the west central and South Pacific per diagnostic monitoring tools and satellite imagery. For instance, as of March 5th the Wheeler index suggests this signal to be at ~160E. We prefer to describe the recent eastward shift as part of a weather-climate or subseasonal event. These typically consist of multiple time scales, one of which is the MJO but others of which are less oscillatory.

Tropical convective forcing has also been getting better organized from the central Indian Ocean to Indonesia during the last few days, centered at about 10S/120E. Hovmoller plots of both total and anomalous OLR suggest this forcing may become a stationary possibly linked to a developing cold event. The global circulation has responded accordingly with anomalous zonal mean easterly flow propagating off the equator into the subtropical atmosphere particularly for the Northern Hemisphere (magnitudes ~minus 10m/s for the latter). Global relative AAM remains very low at about 2 standard deviations below the 1968-1997 reanalysis climatology with its tendency at roughly minus 10 Hadleys. The recent peak of AAM tendency to plus 10 Hadleys was the result of the subseasonal signal discussed above.

As predicted, GSDM Stage 1 best describes the current global circulation. However, regionally from the Asian to North American sector the distribution of circulation anomalies has become very complicated. Animations of daily mean 150mb and 250mb vector wind anomalies show twin anticyclones centered around 115E with downstream troughs near the date line. However, there are also residual anomalies linked to past forcing from the tropical west Pacific including those across the Arctic. From a synoptic view point the ridge that is most probable to be centered ~140-150W in this situation has been split in two; one portion is at the date line with the other over western North America. Most ensemble means (not details) suggested this possibility 7-10 days ago. There is some hint from recent model runs for current Gulf of Alaska trough to slowly move east and southeast into western North America during the next couple of weeks. My own thought a week ago was for an enhanced probability for this process to happen sooner and farther south than the models were showing, allowing a resumption of an active regime for much of the country. This appears less likely in the immediate future.

Whatever the case over the next 7-10 days, I think forecast uncertainty for a week 2-3 prediction for the USA is very high, despite what appears to be good model agreement of a low amplitude trough across western North America and a northward displaced storm track. In fact, I think it probable that a strong synoptic scale trough will impact the western and central USA with the extreme weather typical of GSDM Stage 1 during weeks 2-3. This would mean at least one prominent “southwest flow cyclonic baroclinic event” across the Plains similar to the 2 recent events. Until then, a rather benign weather pattern with warm and dry across the southern states and a bit unsettled across the north can be expected.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion this Friday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann

Brief Update -- Global models may perform like the recent global markets

The dynamical signal with the MJO is centered at ~10S/140E per several monitoring tools including the multivariate EOF Wheeler index and animations of circulation anomalies. Rough calculations of its phase speed give me anywhere from 5-8m/s depending on what variable you use; for example, velocity potential or OLRA. In any case, this recent MJO event has been a very difficult one to monitor and understand. In fact, I think what has happened during the past week is a consolidation between the MJO dynamical signal and westward propagating convection associated with west Pacific SST boundary forcing. The latter is a slower process that started about mid-January 2007 and may be part of the evolution of a transition from El-Nino to La-Nina.

My point in this short writing is I do not think the global models have “much of a clue” of the responses to the above. Global relative AAM tendency is ~plus 10-15 Hadleys per reanalysis data with much of that signal coming from the equatorial and subtropical atmospheres of the Western Hemisphere. Contributions are from the tropical forcing crossing Indonesia and an increase in the global frictional torque (while global relative AAM remains ~minus 2 sigma). I think the global circulation is trending toward GSDM Stage 2. This argues for ridge amplification across the eastern Pacific (~140W given seasonal cycle) during the next few weeks, most likely week 2. The global models (ensembles) do not depict this very well. They generally want to persist a trough across the Gulf of Alaska downstream of the central Pacific blocking.

I would not want to be a person investing money banking on “good” global model performance during the next few weeks. A decline in, for example, the skill of week-2 ensemble model means going into the middle of this month, appears probable (similar to the recent global markets?). Sources would include not only the excitation of a west Pacific wavetrain forced by Rossby wave energy dispersion tied to the east Indonesia/west Pacific tropical forcing, but also seasonal transition. My thought would be for the Gulf of Alaska trough to dig southeast along the USA west coast (impacting ARB) during week 2, then shift into the Rockies leading to a resumption of a stormy regime for much of the USA. This would be similar to recently observed, but with an Arctic cold air source. If this MJO signal stalls, say ~150E, then shifts back to west, this regime may persist for a period longer than 1 synoptic event.

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 a more complete discussion ~Tuesday next week while at ESRL/PSD. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

MJO or not?

