Large scale circulation of the North Pacific Ocean

Abstract

A least squares inversion procedure is used to estimate the large scale cirulation and transport of the subtropical and subpolar North Pacific Ocean from a modern data set of long hydrographic transects. Initially a deep surface of known motion is specified using information derived from abyssal property distributions, moored current meter observations, and basin scale topographic constraints. A geostrophic solution is obtained which conserves mass while devaiting as little as possible in a least squares sense from the initial field. The sensitivity of the solution is tested with regard to changes in the initial field and to the addition of conservation constraints in layers. It is found that about 10 Sv of abyssal water flows northward across 24°N, principally between the dateline and 160°E, in the deepest part of the Northwest Pacific Basin. The flow turns westward across 152°E and then mostly northward again near the Izu-Ogasawara Ridge and the coast of Japan. It then feeds a strong deep anti-cyclonic recirculation beneath the cyclonic subpolar gyre in the Northwest Pacific Basin. The abyssal waters near the western boundary region are found to have a strong component of flow that is upward and across isopycnal surfaces. Here, the abyssal waters complete an important loop in the global thermohaline circulation, entering as bottom water from the South Pacific and returning southward in a less dense and shallower layer. Deep flow into the Northeast Pacific Basin, and circulation within that basin, appear to be weak, making it remote from the main pathway of deep water renewal. The circulation of the subtropical and subpolar gyres dominates transport in the upper layers. The subtropical gyre appears to penetrate to about 1500–2000 m on both sides of the Izu-Ogasawara Ridge, which blocks deeper flow between the Philippine Basin and the Northwest Pacific Basin. The Kuroshio is estimated to carry about 32 Sv northward in the East China Sea. Farther east, as the thermocline slopes upward toward the eastern boundary, the eastward flow is even shallower. In terms of eddy activity, three regimes are observed at 24°N. Peak-to-rough eddy fluctuations in geostrophically balanced sea level diminish from about 40 cm in the west to about 5 cm in the east. Overall, the western boudary of the ocean is about 25 cm higher than the eastern boundary in the 24°N section. Patterns of heat and freshwater flux determined in the North Pacific are in accord with those from air-sea heat flux estimates and hydrological data although the magnitudes are in some cases different. There is large heat loss in the western ocean amounting to about 9.6 × 10 14 W and modest heat gain elsewhere. Heat transport across 24°N is estimated to be 7.5 × 10 14 W. The subpolar ocean has a large excess of precipitation and runoff over evaporation, about 5.6 × 10 5 m 3s −3 north of 35°N, while in the subtropics there is excess evaporation, about 2.7 × 10 5 m 3s −1 between 24°N and 35°N.