Barotropic currents and vorticity in the central North Pacific Ocean during summer 1987 determined from long‐range reciprocal acoustic transmissions

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Abstract
Large‐scale depth‐integrated currents and relative vorticity were measured in the central North Pacific Ocean during summer 1987 using long‐range reciprocal acoustic transmissions between transceivers in a triangle approximately 1000 km on a side. Inverse techniques were used to estimate the depth‐averaged (barotropic) current bihourly at 4‐day intervals from differential travel times. Tidal constituent amplitudes and phases found from the acoustically determined currents agree with those found from current meters and with the tidal models of Schwiderski (1980) and Cartwright et al. (1992), providing confirmation that the tomographically derived barotropic currents are correct within the expected uncertainties. The estimated low‐frequency, large‐scale currents are compared with depth‐averaged currents determined by point measurements using current meters and bottom‐mounted electrometers. Meridional and zonal currents are calculated using the topographic Sverdrup balance with the Fleet Numerical Oceanography Center wind field. The measured time derivative of the areally averaged relative vorticity is shown to be insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude smaller than the observations. The magnitude and variability of the large‐scale currents and vorticity determined from the Semtner and Chervin (1988) eddy‐resolving model of ocean circulation are similar to the direct measurements.