CO as an important high-altitude tracer of dynamics in the polar stratosphere and mesosphere

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd004099, 2004 Correction published 20 August 2004 CO as an important high-altitude tracer of dynamics in the polar stratosphere and mesosphere R. L. de Zafra Institute for Terrestrial and Planetary Atmospheres, State University of New York, Stony Brook, New York, USA G. Muscari Physics Department, University of Rome, La Sapienza, Rome, Italy Received 22 August 2003; revised 18 November 2003; accepted 16 January 2004; published 20 March [1] We present new ground-based measurements of polar stratospheric and mesospheric CO, made with a millimeter-wave spectrometer at Thule, Greenland (76.5 N, 68.7 W). Almost daily measurements were made between 17 January and 4 March 2002 and again between 5 January and 22 February We stress here the retrieval and analysis of CO mixing ratios in the km altitude range, though it can be monitored at lower altitudes as well. Since CO exhibits a strong positive latitude gradient from the summer to the winter pole, it is an excellent tracer for poleward transport from lower latitudes. Moreover, the mixing ratio of CO increases rapidly from 40 km to at least 100 km at midlatitudes, providing a good tracer for high-altitude vertical transport as well. Our profiles indicate that in winter near the poles the CO mixing ratio decreases above 70 km because of subsidence of air and minimal high-altitude photoproduction at high latitudes. Our data also show large variations in mixing ratio and vertical distribution, yielding a good picture of stratospheric and mesospheric dynamics-induced changes on a scale of hours to days. These observations verify that CO serves as an excellent tracer of vortex-related dynamics in the km altitude range, where other information, particularly above 40 km, may be sparse, unreliable, or nonexistent. Our results are in general agreement with analyses of Improved Stratospheric and Mesospheric Sounder (ISAMS) satellite data by Lopez-Valverde et al. [1993, 1996] and by Allen et al. [1999, 2000]. We show the contrast between CO over the summer pole and CO over the winter pole with the aid of trial observations made at the South Pole during the austral summer of Our Thule data indicate that large concentrations of CO generally exist in winter just outside the vortex boundary. The large rapid variations in vertical profile that are found in our data in 2002 appear to be well correlated with vortex position in the lower stratosphere. In 2003 this correlation appeared to be much weaker, but early 2003 was also a period of vortex splitting in the Arctic on three occasions during our observation period, accompanied by generally more complex vortex dynamics. INDEX TERMS: 0341 Atmospheric Composition and Structure: Middle atmosphere constituent transport and chemistry (3334); 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3349 Meteorology and Atmospheric Dynamics: Polar meteorology; 6969 Radio Science: Remote sensing; KEYWORDS: carbon monoxide, mesospheric transport, polar vortex Citation: de Zafra, R. L., and G. Muscari (2004), CO as an important high-altitude tracer of dynamics in the polar stratosphere and mesosphere, J. Geophys. Res., 109,, doi: /2003jd Introduction [2] Carbon monoxide is produced in the Earth s upper atmosphere by the photodissociation of CO 2 [Hays and Olivero, 1970]. Carbon monoxide is interesting within the family of stratospheric/mesospheric trace gases in that its mixing ratio (m.r.) at midlatitudes exhibits a steadily increase with altitude above 25 or 30 km. This increase Copyright 2004 by the American Geophysical Union /04/2003JD becomes quite rapid above 40 km and extends well into the thermosphere (>100 km) [Solomon et al., 1985]. The wide altitude range of distribution and the increase in m.r. into the thermosphere are a result of the large photodissociation energy of CO relative to most other stratospheric trace gases, as well as its high-altitude source. This same robustness against photodissociation has made CO a favorite observational molecule for interstellar molecular radio astronomy since the 1960s, and in fact, some unreported accidental discoveries of atmospheric CO seem to have been made by radio astronomers during early searches for 1of10

