The Connection Between the Lower and Upper Level Quasi-Biweekly Oscillations of the South Asian Monsoon

Among the modes of variability present during the South Asian Monsoon, one of the most prominent is the Quasi-Biweekly mode (QBW). This mode is usually identified by filtering dynamical and thermodynamical fields with a 10 to 20 day band-pass filter, and its spatial structure defined via composite analysis or other statistical techniques. The tropospheric signature of the QBW is an equatorial convective anomaly, near 160E, that propagates northwestward towards the South China Sea in about 8 days, and then to the west over South Asia in the next 8 days (Kikuchi and Wang, 2009).

Perhaps due to the fact that the QBW has been shown to interact with monsoon depression (Goswami, 2003), the interest in understanding the QBW has recently grown. Explanations of its genesis range from dynamics intrinsic to the tropical atmosphere (Chatterjee and Goswami, 2004) to global instabilities that lead to its formation (Krishnamurti and Bhalme, 1976). In most likelihood, both mechanisms are important, but we focus on the second as there is one component of the monsoon system that produces biweekly oscillations seemingly independently of the others.

Such system is the upper tropospheric monsoon anticyclone, evident on the Isentropic Potential Vorticity (IPV) fields. The gyre forms both because of the effect of convection and because of the elevated sensible heating associated with the Tibetan Plateau. Upper level quasi-biweekly oscillations (UQBW) emerge as a consequence of its dynamical instability. In fact, simple shallow water models have shown that the UQBW is intrinsic to the large scale forcing of the anticyclone (Hsu and Plumb, 2000). The question is then: Can the UQBW induce the lower tropospheric QBW?

We present statistical analysis that show a strong correspondence between the UQBW and the QBW. In general the UQBW is related with the breaking of mid-latitude Rossby waves over the Pacific Ocean and the recirculation of its associated potential vorticity around the outer edge of the monsoon anticyclone. Associated with these events, a characteristic lower level QBW signal is observed. We discuss the possible mechanisms by which these two oscillations might be connected, and the possible consequences of this connection.