Since the mid-nineteenth century the Earth’s surface has
warmed, and models indicate that human activities have caused
part of the warming by altering the radiative balance of the
atmosphere. Simple theories suggest that global warming will
reduce the strength of the mean tropical atmospheric circulation.
An important aspect of this tropical circulation is a largescale
zonal (east–west) overturning of air across the equatorial
Pacific Ocean—driven by convection to the west and subsidence to
the east—known as the Walker circulation. Here we explore
changes in tropical Pacific circulation since the mid-nineteenth
century using observations and a suite of global climate model
experiments. Observed Indo-Pacific sea level pressure reveals a
weakening of the Walker circulation. The size of this trend is
consistent with theoretical predictions, is accurately reproduced
by climate model simulations and, within the climate models, is
largely due to anthropogenic forcing. The climate model indicates
that the weakened surface winds have altered the thermal structure
and circulation of the tropical Pacific Ocean. These results
support model projections of further weakening of tropical
atmospheric circulation during the twenty-first century.

Full Article: http://www.nature.com/nature/journal/v441/n7089/abs/nature04744.html

Last Updated: 2007-04-12