The Bangladesh monsoon
The South Asian monsoon has two distinct branches – the eastern branch has caused loss of life across Bangladesh.
Monsoon flooding is a part of everyday life in Bangladesh [Getty Images]
The devastating landslides in Bangladesh, which have killed almost 70 people, are the result of days of torrential rain.
There are reports of up to 400mm of rain falling in just 12 hours at Chittagong – a remarkable total for a region well used to heavy rain.
The rain is linked to the South Asia summer monsoon but the eastern branch of the monsoon has a very distinct character if its own.
Rainfall around the northern Bay of Bengal and Ganges Delta starts to increase as early as May when inflowing warm, humid air is overrun by cold, dry air aloft. This makes for an unstable mix and squally showers result from these ‘westerly disturbances’.
Thunderstorms may also develop in the region when there is a trough of low pressure in the upper atmosphere.
The main thrust of summer rain derives from ‘monsoon depressions’.
These storms, which may affect thousands of square kilometres, form in the northern portion of the Bay of Bengal before tracking west or northwest across India, steered by the easterly winds, high in the atmosphere.
These can form as early as June – as we have seen this week – but rainfall usually peaks in July and August. These features generally occur about twice a month, again when an upper atmospheric trough is present.
Monsoon depressions can often be 1000 kilometres across with a typical lifetime of two to five days. Rainfall totals are usually in the range of 120 to 200mm.
The monsoon rains, which are vital to agriculture across the subcontinent, have been behind schedule across much of central and northern India and western Bangladesh, so far this summer.
But the flood hit eastern areas of Bangladesh and eastern states of India are reeling from this early-season deluge.