Isidoro Orlanski

On the Dynamics of a Storm Track

Abstract An idealized primitive equation model is used to determine the factors controlling the dynamics and maintenance of eddy activity in a storm track. The results show that localized regions of enhanced baroclinicity do not necessarily lead to localization of eddy activity. By studying the energetics of the storm track, it is shown that while baroclinic conversion does indeed correlate with the region of maximum baroclinicity, it is the downstream radiation of energy through the ageostrophic geopotential fluxes which acts as a trigger for the development and maintenance of eddy activity over less baroclinic regions, extending the region of eddy activity much further downstream from the region of high baroclinicity. Examples of eddy life cycles are given that show that convergence and divergence of ageostrophic fluxes can dominate baroclinic and barotropic conversion, especially in regions with weak baroclinicity. Factors that may limit the zonal extent of a storm track are discussed. Evidence of down...

Instability of Frontal Waves

Abstract The stability of the classical Norwegian polar front model is investigated, using a numerical technique to supplement the more precise conclusions which are possible in the limiting cases of zero density difference or zero wavenumber. The feasibility of the numerical technique depends on a careful formulation of boundary conditions at the limits of the frontal zone. The numerical results cover the region of Rossby number (Ro) ≤ 3 and Richardson number(Ri) ≤ 5, but their interpretation is unclear at Ri > 2 and Ro > 1. Unstable waves exist at all wavelengths; Rayleigh shear instability at small Ri, Helmholtz shear instability at large Ro and small Ri, shear instability and geostrophic baroclinic instability simultaneously at small Ro and Ri > 2, and a combination of geostrophic and Helmholtz instability when Ri > 2 and Ro < l (but not too small). The previous conclusion of Kotschin that this frontal model is stable for Ri < 2 is therefore incorrect.

The Influence of Bottom Topography on the Stability of Jets in a Baroclinic Fluid

Abstract The stability of a two-layer model is analyzed using a numerical method taking into account the effect of bottom topography. A jet in geostropic equilibrium exists in the upper layer and baroclinic instability may occur. It is found if the bottom topography has a large amplitude relative to the total depth, that it has a destabilizing rather than a stabilizing influence. Applying the model to the Gulf Stream, it is found that the most unstable disturbances, corresponding to the basic flow upstream from Cape Hatteras, are markedly different in wavelength and period from those corresponding to the basic flow downstream from Hatteras. The baroclinic disturbances in the model are consistent with the limited observational evidence on momentum transfer by Gulf Stream eddies.

Ageostrophic Geopotential Fluxes in Downstream and Upstream Development of Baroclinic Waves

Abstract With the use of a simple primitive equation model, it is demonstrated that the convergence/divergence of ageostrophic geopotential fluxes can be a major source/sink of kinetic energy for both downstream and upstream development of baroclinic waves, and can play a dominant role during the early stages of wave development. It is also shown that both surface friction and β effects lead to an asymmetry in the upstream versus downstream development, with downstream development much stronger. A total group velocity is defined based on ageostrophic fluxes, and its relationship to the rate of wave packet spreading and to convective and absolute instability is discussed.

A New Look at the Pacific Storm Track Variability: Sensitivity to Tropical SSTs and to Upstream Seeding

Abstract There is a fairly well defined stationary wave and storm track response to El Nino SSTs over the Pacific. In this paper, the case is made that this response is a direct result of increased baroclinicity in the central Pacific and that changes in the stationary wave pattern farther east are primarily forced by changes in these transient eddies. There is also a lot of natural variability that is not associated with El Nino. The paper also stresses the point that much of the variability can be understood as forced by variations in the upstream seeding of the storm track. The question of whether these seeding variations should be thought of as chaotic noise or forced by identifiable mechanisms is not addressed. Thus, the claim is that the storm track variability and its feedback to the quasi-stationary circulation depends on two key parameters: mid-Pacific baroclinicity, controlled by SSTs, and the strength of the upstream seeding. The approach is to first examine the effect of storm track seeding by...

Poleward Deflection of Storm Tracks

An analysis of 11 years of European Centre for Medium-Range Weather Forecasts data focuses primarily on the vertically averaged high-frequency transients. The conclusions are discussed in the context of (a) the winter storm track, (b) monthly variability, and (c) interannual variability. (a) Winter storm track: Results show that the pattern of the forcing by the high-frequency eddies along the storm track is highly correlated with the stationary circulation, and the forcing itself is primarily responsible for the location of the trough‐ridge system associated with the stationary flow. The results also clarify the role of wind component covariance terms u9y9 and (y9 2 2 u9 2) in the column-averaged vorticity forcing. The simpler term u9y9 has the well-known effect of intensifying the anticyclonic (cyclonic) tendencies on the southern (northern) side of the jet, thereby producing an increase in the barotropic component of the zonal jet. The (y9 2 2 u9 2) term displays a quadrupole pattern, which is also approximately in phase with the trough‐ridge system associated with the stationary flow. (b) Monthly variability: Eddy activity has been shown to possess a seasonal life cycle, increasing during the early fall and reaching a maximum around the month of November, then decaying for most of the winter months. Month-to-month variations in eddy activity over the Pacific Ocean show that energy levels increase up through November, decreasing thereafter, at the same time the trough‐ridge circulation pattern is intensifying. By December, baroclinicity in the western Pacific has increased substantially, and low-level eddies are found to break by the middle of the ocean. Upper-level eddies start to break well before reaching the west coast of North America, resulting in a displacement of the maximum in (y9 2 2 u9 2) westward from its November position and increasing the trough‐ridge forcing by the high-frequency eddies. (c) Interannual variability: Wintertime eddy kinetic energy is seen to extend further eastward through the Pacific Ocean during the warm phase but displays an abrupt termination during the cold phase. Anomalies in the eddy transient forcing tend to be quite similar to that of the Pacific‐North American pattern itself. The extension of the storm track during the warm phase resembles that of fall conditions and is present in the winter season because the source of low-level baroclinicity

