Sub-mesoscale dynamics in The Southern Ocean

Abstract

The Southern Ocean circulation is dominated by the Antarctic Circumpolar Current (ACC), a quasi-zonal current that encircles Antarctica. Typical features of the ACC are an energetic eddy field and jets that influence both the large scale flow and heat and carbon fluxes and, consequently, impact the climate system. Due to the strong zonal flow and weak stratification of the Southern Ocean, topography steers and influences the ACC. For example, Rossby waves or stationary meanders can be found in the lee of topographic features and the structure of jets and fronts can be modified by topography. ACC dynamics are very complex and understanding these dynamics is crucial, given the Southern Ocean role in the global climate system. The Southern Ocean is an environment where, despite a large nutrient availability, the biological productivity is very low. This biological activity is limited by light irradiance and iron availability. However, there exist several locations in the Southern Ocean where, due to a natural iron fertilisation, phytoplankton blooms can be observed. One such location is the Kerguelen Plateau (KP) region in the south Indian Ocean. Numerous physical mechanisms that drive iron into the euphotic zone of KP waters have been identified. However, in these studies sub-mesoscale dynamics, occurring at horizontal scales of several kilometers, have never been included and their contribution to the iron supply never estimated. These structures have been seen to dramatically trigger an ecosystem response in other parts of the ocean, suggesting that they might represent a significant contribution to Southern Ocean blooms. This thesis is focused on the development and analysis of the first sub-mesoscale-resolving (1/80 resolution) ocean model of the KP area. Resolving sub-mesoscale structures results in an enhancement of vertical velocities and transport, compared to mesoscale-resolving simulations (1/20). Results show that sub-mesoscale fields, such as eddy kinetic energy or vertical velocities, are spatially inhomogeneous. Evidence is presented that this inhomogeneity is strongly related to the topographic features of this region. In particular, it is in part due to internal waves excited by the interaction of the large-scale flow with topography and largely due to an indirect generation by the topography: topography controls mesoscale flows, which in turn generate sub-mesoscale activity. The correlation between mesoscale eddy kinetic energy and strain rate fields with sub-mesoscale vertical velocities suggests a possible new route to parameterise sub-mesoscales in coarser resolution models. The modelled velocity field is used to advect Lagrangian particles. The 1/80 resolution experiments are compared to the 1/20 case, finding that waters reach greater depths at the highest resolution. Built on these Lagrangian experiments is the development of an innovative technique for the study of iron supply, used to contrast the contribution of mesoscales and sub-mesoscales. This technique highlights the sensitivity of iron supply to the horizontal resolution, showing a clear enhancement of iron fluxes (by a factor of 2) at higher resolution. Thus, the vertical motion induced by the sub-mesoscales represents a new process to drive iron into the euphotic waters of the KP region.