Friday, February 23, 2007

MJO and Atmospheric Angular Momentum (AAM) - Reprisal

There is not much “new” information I can add to my posting from 2/20. SSTs generally continue the trend of cooling along the equatorial cold tongue while above average warmth remains from the South Indian Ocean into Indonesia. Above average SSTs also linger from the equatorial date line into the South Pacific. A transition from El-Nino to La-Nina must be monitored during the upcoming months. Such a transition could have implications for the upcoming Atlantic hurricane season.

Tropical convective forcing continues to get better organized across the Eastern Hemisphere. Remembering that we have a slowly evolving situation, many monitoring and diagnostic tools suggest that at least a weak-moderate signal of the MJO is present centered ~10S/80-90E. The development of at least 3 tropical cyclones across the South Indian Ocean during the past week is a response. I think this MJO signal has “stalled”, and that may be part of the evolutionary process of transitioning from El-Nino to La-Nina.

As of February 19th, ESRL/PSD reanalysis data plots of global AAM tendency were still ~minus 50 Hadleys. Contributions were from large negative global mountain and frictional torques as well as deep zonal mean anomalous easterly flow (~5-10m/s) throughout the tropical and subtropical atmospheres. A trade wind surge from the South Pacific into the South Indian Ocean is currently lessening the negative frictional torque and I suspect this may become positive during the next week or so. Finally, about a week ago the general pattern, observed since ~December 1st 2006, of westerly flow being transported out of the subtropics into the northern extratropics resumed. The latter is a characteristic of La-Nina situations and is described by Stage 1 of the GSDM.

I think we are still in GSDM Stage 4 (due to the very large negative global AAM tendency) and it is probable the global circulation will evolve into Stage 1 during the next 1-3 weeks. The East Asian Jet (EAJ) retracted significantly during the past week and it should stay that way. In fact, as some ensemble members and week-2 means show, there may be a surge of cold air into the east Pacific during that period due to this retraction. However, to me this would only be one of those “lulls” for much of the USA (except, of course, the west coast) in an otherwise stormy regime with an Arctic cold air source. What may persist this situation would be a “stationary MJO signal” ~100-120E while flare-ups of convection occur from the west central into the South Pacific. Rossby wave energy dispersions tied to the MJO signal would favor western USA troughs that would subsequently interact with subtropical jets forced by the central/South Pacific signal. My outlook from now through March 2007 posted on February 20th remains unchanged.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post another update on about next Tuesday. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.

Ed Berry

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