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Rui Xin Huang and Qi Wang

Abstract

The communication from the subtropical gyre interior to the Tropics is examined using wind stress datasets and results from an ocean data assimilation system. It is shown that the interior communication can be clarified by a simple interior mass communication rate (IMCR), which can be easily calculated from the Sverdrup function. For the Northern (Southern) Hemisphere the IMCR can be defined as the meridional minimum (maximum) of the Sverdrup function maximum (minimum) at each latitude. The interior communication is closely related to the ENSO cycle, and its rate and pathway have strong interannual–decadal variability.

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Liping Wang and Rui Xin Huang

Abstract

An analytical solution is sought for a wind-driven circulation in the inviscid limit in a linear barotropic channel model of the Antarctic Circumpolar Ocean in the presence of a bottom ridge. There is a critical height of the ridge, above which all geostrophic contours in the channel are blocked. In the subcritical case, the Sverdrup balance does not apply and there is no solution in the inviscid limit. In the supercritical case, however, the Sverdrup balance applies and an explicit form for the zonal transport in the channel is obtained.

In the case with a uniform wind stress, the transport in the β-plane channel is independent of the width of the ridge, linearly proportional to the wind stress and the length of the channel, while inversely linearly proportional to the ridge height. In the f plane with β = 0, the transport is even independent of the width of the channel. In the case with a nonuniform wind stress τx = τ0(1-−cosπy/D), the Sverdrup flow driven by the vorticity input always induces a form drag against the mean wind stress. Now, the transport depends on the width of the ridge but not on the length of the channel.

The model clearly demonstrates how the topographic form drag is generated in a linear barotropic model, which is fundamentally different from the nonlinear Rossby wave drag generation. Here, in this linear model, the presence of a supercritical high ridge is essential in the inviscid limit. The form drag is generated regardless of the flow direction. Besides, the model demonstrates that most of the potential vorticity dissipation occurs at the northern boundary where the ridge is located.

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Wei Wang and Rui Xin Huang

Abstract

Wind stress energy input through the surface ageostrophic currents is studied. The surface ageostrophic velocity is calculated using the classical formula of the Ekman spiral, with the Ekman depth determined from an empirical formula. The total amount of energy input over the global oceans for subinertial frequency is estimated as 2.4 TW averaged over a period from 1997 to 2002, or 2.3 TW averaged over a period from 1948 to 2002, based on daily wind stress data from NCEP–NCAR. Thus, in addition to the energy input to the near inertial waves of 0.5–0.7 TW reported by Alford and by Watanabe and Hibiya, the total energy input to the Ekman layer is estimated as 3 TW. This input is concentrated primarily over the Southern Ocean and the storm track in both the North Pacific and North Atlantic Oceans.

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Qi Wang and Rui Xin Huang

Abstract

A method based on isopycnal trajectory analysis is proposed to quantify the pathways from the subtropics to the Tropics. For a continuous stratified ocean a virtual streamfunction is defined, which can be used to characterize these pathways. This method is applied to the climatological dataset produced from a data-assimilated model. Analysis indicates that in each layer contours of the virtual streamfunction are a good approximation for streamlines, even if there is a cross-isopycnal mass flux. The zonal-integrated meridional transport per unit layer thickness through each pathway varies in proportion to 1/sinθ, where θ is latitude. The vertical-integrated total transport through pathways behaves similarly. Transport through pathways has a prominent decadal variability. Results suggest that in decadal time scales the interior pathway transport (IPT) anomaly may be mainly caused by the wind stress anomaly at low latitude. The western boundary pathway transport (WBPT) anomaly often has a sign opposite to the IPT anomaly, reflecting compensation between the IPT and the WBPT. However, more often than not the wind stress anomaly within tropical latitudes can also be used to explain the WBPT anomaly.

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Liping Wang and Rui Xin Huang

Abstract

A simple barotropic model of abyssal circulation in a circumpolar ocean basin is constructed. In the presence of a sufficiently high ridge, the classical Stommel and Arons theory applies here with very substantial modifications In the case with a point source at one side of the channel and a point sink at the other side of the channel, there is a through-channel recirculation in addition to the flow from the source to the sink. The volume flux of this recirculation is critically determined by the supercriticality of the ridge height. In the case with a uniform sink and point sources and sinks, the circulation is essentially in the Stommel and Arons sense with one major novelty; that is, a through-channel recirculating flow is generated. Both its strength and direction depend critically upon the model parameters. This suggests that the Antarctic Bottom Water formation could drive a substantial amount of westward flow that counterbalances the wind-driven eastward flow. Last, a schematic picture of the abyssal circulation in an idealized Southern Ocean is obtained. The most significant feature is the narrow current along the northern boundary of the circumpolar basin, which feeds the deep western boundary currents of the Indian Ocean and Pacific Ocean and connects all the oceanic basins in the Southern Ocean.

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Wei Wang and Rui Xin Huang

Abstract

Wind energy input into the ocean is primarily produced through surface waves. The total rate of this energy source, integrated over the World Ocean, is estimated at 60 TW, based on empirical formulas and results from a numerical model of surface waves. Thus, surface wave energy input is about 50 times the energy input to the surface geostrophic current and 20 times the total tidal dissipation rate. Most of the energy input is concentrated within the Antarctic Circumpolar Current.

