Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 Contents lists available at SciVerse ScienceDirect Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp An enhancement of the lunar tide in the MLT region observed in the Brazilian sector during 2006 SSW A.R. Paulino a,n, P.P. Batista a, B.R. Clemesha a, R.A. Buriti b, N. Schuch c a ~ Jose dos Campos/SP, Brazil Instituto Nacional de Pesquisas Espaciais, Sao Universidade Federal de Campina Grande, Campina Grande/PB, Brazil c Centro Regional Sul de Ciências Espaciais, Santa Maria/RS, Brazil b a r t i c l e i n f o abstract Article history: Received 1 August 2011 Received in revised form 13 December 2011 Accepted 17 December 2011 Available online 29 December 2011 ~ do Cariri (7.41S, 36.51W), Cachoeira Paulista (231S, 451W) and Meteor radar observations at Sa~ o Joao Santa Maria (29.71S, 53.71W) have permitted estimates to be made of winds and temperature in the mesosphere and lower thermosphere (MLT). Using horizontal winds the lunar semidiurnal tide was determined from January 2005 to December 2008 for these three sites. In January 2006 an unusual enhancement was observed in the lunar tide amplitude at these stations, for meridional and zonal components. During this period, sudden stratospheric warming (SSW) events were observed in the northern hemisphere. Meridional and zonal winds observed in Brazil showed evidence of SSW effects in the MLT region. Moreover, the mesospheric temperatures at Cachoeira Paulista and Santa Maria presented cooling and a semimonthly oscillation, which followed the new–full–new moon phases. These results suggest that the enhancement of the lunar tide could be a response for the SSW in the MLT. & 2011 Elsevier Ltd. All rights reserved. Keywords: Lunar semidiurnal tides Atmospheric tides Sudden stratospheric warming 1. Introduction The atmospheric lunar tide is excited by gravitational action in the lower atmosphere and by movement of the oceans and Earth’s surface. Changes in background winds and temperature gradients in the stratosphere and mesosphere can affect this oscillation when it propagates into the upper mesosphere. Advances in measurements of winds by radars in the mesosphere and lower thermosphere have made possible the study of the lunar tide in this region. In the last three decades, several studies of the lunar semidiurnal tide have been made in different longitudinal sectors (Stening et al., 1987, 1990, 1994, 1997b, 2003; Stening and Vincent, 1989; Stening and Jacobi, 2001; Niu et al., 2005; Sandford et al., 2006, 2007; Sandford and Mitchell, 2007a; Paulino et al., in press). These studies have contributed to the understanding of the structure and characteristics of the lunar tide in the MLT. For example, the vertical wavelengths of the lunar tide changes along the year. Seasonal and year to year variability of the amplitudes and phases have also been observed. Since the lunar tide is excited by a predictable and well-known source, these observed variabilities must represent responses to the changes in the background atmosphere acting upon the lunar tide. Due to the fact that the source of the lunar tide does not n Corresponding author. Tel.: þ55 12 3208 7167; fax: þ 55 12 3208 6990. E-mail address: anaroberta@dae.inpe.br (A.R. Paulino). 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.12.015 change, the determination of the lunar tide in the MLT is an excellent tool to understand the coupling mechanism between the lower and upper atmosphere. The determination of this oscillation can help us understand how the middle atmosphere conditions act upon the tide as it propagates upward. Sudden stratospheric warming (SSW) is the most dramatic meteorological phenomenon in the winter polar region. These events occur mostly in the stratosphere of the northern hemisphere in the form of a temperature enhancement. The warming is generated by a rapid increase of the activity of planetary waves and their interaction with the stratospheric mean circulation. The SSW can be classified as a major warming when the latitudinal mean temperature increases poleward of 601 latitude and an associated zonal mean circulation reversal occurs. Minor warmings occur when the stratospheric temperature increases but the zonal circulation at 10 hPa does not reverse. However, minor events can have all of the same characteristics