|HABYTIME MINI LECTURE 63:|
TROPICAL CYCLONE DEFLECTION 1
METEOROLOGIST JEFF HABY
Tropical systems move according to global wind flow, synoptic influences and even local effects. This writing will focus on a couple of important
global and synoptic influences that impact the path of a tropical system such as a hurricane.
Tropical systems typically develop within the global wind belt called the easterlies. In meteorology, wind direction is the direction the
winds are coming from. Thus, the easterlies produce wind that flows from the east toward the west. Many tropical systems begin by drifting
and moving toward the west. Another tendency is that tropical systems will move gradually away from the equator over time. At first they
may only gradually move away from the equator, if at all. The farther they do get away from the equator, then the faster they tend to move
even farther from the equator. This is because Coriolis deflects to the right of the path of motion and because as a tropical system moves
away from the equator it will have a closer interaction with the westerly winds.
The top diagram below shows the easterlies and the westerlies. The easterlies are closer to the equator while the westerlies are in the middle
latitudes. Often the early portion of a tropical system lifecycle is spent moving toward the west under the influence of the easterlies. Over
time, the tropical system drifts farther away from the equator as it moves toward the west. Once it moves out of the main influence of the
easterlies, the westerlies will pick up the storm and start deflecting it more toward the pole and then will deflect it
towards the northeast.
The bottom diagram shows another powerful influence on tropical cyclone motion and that is the position of the subtropical high pressure
system. The wind flow is generally anticyclonic around a high pressure system. A tropical system will not typically move toward the
center of a high pressure but rather will deflect around its periphery. Large high pressure cells often have somewhat fixed positions.
One such high pressure system is the Bermuda-Azores high in the Atlantic Ocean in summer. South of the high pressure system, a tropical
system will basically follow with the easterlies. As the tropical system moves west it can bend around the periphery of the high pressure
system. This will help send the tropical system farther north and closer to the influence of the westerlies in the middle latitudes.
This writing focused on a couple of major global/synoptic influences on tropical storm path. A later writing will focus more on the local and
storm scale influences that effect tropical storm motion. Each tropical system is unique and will have unique influences that determine
the course it will take. This writing covered a couple of general tendencies that many tropical systems have: moving from the influence
of the easterlies to the westerlies and the deflection around synoptic high pressure systems.