ENSO\u2019s impacts on the tropical Indian and Atlantic Oceans via tropical atmospheric processes: observations versus CMIP5 simulations

Shan He,Shih-Wei Fang,Song Yang,Jin-Yi Yu

doi:10.1007/s00382-020-05247-w

Abstract

This study compares the impacts of the El Niño–Southern Oscillation (ENSO) on sea surface temperatures (SSTs) in the tropical North Atlantic Ocean and the tropical Indian Ocean during 1958–2004. It is found that the tropical atmospheric processes mediating the ENSO impacts are different between the two oceans for two reasons. First, the ENSO-induced anomalous Walker circulation is more extensive over the Atlantic than over the Indian Ocean. As a result, the atmospheric bridge (AB) mechanism is the major contributor to the differences in ENSO teleconnections between the two oceans. Secondly, SSTs in the tropical North Atlantic are under a greater control of the atmospheric thermal forcing than those in the tropical Indian Ocean. Due to these different controls, the tropospheric temperature (TT) mechanism also contributes to the different ENSO teleconnections. When compared with the observations, the mean of thirty-seven models from the Coupled Model Intercomparison Project Phase 5 overestimates the ENSO-induced SST response in the tropical Indian Ocean but underestimates the response in the tropical North Atlantic. The overestimation is brought about by a westward extension of ENSO SST anomalies in the models, which causes the AB mechanism to produce an overly strong impact on Indian Ocean SSTs. On the other hand, the underestimation is caused by a weaker-than-observed sensitivity in the simulated Atlantic SSTs to the thermal forcing produced by the TT mechanism.

Highlights

The neighboring tropical Atlantic and Indian Oceans are influenced by the El Niño–Southern Oscillation (ENSO) in the tropical Pacific Ocean which produces profound climate impacts worldwide (e.g., Rasmusson and Carpenter 1982; Kousky et al 1984; Trenberth et al 1998; Yu et al 2012, 2017; Xie et al 2016)
The Coupled Model Intercomparison Project Phase 5 (CMIP5) models produce asymmetric ENSO responses between the tropical North Atlantic (TNA) and the tropical Indian Ocean (TIO) that are statistically significant at 5% level (Fig. 8), but the simulated asymmetry is opposite from the observed asymmetry
For the simulated TT mechanism, we find that the CMIP5 models reasonably simulate the tropospheric temperature responses to ENSO (Figs. 2c, 3), but the impact of the tropospheric temperature on the SSTA tendency is underestimated in the TNA (Figs. 2d, 3)

Summary

Introduction

The neighboring tropical Atlantic and Indian Oceans are influenced by the El Niño–Southern Oscillation (ENSO) in the tropical Pacific Ocean which produces profound climate impacts worldwide During El Niño (La Niña) events, significant positive (negative) sea surface temperature (SST) anomalies are observed in the tropical Atlantic and Indian Oceans (Enfield and Mayer 1997; Klein et al 1999; Alexander et al 2002; Schott et al 2009). These anomalies typically peak in strength two to four months after ENSO events fully develop (e.g., Klein et al 1999; Wang et al 2004).

Data sets and methods

ENSO inter‐basin teleconnections in the observations

ENSO inter‐basin teleconnections in CMIP5 model simulations

Conclusions