Buoyancy- and Wind-Driven Circulation in an Extended Model of Potential Vorticity Homogenization

Shigeto Nishino Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Japan

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Shoshiro Minobe Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Japan

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Abstract

An analytical model is proposed on the middepth water circulation between the surface wind-driven circulation and the deep buoyancy-driven circulation. The model consists of three and a half layers, with the second and third layers being the middepth layers. The model includes thermohaline processes by allowing diapycnal flows at interfaces between layers, in addition to the potential vorticity homogenization proposed by Rhines and Young. The velocities of diapycnal flow are calculated from the density stratification, which can be given solely by a wind-driven model as a good approximation. The divergence of diapycnal velocity causes a circulation in addition to the wind-driven circulation. Although, in pure wind-driven theories, motion was absent outside the region of homogenized potential vorticity, the present model gives significant currents there. In particular, a prominent eastward flow appears in the third layer along the southern rim of the homogeneous potential vorticity region in the second layer. The flow pattern in the present model is consistent with that estimated diagnostically from the climatological density distribution in the North Pacific. The diagnosed flow field is further supported by the oxygen distribution there.

* Current affiliation: Japan Marine Science and Technology Center, Yokosuka, Japan.

+ Additional affiliation: Frontier Research System for Global Change, Tokyo, Japan.

Corresponding author address: Dr. Shigeto Nishino, Ocean Research Department, Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan.

nishinos@jamstec.go.jp

Abstract

An analytical model is proposed on the middepth water circulation between the surface wind-driven circulation and the deep buoyancy-driven circulation. The model consists of three and a half layers, with the second and third layers being the middepth layers. The model includes thermohaline processes by allowing diapycnal flows at interfaces between layers, in addition to the potential vorticity homogenization proposed by Rhines and Young. The velocities of diapycnal flow are calculated from the density stratification, which can be given solely by a wind-driven model as a good approximation. The divergence of diapycnal velocity causes a circulation in addition to the wind-driven circulation. Although, in pure wind-driven theories, motion was absent outside the region of homogenized potential vorticity, the present model gives significant currents there. In particular, a prominent eastward flow appears in the third layer along the southern rim of the homogeneous potential vorticity region in the second layer. The flow pattern in the present model is consistent with that estimated diagnostically from the climatological density distribution in the North Pacific. The diagnosed flow field is further supported by the oxygen distribution there.

* Current affiliation: Japan Marine Science and Technology Center, Yokosuka, Japan.

+ Additional affiliation: Frontier Research System for Global Change, Tokyo, Japan.

Corresponding author address: Dr. Shigeto Nishino, Ocean Research Department, Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan.

nishinos@jamstec.go.jp

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