Abstract
Marine heatwaves (MHWs) are long-lasting extreme oceanic warming events that can cause devastating effects on warm-water corals and associated ecosystems. The linear trend and interannual variability of MHWs over the Bay of Bengal (BOB) during 1982–2020 are investigated by a high-resolution daily sea surface temperature (SST) dataset. In regions where warm-water coral reefs are concentrated, annual MHW days and frequency significantly increase during 1982–2020, at rates exceeding that of the global mean. The coldest boreal winter season witnesses significant and steady increase trends in MHW days and frequency. In contrast, the trend is insignificant in the climatological warmest season (March to June) south of 15° N in the BOB, mainly due to large interannual variability. El Niño and Southern Oscillation (ENSO) dominates the interannual variability of BOB MHWs, which are highly consistent with the evolution of the mean SST. The negative phase of North Atlantic Oscillation (NAO) also modulates the occurrences of MHWs, especially over the northeastern BOB. The two climate modes synergistically explain about 50~70% of the interannual variances in the BOB’s MHWs. Correlation analysis reveals that south of 15° N in the BOB, the effect of El Niño on MHWs is evident from the boreal autumn of its developing phase to the boreal summer of its decaying phase, along with limited influence from NAO. However, in the northeast of the BOB, the effect of El Niño merely emerges from April to August of its decaying stage. In comparison, boreal winter-to-spring NAO exerts a strong control over March-to-June MHWs in the northeastern BOB. The results suggest that various climate modes may jointly or separately influence MHWs at certain seasons and locations, which is important for the seasonal prediction of MHWs. Indeed, when combining the Niño3.4 mature winter index and boreal winter-to-spring NAO index to build a regression model, it is more effective in reproducing the BOB’s MHW frequency compared to the Niño3.4 index alone.
Highlights
Marine heatwaves (MHWs) are extreme sea surface temperature (SST) warming events lasting for several days, weeks or even months
The trend and interannual variability of the Bay of Bengal (BOB) MHWs during the climatological warmest season lack investigations, which is the motivation of the present study
To highlight the devastating effect of MHWs on coral reef bleaching, we focus the investigation on three regions in which coral reefs are concentrated, with Region 1
Summary
Marine heatwaves (MHWs) are extreme sea surface temperature (SST) warming events lasting for several days, weeks or even months. MHWs are projected to occur more frequently with prolonged days [7–9], further damaging or even destroying the living conditions of marine life. This highlights the importance of investigating the drivers and mechanisms of MHWs, and improving the short-term predictions and long-term projections [10–12]. The mechanisms of individual MHW cases display large differences across regions, but mainly relate to local oceanic and atmospheric conditions, air–sea coupling and climate mode-associated remote teleconnections [6,13–15]. Additional extreme and prolonged warming beyond the high SSTs can cause severe disasters in marine ecosystems, such as the 2010 Andaman sea MHW [28], and may even regulate the Indian summer monsoon [24] and cyclone activities. The rest of this paper is organized as follows: Section 2 describes the definition of MHWs, data and methods; Section 3 shows the climatology and linear trend of the BOB MHWs’ properties; Section 4 investigates the interannual variability of the BOB MHWs and underlying processes; and Section 5 presents the discussion and summary
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