Multidecadal Climate and ENSO’s Seasonal Transitions: The Role of the ENSO Transition Mode

The El Niño-Southern Oscillation (ENSO), a coupled interaction between the ocean and atmosphere in the equatorial Pacific, has exhibited marked changes in recent decades. ENSO events have become increasingly variable in intensity, duration, and phase transition, which can critically modulate the global climate. These changes are primarily attributed to interactions between tropical and extratropical climate systems at interannual timescales. However, how low-frequency climate variabilities drive such changes remains mostly elusive. Understanding these interactions is paramount in a warming world. The ENSO Transition Mode (ETM) is a climatic phenomenon associated with a multidecadal see-saw in sea-level pressure (SLP) variations in the extratropical South Pacific about the dateline during boreal spring. Our findings demonstrate that ETM significantly modulates ENSO’s seasonal transitions from boreal winter to summer, irrespective of the preceding winter ENSO conditions. In its positive phase, ETM-related SLP anomalies reinforce anomalous westerly winds in the central-eastern equatorial Pacific, amplifying boreal winter-to-summer Niño3.4 sea surface temperature anomalies by suppressing upwelling. Case studies of 1997 and 2014 highlight the ETM's substantial influence on ENSO's transition, with the mode enhancing El Niño development in the former and inhibiting it in the latter. To improve ENSO prediction across the spring barrier, we present a real-time monitoring index for ETM. This index can enhance forecasting capabilities by providing valuable insights into ENSO’s potential trajectory. By exploring the relationship between low-frequency climate variability and ENSO, our research aims to enhance the prediction of ENSO seasonal transitions.