Day 12: Sea Surface Temperatures in the Florida Keys
As one might imagine, the latest cold air outbreak in the southeast United States has had a chilling effect on sea surface temperatures (SSTs) in the shallow water areas along the Gulf Coast. SSTs along the coast of Louisiana and in Apalachee Bay are currently running in the upper 30Fs where as south of the Florida Keys, the SSTs are running in the mid-to-upper 70Fs. What is striking about the image below is the strong gradient (or contrast) in the SSTs between the waters immediately north of the keys (low 60Fs) and immediately south of the keys (mid-to-upper 70Fs). Are the keys really large enough to stop such a cold airmass from cooling the waters between Key West, FL and Cuba? The answer is “No”. The real reason for the strong gradient in the SSTs has to do with the depth of the water at each location.
To understand how the depth of the water has an effect on the SSTs let us begin by understanding how a lake freezes. First, let us divide the lake into a bunch of horizontal layers and for the sake of argument let’s assume that each layer is 1 foot thick (in reality each layer is infinitesimally thin). So, if a lake is 10 feet deep in the middle and 1 foot deep along the shore we would have 10 layers in the middle and 1 layer along the shore. When cold air (in this case sub 32F) comes in contact with the warm waters (32+F) of a non-frozen lake, the cold air acts to cool the surface layer of the water. As the surface layer cools it becomes more dense (heavier) than the layer below it. Because the current surface layer is more dense than the layer below it, the current surface layer sinks until it reaches a level that has equal density or it reaches the bottom, whichever occurs first. Since our previous surface layer has now sunk, a new, warmer layer rises to the surface. As this new surface layer interacts with the cold air above the water the surface layer cools, becomes more dense, and then sinks. This process continues over and over until the top layer is able to reach 32F without sinking. At this point the surface layer freezes, and a coating of ice results on the top of the lake. This is why lakes tend to freeze in the shallow portion of the lake first – namely, the places along the shoreline – and then freeze toward the middle of the lake (which is most likely the deepest portion of the lake) – there are fewer layers near the shore that must rise to the surface and cool before a layer can begin to freeze without sinking.
Applying this to the ocean, places that are shallower have fewer “warm” layers beneath the surface that can replace a surface layer that has cooled. The waters north of the Florida Keys are quite shallow, and as such can cool quite quickly. The waters between the Florida Keys and Cuba are deep, which means that in order to change the SST by any appreciable amount, the cold air must remain in place long enough to allow all the layers in the ocean to cool. As you can imagine this can take a long time. Thus, in places where the ocean goes from very shallow to very deep over a short distance, strong SST gradients are often found during cold air outbreaks.
The image above displays the satellite derived sea surface temperatures [SSTs] during the recent cold air outbreak. The image is courtesy of the Cooperative Institute for Meteorological Satellite Studies [CIMSS] blog.
