Climate-driven sea level extremes and height-heat extreme compounds in coastal regions of the tropical east Indian Ocean

– Weiqing Han, Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder –

Résumé :

Low-lying island nations like Indonesia are vulnerable to sea level Height EXtremes (HEXs). When compounded by marine heatwaves, HEXs have larger ecological and societal impact. Here we combine observations with model simulations, to investigate the HEXs and Compound Height-Heat Extremes (CHHEXs) along the Indonesian coasts that border the tropical east Indian Ocean in recent decades. We find that anthropogenic sea level rise combined with decadal climate variability causes increased occurrence of HEXs during 2010-2017. For individual HEX and CHHEX events that occur in specific seasons and years, they are driven by equatorial westerly and longshore northwesterly wind anomalies. For most HEXs, which occur primarily during December-March, downwelling favorable northwest monsoon winds are enhanced which increase coastal Ekman mass convergence and raise sea level; the enhanced monsoon winds however also enhance vertical mixing, limiting surface warming, leading to HEX-only events. For most CHHEXs, equatorial westerly wind anomalies and longshore northwesterlies associated with a negative Indian Ocean Dipole (IOD) and co-occurring La Niña weaken the seasonal southeasterlies as well as upwelling cooling during May-June and November-December, increasing sea level and sea surface temperature (SST) and resulting in compound height-heat events. Our findings emphasize the important interplay between anthropogenic warming and climate variability at interannual-to-decadal timescales in affecting regional extremes.