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42 Why do tropical cyclones occur primarily in the summer and autumn?




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This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.

42 Why do tropical cyclones occur primarily in the summer and autumn?

As described in subject G1), the primary time of year for getting tropical
cyclones is during the summer and autumn: July-October for the Northern
Hemisphere and December-March for the Southern Hemisphere (though there
are differences from basin to basin). The peak in summer/autumn is due to
having all of the necessary ingredients become most favorable during this
time of year: warm ocean waters (at least 26C or 80F), a tropical
atmosphere that can quite easily kick off convection (i.e. thunderstorms),
low vertical shear in the troposphere, and a substantial amount of large-
scale spin available (either through the monsoon trough or easterly waves
- see subject A4)). While one would intuitively expect tropical cyclones
to peak right at the time of maximum solar radiation (late June for the
tropical Northern Hemisphere and late December for the tropical Southern
Hemisphere), it takes several more weeks for the oceans to reach their
warmest temperatures. The atmospheric circulation in the tropics also
reaches its most pronounced (and favorable for tropical cyclones) at the
same time. This time lag of the tropical ocean and atmospheric
circulation is analogous to the daily cycle of surface air temperatures -
they are warmest in mid-afternoon, yet the sun's incident radiation peaks
at noon.

 

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