Tropical Storm Chris is currently off the East coast of the USA and isn't moving very much, despite being in a very favourable environment for deepening we are seeing Chris only slowly develop before it gets caught up in an upper trough later in the week.
We are taught that tropical systems deepen when there is little wind shear to affect the structure of the system and also when they have warm sea surface temperatures of above 27c which allows the heat and moisture to fuel the development. But with Chris, even though it resides in these conditions they are only having a slight effect on the intensity of the system... why?
The main reason is that Chris isn't moving. There is little in the way of steering mechanisms as it remains trapped in a break in the subtropical ridge. When tropical systems don't move they do two things. The first is evaporative cooling of the sea surface. The process of the heavy and intense rainfall on the sea leads to spray, which combined with the strong winds transports the energy into the troposphere above. This process is called evaporative cooling and slowly cools the surface of the sea which reduces the amount of heat and energy available.
The second is far more important and is called upwelling. The strong winds around a tropical system produces stress on the sea surface. As the circulation continues the top level of the sea is moved around and displaced away from the centre of the system and this allows cooler water in the centre to begin to rise to fill this void. In tropical regions we look at a thing called the thermocline layer, this is where the warm tropical waters suddenly turns much cooler at depth and is more shallow than you might expect.
So, there you have it, if tropical systems move very little then it's only a matter of time before upwelling occurs and the sea temperature drops and the system begins to decay due to the lack of available heat.
METEOROLOGIST : MARSH
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