Annual ENSO

Abstract

Abstract Using various observational data, the seasonal cycle of the tropical Pacific is investigated, suggesting the existence of an “annual El Niño–Southern Oscillation (ENSO).” A positive sea surface temperature anomaly (SSTA) appearing off Peru in boreal winter triggers a series of air–sea interactions that consist of westward propagations of positive SSTA, westerly wind anomalies, and negative outgoing longwave radiation anomalies. At the same time, the westerly wind anomaly generates cold temperature anomalies in the off-equatorial region, and they propagate westward as a “cold” Rossby wave, reaching the western tropical Pacific in boreal summer to autumn. A semiresonant condition between the westward propagating component of winds and the first-meridional-mode Rossby wave plays an important role in the amplification. The evolution of cold phase in the latter half of the year is almost a mirror image of the warm phase. From a new viewpoint of the annual ENSO, the ENSO is interpreted as the interaction between two distinct modes of air–sea interaction: the annual ENSO mode and an “interannual ENSO” mode. The eastward-propagating interannual ENSO mode is an air–sea coupled mode, which is triggered by the westerly wind stress anomaly in the western equatorial Pacific and leads to the deepening of the thermocline and the warming of SST in the central and eastern equatorial Pacific. This results in a modulation of the annual ENSO mode with a weaker cold season and stronger warm season owing to less effective upwelling of the cold subsurface water. The decadal variation of ENSO is explained by changes in the relative phase and amplitude of these two modes. The increase in the amplitude of the interannual ENSO mode after the late 1970s favors the appearance of the eastward propagation of ENSO signals.