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: Throughout the Gulf of Mexico open waters, satellite measurements evidenced a clear seasonal variability in the surface chlorophyll concentration. The most important factor controlling this annual cycle is the depth of the mixed layer. Recent studies carried on in subtropical oligotrophic regions suggested that the surface chlorophyll increase may not be systematically associated to a real biomass increase but may results from physiological mechanisms. This finding may be echoed in the Gulf of Mexico as the integrated biomass derived from fluorescence data acquired by the first deployment of profiling floats presents low variability. Despite the increasing amount of observations in the Gulf of Mexico open waters, data is still lacking to infer the seasonal and interannual variability of the chlorophyll vertical structure at a sub-basin scale, as well as the mesoscale variability. Moreover, a strong limitation of observational-based studies remains in the difficulty to provide a synoptic view of all the factors that influence the water column biogeochemical state. In that sense, coupled physical-biological models have been found to be complementary and indispensable tools, especially for understanding the mechanisms controlling the close relationship between physical and biogeochemical processes. In the framework of Cigom consorcium, the biogeochemical model PISCES was coupled to a 1/12 degrees resolution simulation of the Gulf of Mexico circulation. We review the capability of the coupled model to reproduce the main biogeochemical patterns and variability in the basin. The model reveals a more contrasted situation than an unique basin scale chlorophyll pattern. This variability appears to be dependent of sub-regions of the Gulf of Mexico and is strongly affected by the mesoscale dynamic (in particular by Loop Current eddies). We investigate some of the physical-biogeochemical coupling processes associated with the GoM sub-regions and with Loop Current eddies.

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.

Description: Surface chlorophyll concentrations inferred from satellite images suggest a strong influence of the mesoscale activity on biogeochemical variability within the oligotrophic regions of the Gulf of Mexico (GoM). More specifically, long-living anticyclonic Loop Current Eddies (LCEs) are shed episodically from the Yucatan Chanel and propagate westward. This study addresses the biogeochemical response of the LCEs to seasonal forcing and show their role in driving phytoplankton biomass distribution in the GoM. Using an eddy resolving (1/12°) interannual regional simulation based on the coupled physical-biogeochemical model NEMO-PISCES that yields a realistic representation of the surface chlorophyll distribution, it is shown that the LCEs foster a large biomass increase in winter in the upper ocean. The primary production in the LCEs is larger than the average rate in the surrounding open waters of the GoM. This behavior cannot be directly identified from surface chlorophyll distribution alone since LCEs are associated with a negative surface chlorophyll anomaly all year long. This anomalous biomass increase in the LCEs is explained by the mixed-layer response to winter convective mixing that reaches deeper and nutrient-richer waters.