Seminar

Abstract

Observational evidence indicates that the tropics are expanding, but a wide range of expansion rates have been reported, global climate models underestimate the observed trend, and there is no consensus on the cause of this expansion. Here we examine trends in southern edge of the tropics during austral summer. It is shown that the weight of the evidence clearly points to stratospheric ozone depletion as the dominant driver of the tropical summertime expansion over the period in which an ozone hole was formed (1979 to late 1990s), although sea surface temperature and internal atmospheric variability have also contributed (especially since late 1990s). The large role of internal atmospheric variability may explain the underestimate in the ensemble mean trend from climate models. We also show that the wide range of expansion rates is due, in part, to the great variety of metrics used to define the tropical width, with many measuring different aspects of the tropical circulation.

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Darryn Waugh's main research interests are oriented toward understanding dynamics and transport in the atmosphere and oceans. Research in the atmosphere focuses on stratospheric and upper tropospheric dynamics/transport. Improved understanding of, and ability to model, fluid motions in these regions is important for understanding the distribution of trace constituents, such as ozone, and for assessing the impact of human activities on the atmospheric environment. In recent years he has also become interested in the transport in oceans and lakes, and the uptake of anthropogenic carbon.