Coastal upwelling off Peru during normal and El Nino times, 1981-1984

Repeated conductivity, temperature, and depth (CTD) sections were made across the continental\ud margin off Peru at 5°S and 10°S between November 1981 and May 1984, i.e., before, during, and after El\ud Nino of 1982-1983. Coastal sea level at Paita (5°S) and Callao (12°S) began to rise in early October 1982\ud and returned to normal in July 1983. Coastal winds, which are normally favorable for upwelling,\ud remained normal during October 1982; between November 1982 and May 1983 they were weaker than\ud normal at Talara and Paita (5°S) but stronger than normal at Callao (12°S). Between mid-March and\ud mid-April 1983, winds at 5°S were very weak or even unfavorable for upwelling. Repeated CTD sections\ud at 5°S show signs of active upwelling (a negative onshore surface temperature gradient and upward\ud sloping isotherms near the coast) whenever local winds were favorable for upwelling, but not when winds\ud were calm or unfavorable. At 10°S, winds remained favorable for upwelling throughout El Nino. At both\ud 5°S and 10°S, the apparent source depth of upwelling waters, 50-100 m, remained the same before,\ud during, and after El Nino; this layer is normally below the thermocline, but during El Nino it was at the\ud top of the depressed thermocline. The upwelling that occurred during El Nino resulted only in the\ud upwelling of warm, nutrient-poor waters. In May 1983, a CTD section at 10°S indicated that upwelling\ud had ceased there in spite of continued strong, upwelling-favorable winds; this cessation of upwelling is\ud attributed to an alongshore pressure gradient that persisted from mid-March to mid-June 1983, causing\ud onshore geostrophic flow to balance the wind-driven offshore flow in the surface layer