Submesoscale dynamics in presence of freshwater forcing

Georgia Tech Research Corporation, Atlanta GA

Investigators

Abstract

Coastal oceans are dynamic regions where multiple factors including freshwater input from rivers, wind, topography, tides, density gradients, as well as horizontal and vertical mixing influence their circulation and the associated transport of material. Submesoscale flows (of order one kilometer), like small-scale currents, eddies and fronts, are directly influenced by these factors and can strengthen or weaken coastal flows. In the presence of freshwater sources two competing effects can be at play: A freshwater source can, on one hand, enhance density gradients and therefore the generation of ocean fronts, which in turn may deepen the mixed layer, and, on the other, it can stratify the surface ocean and shoal the mixed layer. In some cases, the physical forcing of the wind or tide can reinforce or destroy flow at the submesoscale level, making them difficult to study. This project aims to understand the contribution of freshwater fluxes to submesoscale flow, and of their joint impact on transport and mixing using a regional ocean model. As a case study, coastal waters off Vietnam and the Mekong Delta, where the monsoonal winds are a dominant force and there are large freshwater inputs, will be investigated. This study area has key ecological, social and economic importance. Insights gained from a previous study by the researcher in the Gulf of Mexico will be applied to the Mekong Delta region, exploring possible generalization to other tropical and sub-tropical systems. Furthermore, this project will raise awareness of the existence of coastal submesoscale areas where physical processes are likely to have important, but mostly unexplored, biogeochemical impacts. This project will contribute to a better understanding of cross-scale linkages, from the atmosphere to the land to the ocean or from a kilometer to basin-wide scale. A graduate student supported through this project will gain valuable training in using a regional ocean model configured in idealized and realistic configurations, in-situ data and data-analysis tools. At least one undergraduate student will be introduced to research as part of a Research Experience for Undergraduates (REU) fellowship. Through a collaboration with the Center for Education Integrating Science, Mathematics and Computing at Georgia Tech, a lecture module for grades 6-8 curricula will be developed to promote understanding of near coastal ocean transport in relation to floating pollution, where the concepts of convergence, patchiness and vertical mixing, and the use of models to interpret observations can be introduced. This project is a process-oriented study to investigate numerically the interplay between freshwater fluxes, atmospheric forcing, topography and tides on one hand, and surface submesoscale dynamics, horizontal and vertical mixing, and mixed layer depth on the other, in coastal areas impacted by large freshwater fluxes. The main objective is to verify the existence of a generic framework to attribute submesoscale dynamics to different physical forcings in coastal areas. Formation, distribution and impact of submesoscale circulations will be investigated using a regional ocean model run at 0.5 km horizontal resolution. The focus will be on the southwestern South China and the Vietnam coastal region, where the circulation is characterized by strong seasonal upwelling forced by the monsoonal winds, and by large freshwater inputs due to both intense precipitation pulses during the monsoonal cycle and to the seasonally varying inflow of the Mekong River. Sensitivity simulations will help elucidate the contribution of each forcing (wind, heat fluxes, freshwater fluxes, bathymetry, tides) to transport and mixing in the mixed-layer. Regions where lateral density gradients are externally supplied through riverine or rainfall inputs are "hotspots" of submesoscale activities. In these areas submesoscale processes in general and frontogenesis in particular are not fueled exclusively by the available potential energy stored in the mixed layer, but also by lateral density gradients induced by the freshwater fluxes. Therefore the freshwater fluxes contribute to defining the horizontal and vertical mixing properties, and, at least in part, the seasonal cycle of the submesoscale circulations. The interplay between all possible contributing forcings and the submesoscale circulations, however, is not well understood or characterized. As a result of this project, the contribution of freshwater fluxes to submesoscale circulations and of their joint impact on mixed-layer transport and mixing under varying atmospheric forcing conditions will be better understood. Furthermore, existing parameterizations may be generalized, making them relevant to other coastal regions subjected to large freshwater sources.