Collaborative Research: On the importance of Submesoscale processes for ocean productivity

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). A hierarchy of numerical modeling studies will be performed to examine the processes by which nutrients are advected into the euphotic surface layer of the ocean to support phytoplankton productivity in pelagic regions with shallow pcynoclines. The model experiments are designed to test two competing hypotheses (i) that nutrients are upwelled by mesoscale eddies through eddy-pumping and eddy/wind interaction, vs. (ii) that the ageostrophic vertical motions supported by submesoscale (110 km scale, and O(1) Rossby number) frontal processes are largely responsible for the vertical nutrient fluxes. This proposal aims to extend the newly emerging understanding of submesoscale processes in the upper ocean to exploring their impact on biogeochemical transport and ocean productivity. Intellectual Merit Biogeochemical property fluxes (i.e. the transport of reactive tracers) are affected not only by physics, but also by biological reactions (sources and sinks for the tracers). This project will couple simple biological models with physical models ranging in complexity from the fully nonhydrostatic, three-dimensional (PE) model to the surface-quasigeostrophic (SQG) and semigeostrophic (SG) inversions, to gauge the effects of physical processes on biological productivity. To better understand the contribution of meso- and submeso-scale physics, the team will model both scales simultaneously, delineate between Ro1 (mesoscale) and Ro=O(1)(submesoscale) processes, and ascribe the vertical transport of phytoplankton nutrient to specific scales and processes under various physical scenarios. The pathways of water parcels will be analyzed in conjunction with physical and biological properties (vorticity, velocity, density, nutrient, and light) to gain a Lagrangian view of physical and biological coupling at meso- and submeso-scales. The focus will be on three important sets of questions. (1) What is the contribution of submesoscale processes to vertical nutrient transport in comparison to mesoscale processes? Which physical time scales (meso- or submeso-scale) are most commensurate with the biology and enhance the efficacy of nutrient transport? (2) What is the structure of the vertical velocity associated with different processes and scales? How is this affected by lateral density gradients, mixed layer depth, pycnocline stratification, and surface forcing? (3) How well do the SQG (and the SG and QG) inversions represent the submesoscale vertical velocity structure and transport? Broader Impacts Vertical transport between the pycnocline and surface mixed layer of the ocean is of importance in several biogeocehemical and physical contexts. Hence this study has broad implications. Coupling of biology to physics at submesoscales is relatively unexplored. The findings will help interpret biological observations and determine if indeed 110 km scale physics is relevant for productivity in carbon cycle models. The PIs will link this modeling and analysis work to observations by collaboration with a Japanese group making measurements in the Kuroshio, Norwegian group attempting to interpret high resolution satellite measurements, and an ONR-funded tracer release study of submesoscale lateral mixing. The project will support two postdoctoral researchers who will be trained in modeling and analysis, publication and presentation of results, and collaborative planning activities. The PIs will participate in education and outreach activities through the Ocean Explorium at New Bedford and the Summer Pathways program at Boston University. Women scientists will play an important role in this project and will serve as role models in research and outreach.