Ozone laminae inside the Antarctic vortex ... - Wiley Online Library

Q. J. R. Meteorol. Soc. (2003), 129, pp. 3121–3136

doi: 10.1256/qj.03.19

Ozone laminae inside the Antarctic vortex produced by poleward laments By M. MOUSTAOUI1 ¤ , H. TEITELBAUM2 and F. P. J. VALERO1 Institution of Oceanography, University of California, San Diego, California, USA 2 Laboratoire de M´ et´eorologie Dynamique, Ecole Normale Superieure, Paris, France

1 Scripps

(Received 27 January 2003; revised 5 May 2003)

S UMMA RY Ozone soundings made inside the Antarctic ozone hole exhibit localized ozone increases within thin layers in the lower stratosphere. These structures, called laminae, are explained as poleward laments emerging from the vortex edge and transporting relatively ozone-rich air into levels above the geographical site where the sounding equipment has been launched. It is shown that the interior of the Antarctic vortex is not completely isolated with respect to the poleward air mixing. Filament development follows the occurrence of inward breaking near the vortex edge in a region where isentropes are locally uplifted. It is shown that the circulation associated with such uplift may produce the poleward breaking. The role of that circulation is supported by simple barotropic simulations where development of a poleward lament is reproduced in a realistic circular undisturbed vortex. K EYWORDS: Isentropic uplift Ozone hole Wave breaking

1.

I NTRODUCT ION

Since the discovery of the ozone hole (Farman et al. 1985), many authors have investigated the possibility of the decrease of ozone at lower latitudes through dilution of ozone-poor air originating from regions inside the stratospheric polar vortex. Such dilutions occur mostly in late spring when the vortex breaks up during the nal warming, but some may also occur at earlier stages, depending on the permeability of the vortex edge. Modelling studies of a disturbed vortex (Juckes and McIntyre 1987; Juckes 1989; Waugh 1993; Norton 1994) have shown that the polar vortex remains highly impermeable, except when planetary wave breaking events expel air and transport it equatorward into the surf zone (McIntyre and Palmer 1983). Waugh et al. (1994) showed that the main mechanism inducing the equatorward transport is vortex-edge erosion accompanied by equatorward lament formation. Intrusions of middle-latitude air into the polar vortex are less frequent. However, observations in the northern hemisphere have shown that they may take place (Plumb et al. 1994). Dilution of air from the polar vortex into middle latitudes and intrusion of middle latitude air into the vortex are often associated with breaking Rossby-waves. Wave breaking is a conspicuous phenomenon and probably one of the most important dynamical processes affecting stratospheric dynamics (McIntyre and Palmer 1983). Previous numerical investigations of polar vortex dynamics have shown that the vortex edge is more isolated with respect to poleward than to equatorward mixing of air (Juckes and McIntyre 1987; Polvani and Plumb 1992). Nakamura and Plumb (1994) attributed this asymmetry to the structure of the ow in the circular jet surrounding the polar vortex, which, under most circumstances, is such that waves break uniquely outward. They showed circumstances in which inward wave-breaking occurred, but to do so had to use a circular vortex with an unrealistic ow. To simulate poleward breaking in a realistic ow, Peters and Waugh (1996) considered the evolution of Rossby waves in the upper troposphere, in a jet with a ow that underwent azimuthal variations. ¤

Corresponding author: Center for Atmospheric Sciences, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, MC 0242, La Jolla, CA 92093-0242, USA. e-mail: [email protected] c Royal Meteorological Society, 2003. °