2 Figure 1. A sample of CO line shape changes observed between 24 and 28 January Actual data are shown. Intensity is given in the Rayleigh-Jeans (linear) limit of equivalent black-body radiation temperature. To enhance the visibility of the line shape changes, the spectra have been reduced from 1024 channels to a central span of 512 around the CO line center. The frequency width of a singe channel is 49 khz. See Figure 6 for another view of CO during this period. interstellar CO, before the first deliberate atmospheric measurements by Waters et al. [1976]. [3] Despite its early discovery in the middle atmosphere by millimeter-wave spectroscopy, the sum total of stratospheric-mesospheric observations of CO in the 25+ years between 1976 and 2003 is described in scarcely more than a dozen papers. During much of this time, attention was focused on middle to lower stratospheric photochemistry related to ozone, and CO plays no part in this. The paper by Solomon et al. [1985] remains as the definitive work describing global photochemistry and transport of CO in the middle atmosphere. In that work, a strong seasonal poleto-pole gradient was predicted, with the largest concentration of CO over the winter pole. Some observational evidence supporting strong latitudinal gradients was also given in that paper, and in earlier observations reported by Clancy et al. [1984]. Further evidence of large temporal variations in CO over a midlatitude observing location, tentatively supporting large latitudinal gradients accompanied by complex transport patterns, was given by Bevilacqua et al. [1985]. [4] Recently, interest in CO as a dynamic tracer has revived because of the availability of a near-global data set derived from the Improved Stratospheric and Mesospheric Sounder (ISAMS) on board the Upper Atmosphere Research Satellite (UARS), as well as the development of new satellite-borne instruments for CO measurement, recently launched and undergoing validation, or now awaiting launch. The ISAMS infrared spectrometer produced 180 days of CO measurements at 4.6 mm, spread over about 10 months, before the instrument failed in July of Analyses of subsets of these data have been given by Allen et al. [1999, 2000], in addition to earlier reports of initial results by Lopez-Valverde et al. [1993] and detailed validation studies [Lopez-Valverde et al., 1996]. Allen et al. [1999] used CO observations to examine the synoptic evolution of the Arctic vortex of 1992, and details of mixing at the vortex boundary. Allen et al. [2000] applied ISAMS observations of CO to trace descent within the Antarctic vortex at different latitudes, as well as transport due to wave activity around the vortex. These studies placed major emphasis on the km altitude range. [5] Several difficulties are cited in analysis of the infrared measurements [see, e.g., Lopez-Valverde et al., 1996]: contaminant emission from other atmospheric constituents, weak CO emission close to the edge of the Earth s atmospheric thermal emission, and lack of local thermodynamic equilibrium in the mesosphere for the transitions observed. In contrast, the millimeter-wave pure rotational spectrum of CO is very simple, exhibiting single emission lines starting at 115 GHz (J = 1! 0) and spaced every 115 GHz above that. We have chosen to observe the 230 GHz transition (l = 1.3 mm). The rotational state populations are in thermal equilibrium in the middle atmosphere, there is essentially no interfering overlap from emission lines of other species, and the emission intensity is relatively strong, thus avoiding the problems noted above which complicate quantitative spectroscopy of CO in the near I.R. For the observations reported here, we usually took observations near local noon, although there is no diurnally induced chemical change in the CO mixing ratio throughout the middle atmosphere to at least 100 km in the polar winter and early spring [Solomon et al., 1985] when our observations were taken at Thule. All observed variations in our data over daily to weekly spans in time are therefore driven by horizontal and vertical transport, though these changes may be followed by slower relaxations to local chemical equilibrium. 2. Instrument and Observing Technique [6] The ground-based millimeter-wave spectrometer (GBMS) used for our observations employs a superconducting tunnel junction as a heterodyne detector. A detailed discussion of our observing technique and a description of the instrumental components is given by de Zafra [1995], although in the present case no image side band filter was used. An acousto-optical back-end spectrometer was employed, covering 50 MHz in 1024 readout channels at a nominal 49 khz each. The actual resolution (half width at half maximum) of the narrow-band unit has been measured at 65 khz using a laboratory line source. Tests show that this spectral coverage and resolution allows CO mixing ratio profiles to be retrieved from the observed pressure-broadened line shapes over an altitude range of about 30 to 80 km, where other forms of data giving dynamical insights become increasingly sparse or absent with increasing altitude. [7] Observations were made via sequential 15 min integrations. Corrections for any changes in opacity are made within each 15 min block. Integrations were typically run for a total of 1.5 to 2 hours, depending on tropospheric opacity. This usually resulted in a peak signal/noise ratio well in excess of 100:1, as indicated in Figure 1. At 230 GHz, atmospheric opacity is almost entirely determined by tropospheric water vapor. On a few days when meteo- 2of10

