BAROTROPIC CURRENTS AND VORTICITY IN THE CENTRAL NORTH PACIFIC-OCEAN DURING SUMMER 1987 DETERMINED FROM LONG-RANGE RECIPROCAL ACOUSTIC TRANSMISSIONS

Large-scale depth-integrated currents and relative vorticity were meas ured in the central North Pacific Ocean during summer 1987 using long- range reciprocal acoustic transmissions between transceivers in a tria ngle approximately 1000 km on a side. Inverse techniques were used to estimate the depth-averaged (barotropic) current bihourly at 4-day int ervals from differential travel times. Tidal constituent amplitudes an d phases found from the acoustically determined currents agree with th ose 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 exp ected uncertainties. The estimated low-frequency, large-scale currents are compared with depth-averaged currents determined by point measure ments using current meters and bottom-mounted electrometers. Meridiona l and zonal currents are calculated using the topographic Sverdrup bal ance with the Fleet Numerical Oceanography Center wind field. The meas ured time derivative of the areally averaged relative vorticity is sho wn to be insignificant to the Sverdrup balance. Currents and vorticity calculated using the Sverdrup balance are an order of magnitude small er than the observations. The magnitude and variability of the large-s cale currents and vorticity determined from the Semtner and Chervin (1 988) eddy-resolving model of ocean circulation are similar to the dire ct measurements.