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 eddy in the region with velocities of O(1) m s21. Based upon conservation of heat and mass, the threedimensional mixed layer processes are quantified from the data. During and subsequent to hurricane passage, horizontal advection due to geostrophic velocities is significant in the eddy regime, suggesting that prestorm oceanic variability is important when background flows 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, estimated by reducing flight-level winds to 10 m, indicate a maximum wind stress of 4.2 N m22 and a heat flux of 1200 W m 22 in the directly forced region. The upward heat flux after the passage of the storm has a maximum of 200 W m22 corresponding to a less than 7 m s21 wind speed. Entrainment mixing across the mixed layer base is estimated using three bulk entrainment closure schemes that differ in their physical basis of parameterization. Entrainment remains the dominant mechanism in controlling the heat and mass budgets irrespective of the scheme. Depending on the magnitudes of friction velocity, surface fluxes and/or shear across the mixed layer base, the pattern and location of maximum entrainment rates differ in the directly forced region. While the general area of maximum entrainment is in the right-rear quadrant of the storm, shear-induced entrainment scheme predicts a narrow region of cooling compared to the the stressinduced mixing scheme and observed SST decreases. After the storm passage, the maximum contribution to the mixed layer dynamics is associated with shear-induced entrainment mixing forced by near-inertial motions up to the third day as indicated by bulk Richardson numbers that remained below criticality. Thus, entrainment based on a combination of surface fluxes, friction velocity and shear across the entrainment zone may be more relevant for three-dimensional ocean response studies.

[1]  James L. Franklin,et al.  The NCAR GPS Dropwindsonde , 1999 .

[2]  J. Geisler.  Linear theory of the response of a two layer ocean to a moving hurricane , 1970 .

[3]  J. Price,et al.  Upper Ocean Response to a Hurricane , 1981 .

[4]  Peter G. Black,et al.  Upper ocean response to Hurricane Gilbert , 1992 .

[5]  W. Large,et al.  Open Ocean Momentum Flux Measurements in Moderate to Strong Winds , 1981 .

[6]  D. Marquardt.  An Algorithm for Least-Squares Estimation of Nonlinear Parameters , 1963 .

[7]  R. Elsberry,et al.  A mixed layer model of the oceanic thermal response to hurricanes , 1976 .

[8]  D. Leipper.  Observed Ocean Conditions and Hurricane Hilda, 1964 , 1967 .

[9]  Richard A. Anthes,et al.  Numerical Simulations of the Ocean's Nonlinear, Baroclinic Response to Translating hurricanes , 1978 .

[10]  R. Elsberry,et al.  Vertical Structure of the Ocean Current Response to a Hurricane , 1989 .

[11]  Mark DeMaria,et al.  Sea Surface Temperature and the Maximum Intensity of Atlantic Tropical Cyclones , 1994 .

[12]  George Z. Forristall,et al.  Forced Stage Response to a Moving Hurricane , 1994 .

[13]  Peter G. Black,et al.  Atmospheric Boundary Layer and Oceanic Mixed Layer Observations in Hurricane Josephine Obtained from Air-Deployed Drifting Buoys and Research Aircraft , 1988 .

[14]  J. Kaplan,et al.  Analysis of the Inflow and Air-Sea Interactions in Hurricane Frederic (1979) , 1986 .

[15]  Katsuyuki V. Ooyama,et al.  Scale-Controlled Objective Analysis , 1987 .

[16]  S. Lord,et al.  The Kinematic Structure of Hurricane Gloria (1985) Determined from Nested Analyses of Dropwindsonde and Doppler Radar Data , 1993 .

[17]  B. I. Miller.  RAINFALL RATES IN FLORIDA HURRICANES , 1958 .

[18]  P. Black,et al.  Ocean Response to a Hurricane. Part I: Observations , 1987 .

[19]  Raymond T. Pollard,et al.  The deepening of the wind-Mixed layer , 1973 .

[20]  G. Veronis.  Partition of energy between geostrophic and non-geostrophic oceanic motions , 1956 .

[21]  Richard M. Hodur,et al.  The Interaction between Hurricane Opal (1995) and a Warm Core Ring in the Gulf of Mexico , 2000 .

[22]  Mark D. Powell,et al.  Hurricane Andrew's Landfall in South Florida. Part II: Surface Wind Fields and Potential Real-Time Applications , 1996 .

[23]  C. Rossby,et al.  On the Mutual Adjustment of Pressure and Velocity Distributions in Certain Simple Current Systems, II , 1938 .

[24]  Gustavo Goni,et al.  Effects of a Warm Oceanic Feature on Hurricane Opal , 2000 .

[25]  R. Elsberry,et al.  Near-Inertial Ocean Current Response to Hurricane Frederic , 1987 .

[26]  A. Mariano,et al.  Mean and Near-Inertial Ocean Current Response to Hurricane Gilbert , 1998 .

[27]  J. Turner,et al.  A one‐dimensional model of the seasonal thermocline II. The general theory and its consequences , 1967 .

[28]  A. E. Gill.  On the Behavior of Internal Waves in the Wakes of Storms , 1984 .

[29]  N. Huang.  An Estimate of the Influence of Breaking Waves on the Dynamics of the Upper Ocean , 1986 .

[30]  Frank D. Marks,et al.  Landfalling Tropical Cyclones: Forecast Problems and Associated Research Opportunities. , 1998 .

[31]  E. D’Asaro,et al.  Observations of Persistent Mixing and Near-Inertial Internal Waves , 1986 .

[32]  D. A. Brooks.  The Wake of Hurricane Allen in the Western Gulf of Mexico , 1983 .

[33]  H. Rossby,et al.  A Study of Velocity Profiles Through the Main Thermocline , 1976 .

[34]  H. Inoue.  A least-squares smooth fitting for irregularly spaced data; finite-element approach using the cubic B-spline basis , 1986 .

[35]  J. Deardorff.  A Multi-Limit Mixed-Layer Entrainment Formulation , 1983 .