Asymmetry of Interannual Sea Level Variability in the Western Tropical Pacific: Responses to El Niño and La Niña

Abstract

The western tropical Pacific (WTP) exhibits pronounced large-scale sea level anomalies (SLAs) during El Niño-Southern Oscillation (ENSO) events with amplitudes of~0.1 m, exerting potential threats to many low-lying islands. It is found that the sea level falling in El Niño condition is evidently stronger than the rising in La Niña condition. This SLA asymmetry in the WTP associated with ENSO is investigated in this study, and the underlying dynamic processes are explored. Such asymmetry is most prominent at around 160°E with the SLA response to El Niño being three times stronger than to La Niña, but it is much inconspicuous near the western boundary. Sensitivity experiments of a simplified ocean model suggest that the different structures of surface wind anomalies between El Niño and La Niña conditions are critical in generating asymmetric SLA responses of the WTP. The El Niño's westerly wind anomaly patch locates more to the east than the La Niña's easterly wind patch during the mature stage, and its upwelling effects are accumulated over a wider longitude range and cause stronger negative SLAs in the WTP. Near the western boundary, however, upwelling effects are attenuated by easterly wind anomalies during the mature stage of El Niño. Further analysis reveals that eastern Pacific-type El Niño events are more favorable for generating asymmetric SLAs in the WTP than the central Pacific-type events. Plain Language Summary El Niño-Southern Oscillation (ENSO) is the most important climate variability phenomenon in the tropical Pacific and causes strong interannual sea level anomalies (SLAs) in the western tropical Pacific (WTP). The El Niño's sea level falling is evidently stronger than the La Niña's sea level rising. This difference is most prominent near 160°E with stronger El Niño's sea level falling but becomes much less evident near the western boundary of the Pacific Basin. The different surface wind anomaly structures between El Niño and La Niña are the primary cause. El Niño's westerly wind anomaly center locates more east than La Niña's easterly wind anomaly center in their mature phase, leading to stronger sea level falling in the WTP through westerly wind anomalies over the wide longitude range. The sea level falling is weakened by easterly wind anomalies generated near the western boundary. By contrast, effect of La Niña's easterly wind anomaly strengthens monotonically from east to west, and the produced sea level rising signatures are mainly confined to the WTP. The wind forcing structure of eastern Pacific-type El Niño is more favorable for generating asymmetric SLAs in the WTP than central Pacific-type events.

Special Sea‐Level and Circulation Anomalies in the Philippine Sea During the 2006/2007 and 2009/2010 El Niño Events

“…El Niño events are broadly classified into eastern Pacific (EP) and central Pacific (CP) types with regard to spatial structure, although a mixed type has also been proposed to represent the events with flavors of both types (e.g., Ashok et al., 2007; Kao & Yu, 2009; Kug et al., 2009; Yu & Kim, 2013). Existing studies employed the composite analysis to identify the different impacts of two types of events (Chang et al., 2013; Q. Ren, Li, Zheng, et al., 2020; Shi et al., 2020) and revealed overall stronger signatures of EP events in the western Pacific (Hsin & Qiu, 2012; Tan & Zhou, 2018; X. Wang et al., 2020). However, the composite represents the common features of multiple events (Figure S1a in Supporting Information ) and is sometimes dictated by super events such as the 1997/1998 and 2015/2016 ones, particularly when event samples are few.…”

“…The Philippine Sea (PS) in the northwestern tropical Pacific Ocean is one of the regions with the strongest variability in sea level and upper‐layer circulation (Han et al., 2013, 2017; D. Hu et al., 2015; Qiu & Chen, 2012; Qiu et al., 2021), particularly on interannual timescales linked to the El Niño‐Southern Oscillation (ENSO). During the developing stage of El Niño events, the PS shows sea level falling in response to westerly wind bursts over the western Pacific, with the opposite changes occurring in La Niña condition (Chang et al., 2013; Deepa et al., 2021; Han et al., 2019; Li et al., 2012; Q. Ren, Li, Zheng, et al., 2020; Zhai & Hu, 2012). Super El Niño events such as the 1982/1983 and 1997/1998 ones gave rise to sea level falling near the western tropical Pacific islands by up to 20–30 cm, dramatically increasing the explosion risks of shallow coral reefs and damaging local ecosystems (e.g., Widlansky et al., 2014).…”

Air‐Sea Interaction in the Western Tropical Pacific and its Impact on Asymmetry of the Ningaloo Niño/Niña

“…Ren et al. (2020) showed that the negative skewness of SSH anomalies in the western tropical Pacific is due to asymmetry in the spatial pattern of zonal wind stress anomalies. More specifically, westerly wind stress anomalies that force off‐equatorial upwelling Rossby waves during El Niño are located further to the east compared to easterly wind stress anomalies during La Niña.…”

“…They also pointed out that the magnitude of sea surface height (SSH) anomalies in the western tropical Pacific prior to the Ningaloo Niño is larger than that prior to the Ningaloo Niña. These asymmetries seem counterintuitive since it is known that El Niño, which often co-occurs with the Ningaloo Niña, is typically stronger than La Niña, which often co-occurs with the Ningaloo Niño (Ren et al, 2020;Su et al, 2010), that is, in the western tropical Pacific, negative SSH anomalies associated with El Niño are stronger than positive SSH anomalies associated with La Niña (Figure S1; Ren et al, 2020). Ren et al (2020) showed that the negative skewness of SSH anomalies in the western tropical Pacific is due to asymmetry in the spatial pattern of zonal wind stress anomalies.…”

“…These asymmetries seem counterintuitive since it is known that El Niño, which often co-occurs with the Ningaloo Niña, is typically stronger than La Niña, which often co-occurs with the Ningaloo Niño (Ren et al, 2020;Su et al, 2010), that is, in the western tropical Pacific, negative SSH anomalies associated with El Niño are stronger than positive SSH anomalies associated with La Niña (Figure S1; Ren et al, 2020). Ren et al (2020) showed that the negative skewness of SSH anomalies in the western tropical Pacific is due to asymmetry in the spatial pattern of zonal wind stress anomalies. More specifically, westerly wind stress anomalies that force off-equatorial upwelling Rossby waves during El Niño are located further to the east compared to easterly wind stress anomalies during La Niña.…”