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They separated the poleward breaking into two different types P1 and P2, where the breaking develops in cyclonic and anticyclonic shear respectively. The observational focus of the present paper will be on ozone laminae observed in the interior of the Antarctic polar vortex. Many previous studies have reported the existence, in ozone pro les, of enhanced, and depleted, thin laminae of ozone. Most frequently, they are observed in high northern latitudes during winter and spring (Dobson 1973; Reid and Vaughan 1991; Reid et al. 1993). Most of the laminae have been successfully attributed to transport of air from different origins (e.g., Reid and Vaughan 1991; Reid et al. 1993; Orsolini 1995; Bird et al. 1997; Mariotti et al. 1997; Manney et al. 1998). These authors have primarily investigated laminae in the northern hemisphere, attributing their formation to equatorward laments expelled from the polar vortex. In the present paper we shall present two different instances in which ozone pro les exhibit laminae inside the Antarctic vortex. These laminae are characterized by localized increases in ozone even inside the ozone hole. We shall show that the interior of the Antarctic vortex, at least in the lower stratosphere, is not completely isolated with respect to poleward mixing. We shall show that it is possible that the observed laminae may be caused by poleward laments emerging from the vortex edge, and transporting relatively ozone-rich air into regions well inside the vortex. We shall also show that the development of those laments is induced by localized ow up sloping isentropic surfaces. Furthermore, we shall present simple numerical simulations showing that the circulation associated with such uplifts may produce poleward breaking in a realistic circular undisturbed vortex. In section 2, the two examples of laminae inside the ozone hole are presented, and the role of poleward laments is examined. Section 3 presents numerical simulations reproducing these laments. Finally, conclusions are given in section 4. 2.

R ESULTS FROM OBSERVATIONS

(a) Data and methods The ozone pro les used in this study were obtained from soundings launched from a site at Neumayer (70.7B S, 8.2B W) in the Antarctic. Upper air soundings are carried out routinely once a day between 10 and 12 UTC. They include measurements of pressure, temperature, relative humidity and wind vector. Since March 1992, ozone pro les have been measured about once a week. Usually the pro les start at 2 m above ground (32–42 m above mean sea level), reach altitudes between 15 and 37 km, and have a vertical resolution of about 50 m. Measurements are made by an ozone sensor (VAISALA ECC5A/6A) connected to the radiosonde. During winter and spring, when the Antarctic vortex is well organized, the air above Neumayer is very often located inside the polar vortex, and far from its edge. In general, because of the ozone hole, ozone pro les over Neumayer show decreases in the lower stratosphere. This is not always so, however, and in several instances we found thin layers (laminae), 2–3 km deep, in which the ozone increased and then decreased. To nd the origins of these laminae, and to analyse the detailed structures where laments are resolved, ‘contour advection with surgery’ (CAS) calculations were performed (Norton 1994; Waugh and Plumb 1994). These calculations are initialized with Ertel’s potential vorticity (EPV) contours; then these contours, treated as an idealized tracer, are advected for several days on isentropic surfaces. The elds obtained from CAS calculations will be referred to in this paper as ‘reconstructed EPV’. However, it may be noted that these elds are not strictly EPV elds because the CAS calculations

OZONE-RICH FILAMENTS IN ANTARCTIC STRATOSPHERE

Figure 1.

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Pro le of ozone pressure (mPa), as a function of potential temperature (K) above Neumayer (70.7 B S, 8.2 B W), 23 September 1998 (thick curve) and 30 September 1998 (thin curve).

include no diabatic effects. Nevertheless, these elds are good representations of EPV because the adiabatic assumption is commonly thought to be a good approximation, particularly for advection over periods of less than a week. The wind elds used for the advection were from the reanalysis of the National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/NCAR). Plumb et al. (1994) compared CAS results using for advection analysed winds from the European Centre for Medium-Range Forecasts (ECMWF) and from the US National Meteorological Center (NMC). They found that CAS results are not very sensitive to the source of the analyses. The NCEP data are six-hourly at 17 isobaric levels with horizontal resolution of 2.5 degrees in both latitude and longitude. Some analyses from the ECMWF were also used to analyse atmospheric conditions where laments evolve. The ECMWF data have higher resolution and are at 15 pressure levels. (b) 23 September 1998 Figure 1 (thicker curve) shows the ozone pro le measured over Neumayer on 23 September 1998. There is a distinct ozone-maximum (lamina) around 445 K. In some previous studies (e.g., by Teitelbaum et al. (1996)), such laminae have been explained by vertical transport induced by waves. However, the pro le shown in Fig. 1 is unlikely to be produced by this mechanism because the lamina is localized, and not correlated with the potential temperature. During the pro le measurement, Neumayer was under the interior of the Antarctic polar vortex. In September, the Antarctic vortex is characterized by strong reductions in ozone. An indication of this may be seen in the low absolute values above and below the maximum in the ozone pro le for 23 September 1998 (thicker curve) shown in Fig. 1. In contrast to this, the pro le measured a week later, on 30 September 1998, at Neumayer (the thinner curve in Fig. 1) shows small ozone values