3 Figure 2. The three a priori mixing ratio profiles used as inputs in the Chahine-Twomey line shape deconvolution process. rological forecasts indicated that the vortex edge was approaching or passing over Thule, a second observation was made about 12 hours after the first. Observations of O 3, HNO 3 and N 2 O were interspersed with CO observations and will be described separately with respect to observed correlations. Individual 15 min integrations were averaged together to give a final spectrum from which a vertical mixing ratio profile was retrieved for each day s observation. [8] In Figure 1 we show an example of changes that can take place in the spectral line shape and intensity over the course of a few days as the polar vortex shifts overhead. We have observed changes nearly as large as those shown in Figure 1 in as little as 12 hours. By deconvolving the observed pressure-broadened line shapes against daily pressure and temperature profiles for the atmosphere (see section 3) the mixing ratio profiles that give rise to the observed spectral line shapes can be retrieved, and related to transport of CO within and around the vortex region. 3. Retrieval of Mixing Ratios and Accuracy [9] We have used a modified Chahine-Twomey deconvolution technique to retrieve mixing ratio profiles from the observed pressure-broadened line shapes. Since appreciable CO mixing ratio values extend into the altitude range where Doppler and pressure broadenings are comparable, we have assumed a Voigt line shape. As with other deconvolution techniques, the output from a Chahine-Twomey retrieval can be influenced through the choice of input profile used to start the iterative line shape fitting process. This influence is altitude dependent, tending to be worst toward high altitudes where spectrometer resolution may be insufficient to resolve small amounts of pressure broadening (or also in the present case, where pressure broadening is less than or equal to Doppler broadening), and again toward the lowest practical altitude range, when pressure broadening begins to exceed total spectral bandwidth. In these extremes of high and low altitude, the observational data can exert little corrective influence on the trial starting profile, and retrieval errors are dominated by the latter. [10] The Chahine-Twomey technique, in contrast to the Rodgers optimal estimation method [e.g., Rodgers, 1990; Connor et al., 1995], does not lend itself to a direct means of assessing the residual influence of the a priori or starting profile on the retrieved mixing ratio profile. To assess the influence of various profiles, as well as to minimize the influence of a particular a priori profile, we have used three with quite different characteristics as a function of altitude, as illustrated in Figure 2. Vertical weighting functions must also be defined for the Chahine-Twomey technique. These relate a frequency segment of the pressure-broadened line to an altitude range through the pressure-altitude relation embodied in day-to-day pressure profiles over the observing site. Two sets of weighting functions were used in our standard data processing procedure, along with the three a priori mixing ratio profiles. These were cyclically permuted to give six mixing ratio retrievals, which were then averaged to obtain a daily most likely retrieval, minimally biased by reliance on any single combination of starting profile and weighting function. [11] As stated above, vertical resolution varies with altitude but is also a function of what is being defined: for example, the ability to resolve two distinct layers; the ability to accurately determine the altitude at which the mixing ratio reaches a maximum; the ability to define the true thickness of a layer of some species within the atmosphere. The definition that is perhaps most relevant in the present discussion is that of determining the altitude at which the CO distribution reaches a peak value. [12] Since the expected m.r. profiles for CO in general extend through the mesosphere and into the thermosphere, it is important to carefully assess the retrieval accuracy as a function of altitude, as pressure broadening diminishes and Doppler broadening (from which essentially no vertical m.r. distribution can be retrieved) comes