Saturday, August 05, 2006

Western Pacific "Explosion"

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions (link at the bottom) with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

Global SSTs have changed little since the August 2nd posting. Oceanic Kelvin wave # 3 (since March 2006) continues its slow propagation along the equatorial cold tongue currently located ~165-170W per TAO data. Surface and low-level convergence has increased ~165E tied to the tropical forcing in that region, with easterly wind anomalies ~ 5m/s across the equatorial date line area. This is greatest anomalous easterly surface flow in this part of the world since late June. Global SST information can be obtained from latest TAO data here, ESRL/PSD data here, CPC data,

and BMRC at .

From continuous monitoring and various diagnostic and dynamical tools, there continues to be evidence that the weather-climate system is tilting toward a warm event. In fact, a weak warm event may have already emerged. However, the magnitude of any possible warm event and global impacts are unclear. The following are links to ENSO discussions.

Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and remember that official USA information on anything related to ENSO comes from CPC.

A major purpose for me putting the effort to do these writings is emphasize the linkage weather and climate (within the GSDM framework). Hence I now feel the need to offer the following.

Since about the mid-1970s a region of the world oceans know as the warm pool (60e-170E, 15S-15N; see for details; also see to generate time series of various indices) has been warming and expanding. After the 1997-98 strong El-Nino, the SSTs across the warm pool have remained generally above average with a trend to shift toward the date line. The point is I am beginning to think not only may there be an ENSO signal from that area, but perhaps a process operating on a slower time-scale arguably attributable to global warming.

Since I monitor the weather-climate situation daily, I think I can loosely defend the notion that MJO variability (~50 day recurrence time scale) has been less frequent during especially the past 5 years counting 2006 (noticeable after the 1997-98 warm event). What we have been observing with greater frequency is tropical convective variability having a ~30 day recurrent time scale. We have also been seeing at times persistent episodes of multiple regions of tropical forcing, with the western Pacific tending to dominate. Without going into any more detail, I would have to be suspicious that a slower term process may have modulated the MJO. This is just one issue of our research. Please see the following plot to get a sense of the above; understanding it is based on the 2 EOFs defining the time series.

I think the weather much of the country has experienced so far this summer has had a “direct” contribution from the slow process discussed above, including the devastating floods along the USA East Coast during late June (to be given attention in our next weather-climate discussion) and the on-going drought (multi-year for some) across the western and central sections. I can also attribute the periods of excessive heat to dynamic contributions from the western Pacific tropical convective forcing (working with other physical processes described by the GSDM).

Continuing from the August 2nd writing, I think there already has been a consolidation of the SST boundary forced tropical convection and the Eastern Hemisphere dynamical signal, centered ~ 10N/140E. Recent three-day averaged OLR anomalies of ~minus 50-70 W/m**2 have been observed along the equator around 150E (see link), with the general region of rainfall extending from Southeast Asia into the Southern Hemisphere east of the date line. Yes, Southern Hemisphere baroclinic wave activity is contributing. The TNWP has become quite active for tropical cyclone development (please see for details). I expected this consolidation (with the enhancement of tropical cyclone activity) to occur, but not as soon. Finally, there is the weak dynamical signal (mostly a convectively coupled Kelvin wave) moving through central Africa about to emerge over the warm equatorial Indian Ocean SSTs. What is that going to do????? (recall the wild card scenario in the August 2nd writing).

Empirical, statistical and numerical prediction tools continue to be inconclusive for useful information about the future evolution of the tropical convection, and that may go on until further notice. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and for the details (and draw your own conclusions). These tools generally rely on a moderate to strong MJO signal, which is nearly non-existent at this time. I would like to think there would be at least some kind of respectable MJO during the upcoming Northern Hemisphere cold season following the seasonal cycle. We still have been seeing “some of that”.

Time-latitude sections of 200mb zonal mean zonal wind anomalies indicate there has been an increase in the westerly flow throughout the equatorial and north tropical atmospheres during ~ last 5-7 days. Zonal mean westerly anomalies are ~5m/s. Anomalous zonal mean easterlies continue around 35N and 25S, with westerlies on their poleward flanks (magnitudes generally 5-10m/s). The poleward flank westerly anomalies are believed to have resulted from meridional propagation of previously equatorial westerlies starting in June due to interactions with a mountain-frictional torque index cycle. This event contributed to the excessive East Coast rainfall and goes back to the linkage of a global warming signal with a synoptic event.

This distribution of zonal mean anomalies still has some symmetry to it, implying there is important tropical forcing (even though the signals are weak). The increase of zonal mean anomalous westerly flow across the tropics is attributable to the consolidation of the tropical forcing across the TNWP and particularly the equatorial flare-up around 150-160E.

Tropospheric global relative AAM is about minus 10 Hadleys based on the 1968-1997 reanalysis climatology through August 1st. The global (remember) mountain and frictional torques are ~ plus 15 and minus 10 Hadleys, respectively. The AAM tendency (which is the fundamental basis of our GSDM) has still not been updated; however, I think it is strongly positive. I believe this is so due to the global mountain torque and the rapid increase of upper tropospheric anomalous equatorial westerly flow during the last few days.

One contribution to the positive global mountain torque is from the East Asian topography. Recent baroclinic wave energy packets/fast Rossby wave energy dispersions interacting with the region of tropical convective forcing across the TNWP have been phased to favor positive mean sea level pressure anomalies along the east slopes of the East Asian north-south mountain barriers. Another contribution is starting to come from the Andes Mountains. For those who are astute on the GSDM and the Earth-Atmosphere AAM budget, there are “mixed signals”. For instance, if GSDM Stage 1 has best describes the recent global circulation, the global frictional torque should be positive. Recent anomalously low sea-level pressures across the tropics (along with a negative tropical mountain torque – that is a little better) have contributed to surface westerlies (including the western Pacific) across these areas, with additional contributions from Southern Hemisphere storm activity.

