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

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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 f... 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 f...

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