In the present paper, we show long-term and seasonal behavior of the SAO, AO, and QBO as well as their interrelationship over a Brazilian low-latitude station. Since studies of these influential oscillations from the southern hemisphere are very rare, our investigation with an adequate database is important in the perspective of present state of knowledge in this field.
The observed asymmetric MSAO peaks in the lower mesopause region, i.e., the higher amplitude during the first cycle than the second cycle in the composite monthly mean zonal wind profile (Figure 2) is similar to the earlier results from the low-latitude middle atmosphere (Garcia et al. 1997; Hirota 1978; Li et al. 2012; Venkateswara Rao et al. 2012; Day and Mitchell 2013).
Seasonal asymmetry of the SSAO structure is explained as larger planetary wave activity in the northern hemisphere winter according to Delisi and Dunkerton (1988), which was later extended to describe MSAO asymmetric behavior by Garcia and Clancy (1990). Burrage et al. (1996) pointed out that a combination of the MSAO with different annual oscillations may also cause asymmetry of the MSAO. They also cautioned that a combination of the MSAO with the MQBO may also cause such asymmetry if the time series is not sampled for long enough. It is important to mention that although the composite profile reveals seasonal asymmetry of the MSAO, the individual profile of each year does not always show such behavior (e.g., Figure 4) indicating such asymmetric behavior to be intermittent.
The mean amplitude of the MSAO over the observation period shows a gradual decrease with altitude, while the MAO amplitude decreases up to 87 km and subsequently increases, which behavior agrees very well with the past investigations (Batista et al. 2004; Li et al. 2012) using long-term meteor wind observations over low-latitude sites, although our results generally exhibit higher amplitudes (23 m/s for SAO and 21 m/s for AO). High MAO amplitude at higher mesopause region is believed to be caused by the existing strong annual winter easterly wind as evident in Figure 1 associated with enhanced dissipation of the westward propagating waves during winter. Considerable interannual variability of the oscillations as seen in the present study is consistent with the past investigations (Sridharan et al. 2007; Venkateswara Rao et al. 2012). The westerly (easterly) winds during solstices (equinoxes) are quite consistent with most of the past studies. However, we find higher magnitude of the westerly wind during solstices as compared to the easterly wind during equinoxes unlike a few past studies which reported opposite behavior (Burrage et al. 1996; Garcia et al. 1997; Buriti et al. 2008; Venkateswara Rao et al. 2012). The amplitude of the QBO is found to be significantly weak (<6 m/s) as compared to other two oscillations. Our observed MQBO amplitude is consistent with a number of past studies (Ratnam et al. 2008; Venkateswara Rao et al. 2012; Li et al. 2012; De Wit et al. 2013), although SQBO amplitude is significantly smaller than those studies. The QBO amplitude is higher during 2001 to 2006 in the mesosphere. In the stratosphere the QBO is not found to be very prominent most of the time except in 2005 to 2006 when it is appreciable. Such intermittent nature of QBO activity was also reported by earlier observations (Ratnam et al. 2008; Venkateswara Rao et al. 2012; De Wit et al. 2013). Overall, mesospheric oscillation amplitudes are higher than the stratospheric.
The MSAO variation in the deseasonalized-AO zonal wind at 81 km shows almost the same phase relation (approximately 1 month difference) with the SSAO at 10 hPa similar to the finding of Li et al. (2012) based on northern low-latitude observations. The AO obtained from the deseasonalized-SAO zonal wind in the stratosphere and mesosphere shows westerly phase during winter and easterly phase during summer at lower mesopause. Considerably higher amplitude (>10 m/s) of the AO signifies its role in determining the preferential zonal direction of wave propagation over various seasons of the year through filtering. We observe an opposite phase relationship of the QBO in the mesopause and mid-stratosphere, supporting the results of a number of past investigations (Burrage et al. 1996; Sridharan et al. 2007; Li et al. 2012). It is believed that the gravity waves are filtered through the SQBO and dissipate in the mesosphere region causing the generation of the equatorial MQBO (Baldwin et al. 2001). Most recently, De Wit et al. (2013) examined the role of selective filtering of the gravity waves by the SQBO to drive the MQBO using long-term meteor wind data over Ascension Island (8°S, 14°W). Our observed period of the QBO is approximately 2 to 3 years. Earlier, Li et al. (2012) found the QBO period in the stratosphere and mesosphere of approximately 22 to 24 months. Burrage et al. (1996) obtained the period of the QBO around 2 years in the middle atmosphere. Using MF radar observations over an equatorial station, Sridharan et al. (2007) showed the QBO period of approximately 2 to 3 years. Therefore, the results of the past investigators are quite consistent with the present finding. It is believed that there may be a possible role of the solar cycle in lengthening the period of the SQBO (Mayr et al. 2006).