of major warming but with less intensity. These events affect strongly the entire middle atmosphere; causing variations in the thermal and dynamical structure within the MLT region (see Matsuno, 1971; Schoeberl, 1978; Hoffmann et al., 2002; Vineeth et al., 2009). Some studies have shown evidence of effects of SSW events in the MLT region, mainly through changes in the behavior of the winds and temperature. Frequently, studies have reported that SSW events lead to a decrease in the zonal wind, or even to a wind reversal (e.g., Muller et al., 1985; Schminder and Kuerschner, 1990; Hoffmann et al., 2002). Furthermore, changes in the 98 A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 meridional winds have also been observed (e.g., Muller et al., 1985; Hoffmann et al., 2002; Jacobi et al., 2003). Effects of stratospheric warming on tidal propagation have been reported in some studies. Hoffmann et al. (2007) observed an increase of the solar semidiurnal tide amplitudes after a SSW (January 2006) at Andenes (691N, 161E). Sridharan et al. (2009) verified an enhancement of the solar semidiurnal tide amplitudes over Tirunelveli (8.71N, 77.81E) during the days of major warming in 1998–1999 and 2005–2006. Effects of a Northern SSW have also been observed in the southern hemisphere. Lima et al. (in press) observed an unusual behavior of the quasi-two-day wave in the MLT region at Sa~ o Joa~ o do Cariri (7.41S, 36.51W) during a 2006 SSW event. Chau et al. (2009) observed that the equatorial vertical E  B drifts at Jicamarca (11.951S, 76.81W) showed a unique and distinct daytime pattern during a January 2008 minor warming event. Anderson and Araujo-Pradere (2010) found that for both January 2003 and January 2004 SSW events, the semidiurnal signature in the E  B drift first appeared in the Peruvian sector and then was observed in the Philippine sector 3 days later. Counter electrojet (CEJ) events during the northern winter months of low solar activity years were suggested to be the result of the modified wind system in the ionosphere associated with stratospheric warming events (Rastogi, 1999). CEJ can be explained by an additional current system driven by a solar or lunar semidiurnal tide (Stening, 1989, 1992; Stening et al., 1996). In addition, Stening et al. (1997a), using numerical simulations, showed that lunar semidiurnal tide amplitudes presented significant changes due to SSW events, even in the southern hemisphere. More recently, Fejer et al. (2010) observed that perturbations in the equatorial electrojet during sudden stratospheric warming are related to an enhancement of the lunar tides. In the present study, we use meteor wind and temperature data measured at three Brazilian sites, in order to examine an enhancement of lunar semidiurnal tide amplitudes, which occurred during a 2006 SSW. 2. Methodology In this study we consider horizontal winds measured in the MLT region by All-Sky interferometric meteor radars (SkiYmet) at three Brazilian stations: Sa~ o Joa~ o do Cariri (7.41S, 36.51W), Cachoeira Paulista (231S, 451W) and Santa Maria (29.71S, 53.71W). The data were recorded from January 2005 to December 2008. The meteor radars use five two-element Yagi receiving antennas and one three elements Yagi transmitter antenna (see Hocking et al. (2001), for further details). These radars operate at 35.24 MHz and transmit 2144 pulses per second. The number of useful meteor detected per day is typically between 1000 and 3000. The sampling of meteor changes during the day, where the maximum detection is observed at 6:00 local time and the minimum is around 18:00 local time. The vertical distribution of meteor seems to be a normal function with maximum concentration at around 90 km. The number of useful meteor detections makes it possible to obtain hourly wind value in 3 km bins from 82 to 107 km (Clemesha et al., 2001). In the present work, the meridional (northward) and zonal (eastward) winds were estimated in hourly time bins for seven atmospheric height intervals of 4 km thickness, with a height overlap of 0.5 km, centered at 81, 84, 87, 90, 93, 96, 99 km. Daily mean temperatures derived from meteor decay times from Cachoeira Paulista and Santa Maria were also used. The temperature was estimated from the height variation of the meteor decay time at the peak of the meteor layer at  90 km (see Hocking et al., 2001). Monthly mean tidal amplitudes and phases were calculated for each height using the least squares method of Malin and Schlapp (1980), where the winds are assumed to consist of a mean wind and sinusoidal solar (diurnal, semidiurnal and terdiurnal components) and lunar tidal (semidiurnal component) oscillations, as can be seen from the following equation:     2p 2p V ¼ V 0 þA1 cos tf1 þ A2 cos tf2 24 12     2p 2p þA3 cos ð1Þ tf3 þ A4 cos tf4 , 8 12 where V0 is the mean wind, t is the solar time and t is the lunar time. The lunar time is given by t ¼ tn and n is the lunar age, which is equal to 0 at the new moon (e.g., Stening et al., 1994, 2003; Sandford and Mitchell, 2007b; Paulino et al., in press). The mean temperatures at the pressure level of 10 hPa ð  32 kmÞ at 901N, the longitudinally averaged zonal wind for 601N at the pressure level of 10 hPa, and their corresponding historical means were obtained from the National Center for Environmental Prediction (NCEP). 3. Results and discussion Fig. 1 presents the monthly mean amplitudes of the lunar semidiurnal tide between January 2005 and December 2008. Meridional (left side) and zonal (right side) components are shown for Sa~ o Joa~ o do Cariri (top panel), Cachoeira Paulista (middle panel), and Santa Maria (bottom panel) from 81 to 99 km height. White spaces between the solid lines indicate periods without data as a result of a malfunction in the radars. The mean amplitudes in January 2006 showed an unusual amplification at all sites described above. The enhancements of the tidal amplitudes were more evident for the meridional component at Sa~ o Joa~ o do Cariri and Santa Maria, and for the zonal component at Cachoeira Paulista and Santa Maria. In this case, the uncertainties of the lunar amplitudes calculated by Malin and Schlapp (1980) method were less than 1% in January 2006. The monthly mean amplitudes of the lunar semidiurnal tide in the MLT region have been observed at similar latitudes for different years. Sandford and Mitchell (2007a) obtained at Ascension Island (81S, 14.41W), using data from 2001 to 2005, mean amplitudes of  3:5 m=s for the meridional and  2:5 m=s for the zonal wind in January. Whereas at Jakarta (6.41S, 106.71E) from 1993 to 1997 the mean amplitudes were 5 m/s for meridional and 1 m/s for zonal components in January (see Stening et al., 2003). At Cachoeira Paulista between 2000 and 2008, the mean amplitudes in January were observed to be around 3 m/s for both components (see Paulino et al., in press). Schlapp and Harris (1993) using mesospheric wind data from 1985 to 1990 for Adelaide (351S, 1381E) verified that the mean amplitudes in January were about 3 m/s for meridional wind and 1 m/s for zonal wind. Stening et al. (1994) analyzing 2 years of data for Adelaide reported that the lunar semidiurnal tide at 94 km was 4 m/s for the meridional and 3 m/s for the zonal wind in 1985–1986. In 1987–1988 they observed amplitudes around 2 m/s for the meridional and 1 m/s for the zonal wind. Between January 2002 and October 2003 over Adelaide, Niu et al. (2005) obtained mean amplitudes of about 4 m/s and 3 m/s for meridional and zonal winds, respectively. Seasonality and year to year variability of the lunar tide amplitudes have been observed at several stations. Large amplitudes of the lunar tide can also be observed in other months. For example, in December 2007, the lunar tide in the zonal wind at Sa~ o Joa~ o do Cariri was  9 m=s. In that case, the lunar tide had the same magnitude as in January 2006 at 90 km. However, simultaneous amplifications at the three stations and in both components have not been observed except in January 2006. A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 99 Fig. 1. Monthly mean amplitudes of the lunar semidiurnal tide from January 2005 to December 2009 over Sa~ o Joa~ o do Cariri (top panel), Cachoeira Paulista (middle panel) and Santa Maria (bottom panel). Left side shows zonal and right side shows meridional components. White spaces between the solid lines indicate periods without data. Fig. 2. Monthly mean amplitudes of the solar semidiurnal tide for each month over Sa~ o Joa~ o do Cariri (top panel), Cachoeira Paulista (middle panel) and Santa Maria (bottom panel). White spaces between the solid lines indicate periods without data. 100 