Abstract
The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.
Highlights
The Indian Ocean Basin (IOB) mode is the first mode of the interannual variability of sea surface temperature in the tropical Indian Ocean (TIO), which is characterized by basin-warming/cooling (Klein et al 1999; Yang et al 2007)
Over the past 60 years, five warming and one cooling events occurred during the co-occurrence of Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO), five warming and four cooling events during the pure ENSO, and only one warming event occurred during the pure IOD (Fig. 1b)
The thermocline deepened and sea surface height (SSH) rose in the west, while the thermocline shallowed and SSH dropped in the east through the modulation of oceanic planetary waves and zonal heat transport, which were triggered by the equatorial easterly anomalies and anticyclonic wind stress curl anomalies (Fig. 2b; Feng et al 2001; Vinayachandran et al 2002; Rao et al 2002; Nagura and McPhaden 2010)
Summary
The Indian Ocean Basin (IOB) mode is the first mode of the interannual variability of sea surface temperature in the tropical Indian Ocean (TIO), which is characterized by basin-warming/cooling (Klein et al 1999; Yang et al 2007). The SST warming in the SWTIO can sustain through the summer even as El Niño dissipates (Xie et al 2002; Annamalai et al 2005), which induces an increase in atmospheric convection and precipitation over the southern TIO (Wu et al 2008; Wu and Yeh, 2010). Basin-wide warming occurred in the TIO in 2020, which followed an extreme positive IOD instead of an El Niño event (Fig. 1a).
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