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between 420 K and 520 K, with no maximum. Ozone laminae have been observed inside the Arctic vortex by Bird et al. (1997) and Manney et al. (1998). However, so far as we know, ozone laminae inside the Antarctic ozone hole have not been reported previously. Figure 2 shows EPV contours from NCEP used to initialize CAS calculations on 16 September 1998, at 445 K (Fig. 2(a)), and ‘reconstructed EPV’ elds by the CAS method for each 6 hours in the period between 12 UTC 22 September and 12 UTC 23 September 1998 (Figs. 2(b) to (f)). Although the winds used for advection have a relatively low resolution, the CAS calculations are able to reconstruct elds at much ner scales. Furthermore, the CAS results are less sensitive to spatial than to temporal resolution (Methven and Hoskins 1999). The resolution used here was suf cient to achieve accurate results. At 12 UTC 22 September 1998 (Fig. 2(b)), the structure of the reconstructed EPV distribution is characteristic of a poleward Rossby-wave-breaking event taking place near the vortex edge in regions between 300B E and 330B E (going clockwise round the south pole from the Greenwich meridian, so that Neumayer is at longitude 352B E). The behaviour of the reconstructed contours and the evolution that follows (Figs. 2(c) to (f)) indicate that the breaking event does not produce a large intrusion, in that the outer boundary (OB, the equatormost located contour) of the vortex edge is not displaced toward the pole. Peters and Waugh (1996) showed an example in the northern hemisphere upper-troposphere (their Fig. 16) where only inner contours experience distortion to form laments. As time evolves, the breaking leads to a lament that emerges from the vortex edge and extends to regions located inside the vortex. The lament connection to the vortex edge experiences a clockwise rotation, while the lament becomes thinner and tilts upwind. At 12 UTC 23 September 1998 (Fig. 2(f)), the lament covers regions around Neumayer. This lament transports air which is relatively ozone-rich and produces the lamina observed at 445 K in the ozone pro le shown in Fig. 1. Let us now examine the situation at 12 UTC 22 September 1998. Poleward distortions of the inner reconstructed EPV contours (Fig. 2(b)) show wave breaking, but the lament is not yet well developed. The eld of geopotential height of the 445 K surface (Fig. 3(a)), based on ECMWF analyses, shows that the isentropic surface was raised in the region of the wave breaking. Teitelbaum et al. (1998) showed a similar situation, but then a tongue of EPV was expelled from the OB of the vortex edge and transported towards the equator. Also based on ECMWF analyses for 22 September, Fig. 3(b) shows wind vector and EPV for the 315 K isentropic surface (near the tropopause). We note that an anticyclonic maximum of ¡3:5 potential vorticity units (PVU: 1 PVU D 10¡6 m2 K kg¡1 / near the tropopause lay beneath the highest part of the 445 K isentropic surface (Fig. 3(a)). Hoskins et al. (1985) have demonstrated that such anomalies near the tropopause may produce an upward displacement of isentropes above them. Figure 3(c) shows the corresponding EPV distribution at 445 K. In the region where the isentropic surface is highest, the vortex edge is dilated, so that the EPV contours at the OB of the vortex edge are displaced equatorwards and those marking the inner boundary (IB) are displaced polewards. The distortion of the EPV contours exhibits an upwind tilting as a consequence of a cyclonic shear. We note that the structure of the EPV eld at 445 K (Fig. 3(c)) is quite different from that found at 315 K (Fig. 3(b)). At both levels, the inward distortion of EPV contours is produced by wave breaking. However, at 445 K the breaking involves only inner contours (Fig. 2(b)), whereas at the 315 K level the whole edge of the stratospheric air is displaced towards the pole. In the next example, in subsection (c), we shall show that the circulation associated with the isentropic uplift may explain the occurrence and the location of the inward lament.


