Oceanography Not-So-Fun Fact

I suppose most everyone knows that warm ocean temperatures are necessary for the formation and intensification of hurricanes, but the real story is more complicated, and very interesting.

It's necessary for sea surface temperatures to exceed 26 C to sustain a hurricane. The surface temperatures of the entire Gulf of Mexico are well above this value in the late summer and fall. On the other hand, hurricanes stir up the water, mixing warm surface waters with colder waters just below. By the time the eye of the hurricane (which is what matters) "sees" the surface temperature, the mixing may result in temperatures than 26 C. Therefore, in order for hurricane intensification to be favored, the warm waters must extend deep below the surface, and satellite sea-surface temperature maps, which only show the very skin of the ocean, aren't particularly useful to predict hurricane intensification.

Satellite altimetry, which measures sea surface height, is very useful on the other hand. Warmer waters, being less dense, "float higher" than colder waters. In order to show up in the altimetry the warm waters have to be of significant depth, on the order of tens of meters. In the Gulf of Mexico deep warm water can be found primarily in the Loop Current, which enters the Gulf between the Yucatan and Cuba, looping around Cuba to form the Florida Current and ultimately the Gulf Stream.

Along the way the Loop Current usually form a big loop out into the Gulf. Sometimes this loop gets pinched off, forming a ring or eddy that spins off into the middle of the Gulf and beyond. Like the current, the rings are deep, so this can leave large patches of warm deep water in the path of hurricanes headed for the Gulf coast. In the image below you can see something called the Tropical Cyclone Heat Potential, computed from altimetry data. The big blob in the middle of the gulf is a warm ring that has pinched off the Loop Current. Superimposed on this is hurricane Katrina's extremely unfortunate trajectory, color-coded so that you can see it intensifying. You can also see why it deintensified slightly before coming ashore.


Thanks to NOAA/AML

I've been looking at some altimetry data for hurricane Rita (now a category 4) and trying to interpret it. So far it looks pretty bad, but I'll post something later.