Nonlinear modes of decadal and interannual variability of the subsurface thermal structure in the Pacific Ocean

The nonlinear principal component analysis, a neural network technique, is applied to the observed upper ocean heat content anomalies (HCA) in the Pacific basin from 1961 to 2000. By applying the analysis to high‐passed and low‐passed data, nonlinear interannual and decadal modes are extracted separately. The first nonlinear interannual mode is mainly characterized by the El Niño‐Southern Oscillation (ENSO) structure in the tropical Pacific, with considerable asymmetry between warm El Niño and cool La Niña episodes; for example, during strong El Niño, the negative HCA in the western tropical Pacific is much stronger than the corresponding positive HCA during strong La Niña. The first nonlinear decadal mode goes through several notable phases. Two of the phases are related to decadal changes in the La Niña and El Niño characteristics, revealing that the decadal changes for La Niña episodes are much weaker than the changes for El Niño episodes. Other phases of the decadal mode show a possible anomaly link from the middle latitudes to the western tropical Pacific via the subtropical gyre. The decadal changes in the HCA around 1980 and around 1990 were compared and contrasted.