Variation in the Atmospheric Circulation over Asia and the Western Pacific Associated with the 40-day Oscillation of the Indian Summer Monsoon

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Abstract
Through use of the outgoing longwave radiation (OLR) and 700mb height fields for 1979, an investigation was conducted of the variation of the atmospheric circulation over Asia and the western Pacific associated with the 30-60 day variation of the Indian summer monsoon. Results of the analysis of the OLR indicate that convection over the northwestern Pacific along 15°N is active slightly before the active phase of the Indian monsoon over central India, while there is a decrease in convective activity prior to the break phase. This active convection (between 140°E and 150°E) propagates southward with a period of 40 days from 15°N to the equatorial zone. The cloudiness to the north of the Tibetan Plateau (around 55°N, 75°E) and over the subtropical high region (southeast of Japan) reaches a minimum slightly before the active phase of the Indian monsoon. Conversely, cloudiness reaches amaximum prior to the break phase of the Indian monsoon. From the results of the analysis of the 700mb geopotential heights, it was found that to the north of the Tibetan Plateau the maximum cloudiness corresponds to the stagnation of the trough while the minimum cloudiness is associated with the development of the ridge. The position of the subtropical high shifts northward (along 25-30°N) slightly before the active phase of the Indian monsoon and shifts southward (along 10-20°N) slightly prior to the break phase. The phase of the 30-60 day variation to the north of the Tibetan Plateau precedes that of the subtropical high region. These results suggest a close association among the 30-60 day variation of the Indian monsoon, the convection over the western Pacific, the westerly-wave movements over the Asian continent and the subtropical high in the western Pacific. These large scale interactions affect the weather regime around Japan during the Baiu season. The meridional position of the Baiu front (east of 130°E) around Japan shifts from 40°N, when the subtropical high shifts northward, to 30°N when the subtropical high shifts southward. This is due to the variation of the meridional position of the subtropical high, which is associated with the 40 day variation of the monsoon. The meridional position of the Mei-yu front (west of 130°E), in contrast, does not exhibit a cyclical variation.