Quasi-Biennial and Quasi-Triennial Oscillations in the Summer Monsoon Rainfall of the Meteorological Subdivisions of India

Abstract

The summer monsoon rainfall time series for 29 subdivisions of India from 1951 to 1991 (41 years) were subjected to maximum entropy spectral analysis and showed significant periodicities in a wide range, including quasi-biennial oscillation (QBO 2–3 years) and quasi-triennial oscillation (QTO 3–3.9 years). After filtering out periodicities of 4.0 years or more, the residual series, considered as representative of QBO and QTO only for each series, could be separated into 4 categories (viz., category A of 10 subdivisions having only one strong QBO peak but no QTO, category B of 4 subdivisions having two strong QBO peaks but no QTO, category C of 4 subdivisions having no strong QBO or QTO, and category D of 11 subdivisions having a strong QTO). The 50-mb zonal wind showed two strong QBO peaks at 2.33 years (28 m) and 2.75 years (33 m). A comparison of stratospheric (50 mb) westerly wind and rainfall maxima showed that for many subdivisions, maximum rainfall was associated with the increasing westerly phase of zonal wind and droughts were associated with the easterly phase. A loose relationship with El Niño Southern Oscillation was also noticed for droughts in some subdivisions (mostly western India), and the two effects (50-mb wind and ENSO) seem to be operating independently of each other.