Marine Heatwaves in the Arabian Sea: Drivers and Impacts on Atmospheric Circulation and Extreme Precipitation

Abstract

Marine heatwaves (MHWs) threaten marine ecosystems and significantly impact weather patterns. In the Arabian Sea, summer MHWs are of particular concern due to their potential impacts on the Indian summer monsoon, a lifeline for nearly a billion people. However, the drivers of these MHWs and their influence on atmospheric circulation and monsoon rainfall remain poorly understood. Using satellite observations, reanalysis datasets, and numerical model experiments, we investigate the key drivers of MHW events and assess their impacts. When SST warming trends are retained, the eastern and northern Arabian Sea emerge as MHW hotspots, showing rapid increases during 1982-2023, largely due to anthropogenic warming. On detrending the SSTs to remove the influence of anthropogenic warming on individual MHWs, we find that most MHWs are short-lived (lasting <= 20 days) and are initiated by enhanced surface shortwave radiation and reduced latent heat loss associated with the suppressed convection phase of the Boreal Summer Intraseasonal Oscillations (BSISOs). Interannual SST anomalies, including ENSO and Indian Ocean Dipole (IOD), further modulate the year-to-year MHW variability. Conversely, the warm SSTs during MHWs exert strong atmospheric feedbacks. MHWs in the eastern Arabian Sea drive cyclonic winds, intensify moisture convergence and increase the risk of extreme precipitation along the southwest coast of India. In the northern Arabian Sea, MHW-induced cyclones trigger intense rainfall over northwestern India and Pakistan, contributing to extreme events like the 2022 Pakistan floods. These findings improve our capacity to predict Arabian Sea MHWs and assess their risks, offering significant socio-economic and ecological benefits.

Figures (10)

• Figure 1: Linear trends of (a) MHW frequency, (b) MHW duration, (c) MHW cumulative intensity, and (d) SST over the North Indian Ocean during boreal summer monsoon season (June--September) from 1982 to 2023, based on satellite-observed daily SST data. Stippled regions indicate trends that are significant at the 95% confidence level. Black boxes denote the regions with a large increase in MHWs over the Arabian Sea: the eastern Arabian Sea (EAS; 8$^\circ$N-18$^\circ$N, 62$^\circ$E-77$^\circ$E) and northern Arabian Sea (NAS; 18$^\circ$N-25$^\circ$N, 59$^\circ$E-73$^\circ$E).
• Figure 2: (a) Cumulative intensity of MHWs in the eastern and northern Arabian Sea, averaged for each summer (June--September), from 1982 to 2023. (b) Same as (a), but using detrended SST to identify MHWs. (c) as in (b), but with the influences of ENSO and IOD removed from the detrended SST (see text). (d,e) As in (b), but with (d) interannual (> 365 day) and (e) intraseasonal (10–90 day) SSTA removed from the detrended SST. Panels (c--e) use the same $90^{th}$ percentile threshold as in panel (b) to identify MHWs. Contributions from longer-lasting MHWs (duration > 20 days) are hatched.
• Figure 3: Composite of (a) SST anomalies on the day of MHW peak intensity, (b) mixed layer heat budget during MHWs, and (c) anomalous contributions from various components of net air-sea heat fluxes, for MHWs in the eastern Arabian Sea. Panels (d--f) are the same as (a--c), but for the northern Arabian Sea. SSTA is from OISST. Stippling indicates values significant at the 95 % confidence level based on a t-test. Mixed layer heat budget is from ECCO reanalysis, where $Q_{net}$ represents the net air–sea flux, corrected for shortwave radiation exiting the mixed layer. Advection and diffusion terms include both horizontal and vertical contributions. The composite plots are centered on the peak intensity of each MHW (day 0), with negative days representing the developing phase and positive days representing the decaying phase of the MHWs.
• Figure 4: Evolution of anomalous net air-sea heat flux (color), outgoing longwave radiation (OLR; purple contours---positive solid and negative dashed; units: $\mathrm{Wm^{-2}}$), a proxy for tropical atmosphere deep convection, and surface winds (vectors) associated with MHWs in the eastern Arabian Sea (black box). Panels show (a) day -3, (b) day 0, and (c) day 3, with day 0 marking the MHW peak. Stippling indicates values significant at the 95 % confidence level based on a t-test. Panel (d) shows the climatological boreal summer (JJAS) mean SST and surface winds over the Arabian Sea. Fluxes are from ERA5 reanalysis, SST is from OISST, and surface winds are from the Cross-Calibrated Multi-Platform (CCMP) Ocean Surface Wind Vector Analyses.
• Figure 5: The anomalous net air-sea heat flux (color) and 850 hPa winds (vectors) for BSISOs occurring (a,d) with MHWs and (b,e) without MHWs, along with their differences (c,f). BSISOs are centered around their SST maxima, with day 0 marking the peak SST. The SST maximum is considered over the eastern Arabian Sea (black box). Panels (a-c) display conditions prior to the SST maxima (days -5 to -1), while panels (d-f) show conditions after the SST peak (days 1-5). Stippling indicates values significant at the 95 % confidence level based on a t-test. Air-sea fluxes and winds are from ERA5, while SST is from OISST dataset.