Friday, August 19, 2011

Big storms slipping toward Earth’s poles


Mid-latitude storm tracks are major weather patterns that account for the majority of precipitation in the globe’s middle latitudes, which includes most of the heavily populated areas of North America, Eurasia, and Australia.
Due to atmospheric circulation and the dynamics of weather systems, these bands of low pressure form repeatedly in the same locations.
Apart from being meteorologically important, they’re also major players on the climate scene—clouds in these regions are responsible for reflecting much of the incoming solar radiation that is bounced back to space before penetrating Earth’s atmosphere.

Many climate models have predicted that the positions of these storm tracks would slowly migrate toward the poles, but so far this trend had not been detected.
However, analysis of 25 years worth of data from the International Satellite Cloud Climatology Project now indicates that this shift is probably already taking place.

The International Satellite Cloud Climatology Project (or ISCCP) operates a network of geostationary and polar orbiting satellites that have been collecting data on clouds since 1983.
A team of researchers carefully analyzed data for Northern and Southern Hemisphere storm tracks in the Atlantic and Pacific Oceans to look for trends in storm track positions.
(The Indian Ocean could not be included because of issues with satellite coverage.)
The results indicated a slight poleward shift of the storm tracks.

These satellites have known data issues: measurement changes when new satellites came online, lower data quality at the “seams” between coverage from different satellites, etc.
So the authors tried several different analysis techniques to test the robustness of the observed trend.
Each technique decreased the rate of the observed poleward movement somewhat, but the general trend remained.

That’s mainly interesting because it had been predicted by many climate models.
But the data also shows something that may be much more important, though there are some considerable uncertainties involved.
The satellite observations also show a roughly two-to-three percent reduction in total cloud cover since 1983.
This occurred through a large decrease in low-level cloudiness, and it came despite a slight increase in high-level cloudiness.

Both of these changes act as positive feedbacks to warming, and, as we recently covered, cloud feedbacks are among the largest sources of uncertainty in temperature projections.
High-level cirrus clouds aren’t thick enough to reflect much incoming solar radiation, but the increase in water vapor means more trapping of outgoing infrared radiation (the greenhouse effect).
Most reflective action is in the low-level clouds, so a decrease there means more incoming solar radiation penetrating to the Earth’s surface.

Although the same models that predict the poleward movement of storm tracks also predict reductions in total cloud cover, the paper is heavy on caveats here.
The most interesting data comes at the limits of detection for these satellites, making it unclear how robust the signal is.
Like the storm track positions, the trend is consistent among the regions studied, though.
In addition, satellite observations of atmospheric radiation fluxes (from the Earth Radiation Budget Experiment) corroborate the changes in cloud behavior.

Studies like this underscore the importance of Earth-monitoring satellites operated by NASA and the ESA.
Global climate data isn’t easy to come by, and the analysis is often difficult under the best of circumstances.
Increasingly accurate projections require the kind of data only these satellites can provide.

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