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Ocean warming, acidification, deoxygenation and reduced productivity are widely considered to be the major stressors to ocean ecosystems induced by emissions of CO2 . However, an overlooked stressor is the change in ocean circulation in response to climate change. Strong changes in the intensity and position of the western boundary currents have already been observed, and the consequences of such changes for ecosystems are beginning to emerge. In this study, we address climatically induced changes in ocean circulation on a global scale but relevant to propagule dispersal for species inhabiting global shelf ecosystems, using a high-resolution global ocean model run under the IPCC RCP 8.5 scenario. The ¼ degree model resolution allows improved regional realism of the ocean circulation beyond that of available CMIP5-class models. We use a Lagrangian approach forced by modelled ocean circulation to simulate the circulation pathways that disperse planktonic life stages. Based on trajectory backtracking, we identify present-day coastal retention, dominant flow and dispersal range for coastal regions at the global scale. Projecting into the future, we identify areas of the strongest projected circulation change and present regional examples with the most significant modifications in their dominant pathways. Climatically induced changes in ocean circulation should be considered as an additional stressor of marine ecosystems in a similar way to ocean warming or acidification.

Marine heatwaves (MHWs) represent anomalously warm temperature conditions of seawater that may affect marine life and ocean biogeochemistry. Under such conditions, phytoplankton communities may modify their structure and functions, and their resilience is not assured. This study characterizes the impact of MHWs on the phytoplankton spring bloom in the North‐Western Mediterranean Sea. Here, we synergistically combine autonomous observations from BioGeoChemical‐Argo floats, satellite‐based and marine ecosystem model data, and show that MHW events occurring during winter drastically inhibit phytoplankton carbon biomass in spring by up to 70%. Such reduction is related to the enhanced stratification of the water column under MHWs which hinders the renewal of nutrients from deep‐ocean reservoirs, thus preventing surface phytoplankton from blooming. This process negatively impacts particulate organic carbon stocks within the mixed layer, while severe events cause an earlier shift of phytoplankton phenology that provokes changes in zooplankton biomass distribution. Plain Language Summary: Marine heatwaves (MHWs) are described as an abnormal and prolonged increase of ocean temperatures. These events may occur in all the oceans, and are becoming more frequent than before. Such increase in water temperature might not be tolerated by organisms, which must need to adapt themselves to the new environmental conditions. Consequently, marine ecosystem health is endangered. Here, we analyze the effect that MHWs have on the growth of small algae called phytoplankton. Phytoplankton are vital microscopic organisms for ecosystems that use sunlight to produce organic carbon through photosynthesis. At middle and high latitudes, phytoplankton massively grows (i.e., blooms) once a year at the sea surface, and introduces a major carbon flux into the ecosystem that sustains larger animals. Through combining observations acquired by different platforms (satellites, autonomous in situ BioGeoChemical‐Argo robots, and ecosystem models), we comprehensively study how MHWs affect phytoplankton carbon production during spring blooms and trophic chains at mid‐latitudes. Results show that MHW events occurring in winter lead to a large decrease in phytoplankton carbon biomass (up to 70%) in spring. Winter MHW events, driven by local atmospheric conditions, intensify water column stratification thus hindering the deep‐ocean nutrient transport to the surface, which is essential for phytoplankton to bloom. Key Points: Marine heatwaves (MHWs) intensify water stratification leading to reduction in nutrient supply which inhibits surface phytoplankton spring bloomMHWs lead to a phytoplankton community shift toward smaller cells, while increasing the transparency of surface watersMHWs decrease carbon stocks within the mixed layer, while intense ones shift phytoplankton phenology and affect zooplankton [ABSTRACT FROM AUTHOR]

