Northern Hemispheric Trends of Pressure Indices and Atmospheric Circulation patterns in Observations, Reconstructions, and Coupled GCM Simulations
Raible CC, Stocker TF, Yoshimori M, Renold M, Beyerle U, Casty C, Luterbacher J
J. Climate
submitted Feb 2004


Abstract: The decadal trend behavior of the Northern Hemisphere atmospheric circulation is investigated utilizing long-term simulations with different state-of-the-art coupled general circulation models (GCMs) for present day climate conditions (1990), reconstructions of the past 500 years, and observations. The multi-model simulations show that strong positive winter North Atlantic Oscillation (NAO) trends are connected with the underlying sea surface temperature (SST) and exhibit a SST tripole trend pattern and a northward shift of the storm track tail. Strong negative winter trends of the Aleutian Low are associated with SST changes in the El Ni\~no Southern Oscillation (ENSO) region and a westward shift of storm track in the North Pacific. The observed simultaneous appearance of strong positive NAO and negative Aleutian Low trends is very unlikely to occur by chance in the unforced simulations and reconstructions. The positive winter NAO trend of the last 50 years is not statistically different from the level of internal atmosphere-ocean variability. The unforced simulations also show a strong link between positive SST trends in ENSO region and negative Aleutian Low trends. With much larger observed SST trends in the ENSO region, this suggests that the observed negative Aleutian Low trend is possibly influenced by external forcing, e.g., global warming, volcanism, and/or solar activity change.

KeyWords Plus: ATMOSPHERIC CIRCULATION TRENDS, NORTH ATLANTIC OSCILLATION, ALEUTIAN LOW, STATIONARY WAVE ACTIVITY, EDDY ACTIVITY, OCEAN, SEA SURFACE TEMPERATURE

Addresses:
Raible C.C., Stocker T.F., Yoshimori M., Renold M., Beyerle U., Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstr. 5, CH-3012 Bern, Switzerland.
Casty, C., Luterbacher, J., Institute of Geography, University of Bern, Hallerstr. 12, CH-3012 Bern, Switzerland.

Reprints (after having been accepted):
Raible CC, Climate and Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, raible@climate.unibe.ch