Recent weather disasters: Are jet streams to blame?

Johannesburg -  In Pakistan there are drowned homes and millions of lives set adrift by floods, in Russia wheat crops have been shrivelled by drought and devoured by fire. Some scientists think the floods and the fires could be linked.

This time it seems the culprit is not climate change, but a persistent pattern of waves in the high-altitude flows of air called jet streams.

Yet Harry Hendon, of the Centre for Australian Weather and Climate Research, and a member of the global panel of scientists on monsoons, said he was not sure that the changes in the jet streams alone could have caused the quantity of rain dumped on Pakistan - the panel was in discussion and would release its assessment.

Here is what some scientists have said.

The jet stream link

Jet streams play a fundamental role in the earth's weather and are used by forecasters to predict storms and cyclones and their intensity, says the "Understanding Weather" section on the website of the British Broadcasting Corporation (BBC).

Jet streams move from west to east, and can also move north and south, pushing excess heat from the equatorial regions towards the poles, "and in turn bring cold polar air southwards".

In the northern hemisphere summer of 2010, meteorologists picked up a persistent pattern of waves in the jet stream, which had also formed during the preceding three summers.

"This jet stream pattern led to flooding in the UK during June and July 2007, and to further wet summers over western Europe in both 2008 and 2009," said Michael Blackburn, a research scientist at the National Centre for Atmospheric Science at the University of Reading in the UK, whose suggested links between the Russian drought and the Pakistan floods appeared in the New Scientist on 10 August.

"We do not know the origin of the persistent pattern of waves in the jet stream this summer," he said. "Such patterns are part of the natural variability of the atmosphere that lead to weather variations over weeks, months and seasons."

The waves "appear as a succession of northward and southward meanders of the jet stream. The northward meanders are ridges of high pressure that give cloudless skies and, in summer, hot temperatures at the ground. Conversely, the southward meanders are troughs of low pressure that give cloud and rain."

Meteorologists picked up a "large and persistent northward meander of the jet stream" over western Russia, which drew warm air far northward from the Mediterranean Sea and formed a "so-called 'blocking' anticyclone, with record high temperatures across the region," said Blackburn.

Cyclones are regions of low pressure in which the winds spin anticlockwise in the northern hemisphere and clockwise in the southern hemisphere; an anticyclone is a high pressure system in which the winds circulate in a direction opposite to cyclones.

In the eastern half of the northern hemisphere, meteorologists found that cold air had been drawn southward towards the Asian monsoon region. "This has moved the jet stream further south than normal, in a trough that has crossed the mountains over northern Pakistan, intensifying the monsoon rains there," said Blackburn.

The trough strengthened in the last week of July, at the same time that "a monsoon depression moved west from the Bay of Bengal, taking moisture-laden air across northern India and into northern Pakistan, beneath the jet stream trough," he explained.

"It appears to be this conjunction of events that led to the intense rain over northern Pakistan on 28-30 July, and the subsequent flooding that has spread south along the Indus River."

Not just the trough

However, Hendon noted that "the blocking ridge and associated trough to the north of Pakistan was well established before the floods." He said he felt that a combination of an unusually warm Indian Ocean - La Niña - with an active monsoon and an "episode of the monsoons known as an 'intraseasonal variation', caused the rather heavy downpour in Pakistan".

An intraseasonal event originates as a "region of large-scale enhanced convection over the equatorial Indian Ocean". It then moves into the monsoon and causes the rain to fall in some parts, while simultaneously suppressing it in others.

This event, known as a Monsoon Intraseasonal Oscillation (MISO), usually moves between 20 and 25 degrees north, but this time it moved to 30 degrees and even further north, said Hendon, agreeing that this could have been caused by the "blocking anticyclone" identified by Blackburn.

Hendon said it was not yet clear "how well we can predict such events, and ... how such events are preconditioned on such things as La Nina, and even the enormous ridge causing the heat wave in Russia - this is what our panel wants to assess."

Blackburn said the "weather in other regions has also been affected by the disturbed jet stream; further east, intense rain over parts of China at the start of August appears to have been associated with the 'next' trough in the pattern, while earlier hot weather in Japan was associated with a ridge still further east," he pointed out.

"West of the Russian anticyclone the trough over western Europe strengthened and extended to the Mediterranean in early August, leading to intense rain and flooding over eastern Germany on the 6th[of August]".

Climate change

Scientists are cautious in their response to suggested links between the atmospheric anomalies and climate change. "It is not possible to attribute individual extreme weather events such as the Russian heat-wave or Pakistan floods to global warming," Blackburn said.

Nevertheless, higher temperatures mean warmer air, which holds more water vapour, so peak rainfall can be expected to increase as temperatures rise.

R. Krishnan, a scientist at the Indian Institute of Tropical Meteorology, who has been studying the impact of climate variability on the Asian monsoon, said a long record of heavy rainfall events over Pakistan would have to be examined before any conclusion about the role of climate change could be drawn.

Floods

Factors on the ground must also be taken into account in any assessment. "In the case of Pakistan, hydrologists have drawn attention to changes in river management and water use over time," Blackburn said.

"These affect the severity of flooding resulting from any given amount of precipitation - in addition, the scale of human impact, and therefore the scale of the emergency relief and longer-term recovery, increase with increasing population," he pointed out.

"Pakistan, in common with many developing countries, has seen its population double over the last 30 years, following annual increases of around 3 percent in the 1970s."

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