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Jia Wang and Moto Ikeda

Abstract

A three-dimensional, primitive equation model is applied to the ocean mesoscale eddies and unstable baroclinic waves across a density front in a channel under a very low viscosity environment. Current meanders are well produced. The unstable baroclinic waves are examined for flat, positive (same sense as isopycnal tilt) and negative sloping bottoms. The growth rates with flat, gentle, medium, and steep slopes and with different wavelengths (wavenumbers) are discussed. A positive slope clearly suppresses the meandering wave growth rate whose maximum slightly shifts to a lower wavenumber compared to the flat bottom. A gentle negative slope, however, favors the wave growth with the maximum shifting toward higher wavenumber. When the negative slope becomes steeper, the growth rate significantly decreases correspondingly.

Furthermore, a diagnostic analysis package for the pressure tendency and vertical velocity equations, analogous to the approaches in meteorology (ω equation and Q-vector method), is developed for the first time to reveal the physical processes and mechanisms of the unstable wave propagation in the midlatitude ocean. The weaknesses and strengths of these two diagnostic approaches are evaluated and compared to the model results. The Q-vector method is superior to the quasigeostrophic ω equation for diagnosing the vertical motion associated with the mesoscale dynamics from a hydrographic CTD array because the former has no phase error.

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Jia Wang and Lie Yauw Oey

Abstract

Previous in situ observations and modeling studies have indicated that, through mass and momentum exchanges across the shelf edge, the Kuroshio can significantly influence the shelf currents of the East China Sea (ECS). Here, instead of localized observations, this study uses 25 yr of drifter data, supported by satellite and other data to identify seasonal cross-shelf exchanges along the entire shelf edge. The authors show that Kuroshio meanders onshore from fall to winter and offshore from spring to summer, with the largest amplitude northeast of Taiwan. The influence is limited to the shelf edge when the Kuroshio meanders offshore in spring and summer. By contrast, strong onshelf intrusions and cross-shelf exchanges occur when the Kuroshio meanders onshore in fall and winter. Drifters intrude onshelf northeast of Taiwan and spread as far north as 30°N against the strong northeasterly wind. The forcing on the shelf is identified as a northward downsloping of the sea level that is steepest north of Taiwan at 25°–28°N, but which is 3 times weaker farther north. The vorticity budget computed from a numerical model indicates that intrusion during fall and winter is primarily a result of balance between onshelf advection of ambient potential vorticity and vorticity production by the along-isobath pressure gradient acting on the changing mass of water column across the continental slope.

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Jia Wang, Lawrence A. Mysak, and R. Grant Ingram

Abstract

The summer ocean circulation in Hudson Bay is studied numerically using the Blumberg-Mellor model with a 27.5 km × 27.5 km horizontal grid and a realistic bottom topography. In the control run 1) monthly climatological forcing fields of wind stress, oceanic inflow/outflow, and salt and heat fluxes are used. In addition, results are presented for a number ot sensitivity experiments: 2) no topography (otherwise conditions are identical to the control run), 3) no wind forcing, 4) no oceanic inflow/outflow, 5) no heat and salt fluxes, 6) no temperature and salinity variations, and 7) without the nonlinear terms.

While the overall simulated circulation in Hudson Bay is cyclonic, the strong steering of the flow by the bathymetry is particularly noticeable. Mesoscale topographic gyres are simulated, and the separation of the coastal current due to topographic bumps occurs in several locations. The simulated circulation also has well-developed vorticity features and narrow, density-driven coastal jets along the western, southern, and eastern shores of Hudson Bay, which enhance the wind-driven alongshore current. From various sensitivity experiments, it is estimated that the total transport of 0.55 Sv (Sv ≡ 106 m3 s−1) is made up of a 0.23 Sv wind-driven transport, a 0.12 Sv density-driven transport, and a 0.2 Sv inflow/outflow induced transport. It is also found that the wind-driven circulation in Hudson Bay shows a recirculation, whereas the density-driven and inflow/outflow induced transports do not.

A one-dimensional version of the model is also used to simulate the thermohaline vertical structure over a seasonal cycle. In particular, the observed deepening of the mixed layer in fall is reasonably well reproduced by the model.

