While the seasonal changes in the coastal ocean and Monterey Bay are important, longer term interannual variations, principally "El Niño" events, also affect local physical and biological systems. El Niño is a warming of nearshore waters of the eastern Pacific, caused by relaxation of the trade winds in the equatorial Pacific. Cessation or weakening of the trade winds allows the sea surface, which usually tilts upward by about 1 m from east to west, to relax. This is accomplished as an eastward propagating pulse or Kelvin wave that takes several months to transit the equatorial Pacific (Apel 1987, Norton and McLain 1994). The wave propagates poleward along the coast of Central and North America and eventually is observed locally as warmer surface waters and higher than normal sea level. Local temperature anomalies up to 5°C (Breaker 1989) and sea level anomalies of up to 20 cm occur more or less periodically at 3- to 5-year intervals. For example, Breaker (1989) identified four El Niño episodes in the MBNMS between 1971 and 1985 (1972, 1976, 1978 and 1982). Locally, the 1972 El Niño was accompanied by a deepening of the mixed layer to 100 m, and heavy rainfall.
Section III. Wind, Upwelling and Oceanographic Seasons