The life cycle of submesoscale eddies generated by topographic interactions

Abstract

Persian Gulf Water and Red Sea Water are salty and dense waters flowing at intermediate depths in the Gulf of Oman and the Gulf of Aden, respectively. Their spreading pathways are influence by mesoscale eddies that dominate the surface flow in both semi-enclosed basins. In situ measurements combined with altimetry indicate that Persian Gulf Water is stirred in the form of filaments and submesoscale structures by mesoscale eddies. In this paper, we study the formation and the life cycle of intense submesoscale vortices and their potential impact on the spreading of Persian Gulf Water and Red Sea Water. We use a primitive-equation three-dimensional hydrostatic model at a submesoscale-resolving resolution to study the evolution of submesoscale vortices. Our configuration idealistically mimics the dynamics in the Gulf of Oman and the Gulf of Aden: a zonal row of mesoscale vortices interacting with north and south topographic slopes. Intense submesoscale vortices are generated in the simulations along the continental slopes due to two different mechanisms. First, intense vorticity filaments are generated over the continental slope due to frictional interactions of the background flow with the sloping topography. These filaments are shed into the ocean interior and undergo horizontal shear instability that leads to the formation of submesoscale coherent vortices. The second mechanism is inviscid and features baroclinic instabilities arising at depth due to the weak stratification. Submesoscale vortices subsequently drift away, merge and form larger vortices. They can also pair with opposite-signed vortices and travel across the domain. They eventually dissipate their energy via several mechanisms, in particular fusion into the larger eddies or erosion on the topography. Since no submesoscale flow clearly associated with the fragments of Persian Gulf Water was observed in situ, we modeled Persian Gulf Water as Lagrangian particles. Particle patches are advected and sheared by vortices and are entrained into filaments. Their size first grows as the square root of time: a signature of the merging processes. Then, it increases linearly with time, corresponding to their ballistic advection by submesoscale eddies. On the contrary, without intense submesoscale eddies, particles are mainly advected by mesoscale eddies; this implies a weaker dispersion of particles than in the previous case. This shows the potentially important role of submesoscale eddies in spreading Persian Gulf Water and Red Sea Water.

Structure and Dynamics of the Ras al Hadd Oceanic Dipole in the Arabian Sea

“…After exiting the marginal seas, the RSW and PGW outflows mix with the surrounding water masses in the Gulf of Aden and in the Gulf of Oman, respectively, then, the RSW plume stabilizes at 600-1000 m and the PGW plume at a 250-350 m depth [15][16][17]. Due to the influence of mesoscale eddies or bottom topography [4,[18][19][20], the PGW or RSW outflows are destabilized and they often shed smaller-scale eddies (of radii ranging between 5 and 20 km). These eddies, formed at a depth near the coast, drift away into the open ocean, carrying their warm and salty waters (see [3,21,22] for the small RSW eddies, and [23][24][25] for the PGW eddies).…”

“…Vertical mixing was parameterized with the KPP (K-profile parameterization) scheme [32]. This numerical model has been described and validated in [18].…”

Observations of a Deep Submesoscale Cyclonic Vortex in the Arabian Sea

“…This strip subsequently rolled up into small cyclones in the lee of the cape, possibly via inertia and the D'Asaro mechanism (D'Asaro, 1988) and/or by centrifugal and barotropic shear instabilities (Srinivasan et al, 2019). Frictional generation of vorticity near the coast has been intensively studied, using idealized (Dong et al, 2007; Morvan et al, 2019; Srinivasan et al, 2019) and realistic (Contreras et al, 2019; Dong & McWilliams, 2007; Gula et al, 2019, 2015; McWilliams, 2019; Vic et al, 2015) numerical simulations. It has been identified as an efficient mechanism to form deep submesoscale vortices in gradient wind balance, with high values of | ζ / f | (Srinivasan et al, 2019), and a lens‐like shape (Morvan et al, 2019).…”

Numerical Study of the Exchange Flow of the Persian Gulf Using an Extended Total Exchange Flow Analysis Framework

“…PGW is leaving the Gulf as a bottom current and Indian Ocean Surface Water (IOSW) is entering through the Strait of Hormuz to compensate the volume loss. The PGW stratifies in relatively shallow depth into the Gulf of Oman ( 250 m), mixes with ambient water due to mesoscale and submesoscale eddies (Morvan et al, ; Vic et al, ), and spreads into the Indian Ocean where PGW signatures can be found, for example, in the Bay of Bengal (Jain et al, ). Previous measurements showed that the outflow of the PGW does not follow a significant seasonal cycle and is relatively steady with only small variabilities one time scales of 2–3 weeks (Johns et al, ; Swift & Bower, ; Pous et al, ).…”