to dominate the line shape. The accuracy of the retrieval procedure was assessed by synthesizing pressure-broadened spectral lines from three rather different assumed CO profiles, shown in Figure 3. Random noise was added at a level consistent with that in our typical spectra. These synthesized spectra, for which the true profiles are exactly known, were then deconvolved using the same weighting functions and starting profiles described above and in Figure 2. The results are shown in Figure 3, along with the absolute difference between true and retrieved profiles as a function of altitude. Despite the use of radically different starting profiles, the retrievals from all trials are closely clustered (indicated by the dashed line envelope of all retrievals) over the full altitude range. Between 30 and 70 km the absolute error of retrievals almost never exceeds 0.5 ppmv. Between 70 and 80 km, however, two of the three starting profiles lead to significant overestimations and underestimations of the correct mixing ratios. In panels 1 and 2, ability to retrieve the altitude at which the true CO mixing ratio peaks is seen to be within 2 km. However, in panel 3, where a distribution is assumed that has no peak defined within the range where pressure broadening exceeds Doppler broadening, the retrievals begin to fail above 70 km. As seen below, CO in the Arctic winter has its peak mixing ratio well down into the range where an accurate determination of peak m.r. altitude is possible. 3of10

4 m.r. profiles used in Figure 3. As can be seen, errors in m.r. caused by these 10 K errors in assumed temperature profiles are generally small, except at altitudes greater than 70 km. [14] In Figure 5 we offer a combined budget for retrieval uncertainties arising from a priori profiles, weighting functions, and temperature uncertainties as a function of altitude, plotted on the same mixing ratio scale as Figures 3 and 4, with the following explanation and caveats. For each test profile n = 1 through 3 used in Figures 3 and 4, we have defined the following uncertainty function U n (z) = {[R n (z)] 2 + [DT n (z)] 2 } 1/2, where R n (z) are the retrieval errors illustrated in Figure 3 that are contributed by the a priory profiles (Figure 2) and weighting functions used in all our data deconvolutions, and DT n (z) are the retrieval errors arising from arbitrarily assumed ±10 K temperature offsets, taken from Figure 4. These U n have then been combined, with the assumption that they are uncorrelated errors, into an overall error U o (z) ={[U 1 (z)] 2 +[U 2 (z)] 2 +[U 3 (z)] 2 } 1/2, which we have plotted, after smoothing, in Figure 5. Note that the uncertainty function in Figure 5 might be defined in other Figure 3. Three quite different trial distributions of CO, shown as dotted lines in successive panels. A pressurebroadened CO line shape was generated from each with random noise added at the observed intensity. Six retrievals were made for each trial as described in the text. The average of these is shown as a solid line, and the extreme envelope of all retrievals is indicated by dashed lines in each panel. The numerical difference between average and original distributions is shown as a line with open circles in each panel. [13] The retrieval accuracy for CO mixing ratio profiles is also sensitive to assumed pressure and temperature profiles. To minimize these uncertainties, we used daily NCEP satellite data to 45 km, but above this we used monthly T and P profiles for 80 N from the COSPAR International Reference Atmosphere (1986), interpolated in time to specific dates matching our data. Although the upper atmosphere is thus treated rather crudely, tests show that a 1 km error in assumed pressure altitude (larger than we believe present over any range) causes about 1 km vertical offset error in the resultant profile, with essentially no change in mixing ratio other than the vertical shift in profile. Test results for assumed temperature errors of ±10 K at all altitudes between 50 and 80 K are shown in Figure 4 for the same three widely different assumptions for possible CO Figure 4. Temperature sensitivity tests for each assumed profile used in Figure 3. The apparent shifts in m.r. profiles are shown for ±10 K shifts from the true atmosphere at all altitudes above 50 km. (A gradual transition was introduced between 45 and 50 km.) Absolute differences in true and T-shifted m.r. profiles are also shown. 4of10

