Upperocean physical response to hurricanes

Early observations of upper-ocean response to hurricanes have been reported and studied by Leipper (1967), Brooks (1983), Sanford et al. (1987), Shay and Elsberry (1987), Shay et al. (1989; 1998), Brink (1989), Dickey et al. (1998), and Jacob et al. (2000). Price (1981) summarized early hydrographic surveys and found two prominent features of the SST response to hurricanes: (1) hurricane-induced SST cooling increases as a hurricane's moving speed decreases and intensity increases; and (2) the SST response is markedly asymmetrical about the hurricane track. Hurricane-force winds (speeds greater than 33 m s-1) have dramatic effects on the upper ocean. Frequently, near-surface waters cool several degrees Celsius as the mixed layer deepens by tens of meters with the most intense changes occurring under the more intense winds on the right-hand side (left-hand side in the Southern Hemisphere) of the storm track (Hazelworth, 1968; Dickey and Simpson, 1983; Stramma et al., 1986; Sanford et al., 1987). Conversely, downward mixing of heat causes the upper seasonal thermocline waters to warm. High-amplitude, near-inertial internal gravity waves (with vertical displacements of isotherms of a few tens of meters) and currents (around 1 m s-1) are induced, which persist for several days, and geostrophic flows may also be produced (Shay and Elsberry, 1987; Shay et al, 1989, 1998; Dickey et al., 1998b; Zedler et al., 2002). The SST decrease is a result of both enhanced vertical mixing and upwelling induced by a near-inertial response of the oceanic mixed-layer to the asymmetric surface wind stress (Price, 1981; Shay and Elsberry, 1987), as well as flux-induced cooling.

The upper ocean response to a particular hurricane depends on several parameters involving atmospheric and oceanic variables (e.g., Price, 1981; Dickey et al., 1998b). Some of the important atmospheric variables include hurricane size (e.g., radius of tropical storm force winds, radius of hurricane-force winds), strength (wind speed), and transit speed. Intense, slowly moving hurricanes (~4 m s-1) cause the most significant upwelling and the largest SST response (Price, 1981). Local hydrodynamic conditions, i.e., pre-existing stratification and near-inertial currents, also play an important role in the oceanic response to a hurricane, while the vertical distributions of nutrients and phytoplankton are primary factors in defining the resulting biogeochemical response.

Brink (1989) collected current data at depths from 159 to 1059 m at a location on the path of Hurricane Gloria in 1985 in the western North Atlantic and characterized the response of the deep thermocline and the downward propagation of near-inertial energy. Dickey et al. (1998) studied the Bermuda testbed mooring data during the passage of Hurricane Felix in August 1995. It was found that as Hurricane Felix passed the mooring, large inertial currents were generated within the upper layer by the onset of intense localized wind stress and lasted a period of approximately 22.8 hours. The current shear was the greatest at the base of the mixed layer, where deep cooler waters were entrained into the mixed layer resulting in cooling of the surface layer. The mixed-layer depth was about 15 m prior to the arrival of Felix, and deepened to about 45 m within three days after Felix's passage, according to the mooring. The temperature at 25 m decreased by 3.5-4.0°C and the temperature at 45 m increased by about 2.0°C through the mixing process. Temperatures at 71 m and greater depths decreased slightly.

Three-dimensional ocean numerical models were also used to study the physical process (Price, 1981; Price et al., 1994; Prasad and Hogan; 2007). Price et al. (1994) used a three-dimensional model to simulate ocean response to a moving hurricane and found that the ocean's response to hurricanes can be divided into two stages: forced and relaxation. In the forced stage, hurricane-force winds drive the mixed-layer currents, SST cooling by vertical mixing (entrainment), and air-sea heat exchanges (mainly due to loss of latent heat flux). The barotropic response consists of a geostrophic current and an associated trough in sea surface height. The relaxation stage response following a hurricane's passage is primarily due to inertial-gravity oscillations excited by the storm. The mixed-layer velocity oscillates with a near-inertial period, as do the divergence and associated upwelling and downwelling.

Entrainment has been emphasized as the dominant term in lowering the SST beneath a moving hurricane. Based on observations, Jacob et al. (2000) suggested that entrainment at the mixed-layer base generally accounts for ~75% to 90% of the cooling, while the Price (1981) model indicated that ~85% of heat flux into the mixed layer was through entrainment. Only about 10% to 15% of the cooling in the upper ocean is due to surface heat fluxes, which would range between 2000 and 3000 W m-2. Estimations from Jacob and Shay (2003) ranged from ~10% to ~30% in the directly forced region. Horizontal advection is also found to be important in the mixed-layer heat balance during and subsequent to the passage of hurricanes (Price, 1981; Jacob et al., 2000). This contribution is particularly significant in the eddy region, where maximum cooling due to geostrophic advection (-0.69 °C d-1) was as large as the surface heat flux term in the overall heat budget (Jacob et al., 2000).

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