Mesospheric wind disturbances due to gravity waves near the Antarctica Peninsula

JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 7765 7772, doi:10.1002/jgrd.50577, 2013 Mesospheric wind disturbances due to gravity waves near the Antarctica Peninsula Qian Wu, 1 Zeyu Chen, 2 Nick Mitchell, 3 D. Fritts, 4 and H. Iimura 5 Received 5 March 2013; revised 6 June 2013; accepted 14 June 2013; published 29 July 2013. [1] Based on austral winter mesospheric wind observations from three closely deployed Antarctic Peninsula stations (King George Island, Palmer, and Rothera), mesospheric wind disturbances induced by gravity waves are examined. The mesospheric winds to the west of the Antarctic Peninsula below 88 km were affected by gravity waves, while the winds on the east side of the peninsula were unperturbed. The gravity waves are most likely generated by orographic features of the peninsula. Because the strong westerly stratospheric wind filtered out all eastward propagating waves, only westward propagating waves can reach the mesosphere on the west side of the peninsula. This data set shows the strong gravity wave effect on small scales. Hence, the mesospheric wind data at one station may not be representative of the region for global waves like tides. Small local features can greatly affect the mesospheric winds and may impact the interpretation of global waves. High-density deployment of mesospheric wind instruments may be needed in some cases. Citation: Wu, Q., Z. Chen, N. Mitchell, D. Fritts, and H. Iimura (2013), Mesospheric wind disturbances due to gravity waves near the Antarctica Peninsula, J. Geophys. Res. Atmos., 118, 7765 7772, doi:10.1002/jgrd.50577. 1. Introduction [2] It is well known that the Antarctica Peninsula region produces strong gravity wave (GW) activity [Wu and Jiang, 2002; Jiang et al., 2002, 2003; Alexander and Teitelbaum, 2007; Hertzog et al., 2008, 2012; Alexander et al., 2009, 2011; de la Torre et al., 2012]. Recent observations using stratospheric balloons have shown that the GWs carry strong westward momentum flux into the stratosphere [Hertzog et al., 2008]. Mountain waves are generated as a result of the interaction between the strong westerly winds and topography of the peninsula, similar to the process simulated by Chen et al. [2005]. The strong westerly winds, however, also filter out most of the eastward propagating waves, and only the westward waves were allowed to propagate upward into or pass the stratosphere. [3] Hertzog et al. [2008] using stratospheric balloon observations reported unusually large westward momentum flux over the Antarctica Peninsula between 65ºS and 75ºS at 20 km altitude. They attributed such a large momentum flux to the orographic features of the Antarctica Peninsula and mountain waves originating from them. In the mesosphere, 1 National Center for Atmospheric Research, Boulder, Colorado, USA. 2 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China. 3 Department of Electronic and Electrical Engineering, University of Bath, Bath, UK. 4 Gats Inc., Boulder, Colorado, USA. 5 Northwest Research Associates, Boulder, Colorado, USA. Corresponding author: Q. Wu, National Center for Atmospheric Research, PO Box 3000, Boulder, CO 80307-3000, USA. (qwu@ucar.edu) 2013. American Geophysical Union. All Rights Reserved. 2169-897X/13/10.1002/jgrd.50577 statistical analysis of all sky imager mesospheric GWs at Rothera showed mostly westward propagating GWs [Nielsen, 2007]. [4] Beldon and Mitchell [2010] studied GW modulation by the winds at Rothera. However, there is no direct measurement of effects from these westward propagating GW on the mesospheric winds. Particularly, it is not clear how GWs affect the mesospheric winds on small scales. There are detailed studies of the tides and mean winds at Rothera based on MF radar observations [Riggin et al., 2003; Hibbins et al., 2007]. More recently, Sandford et al. [2010] also examined interannual variation of mesospheric winds at Rothera using a meteor radar. Because the observations were limited to one station, it was impossible to determine how representative these observational results were for the region. [5] Since then, more wind instruments have been deployed in the Antarctica Peninsular region. In 2010, a meteor radar was deployed at the Brazilian Comandante Ferraz base on King George Island (KGI). At the same time, a meteor radar has also been operating at the British station Rothera since 2005. Furthermore, a Fabry-Perot interferometer (FPI) was installed in Palmer station, which is capable of measuring mesospheric and lower thermospheric winds. Hence, the configuration of these three observational sites provides high-density coverage over a small geographic area. In this study, we present a unique case study of small scale variations in the mesospheric winds above the Antarctica Peninsula with simultaneous observations from the KGI and Rothera meteor radars and the Palmer station FPI. The goal is to have a better understanding of the GW effects on the mesosphere in this region. The paper is organized as follows: First the data from the three stations are analyzed and presented. Then the GW effect on mesospheric winds is discussed. Finally major findings are summarized. 7765

