Masters student in tropical meteorology at FSU. Raised in Alaskan blizzards, but drawn toward tropical cyclones by their superior PGF.
By: Levi32 , 1:51 AM GMT on April 26, 2007
As June rushes closer, conditions and patterns for the upcoming season are beginning to reveal themselves. Not surprisingly, some of the changes so far are different than what we expected a month ago. The La Nina which we thought would be so strong this summer may not end up as potent. As I stated in a blog a month and a half ago, several key factors were and still are not present to support a strong La Nina. A big one is the SOI, which instead of going positive, which is the normal case during a positive ENSO, is tanking negative, which we normally see during El Nino conditions. This is altering precipitation patterns across the equatorial Pacific regions, causing the ENSO Precipitation Index (ESPI) to rise (now nearing zero again). Here is a look at the latest SST anomalies:
Back in February, cold anomalies showed up in the central Pacific, where the ENSO is thought to make its strongest impression on the SSTs. However, these anomalies have been replaced by normal to warmer than normal waters, and the cooler anomalies reappeared in the eastern Pacific close to South America a month ago. This is not indicative of a coming strong La Nina to me. If there were, the central Pacific would be cold and not warm. To me, this looks more like a neutral pattern with a cold bias factor in the ENSO. This could be both a good and a bad thing for the upcoming hurricane season. On the good side, neutral to El Nino conditions generally increase wind shear and the number of upper-level lows and troughs in the Atlantic. We saw this last year, when the jet stream consistently dipped south and ripped apart many a potential storm during the entire season. The bad news is that a neutral pattern usually produces the most named storms on average in the Atlantic. The classic example of this was of course 2005, when we had 28 named storms. The ENSO was neutral that year. So, the key will be how the upper pattern sets up over the Northern Hemisphere over the next couple months. The position of the Bermuda High will be closely watched, as well as wind shear anomalies, which have been fluctuating up and down over the last several weeks. Another factor has been the ESPI, which moistened up the bone-dry Atlantic quite nicely last month when it went negative. But the ESPI has been trending back up for 2 weeks now, and the results are clear on the water vapor imagery. The Atlantic is bone dry again in most areas.
All these factors will come together during May and June to give us a picture of what the hurricane season could be like. Right now nothing much is certain. I don't believe we will have a strong La Nina this year, and as I said I think a more neutral pattern with a cold bias is likely. I'm sticking with my season predictions that I posted 2 weeks ago until I see a need to change them.
We shall see what happens!
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