J. G. Esler

A New Look at Stratospheric Sudden Warmings. Part III: Polar Vortex Evolution and Vertical Structure

The evolution of the Arctic polar vortex during observed major midwinter stratospheric sudden warmings (SSWs) is investigated for the period 1957‐2002, using 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) Ertel’s potential vorticity (PV) and temperature fields. Timelag composites of vertically weighted PV, calculated relative to the SSW onset time, are derived for both vortex-displacement SSWs and vortex-splitting SSWs, by averaging over the 15 recorded displacement and 13 splitting events. The evolving vertical structure of the polar vortex during a typical SSW of each type is clearly illustrated by plotting an isosurface of the composite PV field, and is shown to be very close to that observed during representative individual events. Results are verified by comparison with an elliptical diagnostic vortex moment technique. For both types of SSW, little variation is found between individual events in the orientation of the developing vortex relative to the underlying topography; that is, the location of the vortex during SSWs of each type is largely fixed in relation to the earth’s surface. During each type of SSW, the vortex is found to have a distinctive vertical structure. Vortex-splitting events are typically barotropic, with the vortex split occurring near simultaneously over a large altitude range (20‐40 km). In the majority of cases, of the two daughter vortices formed, it is the ‘‘Siberian’’ vortex that dominates over its ‘‘Canadian’’ counterpart. In contrast, displacement events are characterized by a very clear baroclinic structure; the vortex tilts significantly westward with height, so that the top and bottom of the vortex are separated by nearly 1808 longitude before the upper vortex is sheared away and destroyed.

Transport and mixing of chemical air masses in idealized baroclinic life cycles

[1] The transport, mixing, and three-dimensional evolution of chemically distinct air masses within growing baroclinic waves are studied in idealized, high-resolution, life cycle experiments using suitably initialized passive tracers, contrasting the two well-known life cycle paradigms, distinguished by predominantly anticyclonic (LC1) or cyclonic (LC2) flow at upper levels. Stratosphere-troposphere exchange differs significantly between the two life cycles. Specifically, transport from the stratosphere into the troposphere is significantly larger for LC2 (typically by 50%), due to the presence of large and deep cyclonic vortices that create a wider surf zone than for LC1. In contrast, the transport of tropospheric air into the stratosphere is nearly identical between the two life cycles. The mass of boundary layer air uplifted into the free troposphere is similar for both life cycles, but much more is directly injected into the stratosphere in the case of LC1 (fourfold, approximately). However, the total mixing of boundary layer with stratospheric air is larger for LC2, owing to the presence of the deep cyclonic vortices that entrain and mix both boundary layer air from the surface and stratospheric air from the upper levels. For LC1, boundary layer and stratospheric air are brought together by smaller cyclonic structures that develop on the poleward side of the jet in the lower part of the middleworld, resulting in correspondingly weaker

Excitation of Transient Rossby Waves on the Stratospheric Polar Vortex and the Barotropic Sudden Warming

The excitation of Rossby waves on the edge of the stratospheric polar vortex, due to time-dependent topographic forcing, is studied analytically and numerically in a simple quasigeostrophic f-plane model. When the atmosphere is compressible, the linear response of the vortex is found to have two distinct components. The first is a spectrum of upward-propagating waves that are excited by forcing with temporal frequencies within a fixed “Charney–Drazin” range that depends on the angular velocity at the vortex edge and the vortex Burger number. The second component of the response is a barotropic mode, which is excited by forcing with a fixed temporal frequency outside the Charney–Drazin range. The relative magnitude of the two responses, in terms of total angular pseudomomentum, depends on the ratio of the horizontal scale of the forcing to the Rossby radius. Under typical stratospheric conditions the barotropic response is found to dominate. Nonlinear simulations confirm that the linear results remain relevant to understanding the response in cases when strongly nonlinear Rossby wave breaking ensues. It is shown that a sudden warming, or rapid increase in vortex angular pseudomomentum, can be generated at much lower forcing amplitudes when the barotropic mode is resonantly excited compared to when the upwardpropagating waves are excited. A numerical simulation of a “barotropic sudden warming” due to excitation of the barotropic mode by a relatively weak topographic forcing is described.

Stratospheric Sudden Warmings as Self-Tuning Resonances. Part I: Vortex Splitting Events

AbstractThe fundamental dynamics of “vortex splitting” stratospheric sudden warmings (SSWs), which are known to be predominantly barotropic in nature, are reexamined using an idealized single-layer f-plane model of the polar vortex. The aim is to elucidate the conditions under which a stationary topographic forcing causes the model vortex to split, and to express the splitting condition as a function of the model parameters determining the topography and circulation.For a specified topographic forcing profile the model behavior is governed by two nondimensional parameters: the topographic forcing height M and a surf-zone potential vorticity parameter Ω. For relatively low M, vortex splits similar to observed SSWs occur only for a narrow range of Ω values. Further, a bifurcation in parameter space is observed: a small change in Ω (or M) beyond a critical value can lead to an abrupt transition between a state with low-amplitude vortex Rossby waves and a sudden vortex split. The model behavior can be fully u...

Baroclinic Wave Breaking and the Internal Variability of the Tropospheric Circulation

Abstract A simple model of the tropospheric circulation, based on a 10-level primitive equation model, is forced by linearly relaxing the potential temperature toward an idealized, zonally symmetric equilibrium field. The model equations are integrated in time until a statistically steady state is obtained. The local relationship between the state of the background flow, the direction of wave propagation, and subsequent wave breaking at the tropopause level is then investigated. Maps of potential vorticity (PV) on isentropic surfaces are analyzed and all four different types of wave breaking described recently by Peters and Waugh are shown to occur. It is found that cyclonic wave breaking events are usually initiated by poleward fluxes of wave activity, and anticyclonic events by equatorward fluxes. Composites are then used to show that equatorward fluxes are associated with a jet that is locally broad and weak, with relatively strong isentropic PV gradients to its equatorward flank. By contrast, poleward...

