Dynamics of distinct intraseasonal oscillation in summer monsoon rainfall over the Meghalaya–Bangladesh–western Myanmar region: covariability between the tropics and mid-latitudes

Abstract

Detailed spatiotemporal structures for the submonthly-scale (7-25 days) intraseasonal oscillation (ISO) in summer monsoon rainfall and atmospheric circulation were investigated in South Asia using high-quality rainfall and reanalysis datasets. The Meghalaya-Bangladesh-coast of the western Myanmar (MBWM) region is the predominant area of submonthly-scale ISO in the Asian monsoon regions. The distinct rainfall ISO is caused by a remarkable alternation of low-level zonal wind between westerly and easterly flows around the Gangetic Plain on the same timescales. In the active ISO phase of the MBWM, a strong low-level westerly/southwesterly flows around the plain and a center of cyclonic vorticity appears over Bangladesh. Hence, a local southerly flows toward the Meghalaya Plateau and there is strong southwesterly flow towards the coast along southeastern Bangladesh and western Myanmar, resulting in an increase in orographic rainfall. Rainfall also increases over the lowland area of the MBWM due to the low-level convergence in the boundary layer under the strong cyclonic circulation. The submonthly-scale lowlevel wind fluctuation around the MBWM is caused by a westward moving n = 1 equatorial Rossby (ER) wave. When the anticyclonic (cyclonic) anomaly related to the ER wave approaches the Bay of Bengal from the western Pacific, humid westerly/southwesterly (easterly/southeasterly) flows enhance around the Gangetic Plain on the northern fringe of the anticyclone (cyclone) and in turn promote (reduce) rainfall in the MBWM. Simultaneously, robust circulation signals are observed over the mid-latitudes. In the active phase, cyclonic anomalies appear over and around the TP, having barotropic vertical structure and also contributing to the enhancement of low-level westerly flow around the Gangetic Plain. In the upper troposphere, an anticyclonic anomaly is also observed upstream of the cyclonic anomaly over the TP, having wavetrain structure. The mid-latitude circulation around the TP likely helps to induce the distinct ISO there in conjunction with the equatorial waves. Thus, the distinct ISO in the MBWM is strongly enhanced locally (*500 km) by the terrain features, although the atmospheric circulation causing the ISO has a horizontal scale of *6,000 km or more, extending across the whole Asian monsoon system from the tropics to mid-latitudes.

Effects of the South Asian Monsoon Intraseasonal Modes on Genesis of Low Pressure Systems over Bangladesh

“…Fujinami et al (2011) found that the rainfall spectrum in Bangladesh has a pronounced peak at about 14 days, while no statistically significant peak appears within the 30-60-day period. They also reported that the interannual variability of the total summer monsoon rainfall over Bangladesh is correlated significantly with the 7-25-day rainfall variance, suggesting that the QBW controls the interannual variability of the total seasonal rainfall in the country.…”

“…These two ISO modes have differing spatiotemporal structures. The BSISO mode exhibits northward propagation of convection and circulation anomalies from the equatorial Indian Ocean (e.g., Yasunari 1981;Murakami et al 1984;KemballCook and Wang 2001;Lawrence and Webster 2002;Jiang et al 2004;Wang et al 2006), whereas the QBW mode is characterized by westward propagation of convection and circulation anomalies from the western Pacific (e.g., Krishnamurti and Ardanuy 1980;Chen and Chen 1993;Yokoi and Satomura 2005;Hoyos and Webster 2007;Kikuchi and Wang 2009;Fujinami et al 2014).…”

“…The area around Bangladesh exhibits a distinct QBW during summer monsoon rainfall (e.g., Ohsawa et al 2000;Murata et al 2008;Fujinami et al 2011Fujinami et al , 2014Sato 2013;Hatsuzuka et al 2014). Fujinami et al (2011) found that the rainfall spectrum in Bangladesh has a pronounced peak at about 14 days, while no statistically significant peak appears within the 30-60-day period.…”

“…Previous studies have revealed that the distinct QBW in rainfall over Bangladesh is caused by the interaction between the low-level zonal wind fluctuation around the Ganges Plain and the topographical features around Bangladesh (Fujinami et al 2011. In the active (break) phase of the QBW over Bangladesh, a low-level westerly/southwesterly (easterly/southeasterly) dominates along the plain.…”

“…The strong westerly/southwesterly flow enhances the orographic lifting of low-level moisture over the windward area of the Meghalaya Plateau and Chittagong Hill Tracts during the active phase, which results in a significant increase in rainfall over the country. Recently, Fujinami et al (2014) demonstrated that the QBW-scale low-level zonal wind fluctuation is controlled by a westward-moving n 5 1 equatorial Rossby wave with a wavelength of ;6000 km. However, their analyses were based on all active peaks without consideration of LPS and non-LPS cases.…”