Hurricanes play an especially critical role in determining the climate of the seaboard of the eastern United States--annual peaks in tides, winds, and rainfall are often reached during hurricanes. Yet, from year to year, strong variations exist in the number and intensity of storms which actually make landfall.

Formation
 
 

Hurricanes originate as storms over the equatorial western coast of Africa often associated with waves in the atmosphere (easterly waves). Their subsequent propagation is westward and slightly northward, carried by the trade winds towards the Gulf of Mexico. As the hurricane moves northwards, it feels the effect of the midlatitude jet stream and is redirected to the east. A typical hurricane track, is shown here.

 
 

The cyclonic flow around the low pressure center of a hurricane. Winds commonly exceed 60 m s-1 (140 mph). Clouds (retrieved from satellite in this figure) are advected by the winds.

 
 

To understand key aspects of hurricanes, such as their growth, decay, and teleconnection with ENSO, it is necessary to understand their vertical structure and thermodynamic couplings. The figure to the right shows two important aspects of hurricanes.  caption a)  Hurricane force winds reach a maximum approximately 60 km from the storm's eye and correspond to a rapidly changing pressure gradient.    caption b)  Low pressure at the hurricane center is maintained by intese heating associated with clouds and rainfall. The convective elements have a deep vertical structure that extends to the tropopause. The continuity of vertical motions is an essential element of the convection. If the hurricane is introduced to an environment with strong vertical shear (such as the southern equatorial Atlantic Ocean), a strong deterent to its maintenence will be introduced.  Moreover, the convection is supported, in part, by a flux of moisture from the ocean surface. If the storm propogates over land or cold oceans, the moisture flux is reduced and the storm will (probably) weaken.

To view a Quicktime 4.0 version of TRMM data on a fly through of hurricane Mitch, click here.