The southern oscillation: Sea surface temperature and wind relationships in a 100‐year data set

Abstract

AbstractThe development of sea surface temperature (SST) and surface wind anomalies during the course of the Southern Oscillation is examined with a 100‐year data set covering most of the Pacific Ocean region. Correlation coefficients of SST and winds with Wright's (1975) Southern Oscillation Index (SOI) are presented for lags and leads up to four seasons relative to the Index. The primary SST response is in the eastern and central equatorial ocean, the regions usually associated with the El Nino phenomenon, but there are important SST and wind changes elsewhere in the Pacific. Significant precursors in the SST include warming (cooling) in the northwest subtropical Pacific up to one year preceding positive (negative) SOI and like‐signed anomalies near New Guinea one season later, in addition to the cooling (warming) along the Chilean coast and in the eastern equatorial region beginning 3 seasons before positive (negative) SOI. (Positive SOI corresponds to pressures higher than average over the south‐east Pacific and lower than average over the western equatorial Pacific.) The major wind response is an area of easterly (westerly) anomalies in the central and western equatorial ocean beginning (in the latter portion) north of the equator 2 seasons before positive (negative) SOI. There is evidence for increased (decreased) westerlies in the southern mid‐latitudes as well as easterly and southerly (westerly and northerly) anomalies in the southeast subtropics 1‐3 seasons before positive (negative) SOI. For both SST and surface winds, little seasonal dependence of the correlation results is found for the equatorial anomalies, but in the Indonesian‐New Guinea region positive correlations are found in the latter portion of the year; mid‐latitude SST anomalies in both hemispheres are restricted to June to August.The mechanisms that determine the response of the SST and winds to the SOI are examined and compared with the factors that control the seasonal cycle in selected regions. Subtropical precursors to positive SOI are consistent with enhanced subsidence, higher anticyclonic vorticity and stronger oceanic gyres. Advection seems to account for the spreading of the SST anomalies westward along the equator. SST anomalies in the Indonesian region may be due to solar radiation changes accompanying cloud cover variations.