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Jia Wang, L. A. Mysak, and R. G. Ingram

Abstract

Hibler's dynamic-thermodynamic sea ice model with viscous-plastic rheology is used to simulate the seasonal cycle of sea ice motion, thickness, compactness, and growth rate in Hudson Bay under monthly climatological atmospheric forcing and a prescribed ocean surface current field. The sea ice motion over most of the domain is driven mainly by the wind stress. Wintertime sea ice velocities are only of the order of 1–5 (× 10−4 m s−1) due to the nearly solid ice cover and the closed boundary constraint of Hudson Bay. However, the velocities rise to 0.10–0.20 m s−1 during the melting and freezing seasons when there is partial ice cover. The simulated thickness distribution in mid–April, the time of heaviest ice cover, ranges from 1.3 m in James Bay to 1.7 m in the northern part of Hudson Bay, which compares favorably with observations. The area-averaged growth rate, computed from the model is 1.5–0.5 cm day−1 from December to March, is negative in May (indicative of melting) and reaches its minimum value of −4.2 cm day−1 (maximum melting rate) in July. During autumn, the main freezing season, the growth rate ranges from 1 to 2 cm day−1. In the model, sea ice remains along the south shore of Hudson Bay in summer, as observed, even though the surface air temperatures are higher there than in central and northern Hudson Bay. A sensitivity experiment shows that this is mainly due to the pile-up of ice driven southward by the northwesterly winds. The simulated results for ice cover in other seasons also compare favorably with the observed climatology and with measurements from satellites. In particular, the model gives complete sea ice cover in winter and ice-free conditions in late summer. A series of sensitivity experiments in which the model parameters and external forcing are varied is also carried out.

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Zhaoyun Chen, Yuwu Jiang, Jia Wang, and Wenping Gong

Abstract

Satellite images show that the Pearl River plume is entrained into the upwelling front in the northeastern South China Sea. To understand the processes and extend to other coastal zones, an idealized numerical model is used to investigate the upwelling dynamics in response to the arrival of the river plume. Upon forcing by an upwelling-favorable wind, the model reproduces the upwelling frontal jet with a stratified water column, which takes the river plume far away from the mouth of the estuary. The river plume introduces additional upwelling and downwelling at its inshore and offshore sides (defined as plume-related secondary upwelling circulation), respectively. For the initially unstratified water column, the plume-related secondary upwelling circulation is stronger and extends to deeper water than for the stratified condition. The surface boundary layer thins and the offshore current intensifies in the river plume. The variations in wind-driven current over the deep-water shelf in different stratified conditions are modulated by the vertical profiles of the eddy viscosity, which are shown by a one-dimensional numerical model. Offshore transport is reinforced when the head of the river plume arrives. Thereafter, it is changed by the cross-shore baroclinic geostrophic component of velocity, due to alongshore density variation by the river plume. The horizontal gradient of stress on the two sides of the river plume is responsible for the plume-related secondary upwelling circulation owing to different stress decay scales inside and outside the river plume.

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Lie-Yauw Oey, Jia Wang, and M.-A. Lee

Abstract

In eastern boundary upwelling ecosystems, substantial variance of biological productivity (~50%) can often be related to physical forcing such as winds and ocean temperatures. Robust biophysical connections are less clear-cut in western boundary currents. Here the authors show that interannual variation of fish catch along the western boundary current of the North Pacific, the Kuroshio, significantly correlates (r = 0.67; p < 0.001) with the current’s off-slope (more fish) and on-slope (less fish) sideways shifts in the southern East China Sea. Remotely, transport fluctuations and fish catch are related to the oscillation of a wind stress-curl dipole in the tropical–subtropical gyre of the western North Pacific. Locally, the current’s sideways fluctuations are driven by transport fluctuations through a feedback process between along-isobath pressure gradients and vertical motions: upwelling (downwelling) during the off-slope (on slope) shift, which in turn significantly enhances (depresses) the chlorophyll-a (Chl-a) concentration in winter and early spring. The authors hypothesize that changes in the phytoplankton biomass as indicated by the Chl-a lead to changes in copepodites, the main food source of the fish larvae, and hence also to the observed variation in fish catch.

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