Marine heatwaves (MHWs), defined as periods of extreme ocean temperatures exceeding climatological thresholds, are significantly impacting marine ecosystems globally. However, research on MHWs and their effects in the Southern Ocean (SO), a crucial component of the Earth's climate system, remains limited. This study addresses this gap by examining the spatiotemporal patterns of MHWs in the SO, their physical drivers, and their effects on marine biogeochemistry, particularly primary production. The SO's unique characteristics, including the strong Antarctic Circumpolar Current (ACC) and interactions with sea ice, make it vital to understand how MHWs impact its complex ecosystem. This study uses high-resolution satellite and model data from 1982 to 2021 to quantify MHW occurrence, characterise their features, analyse their drivers, and explore the consequences of MHWs on primary productivity. The research is crucial for understanding the impact of climate change on the Southern Ocean's carbon cycle and its contribution to global climate regulation.

Previous research has shown the significant impact of long-term ocean warming and increased frequency and intensity of MHWs on marine ecosystems worldwide. Studies have documented ecosystem structure and function changes due to MHWs, including range shifts in marine species, reduced reproductive success, impacts on phytoplankton, coral bleaching, and mass mortality events. The methodologies for identifying MHWs have involved various climatological baselines (fixed or moving), with fixed baselines being more sensitive to long-term changes but commonly used for ecological impact studies. While studies highlight various drivers of MHWs (atmospheric warming, oceanic advection, ENSO), research focusing specifically on the Southern Ocean and its unique physical and biological interactions has been limited. Previous studies have indicated that Antarctic meltwater-driven stratification affects subantarctic phytoplankton; however, the direct impact of MHWs on high-latitude phytoplankton activity needed further investigation. Existing literature suggests a possible link between MHWs and increased biological activity in the Southern Ocean, but a thorough examination is lacking.

This study employed a comprehensive methodology using multiple datasets and analyses to assess the impact of MHWs on the Southern Ocean. Sea surface temperature (SST) and sea ice concentration (SIC) data were sourced from the European Space Agency (ESA) Climate Change Initiative (CCI) and Copernicus Climate Change Service (C3S) daily reanalysis product (1981-2021). Near-surface air temperature (N-SAT) data came from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis. Mixed-layer depth (MLD) data were derived from the Copernicus Marine Service (CMS) Global Ocean Reanalysis and Simulations 12v1 (GLORYS12V1). To estimate primary production, researchers utilized the Carbon-based Production Model (CbPM), incorporating data from MODIS, SeaWiFS, and VIIRS sensors. Analysis of local MHW drivers involved temperature tendency heat budgets from the GFDL ESM2M coupled model (MOM4p1), examining advection, sea-air heat fluxes, vertical mixing, and diffusion. MHW detection followed established criteria, considering SST anomalies above the 95th percentile of a 1982-2012 climatology, duration, and warmth relative to the long-term mean summer temperature (LMST). Trend analysis used ordinary least squares and Theil-Sen estimators, considering the non-normal distribution of data. The Mann-Kendall test, modified for serial autocorrelation, analyzed trends in nutrients and chlorophyll. Convergent Cross Mapping (CCM) was applied to explore causal relationships between Max SSTA, NPP, SIC, and MLD.

The study revealed a significant increase in MHW frequency, duration, and intensity in the Southern Ocean since 1982, with notable regional variations. The largest increases were observed in the Davis Sea and Southwestern Atlantic. Analysis of heat budgets indicated that sea-air heat fluxes and vertical diffusion were the primary drivers of MHWs, while convective vertical mixing acted as a counteracting force. Post-2015 warming air temperatures and reduced sea ice extent amplified MHWs, likely due to decreased upward sensible heat exchange and strengthened downward infrared radiation. A strong correlation between MHWs and enhanced primary production was discovered, especially in subantarctic regions where sea ice melting increased nutrient availability. The increase in primary production was linked to MHW-induced shallower mixed layers, trapping nutrients and enhancing phytoplankton growth. Causal analysis (CCM) demonstrated the significant causal impact of MHWs (maximum SST anomalies) and MLD on Net Primary Production (NPP), with the MLD being most influential in explaining NPP variation. Regional case studies in the Davis Sea and Amundsen-Bellingshausen Sea illustrated the concurrent occurrence of MHWs and high net primary production (HNPP) events, highlighting the role of water column stabilization and nutrient inputs from sea-ice melt. Zonal asymmetry in phytoplankton responses to temperature extremes was observed, with a reduced influence of temperature as a limiting factor in dynamic regions of the ACC.

The findings demonstrate a strong link between MHWs and enhanced biological activity in the Southern Ocean, contrasting with observations in temperate and tropical regions where MHWs often lead to reduced primary production. In the SO's high-nutrient environment, MHW-driven stratification helps keep phytoplankton in the euphotic zone, coupled with increased iron inputs from sea ice melt, boosts productivity. The significant causal link between MHWs and increased NPP underscores the importance of considering extreme events when assessing the Southern Ocean's carbon cycle. The zonal asymmetry in phytoplankton response highlights the complexity of ecosystem responses to climate change. The study supports the hypothesis that short-term MHWs can influence carbon absorption in the SO through the biological carbon pump, although further research is needed to understand long-term interactions and CO2 fluxes.

This study provides critical insights into the relationship between MHWs and primary production in the Southern Ocean, revealing a positive relationship driven by water column stabilization and increased nutrient availability from sea ice melt. The findings highlight the potential for short-term warming events to significantly influence the Southern Ocean's carbon cycle. Future studies should investigate the impact of MHWs on specific phytoplankton functional types and their implications for carbon uptake, fixation, and export to improve our understanding of the global carbon cycle under climate change. Further research focusing on long-term interactions and CO2 fluxes is needed to refine projections of the ocean carbon cycle under climate change.

The study acknowledges limitations associated with the length of the satellite record (1982-2021), making it challenging to completely disentangle multi-decadal climate variability from anthropogenic long-term trends. The use of a single primary productivity model (CbPM) introduces uncertainties; other models may provide different estimations. The complexity of phytoplankton communities and their varied responses to environmental conditions means a single, universal response to extreme temperatures cannot be predicted. Future work should consider species-specific responses, trophic interactions, and the influence of grazing pressure on phytoplankton communities.