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Chuan Jiang Huang, Wei Wang, and Rui Xin Huang

Abstract

The circulation in the equatorial Pacific Ocean is studied in a series of numerical experiments based on an isopycnal coordinate model. The model is subject to monthly mean climatology of wind stress and surface thermohaline forcing. In response to decadal variability in the diapycnal mixing coefficient, sea surface temperature and other properties of the circulation system oscillate periodically. The strongest sea surface temperature anomaly appears in the geographic location of Niño-3 region with the amplitude on the order of 0.5°C, if the model is subject to a 30-yr sinusoidal oscillation in diapycnal mixing coefficient that varies between 0.03 × 10−4 and 0.27 × 10−4 m2 s−1. Changes in diapycnal mixing coefficient of this amplitude are within the bulk range consistent with the external mechanical energy input in the global ocean, especially when considering the great changes of tropical cyclones during the past decades. Thus, time-varying diapycnal mixing associated with changes in wind energy input into the ocean may play a nonnegligible role in decadal climate variability in the equatorial circulation and climate.

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Guihua Wang, Rui Xin Huang, Jilan Su, and Dake Chen

Abstract

The dynamic influence of thermohaline circulation on wind-driven circulation in the South China Sea (SCS) is studied using a simple reduced gravity model, in which the upwelling driven by mixing in the abyssal ocean is treated in terms of an upward pumping distributed at the base of the upper layer.

Because of the strong upwelling of deep water, the cyclonic gyre in the northern SCS is weakened, but the anticyclonic gyre in the southern SCS is intensified in summer, while cyclonic gyres in both the southern and northern SCS are weakened in winter. For all seasons, the dynamic influence of thermohaline circulation on wind-driven circulation is larger in the northern SCS than in the southern SCS. Analysis suggests that the upwelling associated with the thermohaline circulation in the deep ocean plays a crucial role in regulating the wind-driven circulation in the upper ocean.

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Ling Ling Liu, Wei Wang, and Rui Xin Huang

Abstract

Wind stress and tidal dissipation are the most important sources of mechanical energy for maintaining the oceanic general circulation. The contribution of mechanical energy due to tropical cyclones can be a vitally important factor in regulating the oceanic general circulation and its variability. However, previous estimates of wind stress energy input were based on low-resolution wind stress data in which strong nonlinear events, such as tropical cyclones, were smoothed out.

Using a hurricane–ocean coupled model constructed from an axisymmetric hurricane model and a three-layer ocean model, the rate of energy input to the world’s oceans induced by tropical cyclones over the period from 1984 to 2003 was estimated. The energy input is estimated as follows: 1.62 TW to the surface waves and 0.10 TW to the surface currents (including 0.03 TW to the near-inertial motions). The rate of gravitational potential energy increase due to tropical cyclones is 0.05 TW. Both the energy input from tropical cyclones and the increase of gravitational potential energy of the ocean show strong interannual and decadal variability with an increasing rate of 16% over the past 20 years. The annual mean diapycnal upwelling induced by tropical cyclones over the past 20 years is estimated as 39 Sv (Sv ≡ 106 m3 s−1). Owing to tropical cyclones, diapycnal mixing in the upper ocean (below the mixed layer) is greatly enhanced. Within the regimes of strong activity of tropical cyclones, the increase of diapycnal diffusivity is on the order of (1 − 6) × 10−4 m2 s−1. The tropical cyclone–related energy input and diapycnal mixing may play an important role in climate variability, ecology, fishery, and environments.

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Xiaowei Wang, Shiqiu Peng, Zhiyu Liu, Rui Xin Huang, Yu-Kun Qian, and Yineng Li

Abstract

By taking into account the contributions of both locally and remotely generated internal tides, the tidal mixing in the Luzon Strait (LS) and the South China Sea (SCS) is investigated through internal-tide simulation and energetics analysis. A three-dimensional nonhydrostatic high-resolution model driven by four primary tidal constituents (M2, S2, K1, and O1) is used for the internal-tide simulation. The baroclinic energy budget analysis reveals that the internal tides radiated from the LS are the dominant energy source for the tidal dissipation in the SCS. In the LS, the estimated depth-integrated turbulent kinetic energy dissipation exceeds O(1) W m−2 atop the two subsurface ridges, with a dissipation rate of >O(10−7) W kg−1 and diapycnal diffusivity of ~O(10−2) m2 s−1. In the SCS, the most intense turbulence occurs in the deep-water basin with a dissipation rate of O(10−8–10−6) W kg−1 and diapycnal diffusivity of O(10−3–10−1) m2 s−1 within the ~2000-m water column above the seafloor as well as in the shelfbreak region with a dissipation rate of O(10−7–10−6) W kg−1 and diapycnal diffusivity of O(10−4–10−3) m2 s−1. These estimated values are consistent with observations reported in previous studies and are at least one order of magnitude larger than those based solely on locally generated internal tides.

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