Opposite polarities of ENSO drive distinct patterns of coral bleaching potentials in the southeast Indian Ocean

Ningning Zhang,Harry H Hendon,Jens Zinke,Alistair J Hobday,Ming Feng

doi:10.1038/s41598-017-02688-y

Abstract

Episodic anomalously warm sea surface temperature (SST) extremes, or marine heatwaves (MHWs), amplify ocean warming effects and may lead to severe impacts on marine ecosystems. MHW-induced coral bleaching events have been observed frequently in recent decades in the southeast Indian Ocean (SEIO), a region traditionally regarded to have resilience to global warming. In this study, we assess the contribution of El Niño-Southern Oscillation (ENSO) to MHWs across the mostly understudied reefs in the SEIO. We find that in extended summer months, the MHWs at tropical and subtropical reefs (divided at ~20°S) are driven by opposite ENSO polarities: MHWs are more likely to occur at the tropical reefs during eastern Pacific El Niño, driven by enhanced solar radiation and weaker Australian Monsoon, some likely alleviated by positive Indian Ocean Dipole events, and at the subtropical reefs during central Pacific La Niña, mainly caused by increased horizontal heat transport, and in some cases reinforced by local air-sea interactions. Madden-Julian Oscillations (MJO) also modulate the MHW occurrences. Projected future increases in ENSO and MJO intensity with greenhouse warming will enhance thermal stress across the SEIO. Implementing forecasting systems of MHWs can be used to anticipate future coral bleaching patterns and prepare management responses.

Highlights

Acute disturbances to coral reefs and adjacent marine ecosystems are increasing in frequency and severity and altering coral cover and composition of marine benthic communities[1,2,3,4]
Partial correlation patterns show that sea surface temperature (SST) variations in the tropical region of the southeast Indian Ocean (SEIO) are better correlated to El Niño-Southern Oscillation (ENSO) variations that have maximum amplitude in the eastern Pacific, commonly referred to as EP-type ENSO or canonical ENSO31, 32
SST variations in the subtropical region of the SEIO are better correlated to ENSO variations that have maximum amplitude in the central Pacific, commonly referred to as CP-type ENSO, or ENSO Modoki[31, 32] (Supplementary Fig. S1)

Summary

Introduction

Acute disturbances to coral reefs and adjacent marine ecosystems are increasing in frequency and severity and altering coral cover and composition of marine benthic communities[1,2,3,4]. During the recent 2015–16 El Niño event, extremely high summer SSTs in the tropical SEIO (Fig. 1a) were reported to have caused 60–90% of the shallow coral. These tropical-subtropical spatial contrasts suggest that bleaching response patterns in the SEIO are associated with opposite polarities of ENSO. This study determines in which flavour and polarity of ENSO that MHWs preferentially occur and reveals the relative contributions of atmospheric and oceanic processes responsible for these SST extremes. This insight can be used to predict future coral bleaching patterns and prepare management responses[30]

Methods

Results

Conclusion