How Does Upwelling Affect a Strong Tropical Cyclone? (Credit: Ryan Maue, CIRA)

In tropical cyclone development, one of the main sources that aid in the strengthening process is the transfer of heat from the ocean to the atmosphere. The warm sea-surface temperatures acts as a fuel pump by providing the energy needed for further development. The warmer the sea surface temperatures are in a specific area, the higher the chances the tropical cyclone will strengthen. Now, this also means the system has to be propagating through a very conducive environment with little wind shear and no dry air for strengthening to occur. 

The transfer of heat is enhanced through evaporation which is the process of when a liquid undergoes a change into a vapor or a gas. The water from the sea surface is evaporated by the hurricane surface winds. The newly formed water vapor is transported into the troposphere or the layer in the atmosphere where most weather occurs. As the heat is transferred into the atmosphere, the tropical cyclone will move out, leaving behind much cooler waters. This is either done by vertical mixing or by the process known as upwelling. 

Upwelling is when the hurricane surface winds that are blowing cyclonically (counterclockwise) causes the currents at the sea surface to move cyclonically. After this occurs, the Coriolis Force deflects the currents to the right (in the northern hemisphere), and displaces the water outward from the storms center. This allows the cooler water from below to move towards the sea surface to fill the void. Essentially, the tropical cyclone loses its main fuel source. Upwelling typically occurs along coastlines or underneath a slow moving tropical cyclone. 

Currently out in the western Pacific, we have category 3 typhoon Trami. Before Trami went through a weakening phase yesterday, it was a textbook category 5 super typhoon with a mesmerizing sixty mile-wide eye. Once this powerful storm made a slight turn to the north, it started to slow down in speed immensely.  The environment Trami was propagating through was very conducive for further development. However, Trami had nothing in the upper-levels of the atmosphere to steer it towards warmer waters. Due to its slow speed, it started to spin over the same area for a long period of time allowing upwelling to occur. This looks to be the main reason why Trami went through a weakening phase.

When a tropical cyclone slows down and sits over the same area, the fuel source from the warm sea-surface waters starts to run out. It initiates the process of upwelling underneath the tropical cyclone which brings cooler waters to the sea surface. When you have those cooler waters underneath a tropical cyclone, it starts to decouple the core of the system. The picture at the top of the article demonstrates that the water temperatures underneath Trami had cooled by about 10 degrees Celsius yesterday as it spun over the same area. Below is a current look via the Himawari-8 satellite imager of Typhoon Trami. Trami is forecast to track towards the Northeast and bring impacts to the southern Islands of Japan. 
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© 2018 Meteorologist Joseph Marino