The Cyclic and Episodic Transition of Strong El Niño and Implications for South American Precipitation During Their Peak and Decay Stages

Abstract

The mechanisms associated with the transitions of strong El Niño (EN) events and their implications for the South American precipitation were investigated for the 1950–2023 period. Strong EN events exhibit cyclic or episodic characteristics in their transitions. Cyclic EN events are both preceded and followed by La Niña (LN) conditions, whereas episodic EN events are preceded by neutral conditions, with a more uncertain transition following. For cyclic EN, tropical Pacific mechanisms initiates and peak warming in the eastern tropical Pacific from austral winter to early summer. In contrast, for episodic EN, coupled subtropical and tropical Pacific mechanisms, respectively, initiate and peak warming in the central tropical Pacific from autumn to late summer. The Pacific Decadal Oscillation (PDO) mean state modulates EN's decay stage. During the +PDO mean state, cyclones in the eastern subtropical Pacific of both hemispheres sustain the warming of episodic EN, whereas during the −PDO mean state, anticyclones in the eastern subtropical Pacific accelerate the decay of cyclic EN, favouring its transition to an LN. These mechanisms explain why episodic EN initiates earlier, peaks later, is more intense and decays more slowly than cyclic EN. During an episodic EN summer, the strengthened atmospheric circulation maintains the Atlantic Intertropical Convergence Zone (ITCZ) north of the equator, causing persistent negative precipitation anomalies in north–northeastern South America (SA) until the following winter, while positive precipitation anomalies in southeastern SA are driven by south–southeastward moisture transport from equatorial Atlantic. Conversely, during a cyclic EN summer, negative (positive) precipitation anomalies impact north–northwestern (southeastern) SA; however, the anomalous atmospheric circulation and precipitation in SA quickly return to normal conditions in the autumn, and positive precipitation anomalies appear in northern SA in the following winter. Understanding these mechanisms is crucial for predicting EN's future changes and, consequently, their potential socio-economic impacts globally.