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

Abstract

The quasi-biweekly oscillation (QBW) is a dominant intraseasonal mode in summer rainfall over Bangladesh. Active phases of the QBW are often accompanied by low pressure systems (LPSs) such as vortex-type lows. This study investigated the effects of two intraseasonal modes, the QBW and the boreal summer intraseasonal oscillation (BSISO), on the genesis of LPSs over Bangladesh during 29 summer monsoon seasons. Daily lag composites of convection and low-level atmospheric circulation were constructed for active-phase cases with LPSs (LPS case) and without LPSs (non-LPS case) based on rainfall in the QBW over Bangladesh. In the QBW mode, a westward propagation of an anticyclonic anomaly from the western Pacific to the Bay of Bengal (BoB) is common in both cases. However, the anticyclonic center in the LPS case is located slightly to the east of that in the non-LPS case, which results in stronger cyclonic vorticity over and around Bangladesh. In contrast, the BSISO mode shows an opposite phase between the two cases: a cyclonic (anticyclonic) anomaly propagating northward from the equator to the BoB in the LPS case (non-LPS case). In the LPS case, the cyclonic anomaly in the BSISO mode enhances the westerly (easterly) flow over the BoB (Bangladesh) in the active phase, resulting in the enhancement of cyclonic vorticity over the northern BoB and Bangladesh, in cooperation with the QBW mode. These results suggest that both the QBW and BSISO modes have significant influence on the environmental conditions for LPS genesis over Bangladesh.

Contrasting Features of Monsoon Precipitation Around the Meghalaya Plateau Under Westerly and Easterly Regimes

“…The synoptic‐scale low under the ER resembles the composite structure of low‐pressure systems (LPSs) such as monsoon lows and depressions that initiate over the head of the Bay of Bengal and move northwestward toward India during summer (Godbole, ; Hunt et al, ; Krishnamurthy & Ajayamohan, ). The frequency and tracks of LPSs depend strongly on the phase of the ISOs (Goswami et al, ; Krishnamurthy & Ajayamohan, ; Hatsuzuka & Fujinami, ). Under the ER (i.e., the break phase of the ISO), which generally corresponds to the active phase of the ISO around central India, LPSs typically form over the head of the Bay of Bengal and move northwestward.…”

“…The synoptic-scale low under the ER resembles the composite structure of low-pressure systems (LPSs) such as monsoon lows and depressions that initiate over the head of the Bay of Bengal and move northwestward toward India during summer (Godbole, 1977;Hunt et al, 2016;Krishnamurthy & Ajayamohan, 2010). The frequency and tracks of LPSs depend strongly on the phase of the ISOs (Goswami et al, 2003;Krishnamurthy & Ajayamohan, 2010;Hatsuzuka & Fujinami, 2017 Journal of Geophysical Research: Atmospheres 10.1002/2016JD026116 India, precipitation tends to decrease around the Meghalaya Plateau, consistent with our results for the ER. Figure 6 shows a latitude-height cross section for the horizontal wind speed, meridional component of water vapor flux, and vertical velocity at the longitude of the Meghalaya Plateau.…”

“…The ISOs typically have large-scale spatial structures and propagating characteristics in the atmospheric circulation fields and convection. Changes in atmospheric circulation due to the ISOs provide contrasting atmospheric environments for the genesis of tropical cyclones and monsoon lows (Goswami et al, 2003;Hatsuzuka et al, 2014;Hatsuzuka & Fujinami, 2017;Kikuchi & Wang, 2010;Krishnamurthy & Ajayamohan, 2010;Yanase et al, 2012) as well as diurnal variation in precipitation and orographic precipitation (Fujinami & Yasunari, 2001;Singh & Nakamura, 2010;Shrestha et al, 2012;Sato, 2013).…”

Mesoscale precipitation systems and their role in the rapid development of a monsoon depression over the Bay of Bengal

“…Recent studies have reported a favourable environment for the genesis of LPSs around South Asia (Yanase et al, 2012;Tsuboi and Takemi, 2014;Ditchek et al, 2016;Hatsuzuka and Fujinami, 2017;Samanta et al, 2018). Humid and convectively unstable environments with high cyclonic vorticity and high sea-surface temperature (SST) favour genesis over the northwestern BoB despite relatively strong vertical wind shear due to the vertical structure of a large-scale Asian monsoon circulation.…”

“…Wang et al, 2006;Kikuchi et al, 2012) and quasi-biweekly oscillation (QBW: e.g. Chattergee and Goswami, 2004;Fujinami et al, 2014), clustering the genesis points and tracks of LPSs between active and break phases of intraseasonal oscillations (Goswami et al, 2003;Krishnamurthy and Ajayamohan, 2010;Yanase et al, 2012;Hatsuzuka and Fujinami, 2017).…”