30 July - 5 August 2012 - orsam

When the western Pacific is especially warm and the central Pacific is especially cool — the latter a
natural, cyclically occurring condition known as La Niña, which has prevailed since 2010 — the
temperature gradient causes changes in atmospheric circulation.
Dry air is pushed westward toward the Horn of Africa, which in 2011 experienced a massive drought.
There also seems to be an eastward ripple effect, interacting with other weather patterns to produce
drought conditions in mid-latitude regions.
La Niña generally produces dry spells in southern North America, but adding a warm-to-cool Pacific
gradient generates what some scientists call ―the perfect ocean for drought,‖ spreading it far and
wide. This occurred between 1998 and 2002, when a similar warm-to-cool Pacific gradient existed
and drought struck the United States and mid-latitude regions worldwide. Another, lesser gradient
occurred in 2007 and 2008, just before another U.S. drought.
The latest warm-to-cool gradient occurred in 2010 and 2011. Martin Hoerling, a research
meteorologist at NOAA‘s Earth System Research Laboratory who coined the ―perfect ocean for
drought‖ term, said it may well help explain the current disaster.
―The 2011 drought in Texas was part of the La Niña effect, and we‘ve carried it on here,‖ he said.
―When background conditions in the tropical Atlantic and Indian Ocean are warm, it leads to the
worst of all possible worlds for droughts in the mid-latitudes. I can‘t confirm that‘s been driving the
conditions we‘ve been seeing, but it‘s an useful first guess.‖
If that dynamic is at work, then fossil fuel pollution is implicated. ―Some part of it is related to extra
water vapor that wouldn‘t be there‖ if not for human greenhouse gas emissions, Funk said. ―If we
didn‘t have all that extra anthropogenic water vapor, the western Pacific would be cooler, and the
gradient wouldn‘t be as great.‖
Bin Guan, a drought specialist at the California Institute of Technology, struck a cautionary note on
early interpretations. ―Drought development is a long, complicated process,‖ he said. ―Its response to
greenhouse gases is more complicated than temperature alone because it‘s a combination of
temperature, precipitation, evaporation, soil moisture, and other conditions.‖
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