Large-Scale Baroclinic Instability of the Mean Oceanic Circulation: A Local Approach

Abstract

Large-scale baroclinic instability is investigated as a potential source of Rossby waves and large-scale variability in the ocean. This baroclinic instability is first reviewed in a 2.5-layer model. As already noticed by several authors, the instability arises in westward surface mean flow when the phase velocities of the two vertical modes are made equal by mean flow influence. This large-scale instability is stronger at low latitudes and thus is likely to happen in the westward return flow of the subtropical gyres. Further investigations with a continuous stratification quasigeostrophic model show that the instability is stronger where the mean flow projects negatively on the second baroclinic mode (imposing positive vertical modes at the surface). The linear stability calculation is then performed on Argo-derived mean flow along with mean stratification data. The results show that the unstable regions are situated at low latitudes in every oceanic basin, in western boundary currents, and in some part of the Antarctic Circumpolar Current. The location of these unstable regions is well correlated with the region of negative projection of the mean flow on the second baroclinic mode. Given that the unstable mode growth times are generally smaller than 6 months at low latitudes, these unstable modes are likely to be observable in satellite altimetry. Therefore, results of the present article suggest that the large-scale instability is indeed a source of large-scale variability at low latitudes.

The Internal Generation of the Atlantic Ocean Interdecadal Variability

“…13a), the largest growths are found over most of the subpolar gyre, along the western boundary current, and near the equator. This picture contrasts with that obtained by Hochet et al (2015) in the longwave and adiabatic limits. Their study, based on observations, shows unstable regions concentrated in the tropics with much weaker instability from middle to subpolar latitudes.…”

“…Other regions in the North Atlantic remain remarkably quiescent (see also Sévellec and Fedorov 2013). In this section, we show that the region of maximum variance east of Newfoundland in our simulations can be predicted by a local linear stability analysis of the mean flow, similar to that of Smith (2007) at the eddy scale and Hochet et al (2015) at the longwave limit. The local approximation represents of course a strong simplification to the full problem as emphasized by Tulloch et al (2011).…”

Interannual Variability of Upper Ocean Water Masses as Inferred From Argo Array

“…Another explanation could be the multiscale and nonlinear propagation of baroclinic disturbance associated with an unstable baroclinic mode (Hochet et al, 2015;O'Kane, Matear, Chamberlain, Oliver, & Holbrook, 2014;O'Kane et al, 2016). In subtropical regions, these baroclinic disturbances could be associated with an unstable mode of propagation.…”

On the stability and spatiotemporal variance distribution of salinity in the upper ocean

“…Hochet et al . [] show recently that these are the regions where the largest fraction of the most unstable mode vertical structure is contained in the second baroclinic mode. The intrinsic SSS variability in the 2–5 year time band observed in the Sargasso Sea and extending into the Gulf Stream is generated by coherent large‐scale salinity disturbances in the upper ocean, here shown in the animation of salinity anomalies at 200 m depth (supporting information).…”