Climatology of the semiannual oscillation of the tropical middle atmosphere

Abstract

Abstract. We have used a variety of satellite, ground-based, and in situ observations to construct a climatology of the semiannual oscillation (SAO) of the tropical middle atmosphere. The sources of data include rocketsonde observations of winds and temperature, MF radar wind observations, and observations of winds and temperatures made from space by the High Resolution Doppler Imager (HRDI) and the Solar Mesosphere Explorer (SME). These data sets provide a generally consistent picture of the SAO, of the relationship between its stratospheric and mesospheric manifestations, and of its apparent modulation by the stratospheric quasi-biennial oscillation (QBO). In agreement with earlier studies, we find that the first cycle of the stratospheric SAO (which begins with the stratopause easterly phase in northern winter) is stronger than the second cycle (beginning with the easterly phase in southern winter). Similar behavior is apparent in the mesosphere, where the easterly phase is stronger during the first cycle than during the second cycle. HRDI and MF radar are capable of observing the seasonal cycle well into the lower thermosphere. Data from these two sources indicate that a strong $AO is present up to about 90 kin, giving way above this altitude to time mean easterly winds with a weak semiannual variation. Between 105 and 110 km, HRDI data indicate the presence of a westerly wind layer with Mmost no seasonal variation. Apparent modulation of the stratospheric SAO by the QBO is found in rocketsonde data, while HRDI and MF radar observations suggest a correlation between the QBO and the easterly phase of the mesospheric SAO. We discuss the implications of these observations for the wave processes that drive the $AO.

Properties of the Long-Term Oscillations in the Middle Atmosphere Based on Observations from TIMED/SABER Instrument and FPI over Kelan

“…There are seasonal asymmetries in SAO: the SAOs at 45 km and 85 km have larger maxima at spring equinoxes than those at autumn equinoxes, while the SAO at 75 km has larger minima at spring equinoxes. This is similar to the results given by Garia et al [45] except at 85 km, where the SAO amplitude derived from SME (Solar Mesosphere Explorer) satellite is considered to contaminate by the diurnal tide. The phases of the SAO in the upper stratosphere (45-50 km) are around the equinox and are mostly in phase with those in the UMLT.…”

“…However, many former studies reported an asymmetric structure of SSAO with a maximum around 10 degrees in the Southern Hemisphere using rocket observations [7,11,44,45]. Moreover, Garcia et al [45] show that the SSAO was strongly influenced by a stronger cross-equatorial advection of mean easterly winds from the Southern Hemisphere summer than that from the Northern Hemisphere winter on a zonal mean basis. This asymmetry between the hemispheres could reflect the meridional asymmetry of the SSAO amplitude in the zonal wind field.…”

Observation of horizontal winds in the middle-atmosphere between 30° S and 55° N during the northern winter 2009–2010

“…When band-B is measured, data above 0.1 hPa are replaced by the HCl line retrievals. The downward progression from the lower mesosphere to the middle stratosphere of the eastward phase (reddish colour) is typical of the winter SAO signal (Hirota, 1980;Garcia et al, 1997). A large variation in amplitude is measured at the stratopause (∼ 1 hPa) where a complete reversal of the wind direction occurred: +20 m s −1 (eastward) in November, −60 m s −1 in January and +20 m s −1 in April.…”

Relation between Equatorial Mesospheric Wind Anomalies during Spring and Middle Atmosphere Variability Modes

“…Observations of inter-annual (year-to-year) variations of winds in the equatorial upper mesosphere (80-100 km) are available for the last decades (Garcia et al 1997;Day and Mitchell 2013;de Wit et al 2013;Kishore Kumar et al 2014). Strong easterly wind anomalies during early spring (so-called Mesospheric Spring Equinox Enhancements, MSEE) are usually observed during the westerly phase of the QBO (hereafter: QBO-West) (Garcia et al 1997), but not in all years. As suggested in Kishore Kumar et al (2014) (referred to as KK14), MSEEs are lacking during QBO-West when the northern hemisphere was subject to strong sudden stratospheric warmings (SSWs).…”

“…In addition, they modulate the semi-annual oscillation (SAO) in the stratopause region and contribute to the wind variations in the mesosphere (Andrews et al 1987;Baldwin et al 2001;Plumb 2002;Fritts and Alexander 2003;Haynes 2005;Shepherd 2007). Observations of inter-annual (year-to-year) variations of winds in the equatorial upper mesosphere (80-100 km) are available for the last decades (Garcia et al 1997;Day and Mitchell 2013;de Wit et al 2013;Kishore Kumar et al 2014). Strong easterly wind anomalies during early spring (so-called Mesospheric Spring Equinox Enhancements, MSEE) are usually observed during the westerly phase of the QBO (hereafter: QBO-West) (Garcia et al 1997), but not in all years.…”