© Eugene S. Takle, 2003
Slowly changing temperature patterns in the tropical Pacific Ocean have
long been known to influence precipitation patterns over specific regions
around the Northern Hemisphere. For instance warm sea-surface
temperatures in the eastern Pacific Ocean, known as El Niño, are
associated with extended periods of excess rainfall in the southern US.
And cool sea-surface temperatures in the same region (known as La Nina)
are correlated with drought in this region. These are naturally recurring
patterns that are not likely related to the buildup of atmospheric
greenhouse gases.
The period from 1998-2002 produced a persistent pattern of cool
sea-surface temperatures in the eastern tropical Pacific Ocean. Although
it existed longer than the normal La Nina, this pattern was not
unprecedented in the climate record. Hoerling and Kumar (2003) noticed,
however, that accompanying this pattern was a highly unusual persistent
and strong sea-surface warming in the western Pacific and Indian Oceans.
Global precipitation patterns during this period showed the expected
drier-than-normal conditions in the southern and southeast US, but also
persistent drought in southern Europe and central-southwest Asia from Iraq
to Pakistan.
Hoerling and Kumar (2003) made 51 runs with three global climate models
using these observed persistent sea surface conditions to see what
peculiarities would develop in the atmosphere and in the climate over
continental areas. They found that these two patterns of ocean
temperatures - warm in the western Pacific and Indian Ocean and cool in
the eastern Pacific - in combination were likely responsible for the
globe-spanning drought (see also Kerr, 2003).
So what was the cause of the unusual warming in the western Pacific that
conspired with a more-or-less normally occurring La Niña to produce such
widespread drought? Hoerling and Kumar (2003) point to evidence from
Knutson et al (1998, 1999) that warming in this region is at least
partially due to increased atmospheric greenhouse gas concentrations. It
is therefore likely that such globe-spanning droughts in the lower
mid-latitudes may be more frequent occurrences in the future.
References
Hoerling, M., and A. Kumar, 2003: The perfect ocean for drought.
Science, 299, 691-694.
Kerr, R. A., 2003: A perfect ocean for four years of globe-girdling
drought. Science, 299, 636.
Knutson, T. R., T. L. Delworth, K. W. Dixon, and R. J.Stouffer, 1999:
J. Geophys. Res., 104, 30,981.
Knutson, T. R., and S. Manabe, 1998: J. Climate, 11, 2273.