5 4. Results [16] Each spectral observation leads to retrieval of a vertical profile. Generally one is available for each midday, except when poor weather conditions or occasional equipment malfunctions made observations impossible. On a few days two observations were made, usually near noon and midnight. In Figures 6a and 7a we have plotted the resulting profiles in the form of contour maps of CO mixing ratio versus altitude and time for the winter/early spring periods of 2002 and Figures 6b and 7b show time sequences of potential vorticity (PV) over Thule at three potential temperature levels corresponding to roughly km, km, and km in altitude. It can immediately be seen that large and rapid changes take place in the amount and altitude distribution of CO up to the highest altitude of available retrievals. Below roughly 50 km, there appears to be much less activity, at least within the values of m.r. that we can accurately track. We show altitudes down to 30 km to emphasize this. Since CO has a strong latitudinal gradient that is subject to vortex dynamics and location and a strong vertical gradient that should be altered by downward transport within the polar vortex, one expects that observed changes in CO should be closely correlated with changes in vortex position over Thule. Closer examination of the two years seems to present quite different behavior when correlated against vortex position in the lower stratosphere, which we will discuss in more detail below. Figure 5. Combined budget for retrieval uncertainties illustrated in Figures 3 and 4. As in Figure 4, the assumption of an arbitrary ±10 K temperature error dominates the uncertainties above 65 km. The horizontal range of Figures 3 and 4 is maintained. See text for further discussion. ways. Note, however, that for z > 65 km, U o (z) is dominated by the chosen uncertainty in the temperature field, as expected from inspection of Figures 3 and 4. We do not believe our main finding, that of large short-term transportinduced changes in CO distribution in and around the Polar vortex, is seriously impacted by the uncertainties as exhibited here, nor would it be altered by a more sophisticated method of combining retrieval uncertainties. [15] CO molecular parameters and line frequency have been taken from Pickett et al. [1998], and the CO pressure broadening coefficient and temperature dependence from Semmoud-Monnenteuil and Colmont [1987], adjusted for a standard air mixture Behavior During 2002 [17] The Arctic stratosphere remained unusually warm throughout January and February of 2002, and the vortex generally remained well away from Thule, with the single exception of days 25 to 36. This is clearly seen from PV contour maps at various altitudes within the lower to middle stratosphere (not shown here). Figure 6a shows that a large vertical displacement of CO occurred well above these levels between approximately days 25 and 36, coinciding with Thule being well within the vortex boundary. PV contour maps at 550 K (22 km) as well as the 550 K time series plot of PV in the lowest panel of Figure 6b indicate the vortex arrived on day 25 26, in close correlation with a sudden change in CO, and departed from Thule on day 34, about 2 days before the sharp change in CO seen above 60 km. Contour maps of PV (not shown) indicate that the vortex boundary remained quite close to Thule at lower altitudes over this 2 day period, so the time lag between low- and high-altitude behavior is easily explained. There is, moreover, an expected broadening of the vortex with altitude, as well as no expectation of a smooth vortex boundary over all altitudes, because of differential motion in various layers. [18] Subsidence of air in the winter polar vortices is a well-known phenomenon (e.g., Greenblatt et al. [2002], and numerous references therein to earlier work). In earlier work on CO, Lopez-Valverde et al. [1996, Figure 10] found averaged mixing ratios at 64 N in early January 1992 to be less than 1 ppmv at 50 km altitude, and about 3.5 ppmv at 60 km, for daytime values, and about an order of magnitude less at night. It thus seems quite unlikely that the sharp downward displacement in the peak values (8 10 ppm at 58 km) could be the result of horizontal advection into the vortex from lower latitudes. [19] In Figure 6b we have used PV values for 2002 from the United Kingdom Meteorological Office analysis, gridded to 3.75 longitude by 2.5 latitude. The increased PV, which clearly marks the vortex position at 550 K in Figure 6b, is also present at 750 K and 1000 K, but PV appears increasingly variable over time at these higher levels, at least prior to approximately day 45. Given the strong simple time correlation between PV at 22 km and CO at km, the apparent variability in PV at intermediate altitudes may be (at least to some extent) a function of worsening reliability in PV derived from the available observational data. It is worth noting that Thule experienced relatively rare high tropospheric humidity and storm conditions on the night of January and again on 5 February (day 36), coinciding closely with the CO changes noted at km in the upper stratosphere and mesosphere as well as the PV changes in the lower stratosphere; that is, changes in sea-level weather, the vortex position in the lower stratosphere, and the concentration of CO in the upper stratosphere-mesosphere all correlate rather tightly with one another, but calculated PVat 750 and 1000 K appear to be less reliable indicators of vortex location. [20] CO in the km range shows some variation between days 25 and 36 but exhibits a sharp rebound to values typical of outside the vortex at days After 5of10