A number of previous investigators reported modulation of the MSAO by the SQBO. In order to investigate the relationship between the MSAO and SQBO, we have shown the monthly mean deseasonalized-AO (~MSAO) zonal winds at 81 km and deseasonalized (~SQBO) zonal mean zonal wind at 10 hPa level in Figure 8. It can be noted that the strongest easterly phases of the MSAO as observed in early 2000, early 2002, late 2004, and early 2008 are accompanied by the westerly (2000, 2004, and 2008) and easterly (2002) phases of the SQBO. Garcia et al. (1997) reported a strong modulation of the easterly MSAO phase by the westerly SQBO phase and weak modulation while the SQBO phase is easterly and they explained this phenomenon by gravity waves filtering by the SQBO with the help of a modeling study which was supported by a number of other studies (Burrage et al. 1996; Baldwin et al. 2001; Ratnam et al. 2008; Day and Mitchell 2013). Our result is partially consistent with the findings of these investigators (during 2000, 2004, and 2008). In contrast to the investigators mentioned before, Li et al. (2012) found strong easterly MSAO phase at the time of strong easterly SQBO phase and weak easterly MSAO while SQBO phase is westerly. Also Ratnam et al. (2008) reported strong easterly MSAO phase during easterly SQBO phase for a few cases. In fact, such a phenomenon is also found in the present study during certain times, e.g., 2005. Additionally, we find weakest easterly of the MSAO during early 2003 and early 2006 while the SQBO is westerly similar to Li et al. (2012). The maximum easterly amplitude of the MSAO is obtained in 2002 which is also observed by the most recent study of Day and Mitchell (2013) using observations over Ascension Island.
We observe strong westerly phases of the MSAO in 2002 and 2005 with simultaneous easterly phases of the SQBO which is in agreement with the filtering of the eastward propagating gravity waves by the easterly SQBO and dissipation of the same in the mesosphere. A strong westerly phase of the MSAO can also be driven by the eastward propagating ultrafast Kelvin wave which originates in the troposphere and reaches the mesosphere after getting filtered in the stratosphere by the easterly SQBO. Dissipation of the ultrafast Kelvin wave can provide eastward forcing to the mean flow in the mesosphere which may contribute to the westerly phase of the MSAO (Dunkerton 1982).
It is interesting to note that there is an enhancement of all the oscillation components during 2002 in the mesosphere (Figure 4) but not observed in the stratosphere. The enhancement of the easterly phases of all oscillations is followed by the strong westerly phases in 2002. This phenomenon can be attributed to seasonally dependent wave breaking/dissipation with westward propagating dominant waves (Rossby waves, gravity waves, tides) during early 2002 and eastward progressing waves (Kelvin waves, gravity waves) during late 2002. As this phenomenon is not present in the stratosphere, the waves can be deemed as enhanced dissipation in the mesopause probably due to existing instability. Another interesting feature can be pointed out in Figure 4 is the almost periodic enhancement of the easterly phase of the MAO in early 2002, 2004, 2006, and 2008, which indicates a QBO modulation of the MAO. Recently, Ratnam et al. (2008) showed modulation of the MSAO by the MQBO as a result of interaction between these two oscillation components. They explained the relationship between the MSAO and MQBO in terms of their common driving source, i.e., gravity waves which may hold true for the present observed modulation of the MAO by the MQBO also. It should be mentioned that the diurnal tide also shows amplitude maxima (Figure 5) during March/April in the years 2002, 2004, 2006, and 2008, indicating a quasi-biennial modulation. Such quasi-biennial enhancement of the diurnal tide amplitude was also reported in the available literature (Vincent et al. 1998; Wu et al. 2008). Therefore, the diurnal tide as well as gravity waves are supposed to be responsible for such quasi-biennial modulation of the MAO in the present case since these waves are the dominant driver for large scale oscillations in the atmosphere. In this context it can be mentioned that Venkateswara Rao et al. (2012) observed enhancement of the easterly wind during March equinox with a period of approximately 2 years which was termed a quasi-biennial enhancement in their paper. The observed strongest westerly (Figure 4) in all oscillations in mid-2006 in the stratosphere can be attributed to momentum deposition by the eastward propagating gravity waves and Kelvin waves to the background flow.
