Mooring observations of the near-inertial wave wake of Hurricane Ida (2009)

Abstract

The near-inertial wave wake of Hurricane Ida is examined of the basis of horizontal velocity observations acquired from 7 moorings instrumented with acoustic Doppler current profilers deployed across the shelf break, slope, and at the abyssal plain of the Yucatan Peninsula, from 130m to ∼3300m. During the forced stage, background mean-flow consisted on a dominant cyclonic circulation of ∼100km of diameter intensified toward the Yucatan's shelf (topographic constraint) and bounded by anticyclonic vorticity northeastward (north 25°N). In the low frequency band, subinertial signals of ∈[5.5–7.5]day period propagating along the Yucatan shelf break. After the passage of Hurricane Ida, energetic near-inertial oscillations spread away from the storm's track over cyclonic vorticity. The wave's Eulerian frequency increases shoreward and toward the Yucatan's shelf. After Ida's passage, mooring data show a contrasting velocity response: semi-diurnal and diurnal tides are enhanced at the shelf break of the Yucatan Peninsula and near-inertial oscillations at the slope and abyssal plain. The near-inertial kinetic energy is largest to the right of the storm track because of the asymmetric wind-stress and amplified due to vorticity trapping near z=−500m, which is a proxy of the base of the mesoscale structure and where the mean-flow is nearly zero. The blue frequency shifted wave wake propagates downward at ∼57–70mday−1 and horizontally at 23–28kmday−1 leading a downward vertical energy flux of [1.3–1.6]×10−2Wm−2. This represents a 7–9% of the total wind power input to near-inertial oscillations that, ultimately, became available for interior ocean mixing. The results suggest that the most energetic wave packet propagated poleward and downward from a broad upwelling region located near the Hurricane's track. The vertical structure of the near-inertial kinetic energy is described as a sum of the first 12 standing vertical modes and as vertically propagating near-inertial internal waves. The amplification of near-inertial kinetic energy as the wave train propagates through the region of anticyclonic vorticity is consistent with the reduction of the Eulerian frequency (and mean-flow) at depth and the shrinking horizontal wavenumber in a critical layer. This work shows that energetic near-inertial oscillations of vertical wavelength of 850–1280m, penetrate well below the thermocline, and are concentrated to the right of the storm track in a region of anticyclonic vorticity.