A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 In order to investigate if these enhancements were results of the spectral leakage of solar tide components, we calculated the amplitudes of the solar semidiurnal tide for the same time interval. Fig. 2 is similar to Fig. 1, but for the solar semidiurnal tide. In January 2006 the amplitudes did not intensify like the lunar semidiurnal tide. At Sa~ o Joa~ o do Cariri, for example, the amplitude in January 2006 decreased as compared with the amplitudes for the surrounding months. Fig. 2 shows, in a general form, that the variability of the solar semidiurnal tide is different from the lunar semidiurnal tide. Thus, the intensifications observed in the lunar semidiurnal tide were not connected with spectral leakage of the solar semidiurnal tide, due to their very close frequency. Some studies have pointed out that one needs at least 2 years of data to obtain reliable significance in the measured lunar tide (Stening et al., 1997a, 1994; Stening and Vincent, 1989). For this reason, we need to verify carefully the consistency of the calculated amplitudes and phases for January 2006. One way to do this is to investigate whether the January 2006 phases extracted from one year alone present similar trends with altitude to those obtained using the extended data base. Fig. 3 shows Fig. 3. Phase profiles of the lunar semidiurnal tide calculated to January 2006 for meridional (left panel) and zonal (right panel) components over Sa~ o Joa~ o do Cariri (top panel), Cachoeira Paulista (middle panel) and Santa Maria (bottom panel). The black line shows the averaged phase using all January data. phase profiles for meridional (left side) and zonal (right side) components for January 2006 (stars for meridional and filled circles for zonal). The solid line shows an average phase using all January measurements from 2005 to 2009 for Santa Maria and Sa~ o Joa~ o do Cariri. For Cachoeira Paulista January average phases were calculated from 2000 to 2008. The profiles calculated for January 2006 were relatively close to the averaged phase profiles. In some cases, the phases reveal upward propagating wave patterns. As the January 2006 phase profiles were close to the mean profiles, the calculated amplitudes can be considered consistent. The lunar tides can be affected by modifications that occur in the atmosphere when they propagate from lower to high altitudes. SSWs have been discussed as a possible mechanism of lunar tide enhancement, even in the southern hemisphere (Stening et al., 1997a; Fejer et al., 2010). A major stratospheric warming was observed in January 2006, which is a coincident lunar tide intensification interval. Fig. 4(a) shows the mean temperature at 10 hPa (32 km) at 901N between December 2005 and February 2006, and the historical mean temperature (solid line). Panel (b) shows the zonal wind averaged over 601N at 10 hPa in the same period and its respective historical mean. Panels (c)–(e) show the meridional and panels (f)–(h) show the zonal winds for the three Brazilian stations (Sa~ o Joa~ o do Cariri, Cachoeira Paulista and Santa Maria, respectively) at 90 km height. In the present case, the winds represent a 5-day moving average plotted every day. New and full moons are indicated, by solid and open circle, respectively. The polar stratospheric temperature at 10 hPa exhibited three successive peaks at around January 05, 12, and 23, 2006. Only the third one was a major warming event, because it was accompanied by the reversal of zonal wind at 601N from eastward to westward. The three stratospheric warming events occurred between two consecutive new moons. During these events, the meridional wind increased around January 12 and reached  25 m=s at Sa~ o Joa~ o do Cariri,  30 m=s at Santa Maria, and  40 m=s at Cachoeira Paulista. At Cachoeira Paulista and Santa Maria the meridional wind showed a second peak around January 20, which increased from 7 m/s to 30 m/s (Cachoeira Paulista) and from 10 m/s to 20 m/s (Santa Maria). The zonal wind during the SSW events was westward at Sa~ o Joa~ o do Cariri and reached peak magnitude ð  45 m=sÞ around January 26, then rapidly becoming eastward by February 02. However, at Cachoeira Paulista the zonal wind was eastward and reversed from eastward to westward around January 20. After a brief westward excursion ð  15 m=sÞ the zonal wind reversed to eastward by January 28. At Santa Maria the zonal wind was eastward and decreased around January 02 reaching a minimum value of 4 m/s on January 22. The minima for the zonal winds were observed quasisimultaneous with the minimum of the stratospheric zonal wind at 601N. The minimum values of the zonal wind increase from the equator to midlatitudes. Moreover, there is a phase progression from Santa Maria to Sa~ o Joa~ o do Cariri. Hoffmann et al. (2007) concluded that the strength of the wind reversal or weakening of the eastward-directed winds decreases with latitude, as shown here. Changes in the meridional winds and the reversal of the zonal winds have frequently been reported as effects of SSWs in the MLT region. Hoffmann et al. (2002) observed that stratospheric major and minor warming events between 1989 and 2000 were associated with mesospheric wind reversal or weakening of the zonal wind. Other kinds of disturbances and modifications in the zonal and meridional wind have also been reported in the MLT region at different latitudes during SSW events (Muller et al., 1985; Jacobi et al., 1997, 2003). Furthermore, Jacobi et al. (2003) suggested that the SSW effects can be slightly different at different latitudes. Indeed, the weakening of the zonal wind A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 101 followed by an equatorward phase propagation and an enhancement of the meridional wind could be strong evidences of the effects of the 2006 SSW in the MLT over the Brazilian sector. Fig. 5 shows the mean temperature at around 90 km derived from meteor radar for Cachoeira Paulista and Santa Maria. The panels (a) and (b) show the temperatures from December 21, 2005 to February 2006 at the two sites. Panel (c) shows the temperature from December 2003 to February 2004 at Cachoeira Paulista while panel (d) shows the temperature at Santa Maria from December 2006 to February 2007. In this figure new and full moons are also indicated, by solid and open circle, respectively. Between December 2005 and February 2006 the mesospheric temperature at Cachoeira Paulista shows three coolings, with two major coolings occurring on January 01 and February 01, and a minor one on January 13. These coolings were in phase with new, full, and new moons respectively. At Santa Maria three coolings Fig. 4. (a) Stratospheric polar averaged temperature at 901N and 10 hPa ð  32 kmÞ. (b) Zonal wind at 601N and 10 hPa level. Meridional wind at Sa~ o Joa~ o do Cariri (c), Cachoeira Paulista (d) and Santa Maria (e). Zonal wind at Sa~ o Joa~ o do Cariri (f), Cachoeira Paulista (g) and Santa Maria (h). New and full moons are indicated by filled and open circles, respectively. The wind is a 5 day moving average plotted every day. Fig. 5. Mesospheric temperatures derived from meteor radar decay times from December 2005 to February 2006 at Cachoeira Paulista (a) and Santa Maria (b). Mean temperatures from December 2003 to February 2004 at Cachoeira Paulista (c) and from December 2006 to February 2007 at Santa Maria (d). Temperatures were calculated by a 5 day moving average and plotted every day. 102 A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 are also observed close to new, full and new moon phases. The major warming at 901N occurs before the third coolings of Cachoeira Paulista and Santa Maria, which occur from January 28 to February 03 and the minimum temperature at Cachoeira Paulista was lower than at Santa Maria. Hoffmann et al. (2007) also observed that major warming occurs before a mesospheric cooling at Resolute Bay (751N, 951W), Andenes (691N, 161E) and Kuhlungsborn (541N, 111E). The semimonthly structure observed in the temperature, in phase with the new–full–new moons, could be explained by the changes in the mean wind fields caused by the SSW which enhances the lunar tide also causes the mesospheric cooling and this mechanism is more effective when coincident with lunar phase. In other austral summers without SSW [panels (c) and (d)] semimonthly oscillations have not been observed. For example, at Cachoeira Paulista (2003–2004) a monthly oscillation was observed, while at Santa Maria (2006–2007) no defined periodicity is noted. The temperature measurements by meteor radar should be interpreted carefully during a SSW because those rely on the temperature gradient. Although, this