ENSO Signals in Global Upper-Ocean Temperature

Abstract

The time-space evolution of the El Niño-Southern Oscillation in sea surface temperature (SST) and heat storage of the upper 400 m (HS400) for the Pacific, Indian, and Atlantic Oceans is investigated for 13 years (1979–1991). EOF and rotated EOF (Varimax or VRX) analyses are performed using the time series of normalized anomalies for each ocean separately and then for the global ocean. In the Pacific and Indian Oceans, the two dominant EOF modes for both SST and H5400 are associated with ENSO. For SST they account for 49% of the total variance in each mean, while for H5400 they account for over 35% of the total variance in each ocean. In the Pacific Ocean, the first EOF modes for SST and HS400 display peak values during spring-summer of 1983 and 1987. They are characterized by maximum positive loadings (or warmer temperature) in the eastern and central Pacific Ocean straddling the equator. These modes represent the peak phase of El Niño off the west coast of South America. The second modes for SST and HS400 are precursor modes with maximum positive loadings in the western and central Pacific Ocean both on and off the equator. The peak values in HS400 second mode lead peak values in HS400 first mode by 6–9 months. Taken together, SST and HS400 first two modes are associated with slow equatorial and eastward propagation of ENSO signals, more clearly though in HS400 than in the SST. In the Indian Ocean, the dominant EOF modes for SST display peak values in summer-fall of 1983 and summer of 1987, while peak values for HS400 are found during fall of 1982 and summer of 1987. Corresponding positive SST loadings are found in the central Indian Ocean while positive HS400 loadings are found in the western Indian Ocean. Maximum values are both straddling the equator. As in the Pacific Ocean, the second EOF modes for SST and HS400 in the Indian Ocean are precursor modes to the first modes. They occur 9–15 months earlier, with warmer SST in the Arabian Sea, colder SST northwest of Australia, and colder H5400 in the vicinity of the boreal winter ITCZ. Taken together, thew two modes suggest a slow equatorial and eastward propagation of ENSO signals in SST and HS400, very similar to those in the Pacific Ocean. This propagation can he seen extending eastward south of Indonesia and on into the Timor Sea. There the anomalies have the same sign as the anomalies found in the Philippine and Coral Seas in the western tropical Pacific Ocean during the peak phase of El Niño. In the Atlantic Ocean, ENSO is represented by the third SST VRX mode but not at all in HS400. As such, the Atlantic ENSO signal at the equator is a passive response to a globally perturbed atmosphere. It lags by approximately 18 months the dominant Pacific ENSO mode.