Despite Relatively Cool Atlantic Waters Irma Formed Into Major Hurricane: Why?

  • Date: 09/09/17
  • Judith Curry, Climate Etc.

The surprising thing about Irma’s development into a major hurricane was that it developed over relatively cool waters in the Atlantic – 26.5 C — the rule of thumb is 28.5 C for a major hurricane.

Irma became a hurricane on 8/31.  CFAN’s calibrated track probabilities are shown below for ECMWF (top) and NOAA GEFS (bottom).

For reference, the sea surface temperatures (ECMWF operational analysis) is shown below.  Irma formed where SST was about 80F (26.5C).

In a matter of a few hours, Irma became a major hurricane.  The surprising thing about this development into a major hurricane was that it developed over relatively cool waters in the Atlantic – 26.5C — the rule of thumb is 28.5C for a major hurricane (and that threshold has been inching higher in recent years). On 8/31, all the models were predicting a major hurricane to develop, with some hints of a Cat 5.

So why did Irma develop into a major hurricane?  We can’t blame 26.5 C temperatures in the mid Atlantic on global warming.

The dynamical situation for Irma was unusually favorable.  In particular, the wind shear was very weak.

Further, the circulation field (e.g. stretching deformation) was very favorable for spinning up this hurricane.

Irma intensified to Cat 5 on 9/5.  This intensification was picked up by the models on 8/31, but at this point it was mostly a case of ‘right answer for the wrong reason’, since the models hadn’t yet settled down regarding track.

Astonishingly, Irma has hung on to its Cat 5 status through the time of this writing. […]

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