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Jinbo Wang, Matthew R. Mazloff, and Sarah T. Gille

Abstract

The Kerguelen Plateau is a major topographic feature in the Southern Ocean. Located in the Indian sector and spanning nearly 2000 km in the meridional direction from the polar to the subantarctic region, it deflects the eastward-flowing Antarctic Circumpolar Current and influences the physical circulation and biogeochemistry of the Southern Ocean. The Kerguelen Plateau is known to govern the local dynamics, but its impact on the large-scale ocean circulation has not been explored. By comparing global ocean numerical simulations with and without the Kerguelen Plateau, this study identifies two major Kerguelen Plateau effects: 1) The plateau supports a local pressure field that pushes the Antarctic Circumpolar Current northward. This process reduces the warm-water transport from the Indian to the Atlantic Ocean. 2) The plateau-generated pressure field shields the Weddell Gyre from the influence of the warmer subantarctic and subtropical waters. The first effect influences the strength of the Antarctic Circumpolar Current and the Agulhas leakage, both of which are important elements in the global thermohaline circulation. The second effect results in a zonally asymmetric response of the subpolar gyres to Southern Hemisphere wind forcing.

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Jinbo Wang, Michael A. Spall, Glenn R. Flierl, and Paola Malanotte-Rizzoli

Abstract

Linear and nonlinear radiating instabilities of an eastern boundary current are studied using a barotropic quasigeostrophic model in an idealized meridional channel. The eastern boundary current is meridionally uniform and produces unstable modes in which long waves are most able to radiate. These long radiating modes are easily suppressed by friction because of their small growth rates. However, the long radiating modes can overcome friction by nonlinear energy input transferred from the more unstable trapped mode and play an important role in the energy budget of the boundary current system. The nonlinearly powered long radiating modes take away part of the perturbation energy from the instability origin to the ocean interior. The radiated instabilities can generate zonal striations in the ocean interior that are comparable to features observed in the ocean. Subharmonic instability is identified to be responsible for the nonlinear resonance between the radiating and trapped modes, but more general nonlinear triad interactions are expected to apply in a highly nonlinear environment.

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Jinbo Wang, Glenn R. Flierl, Joseph H. LaCasce, Julie L. McClean, and Amala Mahadevan

Abstract

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numerical simulation. The decomposition yields strikingly realistic subsurface fields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example, sea surface temperature (SST) and SSH, and sparse (or climatological) estimates of the regional depth-resolved density. The method could be further elaborated to integrate additional subsurface information, such as mooring measurements.

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Bo Qiu, Shuiming Chen, Patrice Klein, Jinbo Wang, Hector Torres, Lee-Lueng Fu, and Dimitris Menemenlis

Abstract

The transition scale L t from balanced geostrophic motions to unbalanced wave motions, including near-inertial flows, internal tides, and inertia–gravity wave continuum, is explored using the output from a global 1/48° horizontal resolution Massachusetts Institute of Technology general circulation model (MITgcm) simulation. Defined as the wavelength with equal balanced and unbalanced motion kinetic energy (KE) spectral density, L t is detected to be geographically highly inhomogeneous: it falls below 40 km in the western boundary current and Antarctic Circumpolar Current regions, increases to 40–100 km in the interior subtropical and subpolar gyres, and exceeds, in general, 200 km in the tropical oceans. With the exception of the Pacific and Indian sectors of the Southern Ocean, the seasonal KE fluctuations of the surface balanced and unbalanced motions are out of phase because of the occurrence of mixed layer instability in winter and trapping of unbalanced motion KE in shallow mixed layer in summer. The combined effect of these seasonal changes renders L t to be 20 km during winter in 80% of the Northern Hemisphere oceans between 25° and 45°N and all of the Southern Hemisphere oceans south of 25°S. The transition scale’s geographical and seasonal changes are highly relevant to the forthcoming Surface Water and Ocean Topography (SWOT) mission. To improve the detection of balanced submesoscale signals from SWOT, especially in the tropical oceans, efforts to remove stationary internal tidal signals are called for.

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Bo Qiu, Shuiming Chen, Patrice Klein, Hector Torres, Jinbo Wang, Lee-Lueng Fu, and Dimitris Menemenlis

Abstract

Reconstructability of upper-ocean vertical velocity w and vorticity ζ fields from high-resolution sea surface height (SSH) data is explored using the global 1/48° horizontal-resolution MITgcm output in the context of the forthcoming Surface Water and Ocean Topography (SWOT) mission. By decomposing w with an omega equation of the primitive equation system and by taking into account the measurement design of the SWOT mission, this study seeks to reconstruct the subinertial, balanced w and ζ signals. By adopting the effective surface quasigeostrophic (eSQG) framework and applying to the Kuroshio Extension region of the North Pacific, we find that the target and reconstructed fields have a spatial correlation of ~0.7 below the mixed layer for w and 0.7–0.9 throughout the 1000-m upper ocean for ζ in the error-free scenario. By taking the SWOT sampling and measurement errors into account, the spatial correlation is found to decrease to 0.4–0.6 below the mixed layer for w and 0.6–0.7 for ζ, respectively. For both w and ζ reconstruction, the degradation due to the SWOT errors is more significant in the surface layer and for smaller-scale signals. The impact of errors lessens with the increasing depth and lengthening horizontal scales.

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