Pakistan's weather eased Atlantic hurricanes

Jeff Hecht in New Scientist: The stalled weather pattern blamed for disastrous floods in Pakistan and a record heatwave in Russia may have averted disasters elsewhere by putting the North Atlantic hurricane season on hold.

Forecasters had predicted that warm sea-surface temperatures and the onset of the weather pattern known as La Niña would make a busy Atlantic hurricane season this year. In June, Phil Klotzbach and William Gray of Colorado State University predicted 18 tropical storms, with 10 reaching hurricane force and five becoming deadly major hurricanes. The US National Oceanic and Atmospheric Administration forecast similar numbers. Yet this year's hurricane season got off to a very slow start…

La Niña arrived on schedule this year, and sea-surface temperatures were at record highs in July and August in the main area where Atlantic storms develop, says Klotzbach. Yet little happened there, and the Pacific was eerily quiet, with no July cyclones in its eastern half for the first time since 1966. Klotzbach attributes the calm conditions to dry air subsiding over the oceans, denying tropical storms the moisture that powers their growth.

The dry air came from the blocking pattern that stalled the jet stream over Russia and Pakistan, says Judith Curry of the Georgia Institute of Technology in Atlanta. Air rose over Europe and Asia, then descended over the oceans depleted of the atmospheric moisture that fuels hurricanes.

"When the heatwave broke in Russia, that's when hurricanes started forming," says James Done of the National Center for Atmospheric Research in Boulder, Colorado. Four tropical storms formed in the tropical mid-Atlantic from 21 August to 1 September. The first two, Danielle and Earl, both became powerful major hurricanes, with Earl now affecting the US east coast….

Hurricane's Danielle and Earl, August 29, 2010