To understand this, consider a fluid in steady state and under the thermal-wind balance. These baroclinic instabilities may be initiated by the process of 'sloping convection' or 'slanted thermal convection'. It is often considered that baroclinic instability is the mechanism which extracts potential energy stored in horizontal density gradients and uses this "eddy potential energy" to drive eddies. Under certain conditions, slight perturbations of the equilibrium under the thermal-wind balance may increase, leading to larger perturbations from the initial state and thus the growth of an instability. It requires a change of potential vorticity. However, under the thermal-wind balance, a decrease in slope of the isopycnals cannot occur spontaneously. It would also reduce the pressure gradient, leading to an increase in the kinetic energy. A reduction in slope of the isopycnals would lower the center of gravity and therefore also the potential energy. However, this is not the equilibrium of least energy. Under the thermal-wind balance, geostrophic balance and hydrostatic balance, a flow is in equilibrium. Furthermore, this also results in changing horizontal velocities with height as a result of horizontal temperature and therefore density gradients. This implies that isopycnals can slope with respect to the isobars. In a baroclinic fluid, the thermal-wind balance holds, which is a combination of the geostrophic balance and the hydrostatic balance. The parcel will now float up even further, a small perturbation grows into a larger one and a baroclinic instability is formed. However, when a parcel is displaced to location C, it is surrounded by fluid with a higher density than the parcel itself. When a fluid parcel is perturbed from its steady state location A to location B, it will be surrounded by a fluid with a higher density and the parcel will sink down to its original equilibrium location the fluid parcel is now stable. The sizes of baroclinic instabilities and therefore also the eddies they create scale with the Rossby radius of deformation, which strongly varies with latitude for the ocean.Ī schematic on a baroclinic fluid with sloping isopycnals, intersecting with isobars on the Northern hemisphere, showing the process of sloping convection and formation of a baroclinic instability. The intersecting of isobars and isopycnals in a baroclinic medium may cause baroclinic instabilities to occur by the process of sloping convection. For this barotropic case, isobars and isopycnals are parallel. This is in contrast to a barotropic fluid, in which the density is only a function of pressure. The effect of the temperature on the density allows lines of equal density (isopycnals) and lines of equal pressure (isobars) to intersect. In a baroclinic medium, the density depends on both the temperature and pressure. Therefore, they are key in mixing and transport of for example heat, salt and nutrients. Mesoscale eddies are circular currents with swirling motion and account for approximately 90% of the ocean's total kinetic energy. In contrast, flows on the largest scale in the ocean are described as ocean currents, the largest scale eddies are mostly created by shearing of two ocean currents and static mesoscale eddies are formed by the flow around an obstacle (as seen in the animation on eddy (fluid dynamics). It can lead to the formation of transient mesoscale eddies, with a horizontal scale of 10-100 km. A baroclinic instability is a fluid dynamical instability of fundamental importance in the atmosphere and ocean.
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