Bifurcation in Eddy Life Cycles: Implications for Storm Track Variability

By analyzing a number of very high resolution, nonhydrostatic experiments of baroclinic lifecycles, it was concluded that the intensity of the near-surface baroclinic development influences the upper-level wave to such an extent that it could produce cyclonic or anticyclonic wave breaking. Since the final jet position is equatorward or poleward, the position depends on whether the waves break cyclonically or anticyclonically, respectively. The low-level baroclinicity plays a very important role in the outcome of the wave and feedback to the mean circulation. Using a shallow water model the hypothesis that the intensity of the eddy forcing from the lower layers of the atmosphere can have a profound effect on the disturbances of the upper layers is tested. From these experiments the following is concluded. For weak intensities, the strong effective beta asymmetries due to the earth’s sphericity produce anticyclonic wave breaking and a poleward shift of the zonal jet will occur. For moderate forcing, anticyclonic wave breaking occurs and consequently, as before, a poleward shift of the zonal jet will occur. However, there is an important distinction between weak and moderate forcing. In the latter case, the eddy anticyclonic centers are very intense. The influence of the two anticyclones produces a difluence field that will strain the cyclonic vortex along the SW‐NE direction. Consequently, the meridional vorticity flux y9z9 is positive in the north and negative in the south. This process has two effects: thinning the cyclone and producing positive vorticity fluxes on the north, negative fluxes on the south and moving the jet poleward. By increasing the forcing, the cyclone centers become considerably more intense than the anticyclones (CVC) and they are able to deform and thin the anticyclones, thus moving the jet equatorward. This transition is very abrupt; above a threshold amplitude, the life cycle bifurcates to a cyclonic wave breaking. The implications for storm track variability are quite direct. In normal years, at the entrance of the storm track, intense baroclinicity produces CVCs with a slight shift of the jet equatorward. At the last half of the storm track, due to much weaker baroclinicity, anticyclonic wave breaking occurs (AVCs) displacing the jet poleward. The eddies at the entrance of the storm track develop from the baroclinicity of the subtropical jet. Downstream fluxing and weaker surface baroclinicity make the upper-level waves more aloft and barotropic by the middle of the storm track. These waves normally break anticyclonically, enhancing the subpolar eddy-driven jet. In the warm phase of ENSO, more baroclinicity (and subtropical moisture flux) is present in the eastern Pacific Ocean. This enhanced baroclinicity could support more CVCs in the eastern basin, maintaining the subtropical jet further east.

The Quasi-Hydrostatic Approximation

Abstract Second-order expansion of the aspect ratio gives rise to simple equations with a quasi-hydrostatic approximation that perform far better than the classical hydrostatic system in the simulation of moist convection in a mesoscale model. It also suggests that a simple modification to this system may extend the validity of schemes for aspect ratios larger than 1.

On Energy Flux and Group Velocity of Waves in Baroclinic Flows

Abstract A modified energy flux is defined by adding a nondivergent term that involves β to the traditional energy flux. The resultant flux, when normalized by the total eddy energy, is exactly equal to the group velocity of Rossby waves on a β plane with constant zonal flow. In this study, we computed the normalized energy flux for linear wave packets in baroclinic basic states with different vertical profiles. The results show that the normalized energy flux is a good approximation to the group velocity of all parts of the wave packet for the basic states examined. The extension to the nonlinear case is briefly discussed. The magnitude of the fluxes of a downstream developing wave group over the wintertime northern Pacific storm track defined by a regression analysis is computed, and the group velocity defined by the energy fluxes is found to be comparable to the group velocity of propagation of the observed wave packet. The results indicate a very strong component of downstream energy radiation, sugges...

The Evolution of an Observed Cold Front. Part I. Numerical Simulation

Abstract The 48 h evolution of an observed cold front is simulated by a three-dimensional mesoscale-numerical model for a typical springtime synoptic situation over the southeastern United States. The model used in this study employs anelastic equations of motion on a limited-area domain with locally determined inflow/outflow side boundaries. Both the observed and simulated characteristics of the weather system indicate a mature front which intensifies and then weakens over the 48 h period. Moist convection occurs in the form of intermittent squall lines in the observed case; in the numerical simulation, convection develops above and somewhat ahead of the surface front after 24 h as in ensemble of convective cells. An investigation is made of the mesoscale and subsynoptic-scale features of this solution to determine their sensitivity to the inclusion of moisture and to the magnitude of the eddy viscosity used in the numerical simulation. The primary effect of increased eddy viscosity is to reduce somewhat...