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Alberto throws us for a loop

Overnight Tropical Storm Alberto certain threw us all for a loop! Last night Alberto was on his last legs and shear was ripping him apart. However, by early morning there were signs that things were changing. His central pressure had started to level out yet there was a huge new flare of convection. By late this morning Alberto was approaching hurricane strength and looking like a whole new storm after reforming its center near the new convective center.

Any interesting note is that of all the models, only the COAMPS (called COAL in the Atlantic) and GFDL models were forecasting strengthening to this level. The reason for this could be very clear. Both the GFDL and COAMPS are couple ocean-atmosphere models. That means that are supposed to account for the influence that the atmosphere has on the ocean and vice versa. Many of the others ignore this influence, and some are merely statistical and based on past storm.

An interesting question is what caused this rapid intensification. Well, I welcome every to think back to this winter when I wrote my article on rapid intensification caused by the Loop Current. That same mechanism seems to be in play here. A cyclonic wind stress imparted on the sea surface, by a tropical storm or hurricane for example, causes a net transport of water away from the center of the storm. The wind stress pushes water in the direction of the wind, but the Earth's rotation, or the Coriolis effect, causes the water to turn to the right, away from the center of the storm. The result is a net transport away from the center. In order to conserve mass, water must upwell from underneath to replace the water being blown away by the wind. This process is called Ekman pumping.

ekman.gif
Diagram of Ekman pumping

But what does Ekman pumping have to do with the intensity of Alberto? In the ocean, water gets colder with depth as you go down from the surface. The surface layer of warm water is actually very thin compared to the depth of the ocean. Accordingly, the deeper water that is upwelled underneath the storm is colder than the water that was blown away by the wind. This process creates a "negative feedback" since the storm causes the ocean surface to cool. Since warmer water causes a more intense storm and therefor stronger Ekman pumping, a natural balance is formed between the wind and upwelling.

In the case of the Loop Current, Gulf Stream, or any warm water current, the surface layer of warm water becomes several times deep. This limits the effect of the negative feedback created by Ekman pumping. Now instead of quickly upwelling colder water, a storm will upwell warm water from much longer thanks to the deep warm pool provide by the current. With the removal of this negative feedback, the storm will intensify if there are no other limiting factors such as wind shear and other more complicated factors.

A quick look at the sea surface temperatures in the Gulf with show the Loop Current stretching from the Yucatan northward towards the northern Gulf Coast and the back through the Florida Straight.

Because water expands when it is heated, a deeper layer of warm water will causes a water column to expand. This will cause the surface to actually be higher over a warm water current than the surrounding ocean. Using this fact, we can look at sea height anomalies, surface height departures from normal, that are measured using radar altimeters on satellites. In the chart below, the red denotes positive sea height anomalies. This means the sea surface is higher than normal and represents areas of anomalously deep warm water.

These anomalies can also be used in conjunction with sea surface temperatures and buoy data to calculate a quantity called "tropical cyclone heat potential." TCHP is roughly the amount of energy available to a hurricane. Note the high potential value over the Loop Current, and where Alberto is located.

Now that Alberto is moving off of this area of high TCHP, we very well may see him weaken again. As you can see, the Loop Current is a dominant feature in the Gulf of Mexico. We can use satellite and buoy data to construct model representations of the surface circulation in the Gulf. Below is the latest such model run.


Posted by Bryan Woods | Permalink