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Huijun Yang

Abstract

A simple barotropic double gyre–jet ocean circulation model is developed, driven by surface wind. The model consists of a subtropical gyre in the south and a subpolar gyre in the north and a meandering jet between them. Using this ocean model, the water mass exchange between two gyres is investigated by calculating the Lagrangian trajectories of the water column. The results show that the meandering jet will cause strong intergyre exchange, and the exchange will be substantially enchanced when the wind is allowed to migrate north and south. For example, in the standard case adapted after the Gulf Stream, it is found that about 10% subtropical water mass has been transferred into the subpolar gyre in 25 years when the wind is steady; whereas it is increased to about 18% in the same period of time when the wind is migrating annually with a distance 800 km. When the wind is steady, the subtropical water mass enters the subpolar gyre mainly through the western boundary. It flows eastward and then penetrates and spreads into the whole subpolar gyre after arriving at the eastern part due to the strong jet and the subpolar recirculation.

Extensive parameter sensitivity experiments show that when the wind is steady, the transport increases with the width of the jet, and the amplitude and wavenumber of the waves in the jet. The transport also increases with the amplitude of the waves when the wind is allowed to migrate. Other parameter dependence as well as the dependence on the meandering jet in the migrating wind is complicated. Maximum transport occurs when the wind migrates interannually to decadally.

The finite-time Lyapunov exponent has successfully identified many important features of the transport by the ocean circulation, including a central barrier centered along the meandering jet core and chaotic transport regions on both sides of the jet core, the western boundary transport channel, and the eastern transport regions. There are two recirculation regions with zero Lyapunov exponent when the wind is steady.

The mean Lagrangian transport (MLT) formula is derived based on the Lagrangian trajectory calculation. Applying the results to the North Atlantic Ocean, it is suggested that the 25-yr MLT in the North Atlantic is about 4.7 Sv (Sv ≡ 106 m3 s−1) in the standard case and could be as high as 7.5 Sv for other parameters. These results are consistent with the present understanding of the subtropical/subpolar gyre water mass exchange in the North Atlantic that the net exchange due to the gyre circulation mode is about 6.5 Sv. The methods and some results are also applicable to the intergyre exchange between the tropical gyre and subtropical gyre, between the tropical gyre and the equatorial gyre, as well as inter-hemispheric exchange.

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Huijun Yang

Abstract

Recent TOPEX/Poseidon observations show an enhanced (weakened) westward propagation of long planetary Rossby waves at extratropics (Tropics) and they usually propagate faster in the western basin than in the eastern basin in all major oceans. The evolution of a long planetary wave packet in a continuously stratified ocean in response to the various forcing functions is analytically investigated using the wave packet theory. For a wave packet with a large vertical scale, the stratification variation and the vertical shear of the mean zonal current act in concert, causing the wave packet to propagate directly against the mean zonal current—called the counter-Doppler-shift (CDS) effect. It is found that the speed ratio between the zonal baroclinic zonal current and the classic theory increases with the latitude, the eastward zonal current, and the local vertical scale.

The vertical scale of a wave packet plays a critical role in the propagation, the structure, and spatial-scale development of a wave packet. It is found that the β and stratification effects increase (decrease) the vertical spatial scale of a vertically westward (eastward) tilted wave packet. For a wave packet with a large (small) vertical scale, the vertical spatial scale increases (decreases) when the wave packet is tilted westward in an eastward zonal current. The structural change could effectively separate the extratropic oceanic responses into two kinds of systems with two different vertical scales and strengthen the CDS effect, enhancing speeds in western ocean basins. Several analytical solutions for the wave packet are also obtained.

The author concludes that the evolution of a wave packet with a large vertical scale in a zonal current may account for all major features of the sea surface height anomalies observed in the TOPEX/Poseidon data. The possible forcing functions are the atmospheric wind forcing at the sea surface and the ocean topographic forcing on the seafloor, but not the surface cooling or heating.

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Zhengyu Liu
and
Huijun Yang

Abstract

The effect of the annual migration of the wind field on the intergyre transport is investigated in a double-gyre circulation. It is found that the trajectories of the water columns advected by the gyre-scale circulation exhibit a strongly chaotic behavior. The resulted cross-gyre chaotic transport amounts to about one-third of the Sverdrup transport.

The chaotic intergyre transport causes strong mixing between the two gyres. The study with a passive tracer shown that the equivalent diffusivity of the chaotic mixing is at the order of 107 cm2, s−1, comparable to that estimated for strong synoptical eddies in the region of the Gulf Stream. It is suggested that the chaotic transport may contribute significantly to the intergyre exchange.

Further parameter sensitivity studies show that the chaotic transport is the strongest under the migration with frequencies from interannual to decadal, and with the migration distance about 1000 km. Some possible applications of the chaotic transport to the general oceanic circulation are also discussed.

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Huijun Yang
and
Zhengyu Liu

Abstract

The aim of this paper is to renew interest in the Lagrangian view of global and basin ocean circulations and its implications in physical and biogeochemical ocean processes. The paper examines the Lagrangian transport, mixing, and chaos in a simple, laminar, three-dimensional, steady, basin-scale oceanic flow consisting of the gyre and the thermohaline circulation mode. The Lagrangian structure of this flow could not be chaotic if the steady oceanic flow consists of only either one of the two modes nor if the flow is zonally symmetric, such as the Antarctic Circumpolar Current. However, when both the modes are present, the Lagrangian structure of the flow is chaotic, resulting in chaotic trajectories and providing the enhanced transport and mixing and microstructure of a tracer field. The Lagrangian trajectory and tracer experiments show the great complexity of the Lagrangian geometric structure of the flow field and demonstrate the complicated transport and mixing processes in the World Ocean. The finite-time Lyapunov exponent analysis has successfully characterized the Lagrangian nature. One of the most important findings is the distinct large-scale barrier—which the authors term the great ocean barrier—within the ocean interior with upper and lower branches, as remnants of the Kolmogorov–Arnold–Moser (KAM) invariant surfaces. The most fundamental reasons for such Lagrangian structure are the intrinsic nature of the long time mean, global and basin-scale oceanic flow: the three-dimensionality and incompressibility giving rise to chaos and to the great ocean barrier, respectively. Implications of these results are discussed, from the great ocean conveyor hypothesis to the predictability of the (quasi) Lagrangian drifters and floats in the climate observing system.

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