Upper tropospheric divergence has increased significantly across the Eastern Hemisphere centered ~120-130E just north of the equator. This is a direct response to the increased tropospheric heating due to the tropical convection in that region, at times forcing twin upper tropospheric anticyclones (above twin low-level cyclones – baroclinic mode). Equatorial and north tropical westerly wind anomalies are increasing from the date line into northern South America, with magnitudes ~15-25m/s (see plots for details). This westerly flow will help to suppress tropical cyclone development across the North Atlantic for ~ the next 1-2 weeks (hybrids understood).

To summarize, I think we have a weather-climate situation consisting of (for our purposes): 1) generally persistent SST boundary forced enhanced tropical convection across the western Pacific (global warming/ENSO signal?), 2) another dynamical signal of tropical convective forcing coming back around into Eastern Hemisphere from Africa, 3) the consolidation of the previous dynamical signal with (1), 4) a mountain-frictional torque index cycle, 5) rapid extratropical Rossby wave energy dispersions/baroclinic wave packets, and 6) weak or non-existent MJO. Even when all this is combined (linear super-position) any predictive signal is weak at best.

One behavior not on the list is the ~30 day mode of tropical forcing. If I have studied all the Hovmoller plots correctlyI can look at, my suspicion is (2) may be the “real” 30-day mode whereas the dynamical signal across the west Pacific may have been initiated by the Southern Hemisphere. This is in contrast to my last Blog. Whatever the case, this goes to show how weak individual components are and what a tremendous challenge it is to identify signal from literally mostly white noise.

GSDM Stage 1 still best describes the current global circulation/weather-climate situation. At some point I think it is probable the global circulation will transition to Stage 2; however, I am unclear to offer timing that would be useful. Uncertainty is very high, like it or not. The GSDM, numerical models, hybrid statistical/numerical models, etc. cannot do anything about that. Predictive information must be probabilistic.

Week 1 (6-12 August 2006): GSDM Stage 1, possibly transitioning to Stage 2, seems a reasonable option. There is good model agreement of retrogression of the intense USA ridge from the southeast states back into the central part of the country by early next week, then persisting. This would be consistent within the GSDM framework. The storm track should remain across the northern part of the country, meaning an active severe local storm/MCS track would be possible from the Northern/Central Rockies into the Mid/Upper Mississippi Valley/Great Lakes-Ohio Valley. I think we all know where the heat will be, by now. Tropical cyclone development across the North Atlantic looks to be suppressed. Please see for the latest tropical cyclone information.

Week 2 (13-19 August 2006): I am bravely going to stick with my notion from the last posting of a transition to GSDM Stage 2 possibly occurring in spite of the above, suggesting trough development may occur across the Great Lakes/eastern USA while ridge amplification occurs from initially the Great Basin into Alaska. After all, I am bound to get it right if I keep pushing it back a number of times. This synoptic pattern may retrograde going into week 3 (very optimistic!). A trend toward cooler/wetter for the Central and North Central states to at least New England, and a return to the intense heat for the western part of the country (depending on the ridge position) would be probable IF this happens. Only from initial condition information, the NCEP GFS ensemble offers some support to this option. Tropical cyclone development across the North Atlantic may remain suppressed.

Week 3 (20-26 August 2006): Unclear. During August 2004, before the 2004-05 warm event, GSDM Stage 2 was quite robust (please see our weather-climate discussions). Are we going to see that again??? I have concerns the ridge would remain across the Great Basin region, etc., instead of shifting back to ~130-140W with a ~100W trough, due to the warm pool issues.

About 3 months ago I maintained there was not much support from the weather-climate situation to support a “hot/dry” summer for Southwest Kansas. As far as I am concerned, that statement was a BIG BUST. I think the warm pool issues may have gotten the best of me. This has been a summer featuring stronger than normal subtropical ridges across particularly the western and central part of the country meaning periods of widespread beneficial rainfall have been much less likely. In fact, stronger than average western USA ridges have been a problem since ~summer 2000, and may be part of a longer-term signal linked to a warming western Pacific warm pool.

Understanding we could get a surprise event or 2 (and I hope so), the next three weeks still looks like a continuation of above normal temperatures and below normal rainfall for Southwest Kansas. At this point, anybody can make that statement and have the odds on their side to be “correct”. It may take the seasonal transition to fall to increase any opportunity for repeated widespread beneficial rainfall, understanding the climatology here (falls are generally dry). Perhaps we could get a window of opportunity between summer and fall for precipitation.

The time -filtered coherent modes Hovmoller plots of OLR and OLRA are at, velocity potential Hovmollers at , and an animation of velocity potential overlayed on OLRAs are at

Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa, ; other imagery here. Latest tropical cyclone statements can be found from, while the latest 3-day averages of OLR totals and anomalies and other data can be found here (animations of various fields from the operational data) (Global Tropical Hazards Assessment available from this site, along with other useful information) (SST information going back to January 1982). (composite on time series, etc.) (reanalysis AAM plots) (operational AAM plots)

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts (link to our Weather-
Climate discussions) (model performance; please navigate to others)

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from the Storm Prediction Center not only for severe storms, but also fire weather concerns. Finally, the CPC USA Hazards Assessment for offers additional insights not only for possible week 1 high impact weather, but week 2 as well.

I will try to do another posting about the middle of next week. Work is also on-going to post a weather-climate discussion on the ESRL/PSD MJO web site hopefully by about the middle of August (pushed back). I may have to postpone an update to this Blog next week to get a draft of the weather-climate discussion done.

Ed Berry

No comments: