Tuesday, March 27, 2007

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

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