Pakistan floods (late July/August 2010)
Unusually heavy monsoon rainfall over Pakistan
Heavy monsoon rainfall has been causing devastating flooding over Pakistan, particularly over the northern-most provinces. Below you can see an anmination of rainfall from the 23rd to 30th July (in mm/hour):
Pakistan normally expects to receive monsoon rains between the months of July and September, however the quantity and persistence during late July has been most unusual. Most of the heaviest rainfall fell over three days in late July (28th-30th), particularly in the Gilgit Baltisan / Azad Kashmir and Khyber Paktunkhwa regions. The region formerly known as the Northwest Frontier Province (NWFP) recorded July rainfall totals of 179.5% above normal. Even recording stations in northern Punjab received heavy rainfall, with some parts of Islamabad totalling more than 250mm on 30th July. See rainfall data>>
At the same time we've seen very high temperatures and wildfires over western Russia.
Are these events linked?
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Credit: Produced by the University of Reading using TRMM satellite rainfall data
What's causing these extreme events?
Links with the high level jet stream
A ribbon of strong winds high in the atmosphere, called the jet stream, snakes across the northern (and southern) hemisphere and meanders north and south. Sometimes the jet stream gets stuck in one position and this can cause extreme heat and drought in some regions and heavy rainfall in others. For example, it was a persistent and unusually southerly jet stream over Europe in July 2007 that brought heavy rainfall and flooding to large parts of southern England.
This year, the high level jet stream has been in an unusual wave pattern since mid July - with a series of ridges and troughs stretching from western Europe to eastern China. This brought high pressure and dry, very hot conditions over western Russia and caused an intensification of the monsoon rains over Pakistan. The jet stream also has an unusual split structure with a second band of strong winds over northern Eurasia.
A trough in the jet stream, positioned directly over Pakistan, has caused the warm, moist air drawn off the Indian Ocean to rise strongly leading to very heavy rainfall. Near the surface, the monsoon easterly winds have extended unusually far along the Himalayan foothills and into northern Pakistan (see an image of the low level winds>>).
The wavy nature of the jet stream and it's southerly position over Pakistan appears quite unusual, although meteorologists need to look carefully through past records to analyse just how unusual it is. At the moment, they don't know what is causing this unusual jet stream behaviour. The jet stream is a complex phenomenon which exists because of the temperature contrast between warm sub tropical air to the south and cold polar air to the north. It is partly controlled by local weather, but is also influenced by distant conditions (like warm/cold ocean temperatures in the tropics) - this makes it's behaviour hard to predict.
Cool conditions over the tropical Pacific
Over the tropical Pacific Ocean anomalously cool ocean temperatures are developing - known as a La Nina event. This image shows recent sea surface temperatures in the tropical Pacific and shows the developing cool conditions - La Nina event.
Such cool conditions can often be associated with heavier than normal monsoon rains over the South Asian region, however there is no reason to blame it for this particular event just yet. This image shows how Indian monsoon rainfall is related closely to but not exclusively with La Nina and El Nino events in the tropical Pacific.
Is the heavy rain the result of climate change?
Many of the climate models used to predict future climate suggest a wetter monsoon under climate change, although there is disagreement between models on the details. This image (credit: University of Reading) shows rainfall change over Asia by the end of the century from 15 different climate models used in the2007 IPCC report.
Predictions also suggest that rainfall could come in heavier bursts and that the monsoon over India could become more variable - with drier periods followed by heavier periods of rain.
However, it would be wrong to look at a single event, such as the Pakistan floods, and say that it is due to climate change. The world has always experienced extreme weather, what climate change will do is to change the frequency with which such events occur. So what scientists have to do is to look back at observations and see whether extreme events (like heavy rainfall and heatwaves) are becoming more (or less) common. There is a large amount of evidence to show that very warm days and heavy rainfall events are becoming more frequent and that human-induced climate change is playing a part.