Distinguishing Characteristics of Spring and Summer Onset El Niño Events

Abstract

El Niño events can be classified into two categories according to the onset time: the spring (SP) El Niño with onset time from April to June and the summer (SU) El Niño with onset time from July to October. The SP El Niño is a basin-scale phenomenon and is closer to the conventional ENSO. It goes through the earlier and stronger heat build-up process, and the earlier occurrence of westerlies in the equatorial Pacific, which can partly explain its earlier onset time. For SU El Niño, in contrast, the anomalous signals, such as SSTAs, zonal wind anomalies, and subsurface variations, are much weaker, which can be attributed to the weaker accumulation of warm water and shorter duration of positive Bjerknes feedback. During its peak phase, anomalous southeasterlies over the eastern Pacific enhance the wind–evaporation–SST (WES) feedback and impede the development of positive SSTAs there, and then lead to a west shift of SSTA center. Recharge/discharge processes exist in both types of events but are weaker in the SU type, which may be caused by the lack of meridional Sverdrup transports as a result of weak zonal wind anomalies. A heat budget analysis demonstrates that the relative importance of thermocline (TH) and zonal advective (ZA) feedbacks in SP and SU El Niño is different. In SP El Niño, the TH feedback is dominant compared to ZA feedback in both the GODAS and SODA datasets. In SU El Niño, however, these two terms are equally important in GODAS, but not in the SODA dataset.

Effects of Equatorial Ocean Current Bias on Simulated El Niño Pattern in CMIP6 Models

“…A mixed‐layer heat budget analysis is conducted to diagnose how the oceanic dynamical processes behave during ENSO events under the biased equatorial Pacific circulation system. In general, the TH and ZA are two dominant dynamics related to the recharge/discharge mechanism (Jin & An, 1999) that positively contribute to the mixed layer temperature anomalies during the El Niño development (Chen et al., 2016; Kug et al., 2009, 2010; Xu et al., 2020). Following the heat budget equation from Ren and Jin (2013), the TH and ZA terms can be respectively expressed as $$$\mathrm{T}\mathrm{H}=\overline{w}\frac{{T}_{\mathrm{s}\mathrm{u}\mathrm{b}}^{\prime }}{H}$$$ $$$\mathrm{Z}\mathrm{A}=-{u}^{\prime }\frac{\partial \overline{T}}{\partial x}$$$ where the overbar and prime denote the climatology and anomaly.…”

How does the onset time of El Niño events affect tropical cyclone genesis and intensity over the western North Pacific?

“…At the same time, it is highly correlated with the Southern Oscillation index (SOI). Hence, we follow the definitions of Xu and Chan (2001) and Xu et al (2020) to define an El Niño event as a Spring (SP) or Summer (SU) type. When the Niño-3.4 index first exceeds 0.5 C and persists for 5 months, the first month is defined as the onset time of the El Niño events.…”

“…Many previous studies have also suggested that it has large variability on El Niño time scales, which is important in the changes in the rising branch of the Walker circulation over the WNP (Wang, 1995; Xu and Chan, 2001; Horii and Hanawa, 2004; Zhao and Wang, 2016, 2019; Wang et al . 2018; Xu et al ., 2020). At the same time, it is highly correlated with the Southern Oscillation index (SOI).…”

“…The results are slightly different from those of Xu and Chan (2001), but will not influence our conclusions in this paper. Consistent with the previous results, an SP event is usually a stronger warm event and has a longer life span according to the Niño‐3.4 index (Xu and Chan, 2001; Cai et al ., 2019; Xu et al ., 2020). For example, 1982/1983, 1997/1998, and 2015/2016 events are identified as the three strongest El Niño events since 1950 (Becker, 2016), and they are all classified as SP events.…”

“…In contrast, in the SU type, such conditions are more westward (Figures 9g, 10g, 12g) and hence TC are generated more westward. In this study, we adopt the definition of Xu and Chan (2001) and Xu et al (2020) to label the El Niño events based on the onset time, spring (April~June, SP) or summer (July~October, SU) when the El Niño event generated. Different onset times are caused by different environmental conditions, which will affect TC activity in the WNP.…”