Seasonal Evolution of Dominant Modes in South Pacific SST and Relationship with ENSO

A season-reliant empirical orthogonal function (S-EOF) analysis was applied to the seasonal mean SST anomalies (SSTAs) based on the HadISST1 dataset with linear trend removed at every grid point in the South Pacific (60.5o--19.5oS, 139.5oE--60.5oW) during the period 1979--2009. The spatiotemporal characteristics of the dominant modes and their relationships with ENSO were analyzed. The results show that there are two seasonally evolving dominant modes of SSTAs in the South Pacific with interannual and interdecadal variations; they account for nearly 40% of the total variance. Although the seasonal evolution of spatial patterns of the first S-EOF mode (S-EOF1) did not show remarkable propagation, it decays with season remarkably. The second S-EOF mode (S-EOF2) showed significant seasonal evolution and intensified with season, with distinct characteristics of eastward propagation of the negative SSTAs in southern New Zealand and positive SSTAs southeast of Australia. Both of these two modes have significant relationships with ENSO. These two modes correspond to the post-ENSO and ENSO turnabout years, respectively. The S-EOF1 mode associated with the decay of the eastern Pacific (EP) and the central Pacific (CP) types of ENSO exhibited a more significant relationship with the EP/CP type of El Nino than that with the EP/CP type of La Nina. The S-EOF2 mode contacted with the EP type of El Nino changing into the EP/CP type of La Nina showed a more significant connection with the EP/CP type of La Nina.