Annular modes in the extratropical circulation: A global perspective of Southern Hemisphere climate variability

The leading mode of month-to-month variability in the Southern Hemisphere (SH) circulation is characterized by a deep, zonally symmetric or "annular" structure with zonal wind anomalies of opposing sign found near 60S and 40S. While the dynamics and structure of this mode have been extensively documented, it is widely believed that such an annular mode does not exist in the Northern Hemisphere (NH), where longitudinally dependent teleconnections are thought to dominate climate variability. This talk will demonstrate that, in fact, the leading mode of variability in both hemispheres is fundamentally annular and that they are virtually indistinguishable from each other. Both annular modes perturb climate throughout the year in their respective hemispheres from the subtropical trade wind belt to the polar cap. During certain seasons they extend and amplify with height into the lower stratosphere where they modulate tropopause height and total column ozone. Both annular modes are evident across a wide range of frequencies, including evidence that they have both exhibited a trend towards their high index polarity (i.e., falling geopotential height over the polar cap) over the past few decades. While the reasons for this bias are as yet unknown, recent modelling studies suggest it may be an unanticipated response to anthropogenic forcing.