Zoe Louisa Jacobs

The Somali upwelling is the strongest upwelling region globally during its seasonal peak. The intense productivity that occurs during the southwest monsoon (May-September) sustains artisanal and industrial fisheries. However, due to its complex structure and seasonality, and the typically coarse spatial resolution of global climate models, understanding its future fate remains a challenge. Using a high-resolution future climate projection and a size-spectrum model (in the absence of reliable fish catch data), we identify key climate stressors and projected changes in higher trophic levels to understand potential future impacts on Somali fisheries. Overall, the productivity generated by the Somali upwelling is projected to decline by the end of the century. Our results show that the inner coastal zone may experience elevated productivity, potentially due to changes in the prevailing winds and Somali Current. This may indicate potential climate refugia and minimal impacts to the artisanal fishing fleet. However, further offshore in the Great Whirl region dominated by small pelagic fish, there is a projected decline in productivity and biomass, which may impact the industrial fishing fleets that may target this area in future. To overcome challenges in understanding the fate of global upwelling systems, high-resolution models must be employed to more accurately simulate individual systems.

Marine heatwaves (MHWs) are periods of anomalously warm sea temperatures that are becoming increasingly recognised as ocean stressors. Globally, MHWs have impacted marine ecosystems, with significant socioeconomic implications for coastal communities and industries. The unprecedented MHW that occurred in the waters surrounding the UK and Ireland in summer 2023 highlighted a crucial gap in our understanding of the region-specific characteristics of MHWs and their impacts on marine ecosystems, the services they underpin, the blue economy and society. Using diverse expert opinion, we propose 40 priority questions for interdisciplinary research in the UK that must urgently be addressed to prepare for future risks and potential opportunities associated with MHWs. The targeted questions are listed under a broad range of themes. Recommendations to policy makers and those influencing the strategic direction and allocation of funding and resources are also suggested to assist decision-makers towards evidence-based action and guide funding streams.

Regional effects of marine cold spells (MCS, periods of anomalous cooling), their impact on ecosystem biogeochemistry, and link to salinity extremes remain underexplored. A case in point is North Atlantic's Cold Anomaly (CA) region (known as the “cold blob”), which hits record low temperatures during 2014–16 while most of the global ocean warmed. Using up to 42 years of observations, we characterize the CA as a manifestation of both MCS and Fresh Waves (FW, low salinity extremes) and analyze the surface biogeochemical response. We observe a quasiperiodic pattern of MCS from the 1980s and FW (at least) from the 1990s to early 2020s in the CA region with alternations from cool and freshwater to warm and saline conditions. Since 1990s, the CA region appears to be potentially undergoing MCS and FW compound events that are more frequent and prolonged but less intense than other North Atlantic areas. The 2014‐16 CA was among the most widespread and prolonged MCS and FW events associated with a deeper mixed layer and distinct biogeochemical signature, including elevated nutrients and oxygen, an overall increased chlorophyll‐a and intensified ocean acidification. These results suggest that MCS could mitigate certain climate change effects through cooling and enhanced productivity, while exacerbating others such as ocean acidification. We compare 2014–16 CA region effects with those of Pacific's warm blob, identifying contrasting behaviors from physical processes to biogeochemical impacts and discussing a common atmospheric driver. Our findings emphasize the need to further study ecological responses to MCS in the North Atlantic.

Up to now, the UK has avoided major marine heatwaves (MHWs) that cause severe damage to marine ecosystems and the blue economy. However, an unprecedented in its intensity, though short-lived, MHW occurred in UK waters in June 2023. This event sounded an alarm bell, highlighting gaps in our understanding of MHW characteristics and their potential future impacts in the UK. Here, we use a combination of remote sensing data and model output to characterise MHWs and Marine Cold Spells (MCSs) around the UK and the wider North Atlantic, and to assess the potential for concurrent biogeochemical extreme events. Results indicate that across the wider North Atlantic, the UK is not a hot spot for MHWs or MCSs but, regionally, the southern North Sea experiences the most activity. This is also the location of extreme chlorophyll-a concentrations, here termed blue waves (low chlorophyll-a) and green waves (high chlorophyll-a). However, there is not a very pronounced relationship between temperature and chlorophyll-a extremes, which may be impacted by the exact location, drivers and season of occurrence. In contrast, the southern North Sea and English Channel may experience a MHW and low near-bottom oxygen compound events year-round, which, due to the combination of thermal stress and reduced oxygen availability, may negatively impact benthic marine ecosystems. While MHWs in UK waters do not appear to be as long-lasting or intense as other well-documented events around the world, they are projected to increase. Thus, the UK has a unique opportunity to learn from other nations and so develop robust and comprehensive policies to increase preparedness and response capability for future extreme events.