Precipitation response to La Niña and global warming in the Indo-Pacific

Abstract

Recent studies have highlighted the nonlinear rainfall response to El Nino sea surface temperature (SST) events in the Indo-Pacific region and how this response might change over coming decades. Here we investigate the response to La Nina SST anomalies with and without global warming by performing idealised SST-forced experiments with an atmospheric general circulation model. The La Nina SST anomaly is multiplied by a factor \(1 \le \alpha \le 4\) and added to climatological SSTs. Similar experiments using El Nino SST anomalies were previously performed, in which large nonlinearities in the precipitation response were evident. We find that: (i) Under current climatic conditions, as \(\alpha\) increases, the precipitation responds in three ways: the intertropical convergence zone (ITCZ) dries and moves poleward, the maximum precipitation along the equator moves west, and the South Pacific convergence zone (SPCZ) narrows, intensifies, and elongates. For weak (\(\alpha = 1\)) La Nina events, the precipitation anomalies approximately mirror those from the El Nino events along the ITCZ and SPCZ, though there are some marked differences in the central-eastern Pacific. For stronger La Nina events (\(\alpha > 1\)), precipitation responds nonlinearly to SST anomalies, though the nonlinearities are smaller and differ spatially from the nonlinearities in the El Nino runs. (ii) The addition of a global warming SST pattern increases rainfall in the western Pacific and SPCZ, enhances the narrowing of the SPCZ, and increases the nonlinear response in the western Pacific. However, large La Nina events reduce the impact of global warming along the central-eastern equatorial Pacific as the global warming and La Nina SST anomalies have opposite signs in that region. (iii) The response to La Nina SST anomalies is driven primarily by changes in the atmospheric circulation, whereas the response to the global warming SST pattern is mainly driven by increases in atmospheric moisture. (iv) Large changes in La Nina-driven rainfall anomalies can occur in response to global warming, even if the La Nina SST anomalies relative to the warmer background state are completely unchanged.