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1. Introduction Shear-induced destabilization of laminar currents and the subsequent generation of small-scale turbulence represents one of the major drivers of diapycnal mixing in the ocean interior (e.g., Smyth and Moum 2012 ). Motivated by numerous geophysical applications, the problem of instability of vertically sheared parallel flows has long evolved into a broad and active research area of fluid dynamics. Comprehensive reviews of the field can be found in, for example, Peltier and
1. Introduction Shear-induced destabilization of laminar currents and the subsequent generation of small-scale turbulence represents one of the major drivers of diapycnal mixing in the ocean interior (e.g., Smyth and Moum 2012 ). Motivated by numerous geophysical applications, the problem of instability of vertically sheared parallel flows has long evolved into a broad and active research area of fluid dynamics. Comprehensive reviews of the field can be found in, for example, Peltier and
1. Introduction a. Marginal instability This is an examination of the hypothesis that the mean state of continually forced, naturally occurring, stably stratified turbulent flows is generally one that is marginally unstable. The concept of control in a marginal state is introduced by Turner (1973) , who refers to Mittendorf’s (1961) laboratory observations: After the Kelvin-Helmholtz mechanism had led to the breakdown of an interface between two accelerating layers [of different densities
1. Introduction a. Marginal instability This is an examination of the hypothesis that the mean state of continually forced, naturally occurring, stably stratified turbulent flows is generally one that is marginally unstable. The concept of control in a marginal state is introduced by Turner (1973) , who refers to Mittendorf’s (1961) laboratory observations: After the Kelvin-Helmholtz mechanism had led to the breakdown of an interface between two accelerating layers [of different densities
-induced instability of ENSO As shown in Jin et al. (2007) , the modulated noise has a profound effect on the ENSO stability. There, they analytically depicted the ensemble-mean dynamics of the system under second-order closure. Using the approach outlined by Jin et al. (2006) , starting from Eq. (1 ), which describes the recharge oscillator, the variables are split into ensemble mean and departure terms, where 〈 T 〉 and 〈 h 〉 depict the ensemble means and T  ′ and h ′ depict the departure from the means for
-induced instability of ENSO As shown in Jin et al. (2007) , the modulated noise has a profound effect on the ENSO stability. There, they analytically depicted the ensemble-mean dynamics of the system under second-order closure. Using the approach outlined by Jin et al. (2006) , starting from Eq. (1 ), which describes the recharge oscillator, the variables are split into ensemble mean and departure terms, where 〈 T 〉 and 〈 h 〉 depict the ensemble means and T  ′ and h ′ depict the departure from the means for
included in theories and models of the upper ocean. The present work focuses on the importance of submesoscale dynamics for ML restratification. We show that baroclinic instabilities develop at the submesoscale and act to continuously restratify the surface ML. Present models of the upper ocean ignore these dynamics and are likely to underestimate the rate of ML restratification. As an illustration of how these lateral instabilities arise consider the following scenario. A winter storm hits the open
included in theories and models of the upper ocean. The present work focuses on the importance of submesoscale dynamics for ML restratification. We show that baroclinic instabilities develop at the submesoscale and act to continuously restratify the surface ML. Present models of the upper ocean ignore these dynamics and are likely to underestimate the rate of ML restratification. As an illustration of how these lateral instabilities arise consider the following scenario. A winter storm hits the open
1. Introduction The basic mechanisms of baroclinic instability have been explored in the seminal papers of Charney (1947) and Eady (1949) . Wave perturbations are found to grow in mean flows with vertical shear under favorable conditions. Since then, the theory of baroclinic instability has been a cornerstone of the dynamic interpretation of the atmospheric circulation at midlatitudes and holds, therefore, a central position in the teaching of dynamic meteorology. In that context, the so
1. Introduction The basic mechanisms of baroclinic instability have been explored in the seminal papers of Charney (1947) and Eady (1949) . Wave perturbations are found to grow in mean flows with vertical shear under favorable conditions. Since then, the theory of baroclinic instability has been a cornerstone of the dynamic interpretation of the atmospheric circulation at midlatitudes and holds, therefore, a central position in the teaching of dynamic meteorology. In that context, the so
1. Introduction Charney and DeVore (1979 , hereafter CDV ) found a new instability when analyzing a low-order model of barotropic channel flow over topography with multiple equilibria. This topographic instability is associated with an equilibrium state in the presence of orography, zonal mean flow forcing, and surface friction (see also Ghil and Childress 1987 ). As outlined, for example, by Jin and Ghil (1990) , the basic mechanism of this instability is simple. A stationary high resides
1. Introduction Charney and DeVore (1979 , hereafter CDV ) found a new instability when analyzing a low-order model of barotropic channel flow over topography with multiple equilibria. This topographic instability is associated with an equilibrium state in the presence of orography, zonal mean flow forcing, and surface friction (see also Ghil and Childress 1987 ). As outlined, for example, by Jin and Ghil (1990) , the basic mechanism of this instability is simple. A stationary high resides
1. Introduction Eady’s model of baroclinic instability ( Eady 1949 ) contains the instability process in a pure and elegant form. The model with its linear mean wind profile and two rigid lids is quite simple but there is nevertheless sufficient similarity between the model’s unstable modes and baroclinic waves in the atmosphere to make this instability a paradigm for a dynamical atmospheric mechanism of key importance. In turn, many textbooks of dynamical meteorology provide a discussion of
1. Introduction Eady’s model of baroclinic instability ( Eady 1949 ) contains the instability process in a pure and elegant form. The model with its linear mean wind profile and two rigid lids is quite simple but there is nevertheless sufficient similarity between the model’s unstable modes and baroclinic waves in the atmosphere to make this instability a paradigm for a dynamical atmospheric mechanism of key importance. In turn, many textbooks of dynamical meteorology provide a discussion of
1. Introduction Symmetric instability is a type of instability in a shear flow under thermal wind balance in which perturbations normal to the flow direction become unstable when the baroclinicity of the shear flow becomes sufficiently strong in both statically and inertially stable states. Historically, this instability was investigated as the stability of a circular vortex responding to axisymmetric perturbations. Later, the problem was simplified by neglecting the curvature effects and was
1. Introduction Symmetric instability is a type of instability in a shear flow under thermal wind balance in which perturbations normal to the flow direction become unstable when the baroclinicity of the shear flow becomes sufficiently strong in both statically and inertially stable states. Historically, this instability was investigated as the stability of a circular vortex responding to axisymmetric perturbations. Later, the problem was simplified by neglecting the curvature effects and was
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
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
1. Introduction Radiating instability refers to an instability of the mean flow that propagates energy away from the source of instability. It can be contrasted with a trapped instability, the influence of which is confined mainly to the locally unstable region and has no impact on the far field. Previous studies have shown that parallel zonal eastward barotropic jets do not support radiating instabilities ( Talley 1983 ). For these currents, the perturbation energy stays trapped near the mean
1. Introduction Radiating instability refers to an instability of the mean flow that propagates energy away from the source of instability. It can be contrasted with a trapped instability, the influence of which is confined mainly to the locally unstable region and has no impact on the far field. Previous studies have shown that parallel zonal eastward barotropic jets do not support radiating instabilities ( Talley 1983 ). For these currents, the perturbation energy stays trapped near the mean
