Phytoplankton Responses to Mesoscale and Submesoscale Processes in a Tropical Meander Filipe Pereira, Ilson C. A. da Silveira, Amit Tandon, Peter J. S. Franks, Caique D. Luko, et al. Journal of Geophysical Research Oceans, 2024 The mesoscale activity of western boundary currents can induce changes in the oligotrophic conditions of the oceanic subtropical gyres. Off SE Brazil, recurrent meanders in the Brazil Current can modulate the pelagic ecosystem through eddy‐induced upwelling, as well as drive coastal upwelling and cross‐shelf exchanges. Despite their relevance, direct measurements of such meanders are scarce. At the end of the Spring of 2019, we acquired measurements of both physical and biogeochemical properties along a transect through one of these features, with 5–10 km spacing between stations. Displacements of the isopycnals at the meander edges arose from submesoscale filaments along the meander's rim, associated with the signature of shelf water masses. Chlorophyll distributions along the transect indicated phytoplankton responses to the different dynamic regimes in the meander: eddy stirring in the surface mixed layers versus eddy pumping in the subsurface waters, and mesoscale dynamics in the meander center versus submesoscale dynamics in its periphery. Subsurface phytoplankton biomass was enhanced in the meander center, likely due to eddy pumping raising the nutricline into the euphotic zone. Eddy pumping also appears to have contributed to a shift in the phytoplankton community composition to larger organisms. Our results indicate that the physical dynamics create different habitats across the meander, leading to spatial patchiness in the chlorophyll distributions, with different types of organisms in distinct parts of the meander.
Topographically Generated Submesoscale Shear Instabilities Associated with Brazil Current Meanders Caique D. Luko, Cauê Z. Lazaneo, Ilson C. A. da Silveira, Filipe Pereira, Amit Tandon Journal of Physical Oceanography, 2023 The western boundary current system off southeastern Brazil is composed of the poleward-flowing Brazil Current (BC) in the upper 300 m and the equatorward flowing Intermediate Western Boundary Current (IWBC) underneath it, forming a first-baroclinic mode structure in the mean. Between 22° and 23°S, the BC-IWBC jet develops recurrent cyclonic meanders that grow quasi-stationarily via baroclinic instability, though their triggering mechanisms are not yet well understood. Our study, thus, aims to propose a mechanism that could initiate the formation of these mesoscale eddies by adding the submesoscale component to the hydrodynamic scenario. To address this, we perform a regional 1/50° (∼2 km) resolution numerical simulation using CROCO (Coastal and Regional Ocean Community model). Our results indicate that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instability that can trigger the meanders’ formation. Subsequently, this process generates submesoscale cyclones that contribute, along with baroclinic instability, to the meanders’ growth, resulting in a submesoscale-to-mesoscale inverse cascade. Last, as the mesoscale cyclones grow, they interact with the slope, generating inertially and symmetrically unstable anticyclonic submesoscale vortices and filaments. Significance Statement Off southeastern Brazil, the Brazil Current develops recurrent cyclonic meanders. Such meanders enhance the open-ocean primary productivity and are of societal importance as they are located in a region rich in oil and gas where oil-spill accidents have already happened. This study aims to explore the processes responsible for triggering the formation of these mesoscale eddies. We find that incoming anticyclones reach the slope upstream of separation regions and generate barotropic instabilities that eject submesoscale filaments and vortices and can trigger the meanders’ formation. Such results show that topographically generated submesoscale instabilities can play an important role in the dynamics of mesoscale meanders off southeastern Brazil. Moreover, this may indicate that resolving the submesoscale dynamics in operational numerical models may contribute to an increase in the predictability of the regional eddies.
Mixed layer eddies supply nutrients to enhance the spring phytoplankton bloom Iury T. Simoes-Sousa, Amit Tandon, Filipe Pereira, Caue Z. Lazaneo, Amala Mahadevan Frontiers in Marine Science, 2022 Mixed layer eddies resulting from baroclinic instability of fronts convert horizontal buoyancy gradients into vertical stratification, shoaling the mixed layer. In light-limited regimes – high-latitudes – this process can initiate phytoplankton blooms prior to the springtime warming. The question is whether mixed layer eddies can enhance the spring bloom by delivering nutrients from beneath the mixed layer. We couple a submesoscale-resolving model (SUB) with a simple ecosystem model and examine the role of mixed layer eddies on the development of the spring bloom. We compare the SUB simulation to two coarser resolution (10 km) simulations, one that includes a mixed layer eddy parameterization (MLE) and another that prescribes the restratification from SUB and advects the biogeochemical tracers using geostrophic velocities (NVF). The MLE simulates restratification of the mixed layer and bloom onset, but the spring bloom has a deficit of 10–13% in the new production compared to SUB. The NVF has the same restratification as SUB, and with no vertical flux of nutrients, leads to a spring bloom with a 32–40% new production deficit compared to SUB. Submesoscale processes lead to exchange across the mixed layer base, which is not represented in coarse resolution model simulations, even with mixed layer eddy parameterizations. Our results show that nutrients supplied by mixed layer eddies are important to enhance the spring bloom.
Discrepancies between satellite-derived and in situ SST data in the Cape Frio Upwelling System, Southeastern Brazil (23˚S) F. Pereira, M. Bouali, P. S. Polito, I. C. A. da Silveira, R. N. Candella Remote Sensing Letters, 2020 Satellite-derived sea surface temperature (SST) presents discrepancies in both magnitude and temporal variability in the wind-driven coastal upwelling region off the Cape Frio (23°S) in Southeastern Brazil. Level 2 (L2) data from the Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua, and Level 4 (L4) SST products are compared with in situ measurements. Strong positive biases up to 4°C are systematically observed in L4 products during upwelling events and decrease to less than 1°C in non-upwelling days. Lower biases were found in L2 MODIS SST, and the statistical performance of L4 SST products did not present significant improvements in days when high-resolution clear-sky observations from MODIS were available. This indicates that spatio-temporal interpolation and ingestion of microwave-based data sets in L4 SST analyses are likely the main cause of observed discrepancies during upwelling events. Improved L2 SST retrieval algorithms and L4 SST analyses that take into account upwelling-related regional characteristics are required for more accurate satellite SST products over coastal regions.