2. Observations 100 km Figure 1. Locations of three mesospheric wind instruments around the Antarctica Peninsula. KGI meteor radar, Palmer Fabry-Perot Interferometer, and Rothera meteor radar. [6] Figure 1 shows the locations of the three stations near the Antarctica Peninsula. The KGI meteor radar is located at the Brazilian Comandante Ferraz base (62 05 S, 58 23 W). It is a SKiYMET system called Drake Antarctic Agile Meteor radar with a peak power of 30 kw at 36.9 MHz [Fritts et al., 2012]. The meteor radar at Rothera (67 34 S, 68 07 W) is also a SKiYMET VHF system, which operates at 32.5 MHz with a peak power of 6 kw [Day and Mitchell, 2010]. The two systems on average obtain 62 echoes/hr at 90 (plus/minus 1.5) km at KGI and 55 echoes/hr at 90 (plus/minus 3) km at Rothera. The KGI meteor radar data are binned into 3 km altitude grid starting at 81 km and ending at 99 km. The Rothera radar data are binned into uneven grids centered at 81.1, 84.6, 87.5, 90.4, 93.3, and 96.9 km. The horizontal distance between KGI and Palmer is about 430 km. The distance between Palmer and Rothera is about the same. [7] An FPI was deployed at American Antarctica Station Palmer (64 46 S, 64 03 W). The FPI measures the mesospheric winds by monitoring the neutral wind induced Doppler shift in the OH Meniel band line at 892 nm. The FPI instrument has a similar design to an FPI built by NCAR and deployed in Resolute, Canada, which is described by Wu et al. [2004]. The FPI instrument samples four cardinal directions and the zenith with a 3 min integration time at each. The instrument also measures two other emissions of O (557.7 nm and 630 nm). The three-emission measurement cycle lasts for about 1 hour. At each of the cardinal directions, the instrument is pointed toward the airglow layer at 45 elevation angle. For the OH emission, which peaks at ~87 km, the sampling points at the airglow height are 87 km away from the station. Therefore, the data from the northward and southward viewing directions have a separation of 174 km. The same applies to the eastward and westward viewing direction data. The wind errors are less than 3 m/s. [8] Even though the FPI is running daily, data availability is limited by the weather conditions at Palmer. In this paper, we select one day (2 June 2011) when all three instruments have good data for detailed analysis. We also show data from 4 June 2011 for comparisons. Detailed analysis for those data is not possible due to lack of the Palmer observation. Because of the high variability of the mesospheric winds, simultaneous data sets from all three instruments are very important for interstation comparisons. Otherwise, day-to-day variations could be misinterpreted as spatial changes. [9] The data sets from the KGI and Rothera meteor radars during 2 June 2011 are shown in Figure 2. At KGI, both the meridional and zonal winds show a very strong semidiurnal tide (SD) with downward phase progression. At Rothera, above 87 km, the SD tidal features are also very clear. Below 87 km, the SD tide appears to be disrupted with a phase shift in the meridional winds. In the zonal winds, terdiurnal (TD) tidal features appeared below 87 km. [10] The data from 4 June 2011 are shown in Figure 3. On this day, the SD is much weaker at KGI. At the same time, the discontinuity at 88 km persisted at Rothera proving that this feature is not a one-time phenomenon. [11] To have more quantitative results for 2 June 2011, least squares fit to the data sets with mean wind, diurnal, SD, and TD oscillations are performed. The meteor data were interpolated into 1 km altitude grid for easy comparison between the two radar systems as they were binned into slight different altitude. For a better comparison with past results, monthly averaged wind field from the two stations are also calculated and the same least squares fits are also performed. Figure 4 shows the meridional wind profiles of the least squares fit results for both 2 June 2011 observations (solid) and June monthly averaged wind fields (dashed). The red color is for KGI and blue for Rothera. The amplitude profiles of the various tides are shown in the upper panels, while the phases are in the lower panels. [12] For the observations of 2 June 2011, the meridional diurnal tide amplitudes at the two stations are very similar. The SD tide amplitudes are larger at KGI. The phases are nearly identical above 88 