Asymmetry in the Duration of El Niño and La Niña

Abstract

El Niño and La Niña are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. Most El Niños and La Niñas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Niños tend to decay rapidly by next summer, but many La Niñas persist through the following year and often reintensify in the subsequent winter. Throughout the analysis period, this asymmetric feature is evident for strong events in which Niño-3.4 SST anomalies exceed one standard deviation in December. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Niño enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events.

The role of the southward wind shift in both, the seasonal synchronization and duration of ENSO events

“…These composite analyses shown in Fig. 1 are in broad agreement with those reported by Okumura and Deser (2010), where a different atmospheric reanalysis product was used.…”

“…It is generally accepted that there is an asymmetry in the duration of the two phases of ENSO events, with La Niña events lasting longer than El Niño events (Larkin and Harrison 2002;McPhaden and Zhang 2009;Obha and Ueda 2009;Okumura and Deser 2010;Okumura et al 2011;DiNezio and Deser 2014). Given that McGregor et al (2013) proposed that the asymmetries in the southward wind shift (e.g., El Niño event magnitude is strongly related to the extent of the meridional wind movement, while the meridional wind movement during La Niña events remains relatively small The boxplots in Fig.…”

“…It is worthwhile to mention that the modeled Niño-3.4 data had the long term mean removed and the resulting time series was also smoothed with a 3-month binomial filter to be consistent with the observed. Here we identify ENSO events for which the anomalous Niño-3.4 index exceeds one standard deviation, following Okumura and Deser (2010), however rather than focusing only on the December values our events must exceed this threshold for at least 5 consecutive months.…”

Genesis of the 2014–2016 El Niño events

“…Previous studies suggested that the genesis of an extreme El Niño could be attributed to the effects from nonlinear convection (Hoerling et al, 1997;Kang and Kug, 2002), biological feedback (Timmermann and Jin, 2002;Marzeion et al, 2005), tropical instability waves (Vialard et al, 2001), and the feedback form the Indian Ocean (Okumura and Deser, 2010). However, as found in this study, the genesis and development of extreme El Niño events are closely associated with WWB and the variation of the upper ocean heat content.…”

Strong and moderate nonlinear El Niño regimes

“…Oceanic nonlinear advection does not appear essential for the onset of strong EN. The threshold convective response to SST (Ohba and Ueda 2009;Dommenget et al 2012;Choi et al 2013), state dependent noise acting under EN favorable conditions (Lengaigne et al 2004;Jin et al 2007), all potentially producing asymmetric features of ENSO, like its skewness, the asymmetric time evolution of strong EN and La Niña (LN) event (Ohba and Ueda 2009;Okumura and Deser 2010) or the asymmetric pattern of the zonal wind stress along the equator (Kang and Kug 2002). Note that the study of ENSO nonlinearities has been mostly oriented towards explaining the aforementioned statistical features of ENSO, rather than focusing on the development phase of the strong warm events, which could convey other insights on their dynamics and predictability.…”