FY 1996

The water masses and circulation at 10°N in the Pacific

Wijffels, S.E., J.M. Toole, H.L. Bryden, R.A. Fine, W.J. Jenkins, and J.L. Bullister

Deep-Sea Res. I, 43(4), 501–544, doi: 10.1016/0967-0637(96)00006-4 (1996)

The circulation and distribution of water masses near the southern boundary of the North Pacific Basin are described, based on a recent hydrographic survey made at 10°N. A circulation scheme is found, using 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 representativeness of the transport across the onetime section outside of the boundary current. The upper 400 m are dominated by the North Pacific tropical Sverdrup cell, where the net interior flow is to the north, compensated by an equatorward low-latitude boundary flow: the Mindanao Current. The tropical cell is highly baroclinic in the sense that 18 Sv (1 Sv = 1 × 10⁶ m³ s⁻¹) of tropical surface water flows northwards and is returned southward entirely within the shallow tropical thermocline: 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 section. There is essentially zero freshwater divergence over the North Pacific north of 10°N. Critical to the salt balance of the North 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°N has temperatures and salinities characteristic of the 13°C Thermostad found at the equator, suggestive of a link via the eastern Pacific. At depth, the analysis reveals a deep meridional overturning in the North Pacific consisting of the import across 10°N of 8 Sv of bottom waters of Antarctic origin and their subsequent conversion and export as North Pacific Deep Water. Dominating the mid-depth property distribution and circulation is a deep anticyclonic cell which horizontally cycles 8 Sv of deep water through the North Pacific basin. We find that there is little interaction between the North Pacific wind-driven circulation and the abyssal volume, which contrasts with the Atlantic Ocean, in which there is strong communication between the cold and warm water volumes.