Parts of Pakistan and Thailand have seen devastating floods after two weeks of persistent torrential rain. The floods have come at a time when many parts of South Asia expect heavy rainfall as part of the region’s summer monsoon, but it has been particularly heavy for the affected areas and it has come late in the season.
It is usual for monsoon rainfall to vary from year-to-year and for there to be large regional differences in the amount of rain, but what has caused the problems this year?
Observations show there has been a series of low pressure systems passing over Pakistan from northern India over the past two weeks, with little respite in between. This has given no time for water to flow away or seep into the ground, causing a build up of floodwater.
With the rain coming so late in the season, there are indications that the region’s monsoon is continuing beyond its usual length. This means the conditions which persist in a monsoon and bring the heavy rainfall have allowed the low pressure systems to continue to move over the region, bringing the heavy rains with them. It’s not clear why the monsoon would be extended like this, but there is some suggestion that this may be related to knock-on effects from long-term cycles in sea-surface temperatures in the Pacific and Indian Oceans, although it could also be part of the normal year to year variability in the length of the monsoon period.
In 2010, while the flooding was similarly devastating, the reasons behind it were different. This came in the middle of the monsoon season and most of the heaviest rainfall came from a single low pressure system which moved over the region over a period of four or five days. Such low pressure systems aren’t unusual, but in this case it was unusually intense and moved very far to the west – causing the floods. The low pressure system was intense because atmospheric conditions prevailing at the time meant the low pressure dragged in additional warm moist air from the Indian Ocean, intensifying the rains.