Rui Xin Huang

Ventilation of the North Atlantic and North Pacific : subduction versus obduction

Abstract Ventilation in the North Atlantic and North Pacific is examined by analyzing the Levitus climatological data and the Hellerman and Rosenstein wind stress data. Ventilation between the permanent pycnocline and the overlying seasonal pycnocline and mixed layer consists of two physical processes: subduction and obduction. Subduction takes place mainly in the subtropical basin where surface water is irreversibly transferred into the permanent pycnocline below. Obduction takes place in the subpolar basin where water from the permanent pycnocline is irreversibly transferred into the mixed layer above. Veatilation in the North Atlantic and North Pacific can be clarified into four physically different regions: the subductive region, the obductive region, the ambiductive region where both subduction and obduction take place, and the insulated region where neither subduction nor obduction occurs. Although the total subduction rates in these two oceans are comparable, the total obduction rates are considera...

Mixing and Energetics of the Oceanic Thermohaline Circulation

Abstract Using an idealized tube model and scaling analysis, the physics supporting the oceanic thermohaline circulation is examined. Thermal circulation in the tube model can be classified into two categories. When the cooling source is at a level higher than that of the heating source, the thermal circulation is friction-controlled; thus, mixing is not important in determining the circulation rate. When the cooling source is at a level lower than that of the heating source, the circulation is mixing controlled; thus, weak (strong) mixing will lead to weak (strong) thermal circulation. Within realistic parameter regimes the thermohaline circulation requires external sources of mechanical energy to support mixing in order to maintain the basic stratification. Thus, the oceanic circulation is only a heat conveyor belt, not a heat engine. Simple scaling shows that the meridional mass and heat fluxes are linearly proportional to the energy supplied to mixing. The rate of tidal dissipation in the open oceans ...

Real Freshwater Flux as a Natural Boundary Condition for the Salinity Balance and Thermohaline Circulation Forced by Evaporation and Precipitation

Abstract Freshwater flux used as a natural boundary condition for the salinity balance is applied to a primitive equation model of the oceanic general circulation. Instead of the relaxation condition or the virtual salt flux boundary conditions used in many existing models, the real freshwater flux across the upper surface is specified as the vertical velocity boundary condition for the continuity equation, and the salinity flux is set to identically zero at the sea surface. Numerical experiments show that a model with the natural boundary conditions runs smoothly. Much important physics involving the freshwater flux emerge from the new model. The barotropic Goldsbrough–Stommel gyres driven by the precipitation and evaporation, which were excluded in the previous numerical models, are reproduced. In addition, the models results reveal extremely complex structure of the three-dimensional circulation driven by the freshwater flux. In fact, a relatively small amount of freshwater flux drives very strong mer...

Three-Dimensional Structure of the Wind-Driven Circulation in the Subtropical North Pacific

Abstract The subduction rate is calculated for the North Pacific based on Levitus climatology data and Hellerman and Rosenstein wind stress data. Because the period of effective subduction is rather short, subduction rates calculated in Eulerian and Lagrangian coordinates are very close. The subduction rate defined in the Lagrangian sense consists of two parts. The first part is due to the vertical pumping along the one-year trajectory, and the second part is due to the difference in the winter mixed layer depth over the one-year trajectory. Since the mixed layer is relatively shallow in the North Pacific, the vertical pumping term is very close to the Ekman pumping, while the sloping mixed layer base enhances subduction, especially near the Kuroshio Extension. For most of the subtropical North Pacific, the subduction rate is no more than 75 m yr−1, slightly larger than the Ekman pumping. The water mass volume and total amount of ventilation integrated for each interval of 0.2σ unit is computed. The corre...

Interannual variability of the South China Sea throughflow inferred from wind data and an ocean data assimilation product

[1] The Luzon Strait transport, as an index for the South China Sea throughflow, has attracted much attention recently. In this study the interannual variability of Luzon Strait transport is examined, using the Island Rule and results from ocean data assimilation. Transport variability obtained from these two approaches is consistent with each other. Assessment of contribution from each integral segment involved in the Island Rule indicates that wind stress in the western and central equatorial Pacific is the key factor regulating the interannual variability of the Luzon Strait transport, whereas the effect of local wind stress in the vicinity of the Luzon Strait is secondary. Analysis also shows that when the westerly (easterly) wind anomalies in the tropical Pacific break out, the Luzon Strait transport increases (decreases), associated with the variations in the

Global adjustment of the thermocline in response to deepwater formation

The global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta-plane. The signal is carried from ocean to ocean by Kelvin waves, which travel equatorward along western boundaries, eastward across the equator, poleward at the eastern boundaries, and then eastward around the southern tip of continents into the next ocean basin. The interior is filled by Rossby waves emanating from eastern boundaries. Stronger (weaker) deepwater formation induces an upward (downward) motion of the main thermocline in the world oceans. The adjustment is completed on centennial time scales.

Wind Energy Input to the Surface Waves

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.

Wind Energy Input to the Ekman Layer

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.

The Relative Influence of Diapycnal Mixing and Hydrologic Forcing on the Stability of the Thermohaline Circulation

Abstract Scaling analysis of the oceanic thermohaline circulation has been done under two types of surface boundary conditions: (i) Under “relaxation” conditions (sea surface temperature and salinity are relaxed to prescribed values), there is a two-thirds power law dependence of the meridional overturning (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (ii) Under “mixed” boundary conditions (temperature is relaxed to prescribed values and a virtual salt flux condition is used for salinity), multiple equilibria become possible. For a given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing: for each diapycnal diffusivity K, there is a threshold freshwater flux Ec = CK2/3 (C is a constant) below which three modes are possible with one stable thermal mode, one unstable thermal mode, and a stable hali...

An experimental study on thermal circulation driven by horizontal differential heating

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