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Figure 2. Contours of Ertel’s potential vorticity (EPV) (1 PVU D 10¡6 m2 K kg¡1 / at the 445 K isentropic surface in September 1998: (a) from the US National Center for Environmental Prediction (NCEP) used for initialization on 16th, and (reconstructed from contour advection with surgery (CAS)) for (b) 12 UTC 22nd; (c) 18 UTC 22nd; (d) 00 UTC 23rd; (e) 06 UTC 23rd and (f) 12 UTC 23rd. The asterisk on each map denotes the location of Neumayer.

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Figure 3. Fields derived from ECMWF analyses for 12 UTC 22 September 1998: (a) geopotential height (dam) of the 445 K isentropic surface (contour interval 30 dam); (b) wind vectors on the 315 K isentropic surface (the thick and thin curves are EPV contours at 315 K for ¡3:5 PVU and ¡2:5 PVU respectively), and (c) Ertel’s potential vorticity (EPV) (1 PVU D 10¡6 m2 K kg¡1 / at the 445 K isentropic surface (contour interval 5 PVU). The upright crosses denote the location of Neumayer.

(c) 28 September 1997 Figure 4 shows the ozone pro le obtained from measurements over Neumayer almost a year earlier, on 28 September 1997 (thick curve). As it did before (see subsection (b)), the pro le exhibits an ozone maximum, but this time at a lower isentropic level (420 K rather than 445 K). The pro le measured twelve days later, on 10 October 1997, is superimposed on Fig. 4 (thin curve), to emphasize the dramatic difference of ozone concentration at 420 K on 28 September 1997 from normal values at that level (as did the pro le for 30 September 1998 in Fig. 1). Figures 5(a) to 5(f) show the results from CAS calculations at 420 K, at 12 UTC each day in the six-day period ending on 28 September 1997. The CAS calculations were initialized on 21 September 1997. On 23 September (Fig. 5(a)), the vortex edge is


Figure 4.

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Pro le of ozone pressure (mPa) as a function of potential temperature (K) above Neumayer. Thick curve: 22 September 1997, showing ozone maximum at 420 K. Thin curve: 10 October 1997.

displaced equatorwards between 270B E and 330B E. Its outer part then rotates, cyclonically, faster than its inner part (Figs. 5(b) and (c)), and a wave-breaking event begins to develop 25 and 26 September (Figs. 5(c) and (d)). A poleward lament tilting upwind develops thereafter, and extends well inside the Antarctic polar vortex 27 and 28 September (Figs. 5(e) and (f)). This lament originates at the vortex edge and transports relatively ozone-rich air well into the interior of the ozone hole. The lament passes over Neumayer on 28 September (Fig. 5(f)), and results in the ozone increase at 420 K shown in Fig. 4. The CAS calculations at 480 K (not shown) indicate that Neumayer was well inside the Antarctic vortex, with no lament covering it. This is consistent with the relatively normal concentrations of ozone observed at that level (Fig. 4). Figure 6(a) shows the geopotential height (from ECMWF) of the 420 K surface, at 12 UTC 23 September 1997, before the breaking event. At that time, the CAS calculations show an equatorward distortion of the vortex edge in a region between 270B E and 330B E (Fig. 5(a)). As in the previous example, we note that the isentropic surface in that region is raised. We also note that, beneath it, the EPV contours show an anticyclonic anomaly near the tropopause at 315 K (Fig. 6(b)). Again, this anomaly is also associated with a poleward breaking taking place near the tropopause, and vortex edge dilation at 420 K (Fig. 6(c)). From a kinematics viewpoint, wave breaking is associated with deformations in the wind eld that distort different EPV-contours differently. Under the quasigeostrophic approximation, the stream function for the horizontal wind on isentropic surfaces is the Montgomery potential (M D cp T C Á/. Speci cally, a minimum in the eld of Montgomery potential indicates the presence of a cyclonic circulation. Figure 7 shows Montgomery-potential elds at 420 K for 23–28 September 1997. Two reconstructed EPV-contours from CAS calculations, ¡28 and ¡20 PVU, are superimposed. We note

