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Note that as the sea surface temperature increases and the outflow temperature decreases, the minimum sustainable central pressure falls more and more rapidly, until no solution can be found. In this regime, the Carnot cycle runs away due to the heat input associated with isothermal expansion: the lower the central pressure, the more isothermal expansion heating occurs in the inflow, which makes the storm more intense, dropping the central pressure further, etc.

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[[caption]][[bold]]Figure 2:[[/bold]]Minimum sustainable central pressure of tropical cyclones (in millibars) under September climatological conditions. The central pressures of some of the most intense tropical cyclones on record are shown by italicized numbers and crosses.[[/caption]]

The minimum sustainable pressure under August climatological conditions is shown in Figure 2, together with the recorded central pressures of some of the most intense storms on record. Clearly, the potential for hurricanes is very large over much of the tropical oceans, and a small fraction of the storms achieve their potential intensity. Why don't hurricanes occur more often, and why do so few of them reach their potential intensity?

A clue to the first of these questions has been provided by numerical simulations of hurricanes. Models have been developed that are sophisticated enough to simulate cumulus clouds explicitly, at least in an axisymmetric framework. When such models are initialized with a weak vortex, nothing happens: the disturbance dies. But when a sufficiently strong vortex is used to kick off simulation, it grows into a full fledged storm. Thus hurricanes can be regarded as finite amplitude instabilities on a metastable basic state tropical atmosphere-ocean system. Put simply, hurricanes need a trigger. No one really knows why, though there are some promising theories.

Another feature of modelled hurricanes is that given that mature storms develop, they all intensify right to their maximum potential intensity, while nature is seldom so exuberant. Several theories have been proposed to explain this, but none hav been rigorously tested. Among the more promising holds that the reduction in sea surface temperature, caused by the churning of colder water from the depths by the strong winds of hurricanes, reduces the heat input into the storms compared to what is predicted by

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