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Deep Pacific Circulation Controlled by Vertical Diffusivity at the Lower Thermocline Depths

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  • 1 Center for Climate System Research, University of Tokyo, Tokyo, Japan
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Abstract

Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from observations, 0.1 × 10−4 m2 s−1 for the upper layer and 3.0 × 10−4 m2 s−1 for the bottom layer, is adopted. Comparison is made between cases with different vertical diffusivity at middepths. With larger vertical diffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the deep water through more intense diffusion at middepths. The water below the thermocline is warmed and that at the thermocline is cooled for the whole basin. The warmed deep water leads to larger heat loss through the sea surface, causing the enhanced deep-water formation in the deep-water formation region. On the other hand, the cooled thermocline water leads to larger heat gain through the sea surface where the thermocline water outcrops, counterbalancing the larger heat loss in the deep-water formation region. The deep water brought up to the middepths does not further upwell to the sea surface due to the small upper-layer vertical diffusivity, but it flows back to the deep-water formation region, slowly upwelling within the middepths. In this way, the enhanced meridional overturning forms in the deep Pacific. The layered deep Pacific meridional circulation is realistically reproduced when vertical diffusivity is larger at middepths. This circulation yields tracer distributions that compare well with observations. Such a strong deep Pacific circulation does not occur when vertical diffusivity is taken larger at middepths but is held constant below the middepths. For realistic reproduction of the deep Pacific circulation, vertical diffusivity needs to keep increasing with depth beginning at the lower thermocline depths.

Corresponding author address: Dr. Hiroyuki Tsujino, Center for Climate System Research, University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153-8904 Tokyo, Japan. Email: tsujino@ccsr.u-tokyo.ac.jp

Abstract

Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from observations, 0.1 × 10−4 m2 s−1 for the upper layer and 3.0 × 10−4 m2 s−1 for the bottom layer, is adopted. Comparison is made between cases with different vertical diffusivity at middepths. With larger vertical diffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the deep water through more intense diffusion at middepths. The water below the thermocline is warmed and that at the thermocline is cooled for the whole basin. The warmed deep water leads to larger heat loss through the sea surface, causing the enhanced deep-water formation in the deep-water formation region. On the other hand, the cooled thermocline water leads to larger heat gain through the sea surface where the thermocline water outcrops, counterbalancing the larger heat loss in the deep-water formation region. The deep water brought up to the middepths does not further upwell to the sea surface due to the small upper-layer vertical diffusivity, but it flows back to the deep-water formation region, slowly upwelling within the middepths. In this way, the enhanced meridional overturning forms in the deep Pacific. The layered deep Pacific meridional circulation is realistically reproduced when vertical diffusivity is larger at middepths. This circulation yields tracer distributions that compare well with observations. Such a strong deep Pacific circulation does not occur when vertical diffusivity is taken larger at middepths but is held constant below the middepths. For realistic reproduction of the deep Pacific circulation, vertical diffusivity needs to keep increasing with depth beginning at the lower thermocline depths.

Corresponding author address: Dr. Hiroyuki Tsujino, Center for Climate System Research, University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153-8904 Tokyo, Japan. Email: tsujino@ccsr.u-tokyo.ac.jp

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