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Darryn W. Waugh

Abstract

High-resolution simulations of a polar vortex disturbed by quasi-topographic forcing are performed using the method of “contour surgery”, a numerical method for inviscid flows wherein arbitrarily steep vorticity gradients can be formed and wherein scales of motion can vary over an extensive range. Simulations are performed of inviscid, incompressible, barotropic motion on the hemisphere, the sphere, and the plane. Comparisons with a hemispherical pseudospectral simulation show that accurate contour surgery simulations can be performed using a moderate number of contours to represent each hemisphere. Intermodel comparisons show that, although the overall evolution of the flow is qualitatively similar, there are noticeable differences. There is a significant difference between spherical and hemispherical calculations but, surprisingly, a remarkable agreement between spherical and planar calculations when a spatially varying planetary vorticity is used in the planar calculations.

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Thando Ndarana and Darryn W. Waugh

Abstract

A 30-yr climatology of Rossby wave breaking (RWB) on the Southern Hemisphere (SH) tropopause is formed using 30 yr of reanalyses. Composite analysis of potential vorticity and meridional fluxes of wave activity show that RWB in the SH can be divided into two broad categories: anticyclonic and cyclonic events. While there is only weak asymmetry in the meridional direction and most events cannot be classified as equatorward or poleward in terms of the potential vorticity structure, the position and structure of the fluxes associated with equatorward breaking differs from those of poleward breaking. Anticyclonic breaking is more common than cyclonic breaking, except on the lower isentrope examined (320 K). There are marked differences in the seasonal variations of RWB on the two surfaces, with a winter minimum for RWB around 350 K but a summer minimum for RWB around 330 K. These seasonal variations are due to changes in the location of the tropospheric jets and dynamical tropopause. During winter the subtropical jet and tropopause at 350 K are collocated in the Australian–South Pacific Ocean region, resulting in a seasonal minimum in the 350-K RWB. During summer the polar front jet and 330-K tropopause are collocated over the Southern Atlantic and Indian Oceans, inhibiting RWB in this region.

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Dieter Peters and Darryn W. Waugh

Abstract

The characteristics of the poleward advection of upper-tropospheric air are investigated using meteorological analyses and idealized numerical models. Isentropic deformations of the tropopause during Northern Hemisphere winter are examined using maps of Ertel's potential vorticity together with contour advection calculations. Large poleward excursions of upper-tropospheric air are observed during Rossby wave breaking events. These “poleward” breaking events occur in regions of diffluence (over the eastern Atlantic Ocean-Europe region, and over the eastern Pacific Ocean-North America region), and the evolution of the tropospheric air depends on the local, meridional shear: in anticyclonic (or weak cyclonic) shear the tropospheric air tilts downstream, broadens, and wraps up anticyclonically, whereas in cyclonic shear the tropospheric air tilts upstream, thins, and is advected cyclonically. The role of ambient barotropic flow is further examined by considering the flow in two numerical models: a planar, equivalent-barotropic, contour dynamics model and a simplified general circulation model. In both models, the variation of the poleward wave breaking with the zonal and meridional shear is consistent with that in the analyses.

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Darryn W. Waugh and David G. Dritschel

Abstract

The three-dimensional structure of wave propagation and breaking on the edge of polar vortices is examined using a multilayer quasigeostrophic model, with piecewise constant potential vorticity (PV) in each layer. The linear propagation of waves up the edge of a vortex is found to be sensitive to vertical variations in the vortex structure, with reduced propagation if the PV or area of the vortex increases with height; this reduction is dramatic for a cylindrical vortex with increasing PV. The characteristics of the nonlinear evolution and wave breaking is examined using high-resolution contour dynamics simulations and is also found to be sensitive to the vertical structure of the vortex. The amplitude of the forcing required for wave breaking to occur is larger for baroclinic vortices (with PV or area increasing with height) than for barotropic vortices. For cylindrical vortices with PV increasing with height the variation of wave breaking with forcing amplitude is qualitatively different from that of a barotropic vortex. Wave breaking occurs in the upper layers for only a limited, intermediate range of forcing amplitudes: there is no wave breaking in upper layers for weak forcing and for large forcing there is only wave breaking at the bottom of the vortex (i.e., the wave breaking is more vertically confined than for a barotropic vortex). For vortices with both PV and area increasing with height there is again a regime with wave breaking in the upper layers for weak amplitude forcing. However, the characteristics of the filaments produced by the wave breaking in upper layers is different from that in the barotropic case, with the filaments rolling up into a series of small vortices.

