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 low-level 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.

Highlights

  • During the summer monsoon season (June–August), the prevailing westerly/southwesterly winds bring abundant moisture from the ocean to South Asia

  • In the active intraseasonal oscillation (ISO) phase of the Meghalaya–Bangladesh–coast of the western Myanmar (MBWM), a strong low-level westerly/southwesterly flows around the plain and a center of cyclonic vorticity appears over Bangladesh

  • The associated three-dimensional atmospheric circulation and time-evolution features were investigated for the whole Asian summer monsoon system from the tropics to the mid-latitudes using APHRODITE, TRMM-PR, and JRA25 reanalysis datasets

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Summary

Introduction

During the summer monsoon season (June–August), the prevailing westerly/southwesterly winds bring abundant moisture from the ocean to South Asia. The ISO feature allows the submonthly-scale ISO to modulate the total seasonal rainfall and the spatial patterns of circulation and convection around Bangladesh (Fujinami et al 2011; hereafter referred to as FHY11). FHY11 showed the features of low-level atmospheric circulation and OLR fields related to the ISO in Bangladesh for a limited area (5–35°N, 65–115°E) over the South Asian monsoon domain. We provide a detailed picture of the rainfall distribution in the submonthly-scale ISO in relation to topographical features over South Asia. 3. Section 4 gives the spatiotemporal structure of large-scale atmospheric circulation and convection extending from the tropics to the mid-latitudes, related to the distinct submonthly-scale ISO around the Meghalaya (located in northeastern India near the Bangladesh border)–Bangladesh–coast of the western Myanmar region.

Data and methods
Climatological features
Detailed rainfall distribution and its time evolution
Covariability between the tropics and mid-latitudes
Findings
The dynamics of distinct submonthly-scale ISO over the MBWM
Summary
Full Text
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