Description: Ocean color satellite measurements in the Gulf of Mexico open waters evidenced a clear seasonal variability in the surface chlorophyll concentration. Recent investigations in subtropical oligotrophic regions suggested that the surface chlorophyll increase may not be systematically associated to a real biomass increase but may result from physiological mechanisms. This finding may be relevant in the Gulf of Mexico as suggested by bio-optical measurements recently acquired by APEX profiling floats. Despite the increasing amount of observations in the Gulf of Mexico open waters, data are still lacking to regionalize the seasonal and interannual variability of the chlorophyll vertical structure or to investigate how the energetic mesoscale dynamics within the Gulf of Mexico modulate the chlorophyll distribution. To overcome these limitations, we set up an eddy resolving (1/12deg) coupled bio-physical model (NEMO-PISCES) of the Gulf of Mexico. The use of recent sets of observations allowed for a careful evaluation of the vertical distribution of the nutrient and chlorophyll concentrations throughout the different seasons. The analysis of the seasonal variability of the integrated chlorophyll content revealed a much more contrasted and complex behavior than the basin scale homogeneous pattern of variability inferred from modeled and observed surface chlorophyll concentration. We show that the variability of the mixed-layer depth strongly shape the seasonal distribution of the chlorophyll concentration but also its year to year variability. Finally, we investigate some of the physical-biogeochemical coupling processes specific to GoM sub-regions and mesoscale dynamics (in particular the Loop Current and Loop Current eddies).

Description: A systematic study of Benguela Nino and Benguela Nina events during 1958 to 2015 including those that developed before the satellite era (1982) is carried out using an ocean general circulation model in combination with a linear equatorial model. Altogether, 21 strong warm and cold anomalous coastal events are identified among which 6 undocumented extreme coastal events are reported. Results suggest that most of these extreme coastal events including the newly identified ones are linked to remote equatorial forcing via mode 2 equatorial Kelvin waves. The latter propagates after approaching the African coast poleward as coastally trapped waves leading surface temperature anomalies along the Angola-Benguela current system by one month. One to two months before the peak of Benguela Ninos or Ninas usually occurring in March-April, a large-scale wind stress forcing is observed with both local (variations of alongshore coastal wind stress) and remote forcing developing simultaneously. Results further suggest that surface temperature anomalies off Southern Angola and in the Angola-Benguela Front are associated with equatorial dynamics and meridional wind stress fluctuations off the southwestern African coast north of 15 degrees S. Similar mechanisms are observed for Northern Namibia in combination with forcing by local meridional wind stress variations. Plain Language Summary The Benguela upwelling system located in the southeastern Atlantic Ocean supports a large marine ecosystem due to upwelling conditions. Every few years, anomalous warm and cold coastal events occur in the southeastern Atlantic and are detrimental for Angola, Namibia, and South Africa, as they affect fisheries and rainfall like El Nino phenomenon in the Pacific. To study these coastal events from 1958 to 2015, we use the output from a tropical Atlantic simulation in combination with the solution of a simple linear equatorial model. We study the anomalous coastal events including the ones that occurred before the satellite era (before 1982) and examine the role of the local wind forcing and the remote forcing associated with equatorial variability. We describe so far undocumented extreme events occurring from 1958 to 2015. Results suggest that most of the extreme coastal warm and cold events are associated with the propagation of equatorial Kelvin waves along the equatorial waveguide which trigger poleward-propagating coastal trapped waves along the southwestern African coast. One to two months before the peak season (usually March-April) of the anomalous coastal events, a large-scale wind pattern is observed, encompassing both variations of alongshore coastal wind in the southeastern Atlantic and zonal wind along the equatorial Atlantic.

Description: The seasonal and interannual variability of chlorophyll in the Gulf of Mexico open waters is studied using a three‐dimensional coupled physical‐biogeochemical model. A 5 years hindcast driven by realistic open‐boundary conditions, atmospheric forcings, and freshwater discharges from rivers is performed. The use of recent in situ observations allowed an in‐depth evaluation of the model nutrient and chlorophyll seasonal distributions, including the chlorophyll vertical structure. We find that different chlorophyll patterns of temporal variability coexist in the deep basin which thereby cannot be considered as a homogeneous region with respect to chlorophyll dynamics. A partitioning of the Gulf of Mexico open waters based on the winter chlorophyll concentration increase is then proposed. This partition is basically explained by the amount of nutrients injected into the euphotic layer which is highly constrained by the dynamic of the winter mixed layer. The seasonal and interannual variability appears to be affected by the variability of atmospheric fluxes and mesoscale dynamics (Loop Current eddies in particular). Finally, estimates of primary production in the deep basin are provided.