Earlier, Xu et al. (2009) reported prominent SAO, AO, and QBO variability of the diurnal amplitude in the tropical mesosphere near ±20° latitudes using satellite-based observations which is also evident in the present study. The present diurnal tide amplitude shows semiannual variability with maxima during equinoxes (Figure 5). The amplitude peak observed during the March/April equinox is significantly higher than the spring maximum, which supports the findings of a number of past investigations (Vincent et al. 1998; Wu et al. 2008; Xu et al. 2009). Since the MSAO phase is easterly during equinoxes, it can be made out that the westward propagating diurnal tide plays a vital role in driving the MSAO easterly phase during equinoxes by providing significant momentum to the mean flow. It can also be noted that the quasi-biennial enhancement of the tide amplitude in the mesosphere (as mentioned in the previous paragraph) occurs while the MQBO is easterly (Figure 5c), i.e., the SQBO is in a westerly phase which agrees well with the satellite-based observations over the globe by Wu et al. 2008. The westerly phase of the SQBO allows the westward propagating diurnal tide to pass to the mesosphere through wave filtering. Wu et al. (2008) concluded a possible relationship of weak gravity wave filtering and strong migrating diurnal tide filtering associated to westerly phase of the SQBO. Gurubaran and Rajaram (2001) inferred the possible role of interaction between the gravity waves and diurnal tide to alter the strength of the easterly phase of the MSAO. Unfortunately, in the present study, we are unable to verify the role of gravity waves with the present low temporal resolution wind data. The observed period of the QBO in the diurnal tide amplitude is very near to 23 and 37 months (Figure 7) matching partially with the results of Xu et al. (2009) from low latitudes where they found a period close to 24 to 25 months.
The high QTDW kinetic energy concurs with the westerly phase of the MSAO and vice versa (Figure 6a) indicating a high possibility of westward propagating QTDW filtering by the MSAO. On the contrary, the occurrence of the QTDW energy maxima during the MAO easterly phase (Figure 6b) may suggest that the summer mesospheric easterly jet plays a vital role to enhance the QTDW energy since the summer mesospheric easterly jet facilitates barotropic/baroclinic instability causing QTDW amplification (McCormack et al. 2009; Guharay et al. 2013). The summer mesospheric easterly jet is considered to be generated by dissipation of the westward propagating gravity waves which are filtered by the westerly SQBO. Also high QTDW energy is observed during strong easterly phase of the MQBO in early 2002 (Figure 6c), although it is not found in any other time over the observational span. The large QTDW energy may contribute to strengthen the easterly phase of the mean background wind to some extent. However, the actual role of the gravity waves and QTDW to control the easterly wind can only be ascertained with further intensive analysis which is beyond the scope of the present study. Li et al. (2012) found the QBO enhancement of the easterly wind as well as QTDW energy in the mesosphere and indicated a possible contribution of the QTDW towards driving the easterly phase of the MSAO and MQBO. Our results show SAO and AO modulation in the QTDW energy (Figure 8) although the QBO effect is not prominent suggesting its considerable relationship with SAO and AO.