INTERANNUAL VARIABILITY OF SURFACE CURRENTS IN THE TROPICAL PACIFIC DURING 1987-1993

Buoy drifts and current meter records between January 1987 and Decembe r 1993 are used to investigate the interannual variability of the equa torial Pacific currents at a depth of 15 m. The sampling is coarse unt il mid-1988 but more complete afterward, so that the large-scale featu res of the anomaly currents can be documented on the seasonal to yearl y timescale. Using objective analysis, bimonthly current anomalies are mapped between 20 degrees N and 20 degrees S on a 1 degrees x 5 degre es grid, and the error covariance matrix of the analyzed fields are es timated. The current anomalies are primarily zonal, with largest ampli tudes within about 8 degrees from the equator, and they are largely li nked to the El Nino-Southerm Oscillation phenomenon. In particular, br oad, basin-wide westward anomaly currents were encountered during the 1988 La Nina, and strong eastward currents persisted from July-August 1991 to January-February 1992, followed by westward currents from May- June to July-August 1992. An empirical orthogonal function (EOF) analy sis shows that the first EOF of zonal current anomaly is largely unifo rm in the equatorial band, while the next two EOFs describe large-scal e currents of opposite sign across the equator and across 160 degrees W, respectively. The EOFs are rather smooth and the errors on the prin cipal component time series relatively small, which indicates that the sampling is adequate to describe the large-scale, low-frequency zonal current fluctuations. As the dominant EOFs of meridional current are noisy and the relative errors on the principal components larger, the meridional current fluctuations are not as well captured by the data s et. Correlation analysis and a singular value decomposition are used t o investigate the influence of advection by the large-scale, low-frequ ency currents on sea surface temperature (SST) anomalies during 1987-1 993. Although the data set is noisy and other terms play an important- role in the SST anomaly equation, the effect of zonal and, to a lesser extent, meridional advection is seen in much of the central and easte rn equatorial Pacific. The dominant terms are the anomalous zonal adve ction of mean SST, the mean zonal, and, intermittently, meridional adv ection of SST anomalies.