km. From the vertical variations of the SD tide phase, we estimate the vertical wavelength of the SD tide above 88 km is about 45 km (phase changes about 4 hour over 15 km). At Rothera, the SD tide has a minimum at 85 km. And below 88 km, the SD tide phase shifted by about 2.5 hours. The TD tide shows large amplitudes between 90 and 95 km at KGI and 80 to 85 km at Rothera. There are large phase differences between the two sites in the TD tide. [13] The zonal wind tidal amplitudes and phases are shown in Figure 5, which is in the same format as Figure 4. For the 2 June 2011 observations, the diurnal tide has a larger amplitude at KGI between 90 and 95 km. The amplitudes at Rothera are in general smaller, particularly at altitudes (~93 km) where the diurnal tide is larger at KGI. The SD tide amplitudes and phases at the two locations are similar above 88 km. The Rothera SD tide has minimum at 85 km, and the phase below 88 km also shows a phase shift similar to the meridional winds. The KGI SD has a minimum at 84 km and no drastic phase shift below 88 km. The TD tide at Rothera is much larger than that at KGI below 90 km and vice versa above the same height. [14] Because the two stations have different latitudes and longitudes, any interstation differences can be either latitudinal, longitudinal, or a combination of both. We can estimate the expected phase differences for semidiurnal tides with 7766

Figure 2. Mesospheric winds from KGI and Rothera meteor radars. At KGI, both the meridional (+N) and zonal winds (+E) show clear semidiurnal tide from 80 to 100 km. The amplitude of the semidiurnal tide peaks around 96 km (70 m/s). In the Rothera data, the semidiurnal tide is apparent above 88 km in both the meridional and zonal winds. A discontinuity exists at 88 km, below which an 8 h wave appears in the zonal winds. The contour step size is 10 m/s. westward propagating zonal wave number 2 (SW2) and wave number 1 (SW1) between KGI and Rothera. The longitudinal difference is 10. The horizontal wavelength is 180 in longitude for the SW2 and 360 for the SW1. The longitudinal separation between KGI and Rothera would produce phase differences 40 and 20 min in local time for the two kinds of SD, respectively. They are much smaller than phase differences we observed below 88 km. [15] To further explore this issue, another station between the KGI and Rothera is used. The FPI at Palmer station is located between the two stations and provides a key data point for understanding the KGI and Rothera differences. The comparisons of the data set at 87 km from all three stations are plotted in Figure 6. The meridional winds are in the upper panel. The red (blue) colored line is for the KGI (Rothera) winds and Palmer FPI winds from northward (southward) viewing directions are marked as red (blue) diamond symbols with error bars. The meridional winds from the three stations show fairly large discrepancies. The zonal winds are shown in the lower panel with red (blue) line 7767

Figure 3. Mesospheric wind data from KGI and Rothera on 4 June 2011. The figure is in the same format as Figure 2 for 4 June 2011. The discontinuity at 88 km in the Rothera data persists. for KGI (Rothera) winds at 87 km. The Palmer FPI eastward (westward) viewing direction samples are shown as red (blue) diamond symbols. The zonal winds, however, show very good consistency between the Palmer FPI westward (eastward) viewing samples and Rothera (KGI) observed winds. As the FPI samples are 87 km away from the station, the eastward viewing direction measures the region above the peninsula mountain range, whereas the westward viewing direction sampling point is off the west in the ocean at a longitude close to the Rothera station. [16] The implication from this comparison is that the interstation differences between KGI and Rothera are more longitudinal and less latitudinal. Separated by a small longitudinal difference between KGI and Rothera, the interstation differences are significant. 3. Discussions [17] The combined observations from three closely located mesospheric wind instruments in the Antarctica Peninsula showed a large interstation difference in winds between KGI and Rothera below 88 km. The KGI data showed a clear SD tide from 80 to 100 km, whereas the Rothera data showed a similar SD tide only above 88 km. Below 88 km, Rothera zonal winds contain a TD tide, which is absent from the KGI data. With additional Palmer station wind data, it is 7768

Figure 4. Meridional wind least squares fit result of the diurnal, semidiurnal, and terdiurnal tide amplitudes and phases at KGI (red) and Rothera (blue) for 2 June 2011 (solid) and monthly average of June (dashed). The least square fitting errors for the amplitudes are less 3 m/s. The phase errors for the diurnal are less than 1 h, SD less than 0.2 h, and TD less than 1 h. Figure 5. Same as Figure 4 for zonal winds. The errors are similar to the meridional winds. 7769

