The inner core of moving tropical cyclones (TCs) often initiates an asymmetric thermal response in the upper ocean. To document this response, the National Hurricane Research Laboratory (now the NOAA/Hurricane Research Division) conducted an aircraft field program from 1969-80 that, for the first time, simultaneously measured TC structure and ocean thermal structure under TCs.

Analyses of aircraft observations from 19 storms indicate that when the mixed layer depth (MLD) is < 40 m, TCs produce crescent-shaped patterns of cooling. This pattern is confined between the radius of maximum wind (RMAX) and 3 RMAX. The largest SST changes of ~-5C occur in the right-rear quadrant of a storm with little or no SST decreases in the left-front quadrant. The finding of relatively cold water just outside the eyewall implies that air flowing into the eyewall can cause weakening by reducing eyewall buoyancy and enthalpy fluxes.

The radius and magnitude of the SST pattern vary primarily with a TC’s forward speed (U), maximum horizontal wind speed (VMAX) and initial MLD. When the pre-storm MLD is <40 m, the slowest TCs (U <2 m/s), have the largest observed SST change of -5C at RMAX. For TCs with U of 2.0-3.5 m/s, the peak SST increases linearly with U and becomes ~-2.5C for U=3.5 m/s. The radius of the peak SST value increases from 1 to 3 RMAX. When U >=7.5 m/s, the the peak SST change approaches an asymptote of ~-2C and its radius moves outward beyond 3 RMAX.

As VMAX increases from weak wind speeds up to 40 m/s (category 1), the SST decrease reaches a peak value of ~-5C. With additional VMAX increases, the peak values of SST decrease lessen and approach –2C. As the background MLD increases from 40–70 m, SST decreases become smaller. When MLD >70 m, TC-induced SST decreases are negligible for any U or VMAX and hurricanes stronger than category 1 may occur.

Observations from moored buoys within 1.5 RMAX corroborate the findings from the AXBTs. Furthermore, interpretation of the buoy data suggests a three-stage sequence of events. The first stage, occurring 12-24-h in advance of the TC center, is associated with SST changes < 1C. Sea-air surface enthalpy fluxes associated with increasing surface winds appear to dominate the cooling. The second stage occurs in a 2-6-h period around TC passage and consists of an additional rapid SST cooling of 2-4C in the TC’s right-rear quadrant. Here, mechanical mixing from surface waves and due to shear at the base of the mixed layer induced by strong inertial currents, both of which occur in the presence of storm-induced upwelling, appear to be the dominant cooling mechanisms. The third stage is the TC wake, that is dominated by SST oscillations modulated by inertia-gravity waves.