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

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

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

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

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

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

Right on track, but not El-Nino!

Tropical convective forcing has consolidated around 140-160E along the equator while only sporadic convection persists across the South Pacific. In addition, tropical forcing has also been increasing across northern South America and South Africa. The latter is a response to the remnant dynamical signal of the December-early January MJO interacting with the Southern Hemisphere (SH) extratropics.

A strong Rossby wave energy dispersion (RWD) linked to the west Pacific tropical forcing is exciting the positive phase of a western Pacific wave train as I type (with SH symmetry). This pattern resembles the positive phase of the Pacific-North American teleconnection (PNA) but shifted west by about 20 degrees of longitude. As has been expected for about 10 days, bitterly cold Arctic air from central Siberia is currently being transported across the North Pole and will plunge into the CONUS by the end of this work week. The initial surge of cold air should be into Montana, with the Northern Plains, Upper Mississippi Valley and Great Lakes states being impacted the most severely.

So, what is the most probable evolution of our weather-climate situation during the next few weeks, given uncertainty and inadequate model guidance? Specific diagnostic factors to consider include transports and tendencies of relative angular momentum (AAM), the evolving SST and SST anomalies, the subseasonal tropical convective forcing and the blocking becoming established at the northern polar latitudes (with a possible warming of the stratosphere).

Poleward propagation of zonal mean zonal wind anomalies continues, with moderate westerly anomalies throughout the subtropical atmospheres, easterly anomalies around 50N and westerlies again farther north. The global signal of relative angular momentum is quite impressive with positive anomalies of roughly 2 standard deviations as of January 27th based on the reanalysis climatology. However, as discussed in past postings, this westerly flow did not evolve into a strong combined North Pacific jet typical of a warm ENSO. Strong easterly anomalies in the subtropics preceded the convection increase near the date line and may have interferred. Only during the last few days has there been a weak reversal of AAM transports with westerly flow being fluxed equatorward from the midlatitudes.

Looking farther out, a renewed active phase of the MJO may develop from South Africa into the southwest Indian Ocean during roughly weeks 2-3. Coupling with the warm west Paciific SSTs looks probable centered ~10S/160E while at least diurnally intense convection occurs across much of Brasil. This would suggest a return to a GSDM Stage 4-1 (La-Nina like) response meaning zonal mean easterly flow anomalies should re-appear across the deep tropics while the tendency of relative AAM becomes negative. For PNA sector, the large ridge currently developing west of Canada may shift northwest to Kamchatka during week 2 while the above mentioned anomalous subtropical westerly flow ”undercuts” the east Pacific ridge. Going into the middle of February there may be a situation of a cold trough extending from central Canada to just off the USA west coast interacting with a moist subtropical jet.

As mentioned above, the cold regime is on track for most of the USA for week 1. While there is likely to remain a cold air source for especially the northern states weeks 2-3, moderation of temperatures is expected. Intense convective lake effect snow is a good bet for week 1, with only light snow events across the Northern Plains on east. Portions of the Deep South to the east coast will need to be monitored for possible significant wintery precipitation. This whole pattern should shift north and west weeks 2-3, with possibly an active southwest flow storm track across the Plains by the middle of February. Much of the USA west coast should also finally get some decent precipitation weeks 2-3.

Please note: These are probabilistic statements, which we will try to quantify in future posts.

Ed Berry and Klaus Weickmann

El Nino weakening, west Pacific convection not the answer

The following is a posting that was placed on the HMT Blog at

http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/

Please go to that link if you would like to see some figures.

An MJO excited deep convection near the date line in early January 2007 but a "combined" Pacific jet and eastward shifted storm track did not develop. A persistent pattern of subtropical easterly and mid-high latitude westerly flow anomalies may have intervened. Date line convection is now weakening; a portion is shifting southeast into the southern hemisphere and another is shifting west toward the west Pacific. Combined with eastward shifting convective activity over the Indian Ocean, a consolidation of positive convection anomalies around Indonesia has already occurred. This activity may eventually spread over the warmest SSTs currently in the west Pacific, south of the equator.

The current short term amplification and retrogression of the circulation anomalies over the Pacific Ocean will dominate the weather patterns during the next 1-2 weeks. Prospects do not look good for rain along the west coast during this time. The behavior of the circulation beyond week 2 is partially linked with the strength and location of tropical forcing. The Indian Ocean SSTs continue quite warm and we expect convection to redevelop there, possibly aided by a dynamical component from the recent MJO. Convection should also stay active over the west Pacific. If the west Pacific convection dominates (~GSDM Stage 2), a very boring weather pattern may be in the offing for the USA west coast beyond week 2. Increased activity over the Indian Ocean would lead to a better chance for a trough along the west coast and more favorable prospects for the American River Basin (ARB) (~GSDM Stage 4-1).

