Altimeter-derived surface circulation in the large-scale NE Pacific Gyres. Part 2: 1997-1998 El Nino anomalies

Changes in the sea surface heights (SSH) and geostrophic transports in the NE Pacific are examined during the 1997-1998 El Nino using altimeter data, sea level pressure (SLP) fields, proxy winds and satellite sea surface temperature (SST). Most of the signal occurs along the boundaries of the basin from Panama to the Alaska Peninsula. Changes in the SSH and alongshore transports along the boundaries are caused both by propagation of signals from the south (stronger between the equator and the Gulf of California) and by local and basin-scale winds (stronger between the Pacific Northwest and the Alaska Peninsula). Two periods of high SSH occur at the equator, May-July 1997 and October 1997-January 1998. The first coastal SSH signal moved quickly polewards to approximately 24 degreesN in early June, then stalled and moved farther north during transient events in July-September. Large-scale wind forcing combined with the equatorial signals during the second period of high equatorial SSH (Fall 1997) to move the high SSH and poleward transports quickly around the Alaska Gyre. A connection between the boundary currents and the interior North Pacific developed as part of the large-scale response to the basin-scale winds, after changes in the boundaries. Decreases in anomalies of SSH and poleward transports began in January 1998 south of 40 degreesN and in February 1998 farther north. (C) 2002 Elsevier Science Ltd. All rights reserved.