A method for estimating the extent of denitrification of arctic polar vortex air from tracer‐tracer scatter plots

Denitrification occurs through the sedimentation of HNO3bearing polar stratospheric cloud (PSC) particles, and these form at low temperatures during winter in the Arctic region. Significant denitrification leads to an increase in the lifetime of ozone-depleting chlorine species (ClOx), and therefore leads to an increase in the duration and extent of springtime Arctic ozone loss. It has been suggested that slight cooling in the Arctic due to climate change could lead to enhanced denitrification in the future, and this could lead to larger seasonal ozone depletion, despite the projected decline in inorganic chlorine [Waibel et al., 1999; Tabazadeh et al.,

Mechanisms for Wave Packet Formation and Maintenance in a Quasigeostrophic Two-Layer Model

Abstract A quasigeostrophic, two-layer, β-plane channel model is used to investigate the dynamics of baroclinic wave packets. A series of experiments are performed in which an unstable flow is maintained by lower-level Ekman friction and radiative relaxation toward a temperature profile that corresponds to a broad parabolic upper-level jet. The final statistically steady state achieved in each experiment is found to depend on the magnitude of the hyperdiffusivity ν0 and the supercriticality, which is controlled by β. The most important qualitative difference in such states between experiments is found to be the degree to which a waveguide in the upper level is found to develop. The mechanism for this upper-level waveguide development is the mixing effect of the eddies at the flanks of the jet, which leads to a strong potential vorticity gradient at the center of the channel, with well-mixed regions to the north and south. Two distinct regimes with different qualitative behavior are observed and illustrate...

An integrated approach to mixing sensitivities in tropospheric chemistry: A basis for the parameterization of subgrid‐scale emissions for chemistry transport models

The net effect on the global atmosphere of a continuous isolated chemical source is considered under idealized conditions. A general framework is described that allows M i , the steady state global perturbation to the ith species due to the source, to be calculated. This is achieved by exploiting the fact that once the emissions are sufficiently dilute, far from the source, they decay with the timescales of the chemical eigenstates of the background atmosphere. Both M i and the level of excitation of the longer-lived eigenstates are shown to depend on the details of the mixing of emissions near the source. If the details of the dilution of the emissions plume are known, it is also shown that “equivalent emissions” can be calculated. Equivalent emissions are designed so that when diluted instantaneously into the background atmosphere they result in the same global perturbation to each species as the original slowly diluted plume. The framework is then applied to test the sensitivity to mixing of a simple O3-NO x -CO-HO x tropospheric chemistry system. M i is calculated for a NO-CO source of constant strength as the mixing scenario undergone by the emissions is varied. The global increase in O3 due to the source is found to increase with the rate at which emissions are mixed, whereas the global increase in CO is reduced. The equivalent emissions for each plume dilution mechanism are then calculated. In a simple plume box model it is shown that the equilibrium state obtained when the model is forced by emissions that are first diluted in entraining plumes can be reproduced in a standard box model (i.e., with instantaneous mixing of emissions) by the corresponding equivalent emissions. It is argued that the concept of equivalent emissions can be exploited straightforwardly to derive a parameterization of unresolved subgrid plumes in order to reduce systematic error in global models.

A pseudo-spectral algorithm and test cases for the numerical solution of the two-dimensional rotating Green-Naghdi shallow water equations

A pseudo-spectral algorithm is presented for the solution of the rotating Green-Naghdi shallow water equations in two spatial dimensions. The equations are first written in vorticity-divergence form, in order to exploit the fact that time-derivatives then appear implicitly in the divergence equation only. A nonlinear equation must then be solved at each time-step in order to determine the divergence tendency. The nonlinear equation is solved by means of a simultaneous iteration in spectral space to determine each Fourier component. The key to the rapid convergence of the iteration is the use of a good initial guess for the divergence tendency, which is obtained from polynomial extrapolation of the solution obtained at previous time-levels. The algorithm is therefore best suited to be used with a standard multi-step time-stepping scheme (e.g. leap-frog). Two test cases are presented to validate the algorithm for initial value problems on a square periodic domain. The first test is to verify cnoidal wave speeds in one-dimension against analytical results. The second test is to ensure that the Miles-Salmon potential vorticity is advected as a parcel-wise conserved tracer throughout the nonlinear evolution of a perturbed jet subject to shear instability. The algorithm is demonstrated to perform well in each test. The resulting numerical model is expected to be of use in identifying paradigmatic behavior in mesoscale flows in the atmosphere and ocean in which both vortical, nonlinear and dispersive effects are important.

Orographically generated nonlinear waves in rotating and non-rotating two-layer flow

This paper reports experimental observations of finite amplitude interfacial waves forced by a surface-mounted obstacle towed through a two-layer fluid both when the fluid is otherwise at rest and when the fluid is otherwise rotating as a solid body. The experimental apparatus is sufficiently wide so that sidewall effects are negligible even in near-critical flow when the towing speed is close to the interfacial long-wave speed and the transverse extent of the forced wavefield is large. The observations are modelled by a simple forced Benjamin–Davis–Acrivos equation and comparison between integrations of both linear and nonlinear problems shows the fundamental nonlinearity of the near-critical flow patterns. In both the experiments and integrations rotation strongly confines the wavefield to extend laterally over distances only of order of the Rossby radius and also introduces finite-amplitude sharply pointed lee waves in supercritical flow.