An altimetric study of the regional circulation off Peru and Chile during the 1997-1998 El Nino

Fields of sea surface height (SSH) and geostrophic surface currents are used to examine the changes in the surface circulation during the 1997-1998 El Nino off western South America. Over the large-scale (0-55S), TOPEX/POSEIDON (T/P) altimeters show two major periods of raised sea level, extending to 50S or farther: May through July 1997 and October 1997 through January 1998. During these periods of high SSH, poleward surface currents occur in several pulses, each lasting approximately one month. Winds off Chile during the May-July period were mostly downwelling-favorable (poleward) and may have enhanced the high sea levels and poleward transports during the first half of the El Nino period. During the October-January period, winds off Chile were more upwelling-favorable (equatorward), working against the El Nino signal. This is especially true off central Chile, where alongshore transports derived from altimeter data in the 150 km next to the coast became equatorward in December-January, lowering altimeter SSH levels to normal values. An EOF analysis of non-seasonal, 2-D surface transport fields highlights the El Nino period from May 1997 through January 1998, showing the strong flow to the southeast off Peru and poleward transport as far south as 55S, strongest in May-August 1997. Westward anomalies between 40S-50S indicate that the West Wind Drift was weaker than normal near 82W.

Higher resolution SSH fields are formed for smaller regions from a combination of T/P and ERS-2 altimeter data. Off northern Chile, these show strong poleward flow during May-June 1997, in agreement with data collected during IFOP cruises (Blanco et al., 1999). The high resolution fields off central Chile also show the structure of the poleward flow during May-June period. Newly available ERS-2 data extending to mid-1998 allow us to examine the high resolution surface circulation during the second period of high SSH (October-January), when equatorward winds off central Chile opposed the poleward El Nino flow. This will be presented, along with analysis of satellite sea surface temperature fields from the Pathfinder data set, to look at regions of upwelling and downwelling during different phases of the complete event. Comparisons to similar fields in other eastern boundary currents (the California and Benguela Currents) will also be presented.

Blanco, J.L., M.-E. Carr, A.C. Thomas and P.T. Strub, Oceanographic conditions off northern Chile during the 1996 La Nina and the 1997-1998 El Nino: Part 1: Hydrographic conditions. J. Geophys. Res. (submitted) 1999.