Ocean-atmosphere interactions in tropical cyclones

Over the past two decades, there has been increased interest in not only understanding the ocean response to tropical cyclone (TC) forcing, but the impact of the ocean on intensity and structural change in the TC itself. The oceanic response has been primarily studied in the context of the baroclinic wake left by TC's and the forced near-inertial waves as observed in Gilbert (88), Frances, and Ivan (04). This cold wake structure and the negative feedback are due to ocean mixed layer cooling by shear-induced vertical mixing by near-inertial motions forced by TC momentum flux. That is, the strong shears lower the Richardson numbers to below criticality that forces the entrainment of cooler water from the seasonal thermocline that may lead to TC weakening. Notwithstanding, in some oceanic regimes these cold wakes (>2ºC) simply do not develop over deep warm oceanic features. Background oceanic states impact upper ocean thermal response where the warm layers are deeper and currents transport warm water poleward as part of the gyre circulation as suggested by measurements in Isidore and Lili (02), Katrina and Rita (05), and Gustav and Ike (08) over the Loop Current. In these regimes, the wind-driven shears do not develop and essentially keep the oceanic mixed layer warm with high oceanic heat content levels (>100 kJ cm^-2) during TC passage. The implication is a more sustained enthalpy flux to the atmospheric boundary layer, thus representing an important heat and moisture source for deepening of TCs.

In this broader context, recent studies have shown the surface drag coefficient to level off between 28 to 33 m s^-1 at values from 2.4 to 3.4 x 10^-3. For intense TCs, the theoretical ratio of the enthalpy and surface drag coefficient ranges exceeds unity (typically 1.2 to 1.5), and when this ratio is less than unity, studies suggest that TCs cannot reach their maximum intensity. The important point here is that these fluids are coupled if we ever expect a more accurate intensity forecast, the enthalpy fluxes (and aerodynamic transfer coefficients) must be correct.