Air-sea interactions in tropical cyclones

Significant progress has been made in the area of upper ocean responses and air-sea interactions during the passage of tropical cyclones (TC) since the first edition published in 1992. In terms of the upper ocean impacts on intensity, considerable attention has focused on the three-dimensional cold wake structure and the negative feedback due to ocean mixed layer cooling by shear-induced vertical mixing in quiescent oceans. By contrast, in the western parts of the oceanic basins where the ocean state is not at rest, strong currents (e.g., Kuroshio, Gulf Stream) transport warm water poleward as part of the gyre circulation. In these regimes, the upper ocean current shears do not necessarily develop as in regions with shallow ocean mixed layers where significant sea surface cooling often occurs. Transports by these energetic western boundary currents tend to be resistive to shear-induced ocean mixing events since the mixed layer is already deep and the thermal response (or sea surface temperature cooling) tends to be minimized. The implication is since the oceanic mixed layers do not significantly cool during TC passage, there is a more sustained heat flux to the atmospheric boundary layer, thus representing an important mechanism for observed deepening of recently observed severe TCs. In terms of the air-sea interactions, surface waves and sea spray impact the surface drag coefficient. Recent studies have shown the surface drag coefficient to level offbetween 28 to 33 m s^-1 at values from 2.5 to 3.5 × 10^-3. While the measurement uncertainties increase for higher wind speeds, it is clear that the surface drag cannot continue to increase with wind speeds. Moreover, for intense TCs, the ratio of the enthalpy and surface drag coefficient exceeds unity (typically 1.2 to 1.5). When this ratio is less than unity, theoretical studies suggest that TCs cannot reach their maximum potential intensity.