Annular modes in the extratropical circulation. Part II: Trends

The authors exploit the remarkable similarity between recent climate trends and the structure of the "annular modes" in the month-to-month variability (as described in a companion paper) to partition the trends into component s 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 toward the high index polari ty over the past few decades. The largest and most significant trends are o bserved during the "active season" for stratospheric planetary wave-mean fl ow interaction, January-March (JFM), when fluctuations in the AO amplify wi th height into the lower stratosphere. For the periods of record considered , virtually all of the JFM geopotential height falls over the polar cap reg ion and the strengthening of the subpolar westerlies from the surface to th e lower stratosphere, similar to 50% of the JFM warming over the Eurasian c ontinent, similar to 30% of the JFM warming over the NH as a whole, similar to 40% of the JFM stratospheric cooling over the polar cap region, and sim ilar to 40% of the March total column ozone losses poleward of 40 degrees N are linearly congruent with month-to-month variations in the AO index. Sum mertime 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 i s 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 lasses incurred during the previo us active season. Lower-tropospheric geopotential height falls over the Antarctic polar cap r egion are indicative of a drift toward the high index polarity of the South ern 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 sea son centered in November. The most pronounced SE ozone losses have occurred in September-October, one or two months prior to this active season. In bo th hemispheres, positive feedbacks involving ozone destruction, cooling, an d a weakening of the wave-driven meridional circulation may be contributing to a delayed breakdown of the polar vortex and enhanced ozone losses durin g spring.