Diagnostic calculation for circulation and water mass movement in the deep Pacific

Abstract

The steady circulation of the deep Pacific is estimated with a robust diagnostic model, which is internally constrained by hydrographic data. It is shown that the input data should be modified to fit the model in inverse proportion to the Coriolis parameter because a density field inconsistent with the model generates unrealistic geostrophic flows. The model reproduces most of the deep currents previously reported, such as the deep western boundary current east of New Zealand. In addition, as a new feature, the present model diagnoses an anticyclonic circulation around the East Pacific Rise. This circulation is discovered to be associated with a rise of isopycnals at middepth. Tracking of many particles in the diagnosed velocity field reveals that two water masses enter the Southwest Pacific Basin. One is the deep water of the South Indian Basin which enters through a gap to the south of New Zealand. The other is the upper water of the Antarctic Circumpolar Current; this water becomes dense near the Ross Sea and sinks into the deep Southeastern Pacific Basin. The anticyclonic circulation around the East Pacific Rise transports it to the Southwest Pacific Basin. These waters supply comparable volumes to the Southwest Pacific Basin; the residence time is estimated to be 86 years. The deep water in the Southwest Pacific Basin is brought northward rapidly by the deep western boundary current east of New Zealand; it takes only a few decades to move from the east of New Zealand to the North Pacific.