The 3D structure of Mesoscale Eddies in the Northwestern Indian Ocean, and their impact on Submesoscale Dynamics
The 3D structure of Mesoscale Eddies in the Northwestern Indian Ocean, and their impact on Submesoscale Dynamics
Abstract:
In the Northwestern Indian ocean (the Arabian Sea), mesoscale eddies are prominent features of the circulation. They influence biological activity, tracer transport, as well as physical and chemical properties of seawater. In particular, the pathways of salty water masses outflowing from the marginal seas are strongly impacted by the mesoscale eddies in the Northern part of the basin.
In this study, we use Argo float data colocalized with eddies detected using altimetry in the Arabian Sea for the period 2000-2015, to extract anomalies of temperature and salinity related to mesoscale eddies. It allows us to compute a composite 3D structure of mesoscale eddies in the region. These structures are validated with in situ observations and are shown to be representative of the typical mesoscale activity in the region. Using a high resolution primitive equation model, we study the stability of the composite cyclone. We observe that it is unstable with respect to a mixed barotropic/baroclinic instability. The latter deforms the eddy, which eventually evolves into a tripole after about 4 months of simulation. The presence of a critical level for the most unstable mode generates sharp fronts in the surface mixed layer where the Rossby number is large. These fronts then become unstable, and this generates submesoscale cyclones and filaments. Near these fronts, diapycnal mixing causes potential vorticity to change sign locally so that symmetric instability develops in the core of the cyclonic eddy. Despite the instabilities, the eddy is not destroyed and remains a large-scale coherent structure for the last 6 months of the simulation. The Sea Surface Height (SSH) signature of the tripole after one year of simulation is very close to the initial cyclone's one. This indicates that the instabilities observed in the simulation can not be clearly captured in a systematic way using the currently available altimetric and floats data. Looking at SSH only, the composite eddy evolves little, and fairly represents the eddies observed in the Arabian Sea which can live for several months. The study of this simulation thus illustrates the numerous kinds of instabilities which may occur in large cyclonic eddies and can impact the local dynamics in the Arabian Sea, but can not be captured directly by altimetric data.
In this study, we use Argo float data colocalized with eddies detected using altimetry in the Arabian Sea for the period 2000-2015, to extract anomalies of temperature and salinity related to mesoscale eddies. It allows us to compute a composite 3D structure of mesoscale eddies in the region. These structures are validated with in situ observations and are shown to be representative of the typical mesoscale activity in the region. Using a high resolution primitive equation model, we study the stability of the composite cyclone. We observe that it is unstable with respect to a mixed barotropic/baroclinic instability. The latter deforms the eddy, which eventually evolves into a tripole after about 4 months of simulation. The presence of a critical level for the most unstable mode generates sharp fronts in the surface mixed layer where the Rossby number is large. These fronts then become unstable, and this generates submesoscale cyclones and filaments. Near these fronts, diapycnal mixing causes potential vorticity to change sign locally so that symmetric instability develops in the core of the cyclonic eddy. Despite the instabilities, the eddy is not destroyed and remains a large-scale coherent structure for the last 6 months of the simulation. The Sea Surface Height (SSH) signature of the tripole after one year of simulation is very close to the initial cyclone's one. This indicates that the instabilities observed in the simulation can not be clearly captured in a systematic way using the currently available altimetric and floats data. Looking at SSH only, the composite eddy evolves little, and fairly represents the eddies observed in the Arabian Sea which can live for several months. The study of this simulation thus illustrates the numerous kinds of instabilities which may occur in large cyclonic eddies and can impact the local dynamics in the Arabian Sea, but can not be captured directly by altimetric data.