Due to the anthropogenic global sea level rise over the past century, sea level Height Extreme (HEX) events and associated coastal flooding have frequently occurred in many coastal regions around the world. Such HEX events exert strong impacts on coastal communities, especially those in low-lying coastal areas and island nations, which are vulnerable to rising sea level. Some HEX events are accompanied by marine heat waves, anomalous warm sea surface temperature events persisting for several days to months. One example of such events is a HEX that occurred in June 2016 in coastal Indonesia (see the Figure). The co-occurrence of HEXs and marine heat waves leads to large ecological, economic, and social consequences, and is expected to become more common in a warming climate.
This motivated us to explore characteristics and drivers of such HEX and Compound Height-Heat Extreme (CHHEX) events in recent decades, focusing on those in the Indian Ocean along the Indonesian coast. Indonesia is the fourth-most populous country in the world and home to various marine species such as coral reefs; yet its fast sinking rate in a rising sea makes it more vulnerable to climate variability and climate change. Therefore, understanding the behavior and causes of HEXs and CHHEXs are of vital importance.
Using in situ and satellite observations from 1993 to 2018, we have detected fifteen HEX events, with ten of them occurring in the recent 8-year period from 2010 to 2017. Such a remarkable increase in HEXs during that period was also captured by ocean reanalysis data and ocean general circulation models (OGCMs) forced by the observed atmospheric conditions. Through observational analysis and OGCM experiments, we found that both anthropogenic sea level rise and natural decadal climate variability are responsible for the recent increase in the frequency of HEX events. Specifically, from 2010 to 2017, the long-term sea level rise due to human-induced climate change was further boosted by decadal sea level increase, which is induced by the intensified equatorial westerly and longshore northwesterly winds near the Indonesian coast. These winds are themselves associated with the decadal variability of the El Niño-Southern Oscillation and the Indian Ocean Dipole. This idea is further confirmed by a series of climate model simulations.
We have also examined the driving mechanism of individual HEX events using the same framework of observational and modeling approaches. During HEX events, enhancement of equatorial westerly wind and northwesterly wind along the coasts of Sumatra and Java were observed, and the associated increase in surface mass convergence near the coasts largely explains the anomalous sea level rise associated with HEXs. A key factor that explains the difference between HEX alone and CHHEX events is the season of occurrence: most HEXs occur during December-March when the seasonal, downwelling-favorable northwesterly monsoon winds prevail; the surface warming associated with enhanced downwelling is suppressed by surface cooling induced by increased vertical mixing due to enhanced northwesterly monsoon winds. On the other hand, most CHHEXs occur in May-June and November-December, when the strong equatorial westerly and longshore northwesterly wind anomalies associated with the co-occurrence of negative Indian Ocean Dipole and La Niña act against the seasonal southeasterly monsoon winds, reducing seasonal upwelling cooling and vertical mixing, leading to marine heatwaves that accompany the HEXs. Our results demonstrate that the interplay between human-induced climate change and natural climate variability is crucial for changes in the frequency of extreme sea level events and compounding marine heat waves. To achieve successful predictions and risk assessments of regional extreme events, further efforts are needed to accurately monitor major natural climate modes and understand their modulations under a changing climate.
Update on Sep. 13, 2023:
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