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Figure 5. Results from ‘contour advection with surgery’ (CAS) calculations at 420 K and 12 UTC on consecutive September days in 1997: (a) 23rd; (b) 24th; (c) 25th; (d) 26th; (e) 27th; and (f) 28th. The calculations were initialized on 21 September 1997. The asterisk on each map denotes the location of Neumayer.


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Figure 6. Fields derived from ECMWF analyses for 12 UTC 23 September 1997: (a) geopotential height (dam) of the 420 K isentropic surface (contour interval 30 dam); (b) wind vectors on the 315 K isentropic surface (the thick and thin curves are EPV contours at 315 K for ¡3:5 PVU and ¡2:5 PVU respectively), and (c) Ertel’s potential vorticity (EPV) (1 PVU D 10¡6 m2 K kg¡1 / at the 420 K isentropic surface (contour interval 5 PVU). The upright crosses denote the location of Neumayer.

that on 23 September (Fig. 7(a)) the eld of Montgomery potential around 30B W, 75B S indicates the development of a localized cyclonic circulation. As expected, an EPV contour has a similar shape. During 24 and 25 September (Figs. 7(b) and (c)), the cyclonic circulation becomes stronger and displaced eastwards. On 26 September (Fig. 7(d)), the Montgomery-potential eld shows a minimum around 15B W, 75B S, indicating that in this region a localized cyclonic circulation is well developed. This circulation results in a distortion of the PV contours, and a lament emerges from the vortex edge. During the following days (27 and 28 September), the lament develops well inside the Antarctic vortex and tilts upwind (Figs. 7(e) and (f)). Peters and Waugh (1996) have investigated the dependence of the poleward breaking of the ambient ow eld, in the upper troposphere. Using a one-layer model

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Figure 7. Montgomery potential, from analyses of the US National Center for Environmental Prediction (NCEP), for 420 K for consecutive September days in 1997: (a) 23rd; (b) 24th; (c) 25th; (d) 26th; (e) 27th; and (f) 28th. Thick lines are contours from the ‘contour advection with surgery’ (CAS) calculations for ¡20 PVU and ¡28 PVU. Neumayer is located at 70.7 B S, 08.2B W.


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(equivalent-barotropic contour dynamics model), they found that waves propagating along a jet break poleward when they enter a region where the mean ow is weak. They also showed poleward breaking of Rossby waves in more realistic general circulation model (GCM) simulations. They attributed the dynamical origin of these waves to baroclinic instabilities. As a result of both their one-layer and GCM simulations, Peters and Waugh (1996) have explained the wave-breaking mechanism in terms of the structure of barotropic shear in the mean ow. The breaking event in our present study falls into their P1 breaking type. However, for the cases we report here, we attribute the poleward breaking in the polar stratospheric vortex to the ascent of stratospheric air as it moves on an upward-sloping isentropic surface near the vortex edge, associated with an anticyclonic anomaly beneath it near the level of the tropopause. 3.

