Annular modes in the atmospheric general circulation

Abstract

The leading modes of variability of the extratropical circulation in both hemispheres are characterized zonally symmetric or "annular" structures. Whereas the structure and dynamics of the Southern Hemisphere (SH) annular mode have been extensively documented, the existence of an analogous structure in the Northern Hemisphere (NH) has not been recognized. In this thesis we compare the structure and seasonality of the NH and SH modes, document their signature in recent climate trends, and examine the regional climate impacts of the NH annular mode.The structures of the NH and SH annular modes are remarkably similar. Both are characterized by deep, nearly barotropic structures, with geopotential height perturbations of opposing sign in the polar cap region and in the surrounding ring centered near 45 degrees latitude. The annular modes also exhibit a distinctive signature in the tropics. They exist year round in the troposphere, but modulate the strength of the Lagrangian mean circulation in the lower stratosphere only during the "active" seasons for stratospheric planetary-wave-mean flow interaction. The North Atlantic Oscillation may be viewed as a regional expression of the Northern Hemisphere annular mode.The Northern Hemisphere annular mode has exhibited a trend over the past few decades that is clearly reflected in recent climate trends. Virtually all of the wintertime geopotential height falls over the polar cap region and the strengthening of the subpolar westerlies from the surface to the lower stratosphere, ∼30% of the wintertime warming over the NH, and ∼40% of the March total column ozone losses poleward of 40°N are attributable to the trend in the NH annular mode. The SH annular-mode has exhibited an analogous trend at stratospheric levels during the austral spring. Since the Northern Hemisphere annular mode affects the mean wintertime climate and the frequency of occurrence of weather events throughout the Northern Hemisphere, it is argued that its trend over the past few decades has influenced popular perceptions of climate change.