Journal of Atmospheric and Solar-Terrestrial Physics 70 (2008) 764–773 Stratosphere–troposphere interaction associated with biennial oscillation of Indian summer monsoon K. Mohankumar, Prasanth A. Pillai à Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin 682 016, Kerala, India Received 8 September 2007; received in revised form 7 November 2007; accepted 6 December 2007 Available online 2 January 2008 Abstract Quasi-biennial oscillation (QBO) is a predominant phenomenon in the tropical stratosphere and troposphere. The possible interactions between the stratospheric QBO and tropospheric biennial oscillation (TBO) over the Indian monsoon region as well as the equatorial region is investigated using the zonal wind data of 23 vertical levels (1000–1 hpa) from 1960–2002. The structure of lower stratosphere and troposphere are entirely different over the equator and India. In biennial scales, both the stratosphere and troposphere over the Indian region are closely related and winter season QBO is a good predictor of Indian summer monsoon rainfall. r 2007 Elsevier Ltd. All rights reserved. Keywords: Quasi-biennial oscillation; Tropospheric biennial oscillation; Summer monsoon; Stratosphere–troposphere interaction 1. Introduction Quasi-biennial oscillation (QBO) described by Reed et al. (1961), is a remarkable regular oscillation of the zonal wind in the equatorial stratosphere with a periodicity of about 26 months. During the one half of this period, easterlies propagate from upper stratosphere to lower stratosphere and during the other half they are replaced by westerly winds. Several studies reported the presence of biennial oscillation in various atmospheric and oceanic parameters at different regions of the globe including tropospheric winds, temperature, Indian summer monsoon, tropi- cal sea surface temperature (SST), southern oscilla- tion, etc. (e.g., Nicholls, 1978; Rasmusson et al., 1990; Ropelewski et al., 1992; Goswami, 1995; Terray, 1995; Meehl, 1997; Sathiyamurthy and Mohankumar, 2000). This biennial oscillation observed in the tropical regions, especially over the Indo-Pacific region is known as tropospheric biennial oscillation (TBO). The mechanism of TBO is believed to be quite different from that of stratospheric QBO. It is postulated that stratospheric QBO is excited primarily by vertically propagating equatorial wave modes, and these modes can excite a quasi-biennial mean wind response through mechanism of radiative damping to decay with height and thus to transfer momentum to mean zonal flow (Holton and Lindzen, 1972). This mechanism indicates linkage between the tropospheric disturbances and the QBO in the zonal winds of the lower stratosphere. Some theoretical studies like Brier (1978) and Nicholls (1978) presented a different mechanism for TBO, including air–sea interaction in the Indonesian–north ARTICLE IN PRESS www.elsevier.com/locate/jastp 1364-6826/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2007.12.001 à Corresponding author. Tel./fax: +91 484 2353662. E-mail addresses: kmkcusat@gmail.com (K. Mohankumar), prasanthap2@yahoo.co.in (P.A. Pillai).