Ocean heat content helps drive Irma

Contributed by Nick Shay (nshay@rsmas.miami.edu)

Given favorable atmospheric conditions, Irma has intensified to category 5 status in the past few days. Central to this intensity increase is the relatively high ocean heat content values (80-120 kJ cm-2) as shown in the image relative to the projected track of Irma based on satellite remote sensing. As she approaches the Florida Straits, the high heat content water coming through the Yucatan Straits into the Gulf is essentially taking a quick right turn out of the Gulf of Mexico since the Loop Current is now retracted after separating a large warm core eddy in the eastern Gulf. The exiting water from the Gulf of Mexico forms the core of the Florida Current. Given the large current speeds of 2 m s-1 as observed with High Frequency radar measurements, this current advects the warm water through the Straits forming the core of the Gulf Stream further north in the SECOORA footprint. Given both the current strength and depth of the warm water, any ocean mixing induced by hurricane force winds is arrested.  Thus, there is very little sea surface temperature cooling in the remotely sensed signals and the air-sea fluxes are allowing Irma the storm to intensify (i.e. there is more deep warm water to help intensify Irma via the air-sea fluxes).

ohc_aQG3_2017_249_0600_irma_int

Note the white space is the figure is Cay Sal Bank which is very shallow to calculate ocean heat content from our 2.5 layer model. The water is quite warm over that feature and the ocean heat content is not zero.

An ocean experiment is planned using a combination of drifting buoys and APEX-EM floats to measure the ocean response to Irma in collaboration with SIO and AOML scientists. Details will be forthcoming.  However, the predicted track continues to be problematic at the present time due to the weakening of the subtropical ridge over the Atlantic and the possible influence of a short wave trough that is currently digging southward. The bottom line is it is a race to see what happens with the track of this very powerful hurricane that has approached its maximum potential intensity over high ocean heat content water.

The above Irma figure is updated daily with altimetry data and track information on the Atlantic Ocean website:  http://isotherm.rsmas.miami.edu/heat/weba/atlantic.php

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