THE WATER MASSES AND CIRCULATION AT 10-DEGREES-N IN THE PACIFIC

The circulation and distribution of water masses near the southern bou ndary of the North Pacific Basin are described, based on a recent hydr ographic survey made at 10 degrees N. A circulation scheme is found, u sing both the tracer data and a box inverse model. To ensure the best possible realization of the mean state, repeat survey data are used in the boundary current. Historical data are used to check the represent ativeness of the transport across the onetime section outside of the b oundary current. The upper 400 m are dominated by the North Pacific tr opical Sverdrup cell, where the net interior flow is to the north, com pensated by an equatorward low-latitude boundary flow: the Mindanao Cu rrent. The tropical cell is highly baroclinic in the sense that 18 Sv (1 Sv=1x10(6) m(3) s(-1)) of tropical surface water flows northwards a nd is returned southward entirely within the shallow tropical thermocl ine: half in a broad interior flow and half in the southward boundary current. The strongly baroclinic structure of the tropical cell allows for the efficient transport of North Pacific subtropical water masses across the gyre boundary. The tropical cell is responsible for nearly all of the 0.7+/-0.5 PW of heat transported northwards across the sec tion. There is essentially zero freshwater divergence over the North P acific north of 10 degrees N. Critical to the salt balance of the Nort h Pacific basin is a poleward flux of salty subthermocline water which ultimately derives from the South Pacific subtropical gyre. The South Pacific water found along 10 degrees N has temperatures and salinitie s characteristic of the 13 degrees C Thermostad found at the equator, suggestive of a link via the eastern Pacific. At depth, the analysis r eveals a deep meridional overturning in the North Pacific consisting o f the import across 10 degrees N of 8 Sv of bottom waters of Antarctic origin and their subsequent conversion and export as North Pacific De ep Water. Dominating the mid-depth property distribution and circulati on is a deep anticyclonic cell which horizontally cycles 8 Sv of deep water through the North Pacific basin. We find that there is little in teraction between the North Pacific wind-driven circulation and the ab yssal volume, which contrasts with the Atlantic Ocean. in which there is strong communication between the cold and warm water volumes. 1996 Published by Elsevier Science Ltd.