Annular Modes of Extra-tropical Circulation Judith Perlwitz CIRES-CDC, University of Colorado
Outline • Definition and Perspective • The Northern Hemisphere Annular Mode-signatures in atmospheric and surface variables • The stratospheric connection • The Southern Hemisphere Annular Mode • Annular Modes and climate change • Summary
What are modes of variability? • Monthly or annular average of meteorological parameter like temperature, pressure or wind speed taken at observation stations worldwide • Remove long-term mean =>Recurrent spatial patterns. • These patterns, called modes, are believed to be the signatures of distinctive dynamical interactions.
Sea level pressure Northern Hemisphere Annular Mode pattern Time series
Correlation between SLP at each grid point and: SLP Portugal-Island NAO index Zonal wind 55N index Leading EOF SLP index
Miller et al., 2006
The annular modes in atmospheric height fields (Thompson and Wallace, 1998)
Stratosphere-Troposphere Coupling in Positive and Negative Phase of the Annular Mode Lecture, M. Wallace
Temperature 50 hPa [C] NH DEC-FEB SH JUN-AUG
Downward Migration of NH Annular Mode Anomalies(Baldwin and Dunkerton, 2001)
Slow downward migration ofNorthern Hemisphere Annular Mode -like signature due to wave-mean flow interaction Baldwin and Dunkerton, 2001
Different Temporal Behavior of Annular Modes in Northern and Southern Hemisphere(Thompson and Wallace, 2000) Polar cap geopotential heights (north of 70N and 65N) for 50hPa and tropospheric fields respectively
Thompson et al. 2005
Downward Migration of SAM anomalies in 2002 (Thompson et al 2005)
Observed winter trends 1968-1998 (Thompson and Wallace, 1998)
30 year JFM Trend Temperature (K) Zonal Wind (m/s) Lecture M. Wallace
Trends in the SAM 1976-1999 (Thompson and Solomon, 2002)
Observed and modeled trends of 500hPa heights [m]1950 -1999 (Hoerling et al. 2001) • About 50 % of the observed trend can be explained by slow, tropical ocean warming.
Change in tropospheric wave forcing • Change in stratospheric meridional • temperature gradient via • Stratospheric volcanic aerosol • Polar ozone depletion • Increased greenhouse effect Lecture M. Wallace
NAM index in observation and in transient climate change simulations (Shindell et al, 1999) • Significant increase of surface NAM index was found in the Middle Atmosphere (MA)-GCM, but not in a model without full stratospheric dynamics
Strengthening of westerly flow from the stratosphere to the troposphere Cooling in the troposphere occurs one to two month after the maximum cooling in the stratosphere Model response agrees well with observations. Response to stratospheric ozone loss in the Southern Hemisphere (Gillett and Thompson, 2003) Z [m] T [K] Z 500hPa [m] DJF
Summary • Together with El Nino, the Northern and Southern Hemisphere annular modes have emerged as the leading patterns of variability of the global atmosphere. • The annular modes are primarily extra-tropical phenomena with a deep barotropic, zonally symmetric structure. • There is a strong coupling between the tropospheric and stratospheric annular modes. • Future predictions indicate a trend towards positive phase of the Annular Mode indices due to increasing greenhouse effect and stratospheric polar depletion.
References • Lecture M. Wallace: http://jisao.washington.edu/wallace/ncar_notes/ • Miller, R.L., G.A. Schmidt, and D.T. Shindell 2006. Forced annular variations in the 20th century Intergovernmental Panel on Climate Change Fourth Assessment Report models. J. Geophys. Res.111, D18101, doi:10.1029/2005JD006323. • Shindell, D.T., R.L. Miller, G.A. Schmidt, and L. Pandolfo 1999. Simulation of recent northern winter climate trends by greenhouse-gas forcing. Nature399, 452-455, doi:10.1038/20905. • Thompson, D. W. J., and J. M. Wallace, 2000: Annular modes in the extratropical circulation. Part I: Month-to-month variability. J. Climate, 13, 1000-1016. • Thompson, D. W. J., and S. Solomon, 2002: Interpretation of Recent Southern Hemisphere Climate Change. Science, 296, 895-899. • Gillett, N.P., and D.W.J. Thompson, 2003: Simulation of Recent Southern Hemisphere Climate Change. SCIENCE, 302(5643), 273-275, 10.1126/science.1087440. • Baldwin, M. P., and T. J. Dunkerton, 1999: Propagation of the Arctic Oscillation from the stratosphere to the troposphere. J. Geophys. Res., 104, 30 937-30 946. • Baldwin, M.P., and T.J. Dunkerton, 2001: Stratospheric Harbingers of Anomalous Weather Regimes. Science, 294, 581-584. • Hoerling, M.P., J.W. Hurrell, and T. Xu, 2001: Tropical Origins for Recent North Atlantic Climate Change. Science, 292, 90-92. • Thompson, D.W.J., M.P. Baldwin, and S. Solomon, 2005: Stratosphere-troposphere coupling in the Southern Hemisphere. J. Atmos. Sci., 62(3), 708-715. • Thompson, D. W. J, and J. M. Wallace, 1998: The Arctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett., 25, 1297-1300. • Annular Mode web page: http://www.atmos.colostate.edu/ao/