The Seasonal ENSO Transition Mode of the Southern Hemisphere: Definition, Characteristics and Impact

Abstract

Decadal climate variability (DCV) exerts a profound influence on the global climate system by shaping background oceanic and atmospheric conditions. It drives long-term changes in sea surface temperatures (SSTs), monsoon rainfall, and the El Niño–Southern Oscillation (ENSO), the dominant mode of interannual climate variability. While previous studies have primarily focused on Northern Hemisphere drivers, this thesis emphasizes the underexplored role of the Southern Hemisphere in modulating multidecadal variability.

The first part of the thesis introduces a newly identified Southern Hemisphere climate mode, the ENSO Transition Mode (ETM). Using multiple observational and reanalysis datasets, the ETM is shown to manifest as a zonal sea-level pressure dipole in the extratropical South Pacific during boreal spring. This dipole alters subtropical circulation and equatorial Pacific surface winds, influencing thermocline depth and upwelling, and thereby enhancing Niño3.4 SST anomalies during the winter-to-summer ENSO transition. The ETM displays a distinct multidecadal (~50-year) spectral peak and exerts its strongest influence during weak ENSO winters. Case studies highlight its role in amplifying the 1997 El Niño and suppressing the 2014 event. A real-time ETM index is proposed to enhance monitoring and prediction of ENSO transitions.

The second part evaluates ETM representation in CMIP6 historical simulations. While models generally capture ETM’s spatial pattern and boreal spring seasonality, they underestimate its multidecadal variability. Importantly, a subset of “Good ETM models” realistically reproduces the ETM–ENSO linkage and yields more faithful seasonal transitions, whereas “Bad ETM models” exaggerate quasi-biennial variability and exhibit weak ETM–ENSO coupling.

The final part investigates ETM’s influence on global boreal summer monsoons, with emphasis on Indo-African rainfall. Observations and AGCM experiments reveal that the positive ETM phase is associated with significant rainfall deficits, driven by warm equatorial Pacific SSTs and a negative interhemispheric SST gradient. This gradient weakens the ITCZ by reducing cross-equatorial flow, leading to suppressed monsoon rainfall.

In summary, this thesis establishes the ETM as a distinct Southern Hemisphere multidecadal mode with profound impacts on ENSO seasonal transitions and Indo-African monsoons, advancing understanding of coupled climate variability and informing improved prediction.