Upper ocean response to a hurricane

Abstract

The upper ocean response to a moving hurricane is studied using historical air-sea data and a three-dimensional numerical ocean model. Sea surface temperature (SST) response is emphasized. The model has a surface mixed-layer (ML) that entrains according to a velocity dependent parameterization, and two lower layers that simulate the response in the thermocline. The passage of Hurricane Eloise (1975) over buoy EB-10 is simulated in detail. SST decreased 2°C as Eloise passed directly over EB-10 at 8.5 m s-1. Model results indicate that entrainment caused 85% of the irreversible heat flux into the ML; air-sea heat exchange accounted for the remainder. The maximum SST response was predicted to be -3°C and to occur 60 km to the right of the hurricane track. This is consistent with the well-documented rightward bias in the SST response to rapidly moving hurricanes. The rightward bias occurs in the model solution because the hurricane wind-stress vector turns clockwise with time on the right side of the track and is roughly resonant with the ML velocity. High ML velocities cause strong entrainment and thus a strong SST response. Model comparisons with EB-10 data suggest that a wind-speed-dependent drag coefficient similar to Garratt's (1977) is appropriate for hurricane conditions. A constant drag coefficient 1.5 x w-s underpredicts the amplitude of upwelling and the SST response by -40%. Numerical experiments show that the response has a lively dependence on a number of air-sea parameters. Intense, slowly moving hurricanes cause the largest response. The SST response is largest where cold water is near the sea surface, i.e., where the initial ML is thin and the upper thermocline temperature gradient is sharp. Nonlocal processes are important to some aspects of the upper ocean response. Upwelling significantly enhances entrainment under slowly moving hurricanes (≤4 m s-1) and reduces the rightward bias of the SST response. Horizontal advection dominates the pointwise ML heat balance during the several-day period following a hurricane passage. Pressure gradients set up by the upwelling do not play an important role in the entrainment process, but are an effective mechanism for dispersing energy from the ML over a 5-10 day time scale.