The Subtropical/Subpolar Gyre Exchange in the Presence of Annually Migrating Wind and a Meandering Jet: Water Mass Exchange

Huijun Yang Department of Marine Science, University of South Florida, St Petersburg, Florida

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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.

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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