Please see our January 23rd posting for our weeks 1-3 outlook for the rest of the USA (CONUS). The cold regime scenario along with the precipitation concerns expressed in that discussion remain unchanged. Most models and their ensembles have come into good general agreement through at least days 7-10. We will try to do another posting early next week.

Klaus Weickmann and Ed Berry

No fooling around; possibly severe cold regime for a large portion of USA appearing more probable by week 2

Tropical convective forcing has been persistent across the South Pacific along the SPCZ for roughly a couple of weeks. This response was expected given the December 2006-early January 2007 MJO and warm SSTs (~1-2C anomalies) associated with ENSO. The global circulation has been responding with the recent large tendency in global relative atmospheric angular momentum (~30 Hadleys) and the appearance of twin upper tropospheric subtropical anticyclones around 160W. Rossby wave energy dispersion linked to these twin anticyclones (with inter-hemispheric symmetry) may be contributing to the onset of North Atlantic blocking (negative phase of the NAO). There has been poleward propagation of zonal mean westerly wind anomalies resulting in 5-10m/s anomalies at 200mb in the subtropical atmosphere. GSDM Stage 3 best describes the current global circulation pattern. However, as discussed in our January 19th posting, this El-Nino-juiced forcing has been evolving in a more persistent regime that developed just prior to December 1st. The regime has been characterized by transport of westerly momentum out of the tropics into the higher latitudes. The forcing from the South Pacific has not changed this La-Nina like (GSDM Stage 1-2) pattern.

Rossby wave energy dispersions within the regime have led to a revival of tropical convection across the East Indian Ocean into Indonesia. Fast baroclinic wave packets moving through South Asia are interacting with this forcing leading to storm development across the west and central North Pacific. A discontinuous retrogression and amplification of the North American ridge into the Arctic is expected within the next week. While a convective signal in the South Pacific is expected to persist (allowing a subtropical jet across the Deep South at times), tropical forcing may become quite robust near 120E by sometime week 2, in which case, the ridge may retrograde to 150W. The ESRL/PSD and other model ensembles lend support to this scenario. In fact, blocking may develop all across the polar latitudes as we go through February. Another MJO may also develop across the Eastern Hemisphere next month which could lead to a further demise of our warm ENSO event.

The screaming message for a good part of the country is a turn to much colder temperatures by about a week from now. Bitterly cold Arctic air that has been “bottled up” across much of Siberia is likely to plunge into the USA with a few surges, likely centered on the Northern and Central Plains/Upper Mississippi Valley. Strong winds and brutal wind chills will be probable. Best opportunities for significant wintery precipitation should be from the central-southern Rockies into the Tennessee Valley, Deep South into the mid Atlantic States. Lighter snowfall events may occur with the Arctic surges (along with intense convective lake effect snow). This whole precipitation pattern may shift northwest later weeks 2 and 3 (troughs across the Rockies/western states) while temperatures slowly moderate. By around week 3 the ridge should be far enough northwest to allow strong and moist westerly flow (including “undercutting”) to impact the west coast with significant precipitation especially California.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale on which a reversal of the transports could occur

Ed Berry and Klaus Weickmann

El- Nino is trying, but too little too late?

The following is a version of a posting to the HMT forecast at

http://www.esrl.noaa.gov/psd/programs/2007/hmt/forecast/

Figures to go along with the discussion are on that Blog.

The strong MJO that came out of the Indian Ocean in early January has weakened considerably. As expected, it excited convection near the date line and attempted to generate a strong combined Pacific Ocean jet stream. This MJO also helped force the cold regime over the USA that we discussed on 29 December 2006 (on the HMT conference call). However, two factors indicate our scenario of a strong combined jet stream over the Pacific for this winter is now less likely.

First, since about 1 December the atmospheric momentum transports have been moving momentum out of the subtropics and into mid-higher latitudes. This pattern has been so strong that the forcing produced by the MJO over the warm El Nino waters appears only as a small perturbation in a persistent flow regime. Regionally this zonal mean regime is characterized by split flow patterns, especially over the oceans. Convection is currently increasing over the Indian Ocean and Indonesia while a portion of the convection at the date line is moving westward at ~4 m/s as an equatorial Rossby wave. This combination suggests convection will become centered somewhat to the west of its current position during the next 2-3 weeks, possibly ~0/140E. Implications would be for a retrogression of the east Pacific-North American ridge (from week 1) to perhaps 140-150W (weeks 2-3) and a better shot at the subtropical jet undercut scenario favoring precipitation along the USA west coast, especially California.

