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Annular Modes in the Extratropical Circulation
Part II: Trends
David W.J. Thompson, John M. Wallace and Gabriele C. Hegerl
Submitted to Journal of Climate January 1999
Accepted June 1999
Abstract
We exploit the remarkable similarity between recent climate trends and the structure of
the "annular modes" in the month-to-month variability (as described in the companion paper
Thompson and Wallace (1999)) to partition the trends into components linearly congruent with
and linearly independent of the annular-modes.
The index of the Northern Hemisphere (NH) annular mode, referred to as the Arctic
Oscillation (AO), has exhibited a trend towards the high index polarity over the past few
decades. The largest and most significant trends are observed during the "active season" for
stratospheric planetary-wave-mean flow interaction, January-March (JFM), when fluctuations in
the AO amplify with height into the lower stratosphere. From 1968-97, virtually all of the JFM
geopotential height falls over the polar cap region and the strengthening of the subpolar
westerlies from the surface to the lower stratosphere, ~50% of the JFM warming over the
Eurasian continent, ~30% of the JFM warming over the NH as a whole, ~40% of the JFM
stratospheric cooling over the polar cap region, and ~40% of the March total column ozone
losses poleward of 4 N are linearly congruent with month-to-month variations in the AO-
index. Summertime sea-level pressure falls over the Arctic basin are suggestive of a year-round
drift toward the positive polarity of the AO, but the evidence is less conclusive. Owing to the
photochemical memory inherent in the ozone distribution, roughly half the ozone depletion
during the NH summer months is linearly dependent on AO-related ozone losses incurred during
the previous active season.
Lower tropospheric geopotential height falls over the Antarctic polar cap region are
indicative of a drift towards the high index polarity of the Southern Hemisphere (SH) annular
mode with no apparent seasonality. In contrast, the trend toward a cooling and strengthening of
the SH stratospheric polar vortex peaks sharply during the stratosphere's relatively short active
season centered in November. The most pronounced SH ozone losses have occurred in
September/October, one or two months prior to this active season. In both hemispheres, positive
feedbacks involving ozone destruction, cooling, and a weakening of the wave-driven meridional
circulation may be contributing to a delayed breakdown of the polar vortex and enhanced ozone
losses during spring.
Postscript file containing the manuscript and figures
PDF file containing the manuscript and figures
Corresponding author: Dave Thompson (davet@atmos.washington.edu)