Tropical Cyclones and the Thermohaline Circulation

The strength of the thermohaline circulation is controlled in the long term by diapycnal mixing, as shown in the 1920s by Jeffreys. Much more recent research has shown that the mixing is effective only on the "warm" side of the ocean circulation, and that it may be effective even if highly localized in space and time. The energy source for the mixing remains controversial, with winds, turbulence induced by tidal flows, and geothermal heating regarded as promising candidates. Here I argue that global tropical cyclone activity may provide most of the mixing that drives the thermohaline circulation. The highly transient nature of tropical cyclones allows one to estimate with appreciable accuracy the net, vertically integrated heating necessary to restore the observed cold wakes to pre-storm or climatological conditions. This induced net heat flux must be compensated by oceanic export. Using a combination of coupled ocean-atmosphere models of tropical cyclones and satellite-based estimates of cold wake properties, we estimate that the net heat flux induced by tropical cyclones is (1.4 ± 0.7 x 10¹⁵) W. This is comparable to the observed poleward flux of heat by the ocean, implying that tropical cyclones may indeed be important agents for driving the thermohaline circulation. Since tropical cyclone activity is very sensitive to tropical sea surface temperatures, tropical warming may lead to increased oceanic export of heat from the Tropics, stabilizing the tropical climate but destabilizing the climate outside the Tropics.