Figure 6. Comparison of mesospheric winds from Palmer (diamonds, red eastward, and blue westward), KGI (red solid line), and Rothera (blue solid line) at 87 km. The meridional winds do not show good agreement. The zonal wind shows good agreement between the Palmer eastward (westward) viewing wind and KGI (Rothera) measured winds. Hence, the results strongly suggest that the interstation difference between KGI and Rothera is more of a zonal difference than a latitudinal difference. The FPI wind errors are less than 2 m/s. Meteor radar data do not have error values. The standard deviations for the derived winds are about 5 m/s at 87 km altitude. possible to determine that the interstation difference is more of a longitudinal variation. [18] The Antarctica Peninsula is known for strong orographic GW activity as shown in previous stratosphere observations. The strong westerly winds at low altitudes filter out most eastward propagating waves and only the westward ones can reach the mesosphere. Moffat-Griffin etal. [2011] used 8 year radiosonde data from Rothera to study the Figure 7. Mean winds from KGI (red) and Rothera (blue). Both the monthly (dashed) and 2 June 2011 (solid) results are plotted. The results are obtained during the least squares fit for tidal parameter extraction. 7770

stratospheric GWs. They showed that most of the stratospheric GWs propagate westward in June. Aforementioned study by Nielsen [2007] also showed persistent westward propagating mesospheric GWs over Rothera. It is obvious that the GWs that can travel up to the stratosphere and mesosphere are westward propagating at Rothera. Hence, it is likely that the mesosphere west of the peninsula is affected by the GW forcing while the east side is not, which is consistent with our combined observations from KGI, Rothera, and Palmer. [19] Plougonven et al. [2008] simulated how the GWs originating from the peninsula were able to propagate into the stratosphere west of the peninsula under the winds coming from west and north. Upward propagating wave signatures in the vertical winds showed a preferential location to the west of the peninsula. Since the model upper boundary is about 35 km, it did not show directly that GWs propagate into the mesosphere. Nevertheless, in Plougonven et al. [2008], orographic GWs hold intrinsic frequency 18f, horizontal/vertical wavelength 80/8 km. Observations show the GWs are observed in the stratosphere. Thus, it is anticipated that such kind GWs with large aspect ratio and high intrinsic frequency likely propagate to mesospheric height during the season when moderate westerly winds prevails over the polar middle atmosphere. [20] We do not aim at resolving specific GWs in the MLT region, but we do believe the observational investigation results reflect the GWs effect that can be associating to the orographic origin. 4. Effect on the Observation of the SD Tide [21] The localized GW effect on the mesospheric winds below 88 km introduces a longitudinal asymmetry in the winds. Such a longitudinal variation may be misinterpreted as nonmigrating tides, while the winds above 88 km appear unaffected. Hence, more care is needed in analyzing Rothera mesospheric wind data for extracting nonmigrating and migrating tides. [22] Adding to the complication, the migrating SD tide has a transition near 88 km altitude. Below 88 km altitudes, the migrating SD tide is dominated by the (2,2) mode because of its long equivalent depth and the negative temperature gradient [Forbes, 1995]. The (2,2) mode has a very long vertical wavelength. Above 88 km, other modes (e.g., (2,4), (2,5)) are growing. These modes have shorter vertical wavelengths. Consequently, the migrating SD tide often has different vertical wavelengths in the two altitude ranges. [23] When Murphy et al. [2006] calculated migrating and nonmigrating SD tides using MF radar observations from multiple Antarctic stations, including Rothera, they obtained different vertical wavelengths for the migrating SD above and below 88 km. Above 88 km, the vertical wavelength in June is about 57 km, close to that observed on 2 June 2011. Below 88 km, the vertical wavelength is much longer. [24] Since we have seen how GWs change the Rothera mesospheric winds below 88 km on 2 June 2011, it is useful to see how the monthly averaged results compared to the one day observation. The two KGI and Rothera components all show a single very long vertical wavelength. For the June 2 observation, the KGI meridional wind shows two vertical wavelengths above and below 88 km. The June 2 Rothera wind data below 88 km, of course, were affected by the GWs. The data above 88 km at KGI and Rothera yield a 45 km vertical wavelength. That agrees with the wintertime