What Causes Typhoons?
Pre-Conditions for Typhoons
Several atmospheric ingredients must come together for a typhoon to form. Since a typhoon is just another term for hurricane, the same conditions apply for both. There are perhaps seven atmospheric conditions which, if met, could cause a typhoon to form. A pre-existing
disturbance, warm ocean water, low atmospheric stability, sufficient Coriolis force, moist mid-atmosphere, and upper atmosphere
divergence are all important factors for typhoon formation. These conditions are discussed in greater detail in Tropicalweather.net's "How do hurricanes form?"
These factors are important in that tremendous amounts of heat energy is transported from the tropics northward to the higher latitudes. The typhoon is a large heat engine, where great amounts of heat
are being produced from the process of latent heat of condensation. This occurs as water vapor is being evaporated from the ocean surface and condensed into cloud droplets.
Mechanisms to Cause Typhoon Formation
If all of the pre-conditions are met, typhoon formation then becomes possible. There are several types of atmospheric disturbances that can cause a typhoon to develop.
The most common mechanism to cause a typhoon to develop is the monsoon trough. This is an extension of the Inter-Tropical Convergence Zone where cyclonic spin has developed. The ITCZ is a
trough of low pressure created by the convergence of the northeast and southeast trade winds. This "trade wind" trough does not contain the spin to initiate typhoon development. Typhoon ( hurricanes )
are caused mostly by the monsoon trough in six of the seven hurricane / typhoon formation basins of the world.
Another cause of typhoon formation is from the Tropical Upper Tropospheric Trough (TUTT). The upper
air low is cold in nature as compared to its surrounding environment. A typical low pressure center that forms a typhoon starts at the lower levels and is warm compared to its environment. If a TUTT low
remains over the warm ocean waters for several days, sometimes it gradually warms and takes on tropical characteristics. At this point the low "reaches" down to the surface from the upper atmosphere where
the development process transforms.
Another cause of typhoon development is from a front which stalls over tropical waters. Inherently, a front has a cyclonic spin associated with it. If winds
aloft become favorable with little wind shear, showers and storms could become more numerous and cause a typhoon to form.
By far, the monsoon trough causes most typhoons to form in the western Pacific Ocean.