Hydrographic conditions off northern Chile during the 1996-1998 La Nina and El Nino events

The evolution of oceanographic conditions in the upwelling region off northern Chile (18degrees-24degreesS) between 1996 and 1998 (including the 1997-1998 El Nino) is presented using hydrographic measurements acquired on quarterly cruises of the Chilean Fisheries Institute, with sea surface temperature (SST), sea level, and wind speeds from Arica (18.5degreesS), Iquique (20.5degreesS), and Antofagasta (23.5degreesS) and a time series of vertical temperature profiles off Iquique. Spatial patterns of sea surface temperature and salinity from May 1996 to March 1997 followed a normal seasonal progression, though conditions were anomalously cool and fresh. Starting in March 1997, positive anomalies in sea level and sea surface temperature propagated along the South American coast to 37degreesS. Maximum sea level anomalies occurred in two peaks in May-July 1997 and October 1997 to February 1998, separated by a relaxation period. Maximum anomalies (2degreesC and 0.1 practical salinity units (psu)) extended to 400 m in December 1997 within 50 km of the coast. March 1998 presented the largest surface anomalies (> 4degreesC and 0.6 psu). Strong poleward flow (20-35 cm s(-1) ) occurred to 400 m or deeper during both sea level maxima and weaker (10 cm s(-1) ) equatorward flow followed each peak. By May 1998, SST had returned to the climatological mean, and flow was equatorward next to the coast. However, offshore salinity remained anomalously high owing to a tongue of subtropical water extending southeast along the Peruvian coast. Conditions off northern Chile returned to normal between August and December 1998. The timing of the anomalies suggests a connection to equatorial waves. The progression of the 1997-1998 El Nino was very similar to that of 1982-1983, though with different timing with respect to seasons.