Revisiting the Recharge and Discharge Processes for Different Flavors of El Niño

다국어 초록 (Multilingual Abstract)

El Niño‐related sea surface temperature (SST) anomalies over the tropical Pacific Ocean impact global climates, but these impacts differ substantially for conventional cold tongue El Niño (CT El Niño) and the central Pacific El Niño (CP El Niño) events. This study is motivated by the need for a better understanding of the recharge/discharge processes associated with these two different flavors of El Niño. Composite analysis based on improved CT and CP El Niño identification methods applied to the Simple Ocean Data Assimilation demonstrates that the recharge/discharge processes are active during CT El Niño events. In contrast, for CP El Niño events, the recharge/discharge processes do not play a significant role. Prior to a CT El Niño, warm water accumulates over the western Pacific due to off‐equatorial anticyclonic wind stress curl. The onset of a CT El Niño is closely associated with the formation of a cyclonic atmospheric circulation over the northwest Pacific in the winter and spring, which induces westerly wind anomalies in the equatorial western Pacific and initiates eastward warm water transport. This leads to peak warming in the eastern equatorial Pacific the following winter, followed by the poleward discharge of warm water. This quasi‐cyclical behavior provides a measure of predictability. In contrast, the CP El Niño events do not show a precursor subsurface warming signal along the tropical Pacific thermocline. Instead, modest warm SST anomalies appear in boreal summer and peak in the fall, with weak subsurface warming mainly in the fall during CP El Niños. Hence, CP El Niños are less predictable in terms of an equatorial thermocline precursor than CT El Niño events.
The El Niño‐Southern Oscillation (ENSO) is the largest source of year‐to‐year climate variability. ENSO has a pronounced influence on regional and global circulation and precipitation patterns and thus has considerable worldwide socio‐economical impacts. El Niño, the warm phase of ENSO, exhibits modulation in the longitudinal location of its maximum warming, creating what is referred to as ENSO diversity. For conventional El Niño events, maximum surface warming is located in the eastern equatorial Pacific, for which subsurface warming along the tropical Pacific has proven to serve as a predictor several months in advance. Previous studies disagree on whether this subsurface warming is similarly essential for El Niño events that have peak surface warming in the central Pacific. We developed an improved method for identifying these two types of El Niño in an ocean reanalysis product. Using this improved method, we find no clear evidence of a subsurface warming precursor for the central Pacific El Niño events along the equator. This lack of a tropical subsurface precursor limits our ability to predict these types of El Niño events.



The Simple Ocean Data Assimilation system is used to study the composite behavior of eastern‐ and central‐Pacific El Niño events

Tropical Pacific subsurface recharge‐discharge processes are significantly different for cold eastern‐ and central‐Pacific El Niño events

Positive subsurface heat content anomalies are a not a precursor of central Pacific El Niño events, making them far less predictable


The Simple Ocean Data Assimilation system is used to study the composite behavior of eastern‐ and central‐Pacific El Niño events
Tropical Pacific subsurface recharge‐discharge processes are significantly different for cold eastern‐ and central‐Pacific El Niño events
Positive subsurface heat content anomalies are a not a precursor of central Pacific El Niño events, making them far less predictable