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Introduction

Quasi Biennial Oscillation (QBO) in zonal wind is a dominant natural oscillation in the equatorial lower stratosphere. The driving force for the QBO is the vertical transfer of momentum from the troposphere to stratosphere by Kelvin and Rossby-Gravity waves. One of the current problem is the vertical coupling of the QBO to the mesosphere in the up, and to the troposphere below, its influence on the tropospheric weather conditions etc. Several studies have revealed the biennial type signals in meteorological quantities in the troposphere (Angell and Korshover,1964; Terray, 1995; Trenberth, 1975 etc). Biennial oscillation is found in the rainfall over India (Mooley and Parthasarathy, 1984), Indonesia (Yasunari and Suppiah, 1988). A strong biennial component is seen in the ENSO phenomena (Rasmusson et al, 1990; Roplewski et al, 1992) and is proved to be an integral part of the Asia-Pacific climate. This Tropospheric Biennial Oscillation (TBO) has an irregular period of 2-3 years and shows eastward movement.

The cause for the TBO is not clearly known. Several theories and hypothesizes have been proposed to address the cause, development, evolution of TBO. Majority of the theories suggest that the TBO is caused by the interaction of atmosphere-ocean-monsoon systems and extra tropics (Nicholls, 1978; Meehl, 1994, 1997; Chang and Li, 2000, etc). In an entirely different mechanism, TBO is considered as the result of the forcing from the QBO in the lower stratosphere through an unclear stratosphere ­ troposphere interactions. Gray et al (1992) suggested a mechanism by which QBO can alter the deep convective activity over west Pacific. Khandekar (1996, 1998) proposed a conceptual model by which QBO can produce a biennial oscillation in Indian monsoon activity. Using General Circulation Model simulations, Giorgetta et al (1999) investigated the mechanisms by which the QBO could influence the tropical tropospheric circulation in areas of deep convections. Several studies showed that the phase of the QBO influence the weather events in the troposphere. Indian summer monsoon rainfall activity is seems to be directly related with the phase of the QBO (Bhalme et al, 1987, Mukherjee et al, 1985). Gray (1984) identified an association between QBO phase and seasonal incidence of Atlantic Tropical cyclones.

Using zonal wind data from Singapore, Korur and Ponape, Yasunari (1989) suggested that there exists a coherent phase structure between the lower stratospheric zonal wind QBO and biennial oscillations scale zonal wind anomalies in the lower and upper troposphere. He also showed that the QBO in the lower stratosphere is also coupled with that in the sea surface temperature anomalies in the equatorial Pacific. Yasunarišs (1989) results are based on the inferences from the results obtained from only one parameter (zonal wind) over a few stations in the maritime continent and equatorial Pacific regions. It is interesting to know whether this kind of phase coherence is appearing between TBO and QBO over Indian monsoon region and if it is so, whether it is present in other parameter also?  The station selected for this purpose Thumba, is under the influence of both summer and winter monsoons of the Indian peninsula and tropical lower stratospheric QBO.

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