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Darryn W. Waugh and R. Alan Plumb

Abstract

We present a trajectory technique, contour advection with surgery (CAS), for tracing the evolution of material contours in a specified (including observed) evolving flow. CAS uses the algorithms developed by Dritschel for contour dynamics/surgery to trace the evolution of specified contours. The contours are represented by a series of particles, which are advected by a specified, gridded, wind distribution. The resolution of the contours is preserved by continually adjusting the number of particles, and finescale features are produced that are not present in the input data (and cannot easily be generated using standard trajectory techniques). The reliability, and dependence on the spatial and temporal resolution of the wind field, of the CAS procedure is examined by comparisons with high-resolution numerical data (from contour dynamics calculations and from a general circulation model), and with routine stratospheric analyses. These comparisons show that the large-scale motions dominate the deformation field and that CAS can accurately reproduce small scales from low-resolution wind fields. The CAS technique therefore enables examination of atmospheric tracer transport at previously unattainable resolution.

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Ping-Ping Rong and Darryn W. Waugh

Abstract

The evolution of the polar vortex in a shallow-water model with time-independent topographic forcing and relaxation to a constant equilibrium state is investigated for a range of topographic forcing amplitudes. For small forcing amplitudes there are only weak disturbances on the edge of the polar vortex and the vortex area remains constant, whereas for large amplitudes there are cycles where the vortex breaks down and then reforms (and zonal winds vacillate between westerlies and easterlies). Analysis of the mass within potential vorticity (PV) contours shows that these vacillations are due to out-of-phase variations in the mass fluxes across PV contours due to the relaxation and to hyperdiffusion. During the strong vortex stages Rossby wave breaking produces a cascade of PV to small scales, and these small-scale features are eventually eliminated by hyperdiffusion. This causes a decrease in the mass within the high PV contours and ultimately the destruction of the vortex. In contrast, during stages with no vortex there are very weak PV gradients, weak Rossby wave activity, and little cascade of PV to small scales. The vortex, and PV gradients, are then reestablished by the mass fluxes due to the diabatic relaxation term. These results suggest that the horizontal PV structure may play an important role in the vortex breakdown and recovery in three-dimensional models and in the real stratosphere.

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Darryn W. Waugh and William J. Randel

Abstract

The climatological structure, and interannual variability, of the Arctic and Antarctic stratospheric polar vortices are examined by analysis of elliptical diagnostics applied to over 19 yr of potential vorticity data. The elliptical diagnostics define the area, center, elongation, and orientation of each vortex and are used to quantify their structure and evolution. The diagnostics offer a novel view of the well-known differences in the climatological structure of the polar vortices. Although both vortices form in autumn to early winter, the Arctic vortex has a shorter life span and breaks down over a month before the Antarctic vortex. There are substantial differences in the distortion of the vortices from zonal symmetry; the Arctic vortex is displaced farther off the pole and is more elongated than the Antarctic vortex. While there is a midwinter minimum in the distortion of the Antarctic vortex, the distortion of the Arctic vortex increases during its life cycle. There are also large differences in the interannual variability of the vortices: the variability of the Antarctic vortex is small except during the spring vortex breakdown, whereas the Arctic vortex is highly variable throughout its life cycle, particularly in late winter. The diagnostics also reveal features not apparent in previous studies. There are periods when there are large zonal shifts (westward then eastward) in the climatological locations of the vortices: early winter for the Arctic vortex, and late winter to spring for the Antarctic vortex. Also, there are two preferred longitudes of the center of the lower-stratospheric Arctic vortex in early winter, and the vortex may move rapidly from one to the other. In the middle and upper stratosphere large displacements off the pole and large elongation of the vortex are both associated with a small vortex area, but there is very little correlation between displacement off the pole and elongation of the vortex.

