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- Author or Editor: Simon Chang x
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Abstract
An axisymmetric, hydrostatic ocean model containing a rigid bottom and a free surface is constructed to study the barotropic and baroclinic response in the upper and deep ocean to a wind stress corresponding to a stationary tropical cyclone. The numerical model covers a domain of 800 km and 1475 m in r- and z-directions, respectively, with a uniform radial resolution of 20 km and a stretched vertical resolution from 5 to 54 m. The vertical mixing is parameterized based on a local Richardson number and a mixing length.
The model ocean is spun up with the wind stress of Hurricane Eloise. A strong tangential circulation develops that extends to the ocean floor with a maximum speed of 1.2 m s−1 at the surface. The circulation on the r-z plane, which also extends to the ocean floor, oscillates with time with a maximum upwelling of 0.1 cm s−1 at the center. Surface height has a maximum depression of 57 cm. The deep overturning causes density changes deep in the ocean. A maximum temperature decrease of 3°C occurs in the mixed layer at the center; a maximum temperature increase of 0.45°C is found just below the thermocline at a radius of 200 km. The recovery of both the mass and momentum fields is very slow during the spindown. Inertial oscillations dominate in the spindown even in the deep ocean. Adjustments between the momentum and mass fields seem to converge to a state quite different from the prestorm state.
Direct comparison with observations is difficult because the model is only two-dimensional. Nevertheless, recent observations seem to suggest the existence of the barotropic response in the deep mean. The model suggests that the observed rapid response in the deep ocean is caused by the barotropic pressure gradient force, which arises from the storm-induced perturbation of the free surface.
Abstract
An axisymmetric, hydrostatic ocean model containing a rigid bottom and a free surface is constructed to study the barotropic and baroclinic response in the upper and deep ocean to a wind stress corresponding to a stationary tropical cyclone. The numerical model covers a domain of 800 km and 1475 m in r- and z-directions, respectively, with a uniform radial resolution of 20 km and a stretched vertical resolution from 5 to 54 m. The vertical mixing is parameterized based on a local Richardson number and a mixing length.
The model ocean is spun up with the wind stress of Hurricane Eloise. A strong tangential circulation develops that extends to the ocean floor with a maximum speed of 1.2 m s−1 at the surface. The circulation on the r-z plane, which also extends to the ocean floor, oscillates with time with a maximum upwelling of 0.1 cm s−1 at the center. Surface height has a maximum depression of 57 cm. The deep overturning causes density changes deep in the ocean. A maximum temperature decrease of 3°C occurs in the mixed layer at the center; a maximum temperature increase of 0.45°C is found just below the thermocline at a radius of 200 km. The recovery of both the mass and momentum fields is very slow during the spindown. Inertial oscillations dominate in the spindown even in the deep ocean. Adjustments between the momentum and mass fields seem to converge to a state quite different from the prestorm state.
Direct comparison with observations is difficult because the model is only two-dimensional. Nevertheless, recent observations seem to suggest the existence of the barotropic response in the deep mean. The model suggests that the observed rapid response in the deep ocean is caused by the barotropic pressure gradient force, which arises from the storm-induced perturbation of the free surface.
Abstract
An axisymmetric hurricane model and an axisymmetric ocean model are integrated simultaneously for 24 h to investigate the mutual response of the two systems. The feedbacks between the hurricane and the ocean are negative. The weakening of the hurricane in response to the cooling of the ocean's surface by upwelling and mixing results in a lessened response of the ocean. The results suggest that appreciable weakening of a hurricane due to the cooling of the oceanic surface will not occur if it is translating at typical speed.
Abstract
An axisymmetric hurricane model and an axisymmetric ocean model are integrated simultaneously for 24 h to investigate the mutual response of the two systems. The feedbacks between the hurricane and the ocean are negative. The weakening of the hurricane in response to the cooling of the ocean's surface by upwelling and mixing results in a lessened response of the ocean. The results suggest that appreciable weakening of a hurricane due to the cooling of the oceanic surface will not occur if it is translating at typical speed.
Abstract
Free surface effects induced by an idealized hurricane based on observed air–sea variables in Hurricane Frederic are revisited to examine the barotropic and baroclinic response. Over five inertial periods comparisons between a one-layer and a 17-level model indicate a difference of 6–8 cm s−1 in the depth-averaged current and sea level oscillations of 4–5 cm. In a one-layer simulation, the surface slope geostrophically balances the depth-averaged current, whereas the 17-level model simulations indicate a near-inertially oscillating current of 7–8 cm s−1 found by removing the depth-averaged flow from the geostrophic currents induced by the surface slope. Surface undulations are driven by the depth-averaged nonlinear terms in the density equation, that is, [u
Based on fits of the 17 levels of demodulated horizontal velocities at 1.03f (f the Coriolis parameter) to the eigenfunctions, maximum amplitudes of the barotropic and first baroclinic modes are 7 and 58 cm s−1, respectively. The barotropic mode amplitude is consistent with the current found by removing the depth-averaged flow from the geostrophic current that contributes 2%–3% to the energy in the near-inertial wave pass band. Vertical velocity eigenfunctions at the surface indicate that the barotropic mode is at least 50 to 80 times larger than the baroclinic mode. Surface displacements by the barotropic mode have amplitudes of ±4 cm, explaining 90% to 95% of the height variations. The first baroclinic mode contributes about 0.2–0.4 cm to the free surface displacements. The weak barotropic near-inertial current provides a physical mechanism for the eventual breakup of the sea surface depression induced by the hurricane’s wind stress and surface Ekman divergence.
