Interannual variability of surface currents in the tropical Pacific during 1987-1993

Buoy drifts and current meter records between January 1987 and December 1993 are used to investigate the interannual variability of the equatorial Pacific currents at a depth of 15 m. The sampling is coarse until mid-1988 but more complete afterward, so that the large-scale features of the anomaly currents can be documented on the seasonal to yearly timescale. Using objective analysis, bimonthly current anomalies are mapped between 20 degrees N and 20 degrees S on a 1 degrees x 5 degrees grid, and the error covariance matrix of the analyzed fields are estimated. The current anomalies are primarily zonal, with largest amplitudes within about 8 degrees from the equator, and they are largely linked to the El Nino-Southerm Oscillation phenomenon. In particular, broad, 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) analysis shows that the first EOF of zonal current anomaly is largely uniform in the equatorial band, while the next two EOFs describe large-scale currents of opposite sign across the equator and across 160 degrees W, respectively. The EOFs are rather smooth and the errors on the principal 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 set. Correlation analysis and a singular value decomposition are used to investigate the influence of advection by the large-scale, low-frequency currents on sea surface temperature (SST) anomalies during 1987-1993. 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 eastern equatorial Pacific. The dominant terms are the anomalous zonal advection of mean SST, the mean zonal, and, intermittently, meridional advection of SST anomalies.