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

PerspectivesNorth Atlantic Oscillation (NAO)

Zonal Index Cycle

(Annular Mode)

1st Empirical Orthogonal Function of SLP

(Arctic Oscillation)

Walker and Bliss (1932)

Loon and Rogers (1978)

Hurrell (1995)

Rossby (1939)

Namias (1950)

Lorenz (1951)

Kutzbach (1970)

Trenberth (1981)

Wallace and Gutzler (1981)

Regional

(pressure difference between stations in Portugal and Island )

Hemispheric

(zonal mean wind at 55N)

Hemispheric

SLP Portugal-Island NAO index

Zonal wind 55N index

Leading EOF SLP index

Correlation between SLP at each grid point and:

Miller et al.,2006

NAM-signature in atmospheric and surface variables

Lecture M. Wallace

Thompson and Wallace, 2000

The annular modes in atmosphericheight fields (Thompson and Wallace, 1998)

Thompson and Wallace, 2000

Connection to the stratosphere

Stratosphere-TroposphereCoupling in Positive and Negative Phase of the Annular Mode

Lecture, M. Wallace

Temperature 50 hPa [C]

SHJUN-AUG

NHDEC-FEB

Downward Migration of NH Annular Mode Anomalies(Baldwin and Dunkerton, 2001)

Baldwin and Dunkerton,1999

http://web.dmi.dk/fsweb/solar-terrestrial/staff/boc/aoperp.gif

Slow downward migration ofNorthern Hemisphere Annular Mode -like signature due to wave-mean flow interaction

Baldwin and Dunkerton, 2001

The Southern Hemisphere Annular Mode (SAM)

Lecture M. Wallace

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 andtropospheric fields respectively

Unprecedented Southern Hemisphere Major Sudden

Warming in 2002

Thompson et al. 2005

Downward Migration of SAM anomaliesin 2002 (Thompson et al 2005)

http://www.cmdl.noaa.gov/dv/spo_oz/index.php?option=anim

Trends in the Annular Modes and Related Variables

Lecture M. Wallace

Observed winter trends 1968-1998 (Thompson and Wallace, 1998)

30 year JFM Trend

Zonal Wind (m/s) Temperature (K)

Lecture M. Wallace

Trends in the SAM1976-1999 (Thompson and Solomon, 2002)

Thompson and Solomon, 2002

Thompson and Solomon, 2002

Annular Modes andnatural and anthropogenic

climate forcings

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

Miller et al 2006

Miller et al. 2006

Response to stratospheric ozone loss in the Southern Hemisphere (Gillett and Thompson, 2003)

• 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.

Z 500hPa [m]DJF

Z [m]

T [K]

Miller et al. 2006

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. Nature 399, 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/