Anomalous Tropical Instability Wave Activity Hindered the Development of the 2016/17 La Niña

Aoyun Xue , Wenjun Zhang , Julien Boucharel (1) , Fei-Fei Jin

1 LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales

Although the 1997/98 and 2015/16 El Niño events are considered to be the strongest on record, their subsequent La Niña events exhibited contrasted evolutions. In this study, we demonstrate that the extremely strong period of tropical instability waves (TIWs) at the beginning of boreal summer of 2016 played an important role in hindering the subsequent La Niña's development by transporting extra off-equatorial heat into the Pacific cold tongue. By comparing the TIWs' contribution based on an oceanic mixed layer heat budget analysis for the 1998 and 2016 episodes, we establish that TIW-induced nonlinear dynamical heating (NDH) is a significant contributor to the El Niño-Southern Oscillation (ENSO) phase transition in 2016. TIW-induced NDH contributed to around 0.4°C warming per month during the early boreal summer (May-June) following the 2015/16 El Niño's peak, which is found to be an essential inhibiting factor that prevented the subsequent La Niña's growth. A time-mean eddy kinetic energy analysis reveals that anomalous TIWs during 2016 mainly gained their energy from the baroclinic instability conversion due to a strong SST warming in the northeastern off-equatorial Pacific that promoted an increased meridional SST gradient. This highlights the importance of accurately reproducing TIW activity in ENSO simulation and the benefit of off-equatorial SST anomalies in the eastern Pacific as an independent precursor for ENSO predictions.

Domaines: Planète et Univers [physics]

Aoyun Xue, Wenjun Zhang, Julien Boucharel, Fei-Fei Jin. Anomalous Tropical Instability Wave Activity Hindered the Development of the 2016/17 La Niña. Journal of Climate, 2021, 34, pp.5583-5600. ⟨10.1175/JCLI-D-20-0399.1⟩. ⟨insu-03671330⟩