N UMERICAL RESULTS

In this section, we shall present a modelling study showing that poleward laments similar to those simulated from observed velocity elds can also be obtained in a simple model with a forced cyclonic perturbation. (a) The model The model used is simple, and similar to the one used by Juckes and McIntyre (1987). Brie y, it is based on the nondivergent barotropic vorticity equation in the form dQ D ºr 6 .Q ¡ Q0 /; dt

(1)

where Q D 1Ã C f C F is the PV, Ã is the stream function and º is a hyper-diffusion introduced for model stability. The basic ow is a circular vortex where the pro le of the initial PV Q0 is a function of latitude only. In this one-layer model, it is not possible to introduce explicitly a localized isentropic ascent. Alternatively, a localized time-dependent forcing (F ) that produces a cyclonic circulation is prescribed to qualitatively imitate the observed cyclonic perturbation seen in Fig. 7. The initial PV and wind pro les chosen for the numerical experiment are represented by the solid curves shown in Figs. 8(a) and (b). Equation (1) is integrated on the sphere, using a spectral representation with the truncation T-106. The forcing is centred near the vortex edge around 0B E, 60B S. The latitudinal and zonal shapes of the forcing are shown in Fig. 8(c). The forcing has a smooth variation in time: it increases gradually from zero at day 0, reaches its maximum value at day 1.5, and, thereafter, decreases again to zero at day 3. (b) Results Figure 9 shows a sequence of the stream function simulated by the model for different times. Three simulated PV-contours (thick curves) are also superimposed on this gure. As the forcing increases, a localized cyclonic circulation develops and produces a poleward PV-advection to the east of the forcing (Figs. 9(a) to (d)). This advection involves primarily the inner PV-contour. On day 2 (Fig. 9(e)), the cyclonic circulation is more evident in the stream-function eld, which shows a localized maximum in the forcing region. The distortion of the inner PV-contour represents a wave-breaking event beginning to the east of that circulation. The connection of the inner contour to the vortex edge is advected eastward by the ow at the edge, while its tip undergoes a westward advection associated with the cyclonic circulation. This differential deformation results in a thin lament that extends inside the polar

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Figure 8. Meridional pro les used in simulations: (a) normalized potential vorticity (PV) (solid curve denotes weak vortex and dashed curve a strong vortex); (b) zonal wind (m s¡1 / (solid curve denotes weak vortex and dashed curve a strong vortex), and (c) shapes of forcing (solid curve denotes meridional). The dashed curve in panel (c) denotes zonal forcing at longitudes shown in the lowermost scale.

vortex and tilts upwind by day 2.5 (Fig. 9(f)). The behaviour of the stream function and the PV simulated by the simple barotropic model resembles, qualitatively, those seen in the observations (Fig. 7) (a cyclonic circulation corresponds to a minimum in the Montgomery potential). In a modelling study, Polvani and Plumb (1992) showed that, neglecting transients, the onset of breaking and lamentation in a steady ow coincides with the time when a PV contour of a disturbed vortex reaches a stagnation point in a co-rotating frame. They found that this could happen when the forcing amplitude is greater than some threshold. Nakamura and Plumb (1994) showed that breaking occurs when a forcing deforms a PV contour so that the contour reaches a critical point where the mean ow equals the wave-phase velocity. In accord with these workers, we found that poleward breaking does not occur for a small forcing. We also found that breaking depends on


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Figure 9. Stream-function (thin lines, contour interval 0:2 £ 108 m2 s¡1 , value of outer contour 0:4 £ 108 m2 s¡1 / and PV (thick lines, contour interval 0:14 £ 10¡4 s¡1 , value of outer contour ¡1:24 £ 10¡4 s¡1 / for a relatively weak vortex (de ned by the values shown by the solid curves in Figs. 8(a) and (b)) as the simulation proceeded: (a) day 0; (b) day 1; (c) day 1.25; (d) day 1.5; (e) day 2, and (f) day 2.5.

