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Long-lasting local events involving atmospheric behavior can have worldwide consequences

One of the key properties of atmospheric behavior is that long-lasting local events can have worldwide consequences. For example, the warming of the tropical Pacific associated with El Niño not only affects the overlying atmospheric but can influence weather statistics at numerous locations around the globe. Such “teleconnections” result from large-scale dynamical processes and are just one example of how these processes influence atmospheric variability on weekly and longer timescales. Investigating the large-scale processes that make this kind of behavior possible and studying the effects these processes have on climate variations is the focus of CGD's Grant Branstator.

Recently Branstator has been studying a recurring pattern of tropospheric variability that results from large-scale dynamical processes. Called the Circumglobal Waveguide Pattern (CWP), this teleconnection pattern extends more or less completely around the globe and consists of an east-west chain of alternating cyclones and anticyclones that are trapped in the wintertime subtropical jet of the Northern Hemisphere. The CWP has been found to play a significant role in two surprising settings. One is El Niño. Observational analysis indicates that not all El Niño events affect conditions throughout the Northern Hemisphere, but it appears that when they do it is because the CWP has been excited. This tends to happen when El Niño's warm tropical waters are at the western end of their natural range. The other setting has to do with climate change. Working with Frank Selten of the Royal Netherlands Meteorological Institute, Branstator has found that in some global change experiments performed with NCAR's Community Climate System Model, regional changes in climate conditions are largely controlled by the CWP. In these experiments greenhouse gas trends induce tropical rainfall anomalies which in turn excite the CWP, and its presence controls the ensuing midlatitude regional climate shifts.

An important attribute of long-lasting large-scale circulation anomalies is that they influence the distribution and intensity of smaller scale disturbances including synoptic eddies. In a new investigation with CGD postdoc Jeff Yin, Branstator is studying a special class of such eddies, namely those that produce extreme weather events. There is already a basic understanding of the dynamical processes by which large-scale circulation features control the statistics of synoptic systems. The new study is based on the hypothesis that these same processes can be used to understand how the statistics of weather extremes are also impacted by large-scale features. It thus has the potential to explain how short-term climate events or long term trends can affect extreme weather.

General circulation models (GCMs) are complex numerical models that are an effective tool for studying large-scale atmospheric phenomena, but for some applications they are too cumbersome to be effective. For example, optimization problems, in which one seeks the most efficient means of stimulating the climate, cannot be effectively carried out with GCMs. For dealing with questions of this kind, Branstator has collaborated with Andrey Gritsun and Valentin Dymnikov of the Russian Academy of Science. Together they have implemented a suggestion made by C.E. Leith, a former CGD director, to apply the Fluctuation-Dissipation Theorem to climate science. Using this theorem they have succeeded in constructing a linear operator that well-approximates a GCM's response to external stimuli. This operator is now being employed to discover geographical regions where small forcing perturbations can have large effects on global climate.

For additional information, visit my homepage at: http://www.cgd.ucar.edu/cdp/branst/branst.html.