Operational forecasts rely on accurate and timely observations and it is important that the ocean forecasting community demonstrates the impact of those observations to the observing community and its funders while providing feedback on requirements for the design of the ocean observing system. One way in which impact of new observations can be assessed is through Observing System Simulation Experiments (OSSEs). Various satellite missions are being proposed to measure Total Surface Current Velocities (TSCV). This study uses OSSEs to assess the potential impact of assimilating TSCV observations. OSSEs have been performed using two global ocean forecasting systems; the Met Office's (MetO) Forecasting Ocean Assimilation Model and the Mercator Ocean International (MOI) system. Developments to the individual systems, the design of the experiments and results have been described in two companion papers. This paper provides an intercomparison of the OSSEs results from the two systems. We show that global near surface velocity analysis root-mean-squared-errors (RMSE) are reduced by 20-30% and 10-15% in the MetO and MOI systems respectively, we also demonstrate that the percentage of particles forecast to be within 50 km of the true particle locations after drifting for 6 days has increased by 9%/7%. Furthermore, we show that the global subsurface velocities are improved down to 1500m in the MetO system and down to 400m in the MOI system. There are some regions where TSCV assimilation degrades the results, notably themiddle of the gyres in the MetOsystem and at depth in the MOI system. Further tuning of the background and observation error covariances are required to improve performance in these regions. We also provide some recommendations on TSCV observation requirements for future satellite missions. We recommend that at least 80% of the ocean surface is observed in less than 4 to 5 days with a horizontal resolution of 20 to 50 km. Observations should be provided within one day of measurement time to allow real time assimilation and should have an accuracy of 10 cm/s in the along and across track direction and uncertainty estimates should be provided with each measurement. [ABSTRACT FROM AUTHOR]

Accurate prediction of ocean surface currents is important for marine safety, ship routing, tracking of pollutants and in coupled forecasting. Presently, velocity observations are not routinely assimilated in global ocean forecasting systems, largely due to the sparsity of the observation network. Several satellite missions are now being proposed with the capability to measure Total Surface Current Velocities (TSCV). If successful, these would substantially increase the coverage of ocean current observations and could improve accuracy of ocean current forecasts through data assimilation. In this paper, Observing System Simulation Experiments (OSSEs) are used to assess the impact of assimilating TSCV in the Met Office's global ocean forecasting system. Synthetic observations are generated from a high-resolution model run for all standard observation types (sea surface temperature, profiles of temperature and salinity, sea level anomaly and sea ice concentration) as well as TSCV observations from a Sea surface KInematics Multiscale monitoring (SKIM) like satellite. The assimilation of SKIM like TSCV observations is tested over an 11 month period. Preliminary experiments assimilating idealised single TSCV observations demonstrate that ageostrophic velocity corrections are not well retained in the model. We propose a method for improving ageostrophic currents through TSCV assimilation by initialising Near Inertial Oscillations with a rotated incremental analysis update (IAU) scheme. The OSSEs show that TSCV assimilation has the potential to significantly improve the prediction of velocities, particularly in the Western Boundary Currents, Antarctic Circumpolar Current and in the near surface equatorial currents. For global surface velocity the analysis root-mean-squareerrors (RMSEs) are reduced by 23% and there is a 4-day gain in forecast RMSE. There are some degradations to the subsurface in the tropics, generally in regions with complex vertical salinity structures. However, outside of the tropics, improvements are seen to velocities throughout the water column. Globally there are also improvements to temperature and sea surface height when TSCV are assimilated. The TSCV assimilation largely corrects the geostrophic ocean currents, but results using the rotated IAU method show that the energy at inertial frequencies can be improved with this method. Overall, the experiments demonstrate significant potential benefit of assimilating TSCV observations in a global ocean forecasting system. [ABSTRACT FROM AUTHOR]

The stranding of Sargassum seaweed on the Caribbean archipelago beaches constitutes real socio-economic, ecological and health problems. Météo-France currently operates a model of Sargassum drift forecast (called MOTHY) forced by ocean currents from the global analysis and forecasting system GLO12 at 1/12 ° (∼9 km over the Caribbean) operated by Mercator Ocean International (MOi). In order to improve the Météo-France drift forecast, MOi has developed a regional high-resolution ocean forecasting system CAR36 at 1/36 ° (∼3 km) centred in the Caribbean archipelago region. In addition to a finer spatial resolution, this system was designed to resolve higher-frequency signals such as the tidal forcing and to use hourly atmospheric forcing including the inverse barometer effect. Here the added value of the CAR36 system relative to GLO12 is evaluated with particular focus on the reproduction of meso- and sub-mesoscale structures representing key features of the Caribbean region dynamics and therefore Sargassum transport. The realism of the local dynamics was examined with standard statistical validation diagnostics using satellite data (sea surface height, sea surface temperature, Sargassum detection) and drifting buoys, together with more process-oriented diagnostics such as eddy detection and tracking across the domain. GLO12 and CAR36 hindcast simulations were compared over the year 2019. CAR36 showed marginally better scores using OceanPredict diagnostics (https://oceanpredict.org/ , last access: 10 April 2024). The dynamics of a westward-propagating North Brazil Current (NBC) eddy from its entry into the domain to its dissipation were found to be more realistic in CAR36, especially at the end of its lifetime when it collides with the Caribbean archipelago. The transfer of kinetic energy from the eddy dissipating westward into the Caribbean Sea was diagnosed as more pronounced for CAR36, corresponding to filamentary structures crossing the Caribbean archipelago and resulting in part from the friction of the NBC eddy along the islands to the east. Using detection from satellites, aggregation of Sargassum around eddies or along filaments suggests that CAR36 should improve the algae drift forecasts. [ABSTRACT FROM AUTHOR]