SD tide being dominated by the (2,5) mode, which has a vertical wavelength 41 km [Wu et al., 2011; Yuan et al., 2008]. [25] Similarly, Hibbins et al. [2007] also reported a much longer vertical wavelength (~100 km) based on the monthly results from an earlier MF radar at Rothera. That is very close to Rothera monthly results in this study without considering the change in vertical wavelength in the zonal wind at 88 km. The monthly averaged meteor radar meridional wind data at both KGI and Rothera (Figure 4) have the tendency to give a longer vertical wavelength compared to the single day observation. The monthly results from KGI are consistent with the Fritts et al. [2012] analysis of the same data. If the least square fitting is performed on individual days, the phases of the SD tide do not stay the same. Therefore, monthly averaging may have smeared the SD tidal wind pattern. We should point out that in the one day observations, the SD phases from the two stations are almost the same, where the differences from the monthly results are noticeable. That may indicate some fluctuations in the SD tide phase due to GWs or nonmigrating tides. 5. Possible Effect on the Mean Wind [26] The mean winds from the two stations are obtained from the least square fitting. The zonal mean wind above 80 km at Rothera is less eastward in the wintertime compared to the other Antarctica stations at similar latitudes (Syowa, Davis, and Mawson) according to Hibbins et al. [2005]. The Rothera zonal mean winds in Figure 7 are comparable to those of Hibbins et al. [2005]. The Rothera zonal mean winds are less eastward compared to KGI. The eastward zonal mean wind is in accordance with mostly poleward meridional wind in June [Hibbins et al., 2005; Sandford et al., 2010]. The KGI June 2 meridional winds are near zero while the monthly averages are slightly poleward. The Rothera June 2 meridional mean winds are much more poleward than the monthly values. The meridional monthly values at Rothera are less poleward than those from Hibbins et al. [2005]. [27] Sandford et al. [2010] have noted strong interannual variability of mean zonal wind at Rothera in June. Such variability is particularly strong below 88 km. Sandford et al. [2010] suggested that GW momentum flux might be the cause. While the GW momentum flux in June should be westward, the mean wind in the mesosphere is still eastward. The momentum flux is not strong enough to reverse the zonal mean winds. Nevertheless, it is strong enough to cause noticeable interannual variability. It may have made the zonal wind less eastward at Rothera compared to Syowa, Davis, and Mawson as noted by Hibbins et al. [2005]. [28] It would have been very useful to perform a momentum flux calculation as Fritts et al. [2012] on a daily basis. Unfortunately, this type of GW momentum flux calculation can only be done on a monthly basis. As shown in the paper, the monthly results are quite different from the one day observation. Hopefully, future instruments will be able to provide this kind of information. 7771

6. Future Work [29] Similar mesospheric wind features may exist in other places. Hence, it is necessary, at times, to have a high-density deployment of instruments at the locations where strong GW activities are known to occur (e.g., in the Andes Mountains). A single station is not enough to cover these kinds of small scale variations. Espy et al. [2004] used all sky camera image data at Halley station to estimate the momentum flux. Similar analysis on Rothera data would help quantify the GW effect. More simulations of GW effects in the mesosphere near the peninsular region are also needed. 7. Summary [30] The three closely deployed mesospheric wind instruments in the Antarctica Peninsula region at KGI, Palmer, and Rothera showed GW effects in a very small area not reported before. [31] 1. The three instruments showed that the mesospheric winds on west side of the Antarctica Peninsula may be affected by the westward propagating gravity waves, which possibly originate from orographic features of the peninsula. Only the westward propagating GWs were able to propagate to the mesosphere, whereas the eastward propagating GWs are filtered out by strong westerly winds in the region. Consequently, the mesospheric winds on the east side of the peninsula were mostly unaffected. [32] 2. Observations at Rothera showed that the gravity wave effects reach only 88 km. Above 88 km the SD is predominant. The SD tide is consistent with that observed at KGI with a vertical wavelength of ~ 45 km. [33] 3. The winds at Rothera are less eastward than at KGI, perhaps, affected by the westward momentum flux from the gravity waves. [34] The small scale structures in the mesospheric winds reported here may exist elsewhere. 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