AS TORRENTIAL storms visit unprecedented flooding on Pakistan, thousands of kilometres to the northwest Russia burns. The two events are linked by a large-scale pattern of atmospheric circulation which is producing a particularly persistent area of high pressure over Russia. They are also linked in both being the sort of events climate scientists predict more of in a warming world. The immediate cause of the problems is the behaviour of the jet stream, a band of high-level wind that travels east around the world and influences much of the weather below it. Part of the jet stream’s meandering is tied to regular shifts of air towards and away from the pole, called Rossby waves. The Rossby waves set up wiggles in the jet stream, wiggles which, left to themselves, would move westward. Since the jet stream is flowing eastward, though, the net effect of the Rossby waves varies. When the waves are short, they go with the jet's flow and the resultant wiggling heads downstream to the east. When they are long they go against the flow, and the jet’s wiggling is transmitted upstream to the west. In between, there is a regime in which the waves move neither west nor east, and the weather stays put.
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