The stranding of sargassum seaweed on the Caribbean Archipelago beaches constitutes a real socio-economic, ecological and health problem. Météo-France currently operates a model of sargassum drift forecasts (called MOTHY) forced by ocean currents from the global analysis and forecasting system GLO12 at 1/12° (~ 9 km over the Caribbean) operated by Mercator Ocean International (MOi). In order to improve the Météo-France drift forecast, MOi has developed a regional high-resolution ocean forecasting system CAR36 at 1/36° (~ 3 km) centred on the Caribbean Archipelago region. In addition to a finer spatial resolution, this system was designed to resolve some higher frequency signals such as the tidal forcing and to use hourly atmospheric forcing including the inverse barometer effect. Here the added value of the CAR36 system relative to GLO12 is evaluated with particular focus on the reproduction of meso-and sub-mesoscale structures representing key features of the Caribbean region dynamics and therefore sargassum transport. The realism of the local dynamics was examined with standard statistical validation diagnostics using satellite data (Sea Surface Height, Sea Surface Temperature, Sargassum detection) and drifting buoys, together with more process-oriented diagnostics such as eddy detection and tracking across the domain. GLO12 and CAR36 hindcast simulations were compared over the year 2019. CAR36 showed marginally better scores using OceanPredict diagnostics (https://oceanpredict.org/). The dynamics of a westward-propagating North Brazil Current (NBC) eddy from its entry into the domain to its dissipation was found to be more realistic in CAR36, especially at the end of its lifetime when it collides with the Caribbean Archipelago. The transfer of kinetic energy from the eddy dissipating westward into the Caribbean Sea was diagnosed as more pronounced for CAR36 corresponding to filamentary structures crossing the Caribbean Archipelago and resulting in part from the friction of the NBC eddy along the islands to the east. Using detection from satellite, aggregation of Sargassum around eddies or along filaments suggests that CAR36 should be able to improve the algae drift forecasts. [ABSTRACT FROM AUTHOR]

Regional processes play a key role in the global carbon budget. Major ocean CO2 uptake at mid-latitudes counteracts CO2 release in the tropics, which is modulated by episodes of marine heatwaves. Yet, we lack essential knowledge on persistent marine heatwaves, and their effect on the CO2 sensitive areas. Here we show, using a 1985–2017 joint analysis of reconstructions, ocean reanalysis and in situ and satellite data, that persistent marine heatwaves occur in major CO2 uptake and release areas. Average air-sea CO2 flux density changes from persistent marine heatwaves are strongest in the Pacific Ocean with a 40 ± 9% reduction in CO2 release in the tropics linked to ENSO, and a reduction in CO2 uptake of 29 ± 11% in the North Pacific over the study period. These results provide new insights into the interplay of extreme variability and a critical regulating ocean ecosystem service, and pave the way for future investigations on its evolution under climate change. Ocean CO2 uptake at mid-latitudes counteracts CO2 release in the tropics, but we know little about the effects of marine heatwaves that modulate this process. Here, the authors use joint analysis of satellite measurements, in situ observation, reconstructions derived from machine learning algorithms, numerical model of the global ocean, and find that areas where PMHWs most frequently occur coincide with the regions that are the most critical for the oceanic carbon cycle. [ABSTRACT FROM AUTHOR]