FY 1995

The 1991–1993 El Niño in the central Pacific

Kessler, W.S., and M.J. McPhaden

Deep-Sea Res. Pt. II, 42(2–3), 295–333, doi: 10.1016/0967-0645(95)00041-N (1995)

The 1991-1993 El Niño event is described using data from the TOGA-TAO buoy network, concentrating on variability at 140°W where a full suite of temperature, current, and surface meteorological observations were made. The daily time series furnished by the buoy array brings out the conspicuous importance of remotely forced intraseasonal variability in the form of equatorial baroclinic Kelvin waves during the evolution of the 1991-1993 El Niño. Notable variations along 140°W included a major weakening of the Equatorial Undercurrent in late 1991 to early 1992, and a reduction in intensity of monthly period tropical instability waves during the latter part of 1991 compared to previous non-El Niño years. The North Equatorial Countercurrent showed no major signal due to this El Niño, in contrast to earlier warm events. Although anomalies of the South Equatorial Current spanning the equator were in an eastward sense during the height of the event, the result of these changes was that near-surface flow across 140°W between 5°S and 5°N was close to zero, so there was apparently no large eastward transport of surface water past 140°W into the eastern equatorial Pacific. The relative phasing of anomalies of thermocline depth, equatorial undercurrent speed and SST during the warm event of 1991-1992 was somewhat similar to that seen during the 1986-1987 El Niño, although the earlier event was followed by a strong cold (La Niña) event whereas the recent one was not. Uniquely among modern El Niño events, after the 1991-1992 episode appeared to end with a reappearance of the equatorial cold tongue in mid-1992, a second SST warming in the eastern Pacific occurred in early 1993. The present data set is inadequate to fully diagnose the mechanisms of SST change at 140°W, but time series of insolation, air-sea temperature difference and humidity show that local air-sea heat flux variations (either radiative or turbulent) were probably not the primary cause of the SST changes during the El Niño. Similarly, although horizontal advective mechanisms were important contributors to SST signals at certain times, these alone did not account for the major warming and cooling events. Instead, the largest SST variations of the 1991-1992 El Niño can be ascribed to upwelling variations, and a simple parameterization for this process is presented. The major warming and cooling events in the central/eastern equatorial Pacific occurred nearly simultaneously over a wide longitudinal range, indicating that oceanic wave processes could not have been the sole source of the changes.