A study on quasi-biweekly oscillation around Himalaya, Assam and Bangladesh regions

点击翻译按钮获取中文摘要

This study elucidated a process of distinct rainfall fluctuation related to quasi-biweekly Oscillation (QBW) over Himalaya-Assam-Bangladesh (HAB) region, where one of the heaviest rainfall is observed in the world, by means of data analysis and a numerical simulation. QBW in rainfall became a dominant mode of intraseasonal oscillation since 1980s around Bangladesh. QBW in rainfall over HAB region is most active, especially in Meghalaya plateau and the western coast of Myanmar, in land areas of Asian monsoon. In active (break) phase of rainfall in QBW in the HAB region, a low-level westerly/southwesterly (easterly/southeasterly) is dominated there. The large zonal wind fluctuation is induced by westward propagating n=1 equatorial Rossby wave with a wave length of ~6000 km. Rainfall in active phase of QBW often shows a remarkable diurnal variation around the Meghalaya plateau.The variation is caused by diurnal variation of low-level jet around 900 hPa. Thestrong low-level flow transports abundant moisture to the southern slope of the Meghalaya Plateau during the active period, which triggers forced lifting and brings orographic rainfall. However, low-level wind during the inactive period is too weak to induce orographic rainfall. Thus, the distinct QBW in HAB region is strongly enhanced locally (~500Km) by the topographical features although atmospheric circulation causing the ISO has horizontal scale of ~6000 Km.