Friday, February 09, 2007

Slowing atmosphere means more active weather for the lower 48 states?

Global tropical SSTs remain above average across most of the Atlantic into the east Pacific, as well as the South Pacific and much of the South Indian Ocean. Magnitudes are roughly plus 1-2C. Cooler than normal waters are present around much of Indonesia while weak pockets of negative anomalies are starting to appear along the central equatorial cold tongue. The former is due to recent persistent intense tropical thunderstorm activity while the latter is a response to our rapidly collapsing warm event. After the December 2006 to early January 2007 MJO, SSTs have been cooling particularly from about the equatorial date line region to ~140W. This MJO initiated a strong trade wind surge which led to upwelling of cooler subsurface waters as well as generating an upwelling oceanic Kelvin wave. In fact, latest TAO buoy subsurface data indicates negative anomalies ~minus 5C at roughly 50m around 120W. Anomalous cross-equatorial northerly flow has continued this cooling process. We are at the point to be concerned about a La-Nina evolving as we head into boreal summer.

As discussed in past postings, the global circulation has been generally La-Nina like since about late November. Tropical convective forcing from the Indian Ocean dominated during much of the fall and helped to add anomalous zonal mean easterly flow to the subtropics. This contributed to circulation regimes across the PNA sector not consistent with an El-Nino composite (episodes of GSDM Stage 1 and 2 situations). When the tropical forcing shifted into the South Pacific (in a very complicated manner) by late January, a weak ENSO response did occur (GSDM Stage 3). However, it was only a perturbation on an already established base state (one must always pay attention to the initial conditions). Global relative atmospheric angular momentum increased to about 2 sigma above the 1968-1997 reanalysis data climatology, with much of that coming from the Southern Hemisphere. In fact, the anomalous tropical forcing along the South Pacific Convergence Zone actually played a role in establishing the intense ridge into the Arctic observed off the USA west coast late January. This was an unusual situation where there was a cold regime across the USA (lower 48 states) with GSDM Stage 3.

Our warm event peaked in November-December 2006, which may be a trend that started with the 2002-03 warm ENSO. We also see a situation where there may be an alteration of El-Nino-La Nina having a quasi-biennial time scale, suggesting it is now “time” for a cold event. In my own mind I also see complex forcing-response-feedbacks meaning our recent trend of multiple regions of tropical convective forcing that started during 2001-02 has impacted ENSO (which may be a global warming signal). In our recent case, the Indian Ocean forcing during fall, unusually intense for an El-Nino, sent the circulation into La-Nina first. The tropical Pacific SSTs may now be responding afterwards (think about this).

Tropical convective forcing has been shifting back west into Indonesia since late January (weakly projecting onto a convectively coupled equatorial Rossby mode), helping to maintain the cold regime across much of the lower 48 states. Tropical forcing has also been weakening across the South Pacific. We have also been monitoring South Africa and the South Indian Ocean for a re-emergence of a moist MJO. Since the mid January eruption convection along the SPCZ, there has been a weak dynamical signal moving through the Southern Hemisphere. Interacting with the extratropics, tropical forcing has been increasing across South Africa into the Indian Ocean for about the past week (with a couple of tropical cyclones). I think this notion looks reasonable, and there are even a few empirical and statistical tools supporting our thinking. Tropical forcing also remains across the South Pacific and South America (mainly Brasil and the South Atlantic Convergence Zone).

Zonal mean easterly flow has been increasing throughout the tropical and subtropical atmospheres for the last week or so. In fact, the global tendency of relative atmospheric angular momentum dipped to about minus 20 Hadleys ~ February 5th assisted by both negative global mountain and frictional torques. GSDM Stage 4 best describes the current global weather-climate situation, and I think we will transition to GSDM Stage 1 (or a combination of 4-1) during weeks 2-3. Other processes contributing but not discussed in this already much too lengthy posting are the roles of high latitude blocking and a possible late season warming of the stratosphere. The latter will help to keep sea level pressures above normal across the Arctic maintaining a cold air source for the USA (even though temperatures are slightly above average across most of the Arctic, keeping in mind climatology).

At this time most of the CONUS has below normal temperatures with an active storm track from California into the eastern states. During weeks 2-3 I would expect all of this to slowly shift northwest meaning by late this month an active southwest flow storm track across from the Rockies into the Plains would be probable. Cold and moist energetic troughs would be expected to dig into the western and central Rockies and then lift northeast into the Mid/Upper Mississippi Valley. Given the South Pacific signal (and we need to monitor the east Pacific where there are still warm SSTs – at least for now remembering the cold subsurface), a moist subtropical jet with closed lows may interact with this storm track (GSDM 4-1 instead of Stage 1). While late winter storms would occur in the cold sectors, heavy rain and severe storms would be probable for much of the Deep South into the eastern states. In general, this would be an active pattern for much of the lower 48 states. There would be cold to the northwest and warmth for the far southeast. Alaska may stay under the ridge while closed lows may develop near/west of Hawaii. Finally, with variations in amplitude, I could see this type of pattern persisting well into spring.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/3-4/2. I will try to post another discussion next week. The WB(2007) paper on the GSDM is scheduled to appear this month's publication of MWR.

Ed Berry

1 comment:

AlanHaynes said...

Your prediction of the breakthrough of the westerlies into California this past week worked out very well.

The question I have is what were the primary mechanisms that allowed for this breakthrough? I was expecting retrogression of the blocking high over far NW North America/Alaska coupled with the eastward progression of the subtropical upper jet. Instead, the blocking high remained in place and the breakthrough occurred anyway. Is this an unusual scenario? The models sure had a tough time with the details of this transition as well.

Can you elaborate on how this transition was able to occur?