6 Figure 6. (a) Contour plot of CO mixing ratios versus altitude and time for The smallest contour values in ppmv are 0.5, 1.0, 1.5, and 2.0. Above 2 ppmv the contours all increase in steps of 2.0 ppmv. (b) From bottom to top panel: Time sequence of potential vorticity over Thule at potential temperature levels of 550, 750, and 1000 K (21 22, 26 27, and km), respectively. Values on the vertical scales are to be multiplied by 10 8 to obtain Ertel s PV in units of K/m 2 /kg-s. approximately day 35, PV values remained generally low at 550 K (22 km), and high-altitude CO shows fairly steady behavior during this period. Note that there is no indication of long-term subsidence in CO when Thule is outside the vortex; rather, peak CO values during 2002 appear to increase a little in altitude as time goes on. This is opposite to the mild down trend observed in 2003 (Figure 7a). [21] The large and somewhat variable mixing ratios seen in the km range when the vortex was not over Thule are indicative of pockets of CO that build up around the vortex edge as a result of wave-induced meridional transport [e.g., Allen et al., 1999]. These are again noted in the 2003 observations. [22] In Figure 8 we show the CO column density integrated between km for 2002 and Although a decreasing trend is evident over the total period covered, there is hardly any indication of a change in average column density during the period between days of 2002 within the uncertainties of day-to-day measurements. This is consistent with the vertical displacement of CO during this period, as seen in Figure 6a, and again suggests vertical descent with relatively little mixing across the vortex boundary, at least within the vertical region in question. Note also that during the periods of overlapping data for 2002 and 2003, the average column densities are quite similar Behavior During 2003 [23] During the period from day 5 to 53 of 2003, Thule was within the Arctic vortex for varying lengths of time on 5 occasions, judged at the 550 K level of potential temperature in Figure 7b, as well as from PV contour maps (not shown). This was far more often than in Conditions at ground level were also warmer and more unstable than in Again, as in 2002, passages of the vortex boundary over Thule noted in CO at high altitudes were accompanied by storm conditions at sea level. [24] During the 48 day period from days 5 to 53, moreover, the polar vortex elongated and briefly split in two on three occasions centered around 20 January, 31 January, and 15 February and then re-formed as a single vortex within a few days. This strongly perturbed nature of the 2003 vortex, as seen on PV maps at 550 K, is illustrated in Figure 9 for these three periods. 6of10

7 Figure 7. Same as Figure 6, but for [25] The CO contour plot for 2003 (Figure 7a) presents a correspondingly more complex behavior, while failing to show a layer of downshifted CO for any period when the vortex was overhead. This is most striking for the days around 29 January when it appears that much of the CO disappeared from the upper stratosphere and mesosphere. The frequently perturbed vortex may well have disrupted the usual subsidence pattern within the vortex and led to more-than-normal mixing of extravortex air into the vortex, which probably explains the lack of any clear downwardly displaced CO layer during this or other periods when Thule was within the vortex boundary. [26] Turning to Figure 8, there is a relatively small dip in the integrated column density at approximately day 29, and the perception of strong CO loss around day 30 in Figure 8 is seen to be partly an artifact of the contour presentation: Note that the peak mixing ratio in the gap region around 29 January does not fall below 7 ppmv, in sharp contrast to the much greater drop seen at km during the vortex passage in 2002, a feature again consistent with greater lateral mixing across a strongly perturbed vortex boundary that has largely erased the subsidence that normally occurs during prolonged vortex stability. [27] The perturbation in vertical profile, which peaks around January, is notable for its vertical extent as well as the large m.r. of 17 ppmv reached a little below Figure 8. Column densities from integration of daily CO profiles between 30 and 80 km over Thule for 2002 and Each symbol indicates a day when data was taken. The vertical axis runs from 0 to molecules/cm 2. 7of10

