ALBERT

Description: The near-inertial trapping of the wave wakes of two consecutive hurricanes, Gustav and Ike (2008), at the base of the Loop Current is described using observations of a triangular mooring array. Hurricane Gustav translates over the Loop Current twice as fast and leads to a sharper, but equally strong, wind intensity peak than Hurricane Ike. Following the passage of Gustav, near-inertial oscillations in the main thermocline have large horizontal (∼340±45 km) and vertical (∼850 m) scales. The wave kinetic energy propagates down with a vertical group velocity of several tens of meters per day and leads to a downward energy flux of [1.82-7.10]×10 −3 W m −2 .Observations suggest that the dispersion of the wave wakes produced by the hurricanes is driven by Doppler shifting in the upper ∼600 m and the vertical-varying vorticity inside the Loop Current. Near-inertial kinetic energy is enhanced and near-inertial oscillations are circularly polarized in a critical layer at the base of the Loop Current where the group velocity vector is nearly horizontal. The trapping and amplification of the near-inertial wave wakes of two consecutive hurricanes is addressed using a primitive equation numerical model. Numerical results suggest an increase of the near-inertial kinetic energy and vertical shear by a factor of ∼2 near the critical layer due to wave-mean flow and wave-wave interactions. A K-Profile parameterization of these interactions results in banded regions of elevated turbulent kinetic energy dissipation rates of ε ϵ [0.5-6.6]×10 −6 W m −3 for critical Richardson number Ri c = 1, depending on the stratification of the water column. This article is protected by copyright. All rights reserved.

Description: The Yucatan shelf in the southern Gulf of Mexico is under the influence of an upwelling that uplifts cool and nutrient rich waters over the continental shelf. The analysis of a set of high resolution (Δx=Δy≈2.8km) simulations of the Gulf of Mexico show two dominant modes of variability of the Yucatan upwelling system: (1) a low frequency mode related to variations in position and intensity of the Loop Current along the shelf, with upwelling intensified when the Loop Current is strong and approaches to the Yucatan shelf break and (2) a high-frequency mode with peak frequency in the 6-10 days band related to wind-forced coastal waves that force vertical velocities along the eastern Yucatan shelf break. To first order, the strength and position of the Loop Current are found to control the intensity of the upwelling, but we show that high frequency winds also contribute (∼17%) to a net input of cool waters (〈22.5°C) on the Yucatan shelf. Finally, although more observational studies are needed to corroborate the topographic character of the Yucatan upwelling system, this study reveals the key role played by a notch along the Yucatan shelf break: a sensitivity simulation without the notch shows a 55% reduction of the upwelling.