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Eric M. Leibensperger and R. Alan Plumb

and oceanic flows. While such flows are indeed almost two-dimensional (in the sense of being quasi horizontal or quasi isentropic) they are usually also baroclinic. As the isentropic strain effects a cascade of tracer variance to small horizontal scales, the vertical shear tilts such features in the tracer field (see Fig. 2 ) such that the expected ratio of vertical to horizontal scales, in balanced flow, scales as the Prandtl ratio f / N , the ratio of the Coriolis parameter to the buoyancy

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Che Sun

the only large-scale laminar flow one needs to consider on a uniform earth is zonal flow satisfying the thermal wind equation, which he used as the basic flow for his instability model. Unlike the viscous laminar flows in which a transition to turbulence is determined by the Reynolds number, Eady points out that there is no such criterion for the onset of baroclinic turbulence in rotating laminar flows. Indications of a nonviscous laminar state can be found in the numerical simulation of freely

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Francis J. Poulin, Glenn R. Flierl, and Joseph Pedlosky

to arise for various types of stochastic processes, thus implying that it is a robust feature. This work focuses on understanding the stability of time-dependent baroclinic (BC) shear in the context of the Phillips model ( Phillips 1951 , 1954 ; Pedlosky 1987 ). This is a two-layer quasigeostrophic (QG) model on a beta plane, usually in the confines of a zonal channel ( Pedlosky 1964a , b ). This simple geometry is ideally suited for studying atmospheric flow. The applicability of the Phillips

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Lixin Qu and Robert Hetland

1. Introduction Baroclinic instabilities are ubiquitous throughout the global ocean. They release potential energy stored in horizontal density gradients, creating unsteady and evolving motions in the flow at approximately the first baroclinic deformation radius. As such, the Burger number (Bu) associated with baroclinicly unstable flow is Bu = R d L −1 = RoRi 1/2 ~ 1 ( Eady 1949 ; Stone 1966 , 1970 ). Beyond this, there are two general categories of baroclinic instabilities associated

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Zhiyu Liu

waves (the baroclinic tidal waves) are superimposed. 5. Conclusions In this paper we have investigated the dynamic stability of a baroclinic tidal flow observed in a stratified fjord. The principal results are as follows. The observed flow is sometimes found to be stable to Kelvin–Helmholtz instability with a minimum Ri greater than 0.25, but it mostly has a minimum Ri less than 0.25. The Ri c of the observed flow is often approximately 0.25 (e.g., hours 0, 5, 12), but sometimes it is substantially

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Robert D. Hetland

1. Introduction Baroclinic instabilities are a ubiquitous feature of oceanic flows. They occur at large scales within western boundary currents like the Gulf Stream and Kuroshio (e.g., Tulloch et al. 2011 ) and at very small scales within the oceanic surface mixed layer, referred to as the submesoscale (e.g., Boccaletti et al. 2007 ; Capet et al. 2008 ). The primary difference between the very large- and small-scale instabilities is in the Rossby number Ro = U ( fL ) −1 of the flow, where

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M. D. Greenslade and P. H. Haynes

1. Introduction The extratropical lower atmosphere may be understood as a system in which there is a balance between thermal forcing and relaxation processes, which act to drive the flow toward a baroclinically unstable state, and thermal transport by baroclinic eddies. Parts of the extratropical ocean, particularly the Antarctic Circumpolar Current, exhibit a similar balance between surface mechanical and buoyancy forcing and baroclinic eddy transport. There has been great progress in

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Michael A. Spall and Joseph Pedlosky

1. Introduction A necessary condition for the instability of baroclinic flows is that the lateral gradient of the mean potential vorticity must change sign somewhere within the fluid ( Charney and Stern 1962 ; Pedlosky 1964 ). This result is derived from consideration of global integrals of the momentum budget for a mean flow with an assumed perturbation of wavelike horizontal structure. While this constraint has proved quite useful for identifying flows that are likely susceptible to

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Joseph Pedlosky

by including the effects of baroclinicity. Considering the role of baroclinicity in the island circulation problem allows us to discuss the question of the vertical structure of the flow, in particular the vertical structure of the transport from one subbasin to the next when the island imposes a barrier to the free circulation between the subbasins. In a previous study ( Pedlosky et al. 1997 , hereafter PPSH ), one important consequence of the presence of a meridionally extended island was the

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K. H. Brink and J. Pedlosky

no current reversal with depth. This result can be expressed either as a phase locking of the barotropic and first baroclinic modes, or in terms of modes that are required to vanish at depth. Indeed, Wortham and Wunsch (2014) found that modified, bottom pressure decoupling (BPD) modes, which have no horizontal flow at the bottom, are a better representation of oceanic variability than are the conventional modes. Thus, there has been recent interest in effects that might modify the traditional

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