On the variability of the seasonal scale oscillations over Cachoeira Paulista (22.7°S, 45°W), Brazil

Abstract

The characteristics of long period atmospheric oscillations are studied using long-term meteor radar wind observations over a low-latitude Brazilian station, Cachoeira Paulista (22.7°S, 45°W). The semiannual oscillation (SAO) amplitude decreases gradually with altitude in the mesopause region, while the annual oscillation (AO) amplitude exhibits a minimum at mid-mesopause with significant interannual variability. The quasi-biennial oscillation (QBO) obtained by removing the SAO and AO from the zonal wind exhibits an out-of-phase relationship at lower mesopause heights (approximately 81 km) and mid-stratosphere (approximately 10 hPa). There is an evident signature of modulation of the mesospheric SAO (MSAO) and mesospheric AO (MAO) by the QBO. The diurnal tide is surmised to contribute significantly to drive the easterly phase of these oscillations, especially MSAO and MAO through depositing westward momentum to the background wind. The westerly phase of the MSAO is believed to support the growth of the quasi-2-day wave (QTDW) by wave filtering. The QTDW is found to be strong during the easterly phase of the MAO indicating a possible existence of the barotropic/baroclinic instability during summer and a possible role of the QTDW towards westward forcing of the background wind.

On the variability of the diurnal tide and coupling with planetary waves in the MLT over Cachoeira Paulista (22.7°S, 45°W)

“…1, since these long period oscillations have significant role in controlling the background wind. Observed asymmetry of the semiannual oscillation with higher amplitude during winter was also reported by past investigators from various low latitude locations over the globe (Hirota, 1978;Delisi and Dunkerton, 1988;Li et al, 2012;Guharay et al, 2014a). Delisi and Dunkerton (1988) concluded that higher planetary wave activity might have caused the large winter peak of the semiannual oscillation.…”

“…Previous studies by Guharay et al (2014a) from the same station and Li et al (2012) from Maui (20.7°N, 156.4°W) found maximum amplitude of the annual and semiannual oscillations at 81 km considering the probing altitude range of 80-100 km and this is the reason for selecting this altitude bin in Fig. 1, since these long period oscillations have significant role in controlling the background wind.…”

Vertical coupling of atmospheres: dependence on strength of sudden stratospheric warming and solar activity

“…If in addition to the increase in temperature the usual wintertime eastward zonal mean zonal wind at 10 hPa and at 60°N reverses its direction, then the event is called major or else minor (Andrews et al 1987). During SSW events, the interaction between enhanced planetary waves and tides and the interaction between tides and modified middle atmosphere modulate the tidal components, which register their signature in the upper atmospheric parameters (Jin et al 2012;Pedatella and Liu 2013;Guharay et al 2014). Also, nonlinear interaction between PWs and tides produces additional waves at the sum and the difference frequencies of the two original waves (Teitelbaum and Vial 1991) which fall mainly in the tidal domain.…”

Investigation of the intraseasonal oscillations over a Brazilian equatorial station: a case study

“…The mean dynamical state of the equatorial middle atmosphere is significantly controlled by large-scale long period oscillations, i.e., semiannual oscillations (SAO) and quasibiennial oscillations (QBO) (Burrage et al 1996;Baldwin et al 2001;Guharay et al 2014a). These oscillations show downward propagation of their phases with altitude since they are driven by deposition of energy from upward propagating gravity waves, Rossby waves, Kelvin waves, etc.…”