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Darryn W. Waugh and Beatriz M. Funatsu

Abstract

The evolution and structure of stratospheric intrusions into the upper troposphere (UT) over the northern tropical Pacific is examined in terms of both potential vorticity (PV) and ozone (O3). Analysis of 20 years of NCEP–NCAR reanalysis PV shows that the intrusion events have remarkably similar evolution and structure at 350 K, with all events producing narrow tongues of high PV that have an almost north–south orientation and last around 3 days. Nearly all events extend up into the lower stratosphere, but only for a small percentage is there deep downward penetration. The intrusions explain a large amount of the observed variability in upper tropospheric O3 above Hilo, Hawaii, with large values occurring when a tongue of high PV passes over Hilo and low values when Hilo is just upstream of a high-PV tongue. There is also an increase in total column ozone within the PV tongues, but for most intrusions the increase is relatively small. The relationship between deep convection, as diagnosed by satellite observations of outgoing longwave radiation (OLR), and intrusions is also examined. It is shown that transient convection and intrusions in the central and eastern northern Pacific nearly always occur together, with the convection at the leading edge of the PV tongue. This confirms the results of previous studies that have shown a close link between Rossby wave activity and transient convection, and supports the hypothesis that the ascent and reduced static stability due to anomalous PV in the UT initiates and supports the convection.

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Beatriz M. Funatsu and Darryn W. Waugh

Abstract

The connections between intrusions of stratospheric air into the upper troposphere and deep convection in the tropical eastern Pacific are examined using a combination of data analysis, potential vorticity (PV) inversion, and numerical simulations. Analysis of NCEP–NCAR reanalyses and satellite measurements of outgoing longwave radiation during intrusion events shows increased cloudiness, lower static stability, upward motion, and a buildup of convective available potential energy (CAPE) at the leading edge of the intruding tongue of high PV. Potential inversion inversion calculations show that the upper-level PV makes the dominant contribution to the changes in the quantities that characterize convection. This supports the hypothesis that upper-level PV anomalies initiate and support convection by destabilizing the lower troposphere and causing upward motion ahead on the PV tongue. The dominant role of the upper-level PV is confirmed by simulations using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Convection only occurs when the upper-level PV anomaly is present in the simulations, and the relative contribution of the upper-level PV to changes in the quantities that characterize convection is similar to that inferred from the PV inversion calculations.

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William J. M. Seviour, Darryn W. Waugh, and Richard K. Scott

Abstract

The Martian polar atmosphere is known to have a persistent local minimum in potential vorticity (PV) near the winter pole, with a region of high PV encircling it. This finding is surprising, since an isolated band of PV is barotropically unstable, a result going back to Rayleigh. Here the stability of a Mars-like annular vortex is investigated using numerical integrations of the rotating shallow-water equations. The mode of instability and its growth rate is shown to depend upon the latitude and width of the annulus. By introducing thermal relaxation toward an annular equilibrium profile with a time scale similar to that of the instability, a persistent annular vortex with similar characteristics as that observed in the Martian atmosphere can be simulated. This time scale, typically 0.5–2 sols, is similar to radiative relaxation time scales for Mars’s polar atmosphere. The persistence of an annular vortex is also shown to be robust to topographic forcing, as long as it is below a certain amplitude. It is therefore proposed that the persistence of this barotropically unstable annular vortex is permitted owing to the combination of short radiative relaxation time scales and relatively weak topographic forcing in the Martian polar atmosphere.

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