Vertical Structure of the Ocean Current Response to a Hurricane

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  • 1 Department of Meteorology. U.S. Naval Postgraduate School, Monterey California
  • | 2 Hurricane Research Division, NOAA-AOML, Miami, Florida
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Abstract

During the passage of hurricane Norbert in 1984, the Hurricane Research Division of NOAA conducted a Planetary Boundary Layer Experiment that included the deployment of Airborne eXpendable Current Profilers (AXCP). A total of. 16 AXCPs provided for the fist time high-resolution vertical profiles of currents and temperatures in hurricane wind conditions. This study focuses on the vertical structure of the near-inertial baroclinic current excited by the passage of this hurricane.

The transient hurricane-induced currents are isolated from the AXCP profiles in Norbert by subtracting a spatially-averaged current. Near the center of hurricane Norbert, the WKBJ-scaled vertical wavenumber spectra are a decade greater than the Garrett-Munk spectra (GM75). The fist ten linear, baroclinic free modes are calculated from the spatially-averaged Brunt–Väisälä frequency. To allow a more direct comparison with the AXCP observations in the current wind regime, the near-inertial response for the three dimensional velocities is simulated by superposing a hurricane-like wind stress field onto the first ten baroclinic modes. About 70% of the current variance in hurricane Norbert can be explained by a sum of only the first four near-inertial modes. Most of the ocean current variability can be accounted for by the wind stress curl, although the direct effect of the wind stress and the soon divergence do contribute to the observed current variance within 30–60 km from the storm. However, these last two effects rapidly diminish after one inertial period. Although the energy input by the hurricane forcing is spread over all of the vertical wavelengths, most of the energy is contained in the gravest four vertical modes which then govern the dynamics in the wake region.

Abstract

During the passage of hurricane Norbert in 1984, the Hurricane Research Division of NOAA conducted a Planetary Boundary Layer Experiment that included the deployment of Airborne eXpendable Current Profilers (AXCP). A total of. 16 AXCPs provided for the fist time high-resolution vertical profiles of currents and temperatures in hurricane wind conditions. This study focuses on the vertical structure of the near-inertial baroclinic current excited by the passage of this hurricane.

The transient hurricane-induced currents are isolated from the AXCP profiles in Norbert by subtracting a spatially-averaged current. Near the center of hurricane Norbert, the WKBJ-scaled vertical wavenumber spectra are a decade greater than the Garrett-Munk spectra (GM75). The fist ten linear, baroclinic free modes are calculated from the spatially-averaged Brunt–Väisälä frequency. To allow a more direct comparison with the AXCP observations in the current wind regime, the near-inertial response for the three dimensional velocities is simulated by superposing a hurricane-like wind stress field onto the first ten baroclinic modes. About 70% of the current variance in hurricane Norbert can be explained by a sum of only the first four near-inertial modes. Most of the ocean current variability can be accounted for by the wind stress curl, although the direct effect of the wind stress and the soon divergence do contribute to the observed current variance within 30–60 km from the storm. However, these last two effects rapidly diminish after one inertial period. Although the energy input by the hurricane forcing is spread over all of the vertical wavelengths, most of the energy is contained in the gravest four vertical modes which then govern the dynamics in the wake region.

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