8 Figure 10. Comparison of weak emission line measured in summer (24 December 1999) at the South Pole versus strong winter emission line measured at Thule (24 January 2002). Total column densities above 30 km in each case are proportional to integrated areas under curves, measured from zero intensity. 70 km. PV maps for January show nothing unusual at q = 550 K, with the major axis of an only moderately elongated vortex slowly moving clockwise over several days until its western boundary reaches the Thule region about 13 January. [28] The very large peak mixing ratio, 17 ppmv, measured on January 2003 is manifest on both the contour and the column density plots. This is matched in Figure 7a by the 18 ppmv found on January 2002, though in the latter case the vertical distribution is shallower and extends primarily upward of 65 km, with a consequently much smaller column density (see Figure 8). As stated above, we attribute these large mixing ratios (and the 12 ppmv seen at various other times during 2002 and 2003) to pockets of enhanced CO that build up each winter around the outside of the vortex wall as a result of waveinduced meridional transport and entrainment. The return of extravortex air over Thule on 30 January 2003 is marked not only by a large mixing ratio, but by an anomalously high-altitude CO peak on that day only, after which the distribution drops to the commonly found altitude range. The retrieval on 30 January seems robust against misassignment in altitude, and we are fairly confident that the CO peak is significantly higher in altitude on this single day than on any other day sampled during the 2002 or 2003 observing sessions. We again note, as above, that the January period is marked by severe vortex distortion in the lower stratosphere, consistent with strong transport- 8of10 Figure 9. Contour maps for Ertel s potential vorticity at q = 550 K. Top, middle, and bottom panels show the vortex pattern at 1200 UT on 20 January, 31 January, and 16 February 2003, respectively. The Greenwich meridian is at the bottom, the outer boundary is at 40 N, and the star marks Thule. Maps courtesy of the Danish Meteorological Institute.

9 induced atmospheric changes. The value of 12 ppmv is not inconsistent with the range expected at this altitude at lower latitudes in winter, according to calculations by Solomon et al. [1985] (see, particularly, their Figure 3) and may represent the result of rapid northward transport to the Thule region at km in altitude. [29] The PV time series at 550, 750 and 1000 K in Figure 7b show somewhat more correlation with one another than those for 2002, but generally show a poor (or time-displaced) correlation with changes in high-altitude CO. Note for instance that over the period of approximately days 41 48, the time sequence PV at all three levels (Figure 7b) indicates that the vortex was solidly over Thule (also clearly indicated by the PV contour map at 550 K, for instance (not shown)), but in the km altitude range, CO reached mixing ratios as large as 16 ppmv, characteristic of air outside rather than inside the vortex boundary. PV contour maps at 550 K indicate a distorted, triangular vortex on day 42 (11 February), which became strongly elongated by day 44 and began dividing in two during days 45 47, with Thule near the neck still joining the separating segments. By day 47 (see Figure 9) Thule was on the edge of one separate segment, and was positioned significantly outside both segments on days On days 51 53, the segments rejoined to form a highly elongated vortex with Thule again near the edge. It is likely that the strongly perturbed vortex of 2003 was also distorted in its vertical orientation, explaining the poor correlation between the CO mixing ratios at high altitudes and the vortex position in the lower to middle stratosphere. 