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- Author or Editor: Eric Uhlhorn x
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Abstract
In this second part of a two-part study, details of the upper-ocean response within an idealized baroclinic current to a translating tropical cyclone are examined in a series of nonlinear, reduced-gravity numerical simulations. Based on observations obtained as part of a joint NOAA–National Science Foundation (NSF) experiment in Hurricane Lili (2002), the preexisting ocean mass and momentum fields are initialized with a Gulf of Mexico Loop Current–like jet, which is subsequently forced by a vortex whose wind stress field approximates that observed in the Lili experiments. Because of 1) favorable coupling between the wind stress and preexisting current vectors, and 2) wind-driven currents flowing across the large horizontal pressure gradient, wind energy transfer to the mixed layer can be more efficient in such a regime as compared to the case of an initially horizontally homogeneous ocean. However, nearly all energy is removed by advection and wave flux by two local inertial periods after storm passage, consistent with the observational results. Experiments are performed to quantify differences in one-dimensional and three-dimensional linearized approximations to the full model equations. In addition, sensitivity experiments to variations in the initial geostrophic current structure are performed to develop a parameter space over which a significant energy response could optimally be observed.
Abstract
In this second part of a two-part study, details of the upper-ocean response within an idealized baroclinic current to a translating tropical cyclone are examined in a series of nonlinear, reduced-gravity numerical simulations. Based on observations obtained as part of a joint NOAA–National Science Foundation (NSF) experiment in Hurricane Lili (2002), the preexisting ocean mass and momentum fields are initialized with a Gulf of Mexico Loop Current–like jet, which is subsequently forced by a vortex whose wind stress field approximates that observed in the Lili experiments. Because of 1) favorable coupling between the wind stress and preexisting current vectors, and 2) wind-driven currents flowing across the large horizontal pressure gradient, wind energy transfer to the mixed layer can be more efficient in such a regime as compared to the case of an initially horizontally homogeneous ocean. However, nearly all energy is removed by advection and wave flux by two local inertial periods after storm passage, consistent with the observational results. Experiments are performed to quantify differences in one-dimensional and three-dimensional linearized approximations to the full model equations. In addition, sensitivity experiments to variations in the initial geostrophic current structure are performed to develop a parameter space over which a significant energy response could optimally be observed.
Abstract
The ocean mixed layer response to a tropical cyclone within and immediately adjacent to the Gulf of Mexico Loop Current is examined. In the first of a two-part study, a comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy evolution in response to Hurricane Lili’s (2002) passage. Mixed layer temperature analyses show that the Loop Current cooled <1°C in response to the storm, in contrast to typically observed larger decreases of 3°–5°C. Correspondingly, vertical current shear associated with mixed layer currents, which is responsible for entrainment mixing of cooler water, was found to be up to 50% weaker, on average, than observed in previous studies within the directly forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical Water from the cooler, heavier Gulf Common Water, was found to decrease in intensity by −0.18 ± 0.25 m s−1 over an approximately 10-day period within the mixed layer. Contrary to previous ocean response studies, which have assumed approximately horizontally homogeneous ocean structure prior to storm passage, a kinetic energy loss of 5.8 ± 6.4 kJ m−2, or approximately −1 wind stress-scaled energy unit, was observed. By examining near-surface currents derived from satellite altimetry data, the Loop Current is found to vary similarly in magnitude over such time scales, suggesting storm-generated energy is rapidly removed by the preexisting Loop Current. In a future study, the simulated mixed layer evolution to a Hurricane Lili–like storm within an idealized preexisting baroclinic current is analyzed to help understand the complex air–sea interaction and resulting energetic response.
Abstract
The ocean mixed layer response to a tropical cyclone within and immediately adjacent to the Gulf of Mexico Loop Current is examined. In the first of a two-part study, a comprehensive set of temperature, salinity, and current profiles acquired from aircraft-deployed expendable probes is utilized to analyze the three-dimensional oceanic energy evolution in response to Hurricane Lili’s (2002) passage. Mixed layer temperature analyses show that the Loop Current cooled <1°C in response to the storm, in contrast to typically observed larger decreases of 3°–5°C. Correspondingly, vertical current shear associated with mixed layer currents, which is responsible for entrainment mixing of cooler water, was found to be up to 50% weaker, on average, than observed in previous studies within the directly forced region. The Loop Current, which separates the warmer, lighter Caribbean Subtropical Water from the cooler, heavier Gulf Common Water, was found to decrease in intensity by −0.18 ± 0.25 m s−1 over an approximately 10-day period within the mixed layer. Contrary to previous ocean response studies, which have assumed approximately horizontally homogeneous ocean structure prior to storm passage, a kinetic energy loss of 5.8 ± 6.4 kJ m−2, or approximately −1 wind stress-scaled energy unit, was observed. By examining near-surface currents derived from satellite altimetry data, the Loop Current is found to vary similarly in magnitude over such time scales, suggesting storm-generated energy is rapidly removed by the preexisting Loop Current. In a future study, the simulated mixed layer evolution to a Hurricane Lili–like storm within an idealized preexisting baroclinic current is analyzed to help understand the complex air–sea interaction and resulting energetic response.
