A Study on Marine Heatwaves in the Indian Ocean: Spatiotemporal Variability, Drivers, and Future Changes

Abstract

Marine Heatwaves (MHWs) represent extreme climatic phenomena with profound ecological and economic impacts worldwide. These events threaten vital marine habitats such as coral reefs, seagrass meadows, and fish breeding grounds, which are critical to the health of ocean ecosystems and human livelihoods. Despite their significance, the characteristics and physical drivers of MHWs remain insufficiently understood, particularly in the Indian Ocean (IO). This dissertation investigates MHWs in the Indian Ocean (IO), focusing on their spatiotemporal variability and driving mechanisms. MHWs are defined as prolonged periods (≥5 days) of anomalously warm sea surface temperatures exceeding the 90th percentile of a 30-year baseline. The study reveals significant regional variation in MHW characteristics, where severe events (~3°C intensity, ~150 days duration) predominate in the southwestern IO, while moderate to strong events (<3°C, ~100 days) dominate the northern IO. Large-scale climate modes including the IO Basin Mode, IO Dipole, and El Niño-Southern Oscillation significantly modulate MHW patterns across the region. A key focus of this research is to analysis of summer MHWs (SMHWs), where we identify two dominant modes with distinct formation mechanisms. The first MHW mode involves basin-wide warming preconditioned by ENSO decay and sustained by anomalous northeasterlies linked to the Western North Pacific Subtropical High (WNPSH). The second MHW mode exhibits a zonal dipole pattern driven by interhemispheric pressure differences, inducing cross-equatorial winds that influence SST through feedback mechanisms. These MHW modes significantly impact summer precipitation distribution in the Northern IO. Indian summer monsoon strength inversely correlates with MHW occurrence in the northern IO. Strong monsoons enhance cooling mechanisms (wind-driven evaporative cooling and upwelling), inhibiting MHWs. Conversely, monsoon disruptions suppress these cooling mechanisms, may leads to genesis of MHWs. CMIP6 models were employed to evaluate anthropogenic impacts on IO MHWs under different socioeconomic pathways. Models capture observed SST patterns but show regional biases, overestimating MHW durations and underestimating intensities. Bias correction via quantile delta mapping improves projection reliability. Historically (1982-2014), the most intense MHWs occurred in the northern Arabian Sea and Bay of Bengal, lasting 25-30 days annually with 2-3°C average intensities. Under the high-emission scenario (SSP585), although annual frequency decreases due to prolonged multi-year events, MHW intensity and duration surge dramatically. By 2100, SST anomalies are projected to exceed 4°C with MHWs lasting >350 days/year. The choice of baseline for MHW detection significantly impacts risk assessments, where fixed baselines highlight irreversible ecological stress, while moving baselines obscure long-term trends. Future intensification of WNPSH events is expected to drive more frequent and severe summertime MHWs, with the northeastern IO experiencing more extreme events than the western IO, possibly due to differences in cloud-radiation-SST feedback mechanisms. This research enhances understanding of MHW characteristics, mechanisms, and risks in the Indian Ocean, providing valuable insights for adaptation strategies to protect marine ecosystems in a warming world.

Date of Award	29 Apr 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xuan WANG (Supervisor) & Wen ZHOU (External Co-Supervisor)