Role of Nonlinear Atmospheric Response to SST on the Asymmetric Transition Process of ENSO

Abstract

Physical processes that are responsible for the asymmetric transition processes between El Niño and La Niña events are investigated by using observational data and physical models to examine the nonlinear atmospheric response to SST. The air-sea coupled system of ENSO is able to remain in a weak, cold event for up to 2 yr, while the system of a relatively warm event turns into a cold phase. Through analysis of the oceanic observational data, it is found that there is a strong difference in thermocline variations in relation to surface zonal wind anomalies in the equatorial Pacific (EP) during the mature-to-decaying phase of ENSO. The atmospheric response for the warm phase of ENSO causes a rapid reduction of the EP westerlies in boreal winter, which play a role in hastening the following ENSO transition through the generation of upwelling oceanic Kelvin waves. However, the anomalous EP easterlies in the cold phase persist to the subsequent spring, which tends to counteract the turnabout from the cold to warm phase of ENSO.A suite of idealized atmospheric general circulation model (AGCM) experiments are performed by imposing two different ENSO-related SST anomalies, which have equal amplitudes but opposite signs. The nonlinear climate response in the AGCM is found at the mature-to-decaying phase of ENSO that closely resembles the observations, including a zonal and meridional shift in the equatorial positions of the atmospheric wind. By using a simple ocean model, it is determined that the asymmetric responses of the equatorial zonal wind result in different recovery times of the thermocline in the eastern Pacific. Thus, the differences in transition processes between the warm and cold ENSO event are fundamentally due to the nonlinear atmospheric response to SST, which originates from the distribution of climatological SST and its seasonal changes. By including the asymmetric wind responses the intermediate air-sea coupled model herein demonstrates that the essential elements of the redevelopment of La Niña arise from the nonlinear atmospheric response to SST anomalies.

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

“…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.…”

Indian Ocean Variability in the CMIP5 Multimodel Ensemble: The Basin Mode

“…At the mature phase of El Niño (Figure 7), in the SEIO, the prevailing wind shift to northeasterly, a change important for the development of interannual anomalies (Nicholls 1984;Klein et al 1999;Shinoda et al 2004;Ohba and Ueda 2009). The northeast flank of the anti-cyclonic atmospheric circulation, particular in the region west of Sumatra and Java, weakens, and so do the coastal upwelling and surface evaporation (Murtugudde et al 2000).…”

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.…”