The second factor is more relevant for the atmospheric circulation beyond week 3. The onset of the Southern Hemisphere monsoon (also associated with the strong MJO) has produced strong anomalous northerly flow across the equator. This appears to be linked with an amplification and deepening of the cold water (negative anomalies) below the equator in the Pacific Ocean. This cold signal may continue to deepen and eventually reach the surface putting an end to the basin wide aspects of this El Nino.

For the USA, after the storm system and surge of cold air this weekend into early next week, much of the western two-thirds should have a moderating temperature trend. There may be storm development close enough to the east coast that may lead to significant winter weather for inland locations. During weeks 2-3 (~late January-mid February), a return to a cold regime, similar to what has already been experienced for the last 7 days or so, appears probable. However, this time there may also be a source of Arctic air from Siberia (where temperatures are currently lower than minus 50C, but anomalies are still slightly positive) as well as Alaska, meaning possibly more severe cold than observed last week. Additionally, significant winter weather hazards would again be possible from the Rockies into the Plains with heavy rain and thunderstorms (possibly severe) across the Deep South. As discussed above, precipitation may also increase along the California coast especially week 3.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale which a reversal of the transports could occur.

Klaus Weickmann and Ed Berry

The “Truth” lies with Understanding Reality

Tropical convective forcing continues to reorganize across the Eastern Hemisphere. The MJO signal has weakened significantly. The intense thunderstorm activity currently along the South Pacific Convergence Zone (SPCZ) was initiated by the downstream troughs associated with the MJO and not a coherent eastward propagating convectively coupled mode. In fact, the western portion of this tropical forcing is shifting west as a convectively coupled Rossby mode.

While there was a strong signal of the MJO during much of December into early this month, several complex forcing-response-feedback circulation variations occurred. To save space, what I hope is most relevant to the readers is discussed here (comments welcomed). First, there was a strong trade wind surge downstream of the MJO that led to significant cooling of the warm central Pacific SSTs around the date line (anomaly and actual SST tendencies ~negative 1C). In consideration of other monitoring tools such as relative atmospheric angular momentum (AAM) transports, my feeling is that the warm ENSO peaked during December and is now decaying. The recent anomalous low level westerly winds along the SPCZ (weekly mean anomalies ~15m/s centered on the date line around 10S) I feel is transient and not the beginning of a “coupled warm ENSO response” (which is different than what I would have thought about a month ago). Secondly, as was discussed in my posting dated December 15th, a regime transition to cold/wet pattern for much of the USA occurred starting about a week ago (more said below). In fact, a negative phase of the PNA teleconnection index evolved which is consistent with the GSDM Stage 1 response that occurred (and not with the composite warm ENSO signal).

Per ESRL/PSD reanalysis data plots, as of 3 days ago the global tendency of relative AAM was ~ plus 25 Hadleys. As the MJO shifted east, zonal mean westerly flow anomalies initially developed along the equator and then propagated poleward, contributing to this positive tendency. These westerlies are currently in the subtropical atmospheres of both hemispheres with anomalies ~5-10m/s at 200mb (contributing to the current subtropical jet across the southern USA). Hence a nice evolution from GSDM Stage 1-2 has occurred. However, unlike my thinking from about a month ago, it now appears unlikely that a coupled ocean-atmosphere response involving the tropical forcing east of the date line will occur, typical of a warm ENSO (leading to GSDM Stage 3). Instead, this process may happen ~10S/160E during the next couple of weeks and then shift to perhaps 120-140E during February. As I type tropical forcing is once again increasing across the Indian Ocean where SST tendencies during the past week were ~plus 1C, and twin upper tropospheric anticyclones are already appearing.

My outlooks for weeks 1-3 remain unchanged from our January 12th posting. It remains most probable for the trough-ridge-trough pattern from the west/central Pacific to the central USA to persist, with the usual synoptic variations of amplitude. The subtropical jet should continue to undercut the east Pacific ridge leading to split flow along/off the west coast and a cold/wet regime for much of the country particularly east of the Continental Divide. If my notions of the centroid tropical convective forcing shifting back to ~10S/120-140E during February are correct, the above circulation pattern would be expected to do the same allowing locations along the USA west coast to get precipitation especially California (loosely GSDM Stage 4-1).

We are planning on issuing another discussion this Friday, along with a parallel posting on the HMT Blog. The MWR WB paper on the GSDM is expected to appear in the February 2007 issuance.

Ed Berry