Fig. 4

Schematic of the impacts of El Niño and its Combination Mode during a composite El Niño event. Highlighted are the mechanisms associated with El Niño and its Combination Mode that regulate the anomalous low-level circulation in the Indo-Pacific with emphasis on the anomalous low-level North-West Pacific Anticyclone (NWP-AC). The seasonal climatological extent of the warm pool (temperatures above 27.5 °C) is displayed by gray shading, and the seasonal climatological surface wind field is displayed by vectors. Idealized sea surface temperature anomalies are indicated by red (positive) and blue (negative) shading. The anomalous low-level anticyclones (AC) and cyclones (C) are indicated by ellipses. a Quasi-symmetric circulation response during the developing El Niño year (June-July–August: JJA(0) to September–October–November: SON(0)) due to the quasi-symmetric warm pool background state. b Antisymmetric circulation response during the El Niño peak phase (in December-January–February: DJF) and following boreal spring (March–April-May: MAM(1)) due to the seasonal warm pool migration to the Southern Hemisphere (antisymmetric background state). c) Persistence of the anomalous NWP-AC until boreal summer after an El Niño event (June-July–August: JJA(1)). The symmetric anticyclone response due to eastern equatorial Pacific cooling (blue shading), a weak antisymmetric Combination Mode response, local air–sea feedback (cooling on the eastern flank of the NWP-AC and warming on the western flank), and the delayed Indian Ocean warming (capacitor effect) contribute to the persistence and amplification of the anomalous NWP-AC until JJA(1). Figure and modified caption from Stuecker et al. (2015b).