typhoon_tip
Meteorologist
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Re: Last Few Days of May, Probably Quiet
Mon May 21 2007 08:20 PM
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I find it intriguing that most of the Global based numerical models have been tantilizing us with these Caribbean teasers since...really the beginning of March. I do not recall seeing this specific and rather peculiar behavior in the models persist as often during recent springs (going back to 2000 or so), as we are now. May not mean much more than an interesting artifice of the pattern we have been in; most of these have proven red herrings to begin with. Nonetheless, as we near June this should be watched closely.
As things stand now we are stowing oceanic heat content rather nicely as ambient cloud patterns "seem" to have been rather sparse during the last month. This maximizes solar insolation for lacking cloud-based albedo.
As to the season:
My belief is that the subtropical Atlantic will have greater than normal magnitude of ridging to some degree... This assessment is based on two basics assumptions. One being the Equitorial Pacific oceanic SSTs and the El Nino vs La Nina state; the other being some recent 45 day trends in the GLAAM.
Most people are aware of El Nino and La Nina. After spending the winter in an over-advertised weak-moderate warm ENSO phase, conditions began to cool in the Equitorial Pacific waters. Particularly closer to the South American Coast this is true. Dynamical based models at that time began really blowing a strong La Nina pattern horn to emerge during this current spring. That has not happened. Instead, although Nino numbers nearer to the S/A Coast are -1.8, they are barely 0.0 in most locations. The whole field is thus only modestly cool anomalous. Nevertheless, cool anomalous it is and if anything this should continue to evolve; although doing so at a slower pace than the already proven to be zealous dynamical model sets. La Nina times tend to mean a more favorable U/A pattern for the Gulf, Caribbean and adjacent SW Atlantic Basin as a whole, for contributing weaker westerlies and a mean jet structure well north of these areas.
As to the the GLAAM, it may not be as well known. The acronym stands for GLobal Atmospheric Angular Momentum, a scalar measure of the integrated torque in the atmosphere. By torque we obviously mean tortionally, the larger scale wind structure of the westerlies. When the wind field has more N to S component, the torque is greater, and the subsequent result a greater measurement of GLAAM.
Since about early April the GLAAM has shifted below 0.0, which means, negative anomaly in the Globally intergrated torque in the atmosphere. This can be seen graphically at: http://www.cdc.noaa.gov/map/images/reanalysis/aam_total/gltotaam.sig.90day.gif
What this means is that the atmosphere has had a tendency to be more zonal than meridional in the flow, or in other words, west to east. This west to east nature supports the warmer and stronger subtropical ridging as we get later in the summer.
The GLAAM can not outrightly be used as a prognostic tool, but it is still useful for a couple of reasons. One, it is useful as a potential corrective measure when evaluating in situ specific modeling behavior. The other is persistence. We note that going back along the last 90 days there really has been a bias for less meridional flows to verify. Looking at the top panel, we also note that the majority of the negative anomaly distribution has been in the Northern Hemisphere. This might carry some significance in the earlier part of the season, in the sense that as the westerlies continue along their seasonal migration north, should the GLAAM persist negative, this would mean less incursive westerlies into lower latitudes. As we know, a huge stumbling block for westward moving tropical waves are TUTT related (less troughing/less hostile environs). The trouble with using persistence however is that it only works so long as your persisting behavior continues to manifest. If there is big modal shift in the overall paradigm, the persistence based model will demonstrate an abrupt error. ...So, for that there is natively some risk in using this technique.
Summing all this up in simple terms:
The weak La Nina favors deep layer ridging in the Gulf, Caribbean and adjacent SW Atlantic Basin;
Persistent background signal in the Northern Hemisphere suggests a potential for stronger than usual subtropical ridging, which will block potential incursive westerlies from digging to deeper latitudes where they can cause shearing environments.
The first of these two factors is the more widely studied. The second is suppositional, though certainly has its logic. There are other factors also to consider. Namely, SAL, or Saharan Air Layer. This is the milky film that can be seen in some high res satellite imagery of the central and eastern Atlantic. It has been statistically and to some extent physically shown to be a cumulus cloud type suppressor, thus, a tropical cyclone detriment. Last year there was a preponderance of this dust material from off the intra-continental areas of Africa - although it is not known outrightly if this had anything to do with the year as a whole drastically under-performing compared to earlier predictions.
We seem to be setting the table for the predictions for a more active than normal season to actually verify better this year.
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