Deep Pacific circulation controlled by vertical diffusivity at the lower thermocline depths

Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from o bservations, 0.1 x 10(-4) m(2) s(-1) for the upper layer and 3.0 x 10(-4) m (2) s(-1) for the bottom layer, is adopted. Comparison is made between case s with different vertical diffusivity at middepths. With larger vertical di ffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the dee p water through more intense diffusion at middepths. The water below the th ermocline is warmed and that at the thermocline is cooled for the whole bas in. The warmed deep water leads to larger heat loss through the sea surface , causing the enhanced deep-water formation in the deep-water formation reg ion. On the other hand, the cooled thermocline water leads to larger heat g ain through the sea surface where the thermocline water outcrops, counterba lancing the larger heat loss in the deepwater formation region. The deep wa ter 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 thi s way, the enhanced meridional overturning forms in the deep Pacific. The l ayered deep Pacific meridional circulation is realistically reproduced when vertical diffusivity is larger at middepths. This circulation yields trace r distributions that compare well with observations. Such a strong deep Pac ific circulation does not occur when vertical diffusivity is taken larger a t middepths but is held constant below the middepths. For realistic reprodu ction of the deep Pacific circulation, vertical diffusivity needs to keep i ncreasing with depth beginning at the lower thermocline depths.