A mechanism for jet drift over topography

Abstract

The dynamics of multiple alternating oceanic jets has been studied in the presence of a simple bottom topography with constant slope in the zonal direction. A baroclinic quasi-geostrophic model forced with a horizontally uniform and vertically sheared background flow generates mesoscale eddies and jets that are tilted from the zonal direction and drift with constant speed. The governing dynamical equations are rewritten in a tilted frame of reference moving with the jets, and the cross-jet time-mean profiles of the linear and nonlinear stress terms are analysed. Here, the linear stress terms are present because of the zonally asymmetric topography. It is demonstrated that the linear dynamics controls the drift mechanism. Also, it is found that the drifting jets are directly forced by the imposed vertical shear, whereas the eddies oppose the jets, although this is limited to continuously forced dissipative systems. This role of the eddies is opposite to the one in the classical baroclinic model of stationary, zonally symmetric multiple jets. This is expected to be more generic in the ocean, which is zonally asymmetric nearly everywhere.

Tilted drifting jets over a zonally sloped topography: effects of vanishing eddy viscosity

“…We used a two-layer QG model forced with a uniform background flow in the upper layer. In accord with previous studies (Boland et al 2012;Khatri & Berloff 2018), the jets tilt from the zonal direction and drift meridionally. Khatri & Berloff (2018) also showed that the tilted jets are coupled to the imposed background shear via the sloped topography and, in the overall balance, the jets gain energy directly from the background flow and lose energy to eddies.…”

“…In agreement with previous studies (Boland et al 2012;Khatri & Berloff 2018), the jets tilt from the zonal direction and drift meridionally because of the zonally sloped Table 1. Parameter values used in the numerical simulations.…”

“…The top layer experiences a background PV gradient only in the meridional direction, whereas, in the bottom layer, the background PV gradients have both zonal and meridional components (see equation 2.1). The jets tilt in response to the titled mean barotropic PV isolines and drift meridionally to compensate for the PV advection due to the mean jets across PV isolines (Khatri & Berloff 2018). In the statistical equilibrium, the jets gain energy directly from the imposed background flow and lose energy to eddies.…”

“…2011; Boland et al. 2012; Khatri & Berloff 2018 a ). Also, jets in the oceans can merge and disappear under the influence of a nonlinear topography (Thompson 2010; Chen, Kamenkovich & Berloff 2015).…”

Role of Eddies in the Maintenance of Multiple Jets Embedded in Eastward and Westward Baroclinic Shears

“…Reference [29] studied jet formation in a differentially heated rotating annulus experiment and found evidence of direct energy transfer to jets from eddies. Contrary to the upscale energy-transfer scenario, in some flow regimes jets can directly extract energy from the imposed background shear, rather than from the eddies (e.g., Reference [30]).…”