Great Blog from Dr. Lyons - TUTTs & SERs
Posted by JAC on 9/22/2009, 7:36 am
What's Different This 2009 Hurricane Season?

Dr. Steve Lyons, Tropical Weather Expert


A lot of people have asked me why the hurricane season has been so quiet thus far in 2009. Most ask with their own answer included, namely, water temperatures must be pretty cold out there this year. That is NOT the reason, water temperatures are nearly everywhere warmer than average across the tropical Atlantic, Caribbean Sea and Gulf of Mexico!

The "primary" answer, in my opinion, is found high up at around 35-45,000 feet in the upper portion of our tropical weather atmosphere. It is relatively simple to explain, but not to forecast! You see if we look at an average weather pattern in the upper atmosphere across the Atlantic, we find 3 primary features; the "Subtropical Ridge", the "Tropical Upper Tropospheric Trough" (TUTT) and the "Subequatorial Ridge." These features move north, south, east or west in tandem with each other partly in response to mid-latitude west winds dancing in various directions. I show the average tropical upper atmosphere weather features in Figure 1.





FIGURE 1. Average position of upper level weather features in summer/early fall across the tropical and subtropical Atlantic. Black: non-tropical west winds. Yellow:Subtropical Ridge. Red: Tropical Upper Tropospheric Trough (TUTT). Green: Subequatorial Ridge


These three features are very important for hurricane formation, or lack of it. The Subtropical Ridge does usually have weak winds along its axis (one requirement for hurricane development) and hence is a low wind shear zone, but it is also associated with sinking air and is not typically favorable for hurricane development because it does not let disturbances "breathe" very well. The elongated TUTT, always found south of the Subtropical Ridge axis, often has strong upper lows embedded in it that do not favor hurricane development and the south side of the TUTT axis normally has strong west or southwest winds, thus causing high wind shear that is not favorable for hurricane development. The Subequatorial Ridge axis, always found south of the TUTT axis, does favor hurricane development because: 1) it has light winds along it's axis and hence low wind shear, and 2) it is a ventilating axis of high pressure that favors rain and thunderstorm development and hence also favors hurricane development and strengthening. It allows hurricanes to spin up and get strong enough to blast through less favorable upper winds in other locations once it leaves the Subequatorial Ridge axis.

This 2009 hurricane season, we have a very anomalous upper level pattern (Figure 2); the three primary upper level features are displaced well south of average. Partly because mid-latitude west winds are shifted south across the eastern U.S. and western Atlantic, those west winds have displaced the Subtropical Ridge to the south. The TUTT, south of the Subtropical Ridge, is correspondingly shifted south and the Subequatorial Ridge, normally south of the TUTT has been shifted so far south it has either been over northern South America or has been completely missing, yes, missing other than in a transient form, except in the eastern Atlantic Ocean near Africa this year.






Figure 2. As in Figure 1, except for summer/earlyfall 2009


The result is that so far in hurricane season 2009, most of the central and western Atlantic Ocean has been dominated by either sinking air from the Subtropical Ridge or fast winds west or southwest winds associated with the TUTT. The result is fewer hurricanes, more short-lived hurricanes and none that have made it west of 70W longitude.

There are at least three possible reasons for this unusual upper-air pattern, some or all of which may be to blame this 2009 hurricane season:

1) Unusual non-tropical weather that includes a southward extension of non-tropical west winds that have pushed our three important tropical upper-level weather features well south of normal and made it less favorable for hurricane development

2) El Nino, which usually causes a strong Subequatorial Ridge over the east and central Pacific and causes the Atlantic Subequatorial Ridge to be weak, displaced south and east of normal or missing completely

3) Below average showers and thunderstorms over southern Central America and northern South America that have not been able to form or maintain a significant Subequatorial Ridge because there is not enough heating from thunderstorms to make one or keep one in place

It appears that all three of these may be the cause for our lack of a Subequatorial Ridge during hurricane season 2009! It is not over yet, but it appears that hurricane development may be a serendipitous event for the remainder of the fall, at least in the western Atlantic where our Subequatorial Ridge is missing.

It always becomes clearer AFTER the season is over so history will tell us how the 2009 hurricane season turns out. Just remember it only takes one serendipitous hurricane to blast your coast, so always be prepared!

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Great Blog from Dr. Lyons - TUTTs & SERs - JAC, 9/22/2009, 7:36 am
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