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the forcing’s location: the forcing used for the above results, for example, does not lead to poleward breaking when it is located just outside the vortex edge. The amplitude and location of the forcing used in our experiments were chosen from visual inspection of the observations. The breaking also depends on the time dependence of the forcing. At least for the earlier times inspected, breaking does not occur for slow temporal variations. Finally, an experiment was conducted (not shown) using the same amplitude of forcing as in the previous case, but with a stronger vortex. The initial pro les of PV and wind are superimposed in Fig. 8 (dashed curves). These pro les represent a polar vortex with a steeper PV-gradient and higher wind speed at its edge. The results show Rossby waves propagating along the vortex edge, but no production of poleward laments. Obviously, wave breaking may also occur for the stronger vortex if the amplitude of forcing is increased. The above results indicate, at least, that it is more probable for poleward breaking to occur for a weaker vortex, because a smaller-amplitude forcing is required. In the real stratosphere, the vortex edge becomes stronger with altitude, with steeper PV-gradients and stronger winds at its edge. As a consequence, poleward breaking and laments are likely to be relatively more frequent in the lower stratosphere. The laminated ozone maximum inside the ozone hole associated with such laments is then also expected to be limited to the lower stratosphere. The ozone pro les presented, as well as others not shown in this study, indicate that laminae do indeed tend to be observed at lower levels of the stratosphere. 4.

C ONCLUSION

Soundings performed inside the Antarctic vortex generally show very low values of ozone content in the stratosphere. This is not always so, however. In this paper, we have presented two ozone pro les showing thin layers (laminae) in the lower stratosphere containing ozone maxima, even though the launching site (Neumayer) was inside the Antarctic vortex. Ozone laminae have been observed in many previous studies, particularly outside the polar vortex in the northern hemisphere. However, so far as we know, laminae inside the Antarctic ozone hole have not been reported previously. In this study, we have shown that the interior of the Antarctic polar vortex is not completely isolated with respect to poleward mixing. Using CAS calculations, we have found that poleward laments emerging from the vortex edge and transporting ozone-rich air into levels above Neumayer are the causes of the observed laminae. The development of these laments follows Rossby waves breaking poleward near the vortex edge. The breaking is found in regions where isentropic surfaces slope upwards and the vortex edge is dilated. In one instance, we showed that the ascent of air moving in the sloping isentropic surface was produced by a severe poleward breaking that took place at the tropopause and resulted in an anticyclonic anomaly below the region of ascending air. In the other, we showed that the breaking was preceded by development of a localized cyclonic circulation near the vortex edge, in the uplifted region. To the east of this circulation, there were poleward distortions of EPV contours that broke poleward, and resulted in an upwind-oriented lament. This breaking falls into the P1 type described by Peters and Waugh (1996) for the northern hemisphere upper troposphere; although, the cyclonic shear that produced this tilting was associated in the present study with both the mean ow and the cyclonic perturbation. Using a simple barotropic model where a ‘realistic’ circular undisturbed vortex was forced near the vortex edge, we have shown that localized cyclonic circulations such as those found in observations may produce poleward laments. The structure of the ow and the onset of the breaking found in the simulations qualitatively resemble those


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noted from observations. In previous studies, it was found that waves propagating in a zonally varying basic state may break in regions where the basic wind is weak (Peters and Waugh 1996; Swanson et al. 1997; Swanson 2000). The basic state used for the simulations presented in this study, undergoes only meridional variations. Our results show that breaking may also occur in regions where the total wind decreases along the motion. Although the breakings shown in this study are similar to those presented by Peters and Waugh (1996), the origin of the perturbations producing them is different. Peters and Waugh (1996) partly emphasized poleward breaking in the upper troposphere of eddies that develop through baroclinic instability, whereas we have investigated breaking in the lower stratosphere polar vortex, caused by localized uplifts due to anticyclonic anomalies near the tropopause. Nevertheless, some barotropic aspects of the atmosphere turn out to be relevant to the development of breaking, both in our cases reported here and in those of Peters and Waugh (1996). Finally, other simulations were conducted by using a stronger undisturbed vortex, with stronger winds and PV gradients at its edge, while maintaining the same forcing. For such a vortex, no poleward breaking or lament emerged from the vortex edge; and for the breaking to occur, a larger amplitude forcing was required. In the real stratosphere, the polar vortex becomes stronger with altitude, in that, both PV gradients and the jet ow at its edge are stronger. The results shown here indicate that poleward laments tend to be con ned to the lower stratosphere. Consequently, localized ozone increases (laminae) inside the ozone hole may be expected to be relatively more frequent in the lower stratosphere. This is, at least, true for the ozone pro les shown in this study. However, despite the examples presented here, poleward laments are likely to be relatively uncommon in the stratosphere, and the importance of the inward ozone mixing for the ozone hole is questionable. Nevertheless, further observational studies are needed before any de nite conclusion about the importance of the poleward air mixing for the Antarctic vortex can be reached.

