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El Niño Southern Oscillation (ENSO) is the leading interannual coupled climate mode in the tropical Pacific. The seasonal transition of ENSO from boreal winter to the following summer can significantly affect the global climate. One of the major hurdles in understanding the seasonal transition of ENSO is the spring predictability barrier. Here, we show that ENSO’s seasonal transition is modulated by a multidecadal climate mode of boreal spring sea-level pressure (SLP) in the extratropical Southern Hemisphere. This ENSO transition mode (ETM), when characterised by a decrease in SLP and associated clockwise circulation of the surface winds centred over the southeastern subtropical Pacific Ocean, produces westerly anomalies at the equator. These wind anomalies in the equatorial eastern Pacific aid the seasonal warming of Niño3.4 sea surface temperature anomalies (N34SST) from boreal winter to the following summer. The ETM time series shows prominent multidecadal variations at around 50 years. This creates a conducive environment for alternate cold and warm seasonal transitions leading to multidecadal variations in boreal summer N34SST. Thus, ETM provides a physical insight into the seasonal transition of ENSO and leads to a new paradigm for ENSO evolution beyond its peak. This has implications for seasonal ENSO forecasts and decadal climate predictions.
The ENSO Transition Mode (ETM) is a distinct driver of ENSO multidecadal climate variability. The ETM plays an important role in influencing ENSO's seasonal transition from boreal winter to the following summer by affecting zonal winds in the central-eastern equatorial Pacific Ocean during boreal spring. In this study, we show that ETM's spatial pattern and seasonality are robustly captured by the CMIP6 models' historical simulations. However, they struggle to capture its multidecadal variability. Nonetheless, these models effectively depict ETM's influence on equatorial winds, thereby affecting the seasonal transition of ENSO from boreal winter to the following summer (ΔT). We further demonstrate that the models that more accurately represent the relationship between ETM and ΔT (referred to as “Good ETM models”) are those in which ΔT is less influenced by the preceding winter Niño3.4 sea-surface temperature anomalies (N34DJF) and largely influenced by phases of ETM. Additionally, these models better capture the 2–7-year spectral peak observed in N34DJF. On the contrary, Bad ETM models are characterized by a dominant higher-frequency, 2–3-year quasi-biennial peak. Consequently, Good ETM models render ENSO states that align more closely with observations compared with Bad ETM models. This study underscores, through CMIP6 models, the significance of boreal winter ENSO amplitude in influencing how extratropical climate affects the seasonal transitions of ENSO.
The ENSO Transition Mode (ETM) is a distinct Southern Hemisphere mode characterised by a multidecadal see-saw in boreal spring sea-level pressure about the dateline in the Southern Pacific Ocean. ETM significantly influences the equatorial winds and plays a key role in shaping ENSO’s boreal winter-to-summer seasonal transition. This study combines observational data and climate model simulations to examine how ETM affects global rainfall during boreal summer at multidecadal timescales. During its positive phase, ETM is linked to a cooler Northern Hemisphere, especially the Northern Pacific, and a warmer Southern Hemisphere with maximum warming over the Southern Indian Ocean. This creates an unfavourable inter-hemispheric sea surface temperature (SST) gradient, weakening the large-scale atmospheric circulation by reducing cross-equatorial mass flux and decreasing rainfall over the regions such as South Asia and the Sahel. These results underscore the significant impact of the Southern Hemisphere variability on the global climate patterns at multidecadal timescales.
You can read my PhD thesis here:
The Seasonal ENSO Transition Mode of the Southern Hemisphere: Definition, Characteristics and Impact
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
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