Abstract
Free surface effects induced by an idealized hurricane based on observed air–sea variables in Hurricane Frederic are revisited to examine the barotropic and baroclinic response. Over five inertial periods comparisons between a one-layer and a 17-level model indicate a difference of 6–8 cm s−1 in the depth-averaged current and sea level oscillations of 4–5 cm. In a one-layer simulation, the surface slope geostrophically balances the depth-averaged current, whereas the 17-level model simulations indicate a near-inertially oscillating current of 7–8 cm s−1 found by removing the depth-averaged flow from the geostrophic currents induced by the surface slope. Surface undulations are driven by the depth-averaged nonlinear terms in the density equation, that is, [u
Based on fits of the 17 levels of demodulated horizontal velocities at 1.03f (f the Coriolis parameter) to the eigenfunctions, maximum amplitudes of the barotropic and first baroclinic modes are 7 and 58 cm s−1, respectively. The barotropic mode amplitude is consistent with the current found by removing the depth-averaged flow from the geostrophic current that contributes 2%–3% to the energy in the near-inertial wave pass band. Vertical velocity eigenfunctions at the surface indicate that the barotropic mode is at least 50 to 80 times larger than the baroclinic mode. Surface displacements by the barotropic mode have amplitudes of ±4 cm, explaining 90% to 95% of the height variations. The first baroclinic mode contributes about 0.2–0.4 cm to the free surface displacements. The weak barotropic near-inertial current provides a physical mechanism for the eventual breakup of the sea surface depression induced by the hurricane’s wind stress and surface Ekman divergence.
Abstract
An asymmetric nonlinear ocean model is employed to investigate the oceanic response to moving hurricanes. A turbulent kinetic energy budget is used to parameterize the stress-induced vertical mixing. The results show that the ocean's response to a symmetric storm is stronger on the right of the storm track. Although the maximum speed of the induced current under the storm is not sensitive to the storm's translation speed, the speed does have a large influence on the temperature structure and the thermocline depth in the wake. Vertical motions associated with the inertia-gravity oscillations persist in the wake of the storm. A narrow ridge in the thermocline is left in the storm track for fast-moving storms. The results in many respects agree with Geisler's linear solutions. However, vertical mixing produces significant differences in the depth of the thermocline behind the storm.
Abstract
An asymmetric nonlinear ocean model is employed to investigate the oceanic response to moving hurricanes. A turbulent kinetic energy budget is used to parameterize the stress-induced vertical mixing. The results show that the ocean's response to a symmetric storm is stronger on the right of the storm track. Although the maximum speed of the induced current under the storm is not sensitive to the storm's translation speed, the speed does have a large influence on the temperature structure and the thermocline depth in the wake. Vertical motions associated with the inertia-gravity oscillations persist in the wake of the storm. A narrow ridge in the thermocline is left in the storm track for fast-moving storms. The results in many respects agree with Geisler's linear solutions. However, vertical mixing produces significant differences in the depth of the thermocline behind the storm.
Abstract
During the passage of hurricane Frederic in 1979, four ocean current meter arrays in water depths of 100–950 m detected both a baroclinic and a depth-independent response in the near-inertial frequency band. Although the oceanic response was predominately baroclinic, the hurricane excited a depth-independent component of 5–11 cm s−1.
The origin and role of the depth-independent component of velocity is investigated using a linear analytical model and numerical simulations from a 17-level primitive equation model with a free surface. Both models are forced with an idealized wind stress pattern based on the observed storm parameters in hurricane Frederic. In an analytical model, the Green's function (K 0) is convolved with the wind stress curl to predict a sea surface depression of approximately 20 cm from the equilibrium position. The near-inertial velocities are simulated by convolving the slope of the sea surface depression with a second Green's function. The barotropic current velocities rotate inertially with periods shifted above the local inertial period by 1%–2% and the maximum amplitude of 11 cm s−1 is displaced to the right of the track at x = 2R max (radius of maximum winds).
The free surface depression simulated by the primitive-equation model is also about 18–20 cm. The primitive equation model simulations indicate that the vertical mean pressure gradient excites 10–11 cm s−1 depth-averaged currents at x = 3R max. The net divergence and convergence of the horizontal velocities induces vertical deflections of the sea surface. The spatial pattern of the barotropic amplitudes simulated by the numerical and analytical models differ by less than 2 cm s−1 in the region 0 < x < 4R max, which suggests that the barotropic response to the passage of a moving hurricane is governed by linear processes.
Abstract
During the passage of hurricane Frederic in 1979, four ocean current meter arrays in water depths of 100–950 m detected both a baroclinic and a depth-independent response in the near-inertial frequency band. Although the oceanic response was predominately baroclinic, the hurricane excited a depth-independent component of 5–11 cm s−1.
The origin and role of the depth-independent component of velocity is investigated using a linear analytical model and numerical simulations from a 17-level primitive equation model with a free surface. Both models are forced with an idealized wind stress pattern based on the observed storm parameters in hurricane Frederic. In an analytical model, the Green's function (K 0) is convolved with the wind stress curl to predict a sea surface depression of approximately 20 cm from the equilibrium position. The near-inertial velocities are simulated by convolving the slope of the sea surface depression with a second Green's function. The barotropic current velocities rotate inertially with periods shifted above the local inertial period by 1%–2% and the maximum amplitude of 11 cm s−1 is displaced to the right of the track at x = 2R max (radius of maximum winds).
The free surface depression simulated by the primitive-equation model is also about 18–20 cm. The primitive equation model simulations indicate that the vertical mean pressure gradient excites 10–11 cm s−1 depth-averaged currents at x = 3R max. The net divergence and convergence of the horizontal velocities induces vertical deflections of the sea surface. The spatial pattern of the barotropic amplitudes simulated by the numerical and analytical models differ by less than 2 cm s−1 in the region 0 < x < 4R max, which suggests that the barotropic response to the passage of a moving hurricane is governed by linear processes.
