TOGA-TAO and the 1991–93 El Niño Southern Oscillation Event

scale fluctuations . . . T H E EL N~r~o-SOUTHERN OSCILLATION (ENSO) phenomenon is an interannual perturbation of the climate system characterized by aperiodic weakening of the tradewinds and warming of the surface layers in the equatorial Pacific Ocean every 4-7 years. The impacts of ENSO are felt worldwide through disruption of the atmospheric general circulation and associated global weather patterns (Rasmusson and Wallace, 1983; Ropelewski and Halpert, 1987). ENSO also affects the ecosystem dynamics in the Pacific Ocean, particularly the higher trophic levels of the food chain on which fisheries depend (Barber and Chavez, 1983). The widespread and systematic influence of ENSO on the ocean-atmosphere system led to initiation of the Tropical Ocean-Global Atmosphere (TOGA) Program, a 10-y study (1985-1994) of climate variability on seasonal to interannual time scales. Key to the success of TOGA is the accurate determination of basin scale fluctuations in surface winds, sea surface temperature (SST), upper ocean heat content, near-surface currents, and sea level in the tropical Pacific. Measurement of these oceanographic fields is required to describe fully the variability related to ENSO, to understand the physical processes responsible for that variability, and to initialize and verify shortterm climate prediction models. Plans for TOGA in the early 1980s called for an ocean observing system that would rely on an increased utilization of satellite products, in particular for SST, surface winds and sea level, and on the development of a "thin monitoring" array of in situ measurements based on an enhancement of existing capabilities. The in situ array would include specifically a volunteer observing ship (VOS) expendable bathythermograph (XBT) program, a tide gauge network, a drifting buoy pro-