ACKNOW LEDGEMENTS

We acknowledge the ECMWF. NCEP reanalysis data were provided by the NOAA–CIRES Climate Diagnostics Center (CDC). Neumayer ozone soundings were provided by the World Ozone and Ultraviolet Radiation Data Center (WOUDC). This study was supported by NASA Grant NAS5-99120 to the Scripps Institution of Oceanography, University of California, San Diego, USA.

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1996

Comparison between vertical ozone sounding and reconstructed potential vorticity maps by contour advection with surgery. J. Geophys. Res., 102, 6131–6142 Breaking planetary waves in the stratosphere. Nature, 305, 593– 600 The advection of high-resolution tracers by low-resolution winds. J. Atmos. Sci., 56, 3262–3285 The effect of ow asymmetry on the direction of Rossby wave breaking. J. Atmos. Sci., 51, 2031–2045 Breaking Rossby waves in a model stratosphere diagnosed by a vortex-following coordinate system and a technique for advecting material contours. J. Atmos. Sci., 51, 654–673 On the formation of ozone laminae at the edge of the Arctic polar vortex. Q. J. R. Meteorol. Soc., 121, 1923–1941 In uence of barotropic shear on the poleward advection of upper tropospheric air. J. Atmos. Sci., 53, 3013–3031 Intrusions into the lower stratospheric Arctic vortex during the winter of 1991–1992. J. Geophys. Res., 99, 1089–1105

Plumb, R. A., Atkinson, R. J., 1994 Newman, P. A., Lait, L. R., Shoeberl, M. R., Browel, E. V., Simmons, A. J. and Loewenstein, A. J. Polvani, L. M. and Plumb, R. A. 1992 Reid, S. J. and Vaughan, G.

1991

Reid, S. J., Vaughan, G. and Kyro, E. Swanson, K. L.

1993

Swanson, K. L., Kushner, P. J. and Held, I. M. Teitelbaum, H., Moustaoui, M., Ovarlez, J. and Kelder, H. Teitelbaum, H., Moustaoui, M., Van Velthoven, P. F. J. and Kelder, H. Waugh, D. W.

1997

Waugh, D. W. and Plumb, R. A.

1994

Waugh, D. W., Plumb, R. A., Atkinson, R. J., Shoeberl, M. R., Lait, L. R., Newman, P. A., Loewenstein, M., Toohey, D. W., Avallone, L. M., Webster, C. R. and May, R. D

1994

2000

1996 1998 1993

Rossby wave breaking, lamentation and secondary vortex formation: the dynamics of a perturbed vortex. J. Atmos. Sci., 49, 462–476 Lamination in ozone pro les in the lower stratosphere. Q. J. R. Meteorol. Soc., 117, 825–844 Occurrence of ozone laminae near the boundary of the stratospheric polar vortex. J. Geophys. Res., 98, 8883–8890 Stationary wave accumulation and the generation of lowfrequency variability on zonally varying ows. J. Atmos. Sci., 57, 2262–2280 Dynamics of barotropic storm tracks. J. Atmos. Sci., 54, 791–810 The role of atmospheric waves in the laminated structure of ozone pro les at high latitude. Tellus, 48A, 442–455 Decrease of total ozone at low latitudes in the southern hemisphere by a combination of linear and nonlinear processes. Q. J. R. Meteorol. Soc., 124, 2625–2644 Contour surgery simulations of a forced polar vortex. J. Atmos. Sci., 50, 714–730 Contour advection with surgery: A technique for investigating ne scale structure in tracer transport. J. Atmos. Sci., 51, 530–540 Transport out of the lower stratospheric Arctic vortex by Rossbywave breaking. J. Geophys. Res., 99, 1071–1088