5. Seasonal Variation [30] Solomon et al. [1985] predicted a large pole-to-pole gradient in CO, with CO concentration far less over the summer pole. We have no summer observations over Thule, but do have trial data taken with the same apparatus at the South Pole in December (Southern Hemisphere summer) of (Much stronger late winter CO spectra taken in 1999 were spoiled by saturation of the spectrometer response at the line peak, unfortunately not recognized while data was being taken, and are not presented here. Saturation had less effect on the much weaker December data, which we do show.) We present the summer-winter contrast in emission intensities by showing superimposed spectra in Figure 10. The 24 January 2002 Thule spectrum is the same shown in Figure 1, and represents relatively strong emission observed just outside the vortex wall (see also Figure 6), but significantly less than emission from the largest mixing ratios found outside the vortex, for example, during January, or again around 11 February The far weaker summer emission spectrum is representative of several taken in late December 1999 and early January 2000 at the South Pole Station. The strong depletion of CO over the summer pole predicted by Solomon et al. [1985] is clearly borne out by the comparison spectra in Figure Summary [31] In this paper, we have not attempted to discuss the changes in CO that take place below 50 km, choosing instead to stress the large, rapid changes observed in CO in the upper stratosphere and mesosphere during polar winter. Other works cited in the introduction have shown that CO may be fruitfully observed down to 30 km with remote sensing techniques. Our winter data are dominated by the strong emission from the region above 50 km and do not lend themselves to a reliable analysis of the much smaller and more difficult to track changes occurring in the km range. We have included this region in our contour plotting primarily to show that it remains relatively featureless against change down to m.r. values of 1 ppmv or less, with the apparent exception of one short interval within our two observing periods (12 17 January 2003) when the CO m.r. reached 1 ppmv at about 35 km. [32] The vertical profiles of CO that we have tracked in the upper stratosphere and mesosphere over time in 2002 and 2003 not only show large short term variations but also show marked interannual differences in behavior, suggesting that the vertical and meridional transport and intermixing of CO into the vortex region is subject to large dynamically-induced fluctuations, at least during the winter-early spring period studied here. This should make CO a particularly illuminating, though complex, tracer of transport in the polar vortex regions, particularly at high altitudes where other data are often lacking. [33] New satellite-based observations of CO (e.g., by the NASA/JPL improved Microwave Limb Sounder, with a current launch date in June of 2004), as well as the current European ODIN and ENVISAT/SCIAMACHI spacecraft missions, may be expected to reveal new and interesting details of vortex dynamics in the upper stratosphere and mesosphere. In particular, the smaller mixing ratios for CO that occur at lower latitudes, where the upper atmosphere is exposed to longer periods of sunlight, should make any high-altitude episodes of rapid poleward transport in winter particularly easy to see. Conversely, the strong CO depletion in polar summer should also provide a contrast by which to observe larger mixing ratios transported from low latitudes. [34] Long-term climate change is widely postulated to produce larger, and sometimes earlier, effects in Polar latitudes than elsewhere. This may prove to be particularly true in the more ethereal regions of the upper atmosphere than in the lower atmosphere, and the long-term monitoring of CO as a tracer of change in high-altitude transport could become an important tool in the monitoring of long-term climate shifts. [35] Acknowledgments. This research was carried out with a grant from the Italian Space Agency (ASI) and Italy s National Program for Antarctic Research (PNRA). Development of our equipment was funded by earlier grants from NASA s Upper Atmospheric Research Program. We are grateful to Simon Stevenson of the National Science Foundation s Office of Polar Programs and the U.S. Air Force for logistical assistance to and from Thule, and we particularly thank Susan Zager of Veco Polar Resources, Inc., for attending to various logistical needs before and during our time there. We thank Giorgio di Sarra for help and support during the two Thule field campaigns. Curt Trimble took the trial South Pole CO data during a yearlong observing run that produced valuable data on several other trace gases. Anthony Kerr at the National Radio Astronomy Observatory supplied us with an improved very low noise superconducting tunneljunction mixer, which has significantly enhanced our data-taking capability. We have made considerable use of the NASA Goddard Automailer system in tracking the dynamics of the vortex over Thule in 2002 and 2003, and thank L. Lait, P. A. Newman, and M. R. Schoeberl for providing this service for external users. We also thank the Danish Meteorological 9of10

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