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

Abstract

Oceanic multiple jets are seen to possess spatio-temporal variability imposed by varying bottom topography resulting in jets that can drift and merge. The dynamics of multiple jets over a topographic zonal slope is studied in a two-layer quasi-geostrophic model. The jets tilt from the zonal direction and drift meridionally. In addition to the tilted jets, other large-scale spatial patterns are observed, which are extracted using the principal component analysis. The variances of these patterns are strongly influenced by the values of eddy viscosity and bottom friction parameters. The contribution of the tilted jets to the full flow field decreases with decreasing friction and viscosity parameters, and purely zonal large-scale modes, propagating in the meridional direction, populate the flow field. Linear stability analysis and two-dimensional kinetic-energy spectrum analysis suggest that the zonal modes gain energy from ambient eddies as well as from the tilted jets through nonlinear interactions. However, viscous dissipation and bottom friction tend to suppress the nonlinear interactions, which results in the inhibition of the upscale energy transfer from eddies to the zonal modes. These simulations suggest that, in the presence of topography, alternating jet patterns may be sustained through interactions among various large-scale modes. This is different from the classical zonal jet formation arguments, in which direct eddy forcing maintains the jets.

Global cascade of kinetic energy in the ocean and the atmospheric imprint

“…S4 and ( 70 )]. These observations reinforce previous studies ( 31 , 71 , 72 ) indicating that Rossby wave generation does not adequately explain the upscale cascade’s arrest. It is likely that the cascade is attenuated at a few hundred kilometers because of frictional processes that act at these scales in a more isotropic fashion, including eddy damping by wind ( 61 – 63 ) and bottom drag ( 25 ).…”

Linear stability analysis for flows over sinusoidal bottom topography

“…Such a method yields a complex eigenvalue problem with coupled eigenmodes; e.g. Berloff & Kamenkovich (2013) and Khatri & Berloff (2019) used finite-difference discretisation to study the stability of multiple jets. However, this technique is computationally taxing, as it is relatively sensitive to the numerical grid resolution, which can be rather fine for obtaining numerically convergent results.…”

“…Irregularities in bottom topography play a pivotal role in the dynamics of the ocean's circulation (Marshall 1995;Gille, Metzger & Tokmakian 2004). It has been shown that oceanic mesoscale eddies and ubiquitous multiple alternating jets form and exhibit significant variability localised over topographic features (Thompson & Richards 2011;Boland et al 2012;Chen, Kamenkovich & Berloff 2015;Stern, Nadeau & Holland 2015;Khatri & Berloff 2018;Lazar, Zhang & Thompson 2018;Khatri & Berloff 2019). This variation in oceanic flows and flow-topographic interactions affect many processes such as the eddy-induced transport, eddy energetics and ocean ventilation (Thompson 2010;Abernathey & Cessi 2014;Barthel et al 2017;Tamsitt 2017;Youngs et al 2017;Klocker 2018).…”

3D Structure of Striations in the North Pacific

“…Since striations in these regions have different orientations, they are called by different names, such as multiple zonal jets (Berloff et al 2009a;Kamenkovich et al 2009), quasi-zonal jets (Schlax and Chelton 2008;Ivanov et al 2009; van Sebille et al 2011;Ivanov et al 2012;Wang et al 2012), and quasistationary striations (Melnichenko et al 2010), among others. There are various dynamic mechanisms in the eastern Pacific; the influence of the eastern boundary current could lead to the poleward tilting induced by nonlinear radiating instabilities (Wang et al 2012) or modulation by topographic steering associated with the background flow (Melnichenko et al 2010;Chen and Flierl 2015;Belmadani et al 2017) or influenced by eddy viscosity and bottom friction (Khatri andBerloff 2018, 2019). Based on an idealized, eddy-resolving and flat-bottom quasigeostrophic model, Berloff et al (2011) also proposed that the degree of the jet's latency is controlled primarily by bottom friction.…”