The 3D oceanic mixed layer response to Hurricane Gilbert

Upper-ocean heat and mass budgets are examined from three snapshots of data acquired during and after the passage of Hurricane Gilbert in the western Gulf of Mexico. Measurements prior to storm passage indicated a warm core e ddy in the region with velocities of O(1) m s(-1). Based upon conservation of heat and mass, the three dimensional mixed layer processes are quantifie d from the data. During and subsequent to hurricane passage, horizontal adv ection due to geostrophic velocities is significant in the eddy regime, sug gesting that prestorm oceanic variability is important when background Rows have the same magnitude as the mixed layer current response. Storm-induced near-inertial currents lead to large vertical advection magnitudes as they diverge from and converge toward the storm track. Surface fluxes, estimate d by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m(-2) and a heat flux of 1200 W m(-2) in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m(-2) corresponding to a less than 7 m s(-1) wind speed. Entrainment mixing across the mixed layer base is estimated using three bul k entrainment closure schemes that differ in their physical basis of parame terization. Entrainment remains the dominant mechanism in controlling the h eat and mass budgets irrespective of the scheme. Depending on the magnitude s of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the d irectly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme pred icts a narrow region of cooling compared to the the stress induced mixing s cheme and observed SST decreases. After the storm passage, the maximum cont ribution to the mixed layer dynamics is associated with shear-induced entra inment mixing forced by near-inertial motions up to the third day as indica ted by bulk Richardson numbers that remained below criticality. Thus, entra inment based on a combination of surface fluxes, friction velocity and shea r across the entrainment zone may be more relevant for three-dimensional oc ean response studies.