The Distinct Contributions of the Seasonal Footprinting and Charged‐Discharged Mechanisms to ENSO Complexity

Abstract

This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged‐discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric.

The influence of wintertime SST variability in the Western North Pacific on ENSO diversity

“…This is consistent with the existing view that a La Niña event produces a wind stress pattern that tends to charge the equatorial Pacific thermocline (i.e. the positive phase of the recharge-discharge mechanism) and subsequently leads to the onset of an El Niño event, and similarly, to a La Niña from the negative phase (Yu and Fang 2018). We note here that the charging associated with La Niña and the discharging associated with El Niño can be asymmetric in amplitude (Chen et al 2015;Hu et al 2017).…”

Identifying the complex types of atmosphere-ocean interactions in El Niño

“…Kim and Jin 2011). The regressions onto EIT2 slp (figures 2(f)-(j)) show a subtropical Pacific interaction that resembles the seasonal footprint mechanism (Vimont et al 2003, Yu andFang 2018). Previous studies (Yu et al 2010, Yu and Kim 2011 have pointed out that the negative subtropical SLP anomaly pattern first produces anomalous surface southwesterly winds.…”

Enhancing the ENSO Predictability beyond the Spring Barrier

“…Interestingly, the ENSO forecast skill was noticeably reduced after 2000 in terms of the correlation and root mean square error (RMSE) (e.g., failures of forecast in 2012/13 and 2014/15 are good examples; most models predicted a strong ENSO event but these years ended up with a neutral or a weak El Niño conditions). Consistently, the strength of the seasonal footprinting mechanism has intensified during the past 2 decades 15 . The footprinting mechanism can trigger central Pacific types of ENSO events via the subtropical ocean-atmosphere coupling [16][17][18][19][20] .…”

“…The footprinting mechanism can trigger central Pacific types of ENSO events via the subtropical ocean-atmosphere coupling [16][17][18][19][20] . The amplified footprinting mechanism makes the ENSO more complicated and harder to predict because of changes in ENSO frequency and magnitude in the 21 st century 1,15 . On interseasonal time scales, the spring predictability barrier (SPB) is still a large challenge that limits ENSO predictability, which tends to cause the model's forecasting skill to decrease sharply when the prediction is made through spring 21,22 .…”