Characteristics of Low-pressure Systems associated with Intraseasonal Oscillation of Rainfall over Bangladesh during Boreal Summer

Abstract:

Characteristics of low-pressure systems (LPSs) responsible for submonthly scale (7-25
days) intraseasonal oscillation (ISO) in rainfall over Bangladesh and their impact on the
amplitude of active peaks are investigated for 29 summer monsoon seasons. Extreme
and moderate active peaks are obtained based on the amplitude of 7-25-day filtered
rainfall series. By detecting the LPSs that formed over the Indian monsoon region, it
was found that about 59% (62%) of extreme (moderate) active peaks of rainfall are
related to LPSs. These LPSs have horizontal scale of about 600 km and vertical scale
of about 9 km. For the extreme active peak, the locations of the LPS centers are
clustered significantly over and around Bangladesh, accompanied by the maximum
convergence in the southeast sector of the LPSs. After their formation, they tend to
remain almost stationary over and around Bangladesh. In contrast, for the moderate
active peak, the LPS centers are located over the Ganges Plain around 85°E, and the
maximum convergence of the LPSs occurs around their centers. This difference in the
convergence fields is associated closely with the geographical features to the north
and east of Bangladesh and the horizontal scale of the LPSs. These features suggest
that the amplitude of the active peaks in the submonthly scale ISO is modulated by
small differences in the locations of the LPS centers. These findings suggest that
improved predictions of both genesis location and the tracks of the LPSs are crucial to
forecasting seasonal rainfall over Bangladesh.