ORIGINAL PAPER Climatology of stratospheric gravity waves and their interaction with zonal mean wind over the tropics using GPS RO and ground-based measurements in the two phases of QBO Debashis Nath & Wen Chen & Amitava Guharay Received: 7 April 2013 /Accepted: 27 March 2014 # Springer-Verlag Wien 2014 Abstract The present manuscript deals with the spatial dis- tribution of gravity wave activity over the tropics using ten years (2001 – 2010) of CHAllenging Mini Payloads (CHAMP) and Constellation Observing System for Meteorol- ogy, Ionosphere and Climate (COSMIC) Global Positioning System (GPS) Radio Occultation (RO) data and ground-based radiosonde measurements over an equatorial station Singa- pore (1.36°N, 103.98°E) and four tropical stations, Guam (13.48°N, 144.80°E), Palau (7.33°N, 134.48°E) in the north- ern hemisphere, Darwin (12.41°S, 130.88°E) and Pago-Pago (14.33°S, 170.71°W) in the southern hemisphere from Janu- ary 2001–December 2010. It also aims to quantify the differ- ence in wave activity in the two phases of QBO, climatolog- ically. Space-time spectra have been constructed over a lati- tude band of ±10° and decomposing the CHAMP/COSMIC temperature perturbations into symmetric and antisymmetric modes about the equator. Clear signature of equatorial waves with higher wavelength and a constant background of gravity waves (GW) with inertial frequency are prominent in the spectra. Strong GW and mean flow interaction can be seen in the lower stratosphere potential energy density (E P ) and momentum flux with enhanced wave activity during the west- erly (eastward wind) phase of quasi-biennial oscillation (QBO) (WQBO) over the equatorial and tropical stations like Singapore and Palau/Darwin, respectively. From the latitudi- nal distribution of energy density, the occurrence of two-peak structure in energy density can be seen in the middle and lower latitudes with an enhancement during the WQBO phase. The E P associated with GWs are calculated at lower stratospheric (19–26 km) heights and are compared with outgoing longwave radiation (OLR) to correlate the wave events with tropical deep convection during the easterly, i.e. westward wind (EQBO) and WQBO phases of QBO. Clear coherence of convection due to Asian summer monsoon with localized enhancement of wave activity over Western Pacific, South America and African region during the WQBO phase is observed at the lower stratospheric heights. Significant en- hancement is observed during Northern Hemisphere winter months and minimum during summer. The longitudinally elongated portion of E P over the equator is partially affected by Kelvin wave (KW) like disturbances with short vertical scales and also by inertia GW. 1 Introduction The knowledge of gravity wave and zonal mean flow interac- tions is important for understanding the dynamical processes in the equatorial atmosphere including the long-term variations of the propagation characteristics and its impact with the back- ground mean wind. In the equatorial stratosphere, the quasi- biennial oscillation (QBO) can be considered as one of the best examples of wave-mean flow interaction, which can be seen as downward propagating easterly and westerly wind regimes. The period of oscillation varies from 22 to 34 months with a mean around 28 months. Active convection over the equatorial region is mainly responsible for the generation of the atmo- spheric waves over the tropics (Tsuda et al. 1994; Alexander and Pfister 1995; Vincent and Alexander 2000; Alexander et al. 2008b, c; Nath et al. 2009, 2013). The equatorial QBO is mainly driven by the Kelvin wave (KW) (Wallace and Holton 1968; Wallace and Kousky 1968a, b; Lindzen and Holton 1968; Holton et al. 2001; Randel and Wu 2005; Tsuda et al. 1994; Salby et al. 1984) and mixed Rossby-Gravity wave D. Nath (*) : W. Chen Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China e-mail: debashis.narl@gmail.com A. Guharay National Institute for Space Research, INPE, Sao Jose dos Campos, SP, Brazil Theor Appl Climatol DOI 10.1007/s00704-014-1146-7