does not alter the present results. Fig. 6 shows daily variability of the lunar semidiurnal tide amplitudes between December 2005 and February 2006. These amplitudes were calculated using a 30 days window composed by hourly mean winds and this window was shifted by 24 h. The amplitudes for the meridional component were calculated for 84 km at Sa~ o Joa~ o do Cariri (a) and Santa Maria (c), and 87 km at Cachoeira Paulista (b). In relation to the zonal component, the amplitudes were calculated at 96 km for the three sites: Sa~ o Joa~ o do Cariri (d), Cachoeira Paulista (e) and Santa Maria (f). These heights have been selected because of the high amplitudes of the lunar tide as shown in Fig. 1. An evident amplification of the lunar tide was observed during the time of the SSW event for all observed sites and in both components. Furthermore, the maxima amplitudes in January were reached quasi-simultaneously. A phase propagation of the maximum amplitudes is clear for the zonal components from January 18 (Sa~ o Joa~ o do Cariri) to January 23 (Santa Maria). The meridional component seems to have a phase propagation between Sa~ o Joa~ o do Cariri and Cachoeira Paulista, but does not for Santa Maria. This feature could be due to the presence of nonmigranting modes (Pedatella and Forbes, 2010). The period of the lunar tide intensification was practically confined to the last 15 days of January. This last feature, together with the previously stated reasons, leads us to believe that the amplification of the lunar tide could be an effect of the northern hemisphere warming. According to Stening et al. (1997a), an amplification of the lunar tide in the southern hemisphere due to a northern SSW is theoretically possible. They simulated a northern SSW event as an input in their model and observed amplification in the lunar tide at southern latitudes. Due to the fact that the lunar tide excitation mechanisms are precisely specified and unchanged, any changes in the background conditions in the stratosphere and mesosphere can affect the lunar tides in the upper atmosphere winds. In addition, Fejer et al. (2010) observed effects of SSWs in the plasma drifts and equatorial electrojet at Piura (5.21S, 80.61W) and Jicamarca (11.91S, 76.81W) associated with the lunar tide. Therefore, the amplification in the lunar tide discussed in the present work could plausibly be caused by the January 2006 SSWs. Further observations at different sectors and improvement of the theoretical models will be necessary for a better understanding of the influence of northern SSWs events in the lunar tide. 4. Summary Fig. 6. Daily amplitudes of the lunar semidiurnal tide calculated at 24 h intervals using a 30-day sliding fit window. (a) Meridional component at Sa~ o Joa~ o do Cariri (84 km height). (b) Meridional component at Cachoeira Paulista (87 km height). (c) Meridional component at Santa Maria (84 km height). (d) Zonal component at Sa~ o Joa~ o do Cariri (96 km height). (e) Zonal component at Cachoeira Paulista (96 km height). (f) Zonal component at Santa Maria (96 km height). The responses of the local MLT to the January 2006 Sudden Stratospheric Warming in the winds and temperature have been analyzed and discussed. During the austral summer 2005–2006, MLT horizontal winds were measured over three Brazilian sites. The amplitude of the lunar semidiurnal tide during this period showed an unusual enhancement at all sites and most heights in both the zonal and meridional winds. This amplification is not a result of spectral leakage of the solar semidiurnal tide, because its variability was completely different from the lunar tide variability. Phase profiles of the lunar tide calculated for January 2006 at the three sites were similar to the year averages. This justifies our assumption that the simultaneous amplification observed at the three Brazilian sites is significant. A.R. Paulino et al. / Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 97–103 A weakening of zonal wind observed at all sites and an increase of meridional wind observed in January 2006 constitutes strong evidence of SSW effects in the southern MLT. During the SSWs period, three successive coolings in phase with new–full– new moon transitions were observed at Cachoeira Paulista and Santa Maria. 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