Deluges around the Bay of Bengal

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  • Cumulative rainfall figures so far this season in India, colour coded with comparison to normal values. Source: India Meteorological Department.

    NASA's Terra satellite captured this impressive image of then Tropical Cyclone 02B (later Komen) on 29th July. Credit: NASA Goddard's MODIS Rapid Response Team).

    NASA's Global precipitation measurement core observatory managed to project the cross section of Tropical Storm Komen, indicating cloud tops of nearly 10 miles from bands of thunderstorms. Credits: NASA/JAXA/SSAI, Hal Pierce.

  • Deluges around the Bay of Bengal
    01.08.2015 14:11

     

    Heavy rain has been a persistent feature in parts of South Asia and Indochina over the past few weeks due to a combination of tropical cyclones and monsoonal rains, causing severe flooding, landslides and scores of deaths. Whilst this corner of the globe is not unfamiliar with torrential downpours, it is the prolonged nature of rainfall that has been catastrophic in places and has led to a state of emergency being declared in Myanmar.

    To understand why there has been such an inundation of wet weather, we have to consider the broad-scale synoptic situation present at this time of year. During the onset of summer there is a differential heating of the land and sea around the Indian subcontinent, which in turn causes low pressure to form over northern and central India and high pressure in the Indian Ocean. As air is always flowing from high to low pressure in the atmosphere, south-westerly winds are naturally drawn from the Indian Ocean to address the imbalance. Containing large amounts of moisture, these winds provide the catalyst for heavy rainfall between July and September and account for the majority of the annual rainfall in India and neighbouring Bangladesh. Known as the Southwest Monsoon, it is vital for agriculture in the region but also comes at a price as has been seen in recent weeks.

    As can be seen in the image to the left, monsoon rainfall has varied quite considerably so far with a noticeable difference between north-west and south-west states. In the past week it has been Gangetic West Bengal that has seen the heaviest rain due to the presence of a slow-moving low pressure system in the Bay of Bengal that has exacerbated the effects of the monsoon. This depression gave several hundred millimeters of rain to Bangladesh, north-east India and parts of Indochina in the early part of last week before the system deepened further, having been classified as Tropical Storm Komen on Friday.

    Resultant exceptional rainfall totals have led to flash flooding and numerous mudslides, causing deaths in Chittagong (Bangladesh), Odisha (India), northern and western Myanmar along with northern Vietnam. The situation is particularly dire in Myanmar where hundreds of thousands of people are currently displaced and living in temporary camps.

    Meteorological conditions should improve somewhat over the next week as the cyclone weakens, meaning prolonged heavy rain is likely to subside. However, further seasonal rain or showers are to be expected and with a wide area currently inundated by floodwater, even modest rainfall totals may lead to further disruption.

     

    By: Nick Prebble