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Subject: G4) Why do tropical cyclones occur
primarily in the summer and autumn?
Contributed by Chris Landsea
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 fa vorable during this time of year: warm
ocean waters (at least 26°C or 80°F), 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).
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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