Mechanical energy and vorticity balances within the OML under tropical cyclones

The upper ocean presents a significant mechanical energy sink to tropical cyclones (TC). The ocean mixed layer (OML) responsive cooling, due to storm-generated current shear-induced mixing, is clear evidence of this fact. A joint NOAA/NSF sponsored research experiment was executed in 2002 to measure the evolution of the OML mass and current fields under TC forcing. For the first time, in situ observations of the temperature, salinity and horizontal currents were obtained to develop a fairly complete description of the OML mechanical energy and vorticity budgets under such conditions. In particular, estimates of the net total energy exchange across the air-sea interface are now possible in principle.

Observations from Hurricane Lili's passage through the southern Gulf of Mexico indicate a mechanical energy response as complex as the thermal. To the southeast of the Loop Current (LC), energy response is weak due to a deep OML over which the storm-generated current is distributed. Within the LC, post-storm currents are observed to slightly weaken as the storm-generated inertial current has rotated over time to oppose the geostrophically-balanced current. Additionally, the balanced current is also observed to weaken, due to a downwelling of the thermocline and a corresponding flattening of the initial horizontal density gradient. Finally, to the northwest of the LC, currents are observed to be accelerated from practically non-existent before the storm to ~ 0.8 m s-1 after, and are associated with the observed OML cooling of 2-3 ◦C. These results have implications for coupled-modeling efforts that should require, as a general measure of success, total energy conservation in both the atmosphere and ocean.