The Southern Oscillation in the early 1990s

Two aspects of the onset of a warm event in the Southern Oscillation are 1) the subtropical South Pacific High is weakened in early southern winter which weakens subtropical sea level pressure (SLP) gradients, and thus reduces the trade winds and upwelling along the equator, and 2) in the months that follow, and particularly during the mature phase of the warm event during southern summer, the negative SST anomalies in the equatorial cold-water tongue are displaced by positive anomalies and the water along the tropical Peruvian coast usually warms. To highlight the role of Southern Hemisphere subtropical processes in warm events, we focus on the first half of the 1990s. That period has been viewed as a five year warm event, but actually two warm events developed during this period according to the criteria in 1) and 2): in 1991 and 1994. This study emphasizes the importance of interactions between the subtropical and equatorial Pacific in warm events.