Atlantic Hurricane Season About to Get Busy

The North Atlantic has been very quiet since Hurricane Alex in late June. Alex was the strongest storm in terms of wind speed in the month of June since Alma (1966). However, and according to NOAA’s 2010 Updated Atlantic Hurricane Season Outlook issued on 5th August, the Atlantic Basin remains on track for an active season. The three main indicators are:

– Expected continuation of tropical multi-decadal signal. Key components of this signal include an enhanced west African monsoon circulation and above average SSTs in both the lower and higher latitudes of the North Atlantic. In the tropical Atlantic and Caribbean Sea atmospheric aspects of the tropical multi-decadal signal seen since 1995 include reduced vertical wind shear, weaker easterly trade winds, and an extensive area of cyclonic shear at 700-hPa along the equatorward flank of the African Easterly Jet. These atmospheric and oceanic conditions, that are now in place, are conducive to hurricane formation;

– La Niña. It refers to a periodic anomalous cooling of SSTs in the central and eastern equatorial Pacific Ocean. This cooling affects rainfall patterns across the tropical Pacific which, in turn, alters wind patterns so as to reduce the vertical wind shear in the tropical Atlantic and Caribbean Sea. Consequently, La Niña is typically more conducive to increased Atlantic hurricane activity (Gray, 1984);

– Above average SSTs. Currently the SSTs in the tropical Atlantic and Caribbean Sea are 1-2C above average and are expected to be much above-average to near-record during August-October. This warmth is much larger than anywhere else in the global tropics, and is a further indication that climate conditions are conducive for hurricane development in the Atlantic basin.

In the next couple of weeks we should be heading into a more favorable large-scale regime for tropical cyclone formation according to the latest Madden-Julian Oscillation forecasts. Klotzbach (2010) showed that when the MJO is located in Phases 1 and 2 (convectively active over the Indian Ocean), it reduces vertical wind shear in the tropical Atlantic, thereby providing a more conducive environment for formation on a shorter time-scale basis. The GFS ensemble is hinting that the MJO may be amplifying in the Indian Ocean in the next couple of weeks and the weather pattern will also become more conducive for development so we are heading towards a busier second half of the month.

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