Mean and Near-Inertial Ocean Current Response to Hurricane Gilbert

Abstract

The three-dimensional hurricane-induced ocean response is determined from velocity and temperature profiles acquired in the western Gulf of Mexico between 14 and 19 September 1988 during the passage of Hurricane Gilbert. The asymmetric wind structure of Gilbert indicated a wind stress of 4.2 N m−2 at a radius of maximum winds (Rmax) of 60 km. Using observed temperature profiles and climatological temperature–salinity relationships, the background and storm-induced geostrophic currents (re: 750 m) were 0.1 m s−1 and 0.2 m s−1, respectively. A Loop Current warm core ring (LCWCR) was also located to the right of the storm track at 4–5 Rmax, where anticyclonically rotating near-surface and 100-m currents decreased from 0.9 m s−1 to 0.6 m s−1 at depth. The relative vorticity in the LCWCR was shifted below the local Coriolis parameter by about 6%. In a storm-based coordinate system, alongtrack residual velocity profiles from 0 to 4 Rmax were fit to a dynamical model by least squares to isolate the near-inertial content over an e-folding timescale of four inertial periods (IP ≈ 30 h). Observed frequency shifts in the mixed layer currents ranged from 1.03 to 1.05f in agreement with both the backrotated velocity profiles at 1.04f relative to the storm profile (where maximum correlation coefficients were 0.8) and the predicted frequency shift from the mixed-layer Burger number. This frequency was increasingly blue shifted in the upper 100 m to 1.1f, decreasing toward f within the thermocline. Near-inertial currents rotated anticyclonically by 90°–180° in the upper ocean, providing the velocity shear for layer cooling and deepening observed on the right-hand side of the track. A summation of the first four baroclinic modes described up to 77% of this near-inertial current variability during the first 1.75 IP. However, the variance explained by this modal summation decreased to a minimum of 36% after 2.9 IP following passage due to phase separation between the first baroclinic mode and higher-order modes in the mixed layer. Although the response was complicated by the LCWCR, the evolving three-dimensional current structure can be described by linear, near-inertial wave dynamics.