Surface Signatures of Stratospheric Variability and Trends

Columbia University, New York NY

Investigators

Abstract

This project examines the surface climate impacts of trends and variability in the stratosphere. Motivation for the project comes from observations and numerical simulations which suggest that the South Polar ozone hole constitutes an important forcing for the Antarctic climate system, primarily through its impact on the extratropical jet stream. The forcing is in the sense that the cooling of the polar stratosphere due to ozone depletion causes the jet stream to shift poleward, along with the storm tracks and surface westerlies. These poleward shifts have a variety of consequences for surface climate, including impacts on the Southern Ocean and Antarctic sea ice. Similar poleward shifts are expected due to increases in greenhouse gases (GHGs), but while GHG concentrations are expected to increase indefinitely, polar stratospheric ozone is expected to recover as a consequence of regulations against ozone-depleting substances (ODSs). Thus, some cancellation is expected between the surface climate forcing due to the recovery of ozone in the South Polar stratosphere and the global increase of GHG concentrations. The work proposed here has two components, one examining the climatic impacts of trends in stratospheric ozone (specifically the filling in of the ozone hole), and the other looking at the effects of stratospheric variability (in both hemispheres) on surface conditions. Most of the proposed work involves the Whole Atmosphere Community Climate Model (WACCM), a version of the Community Earth System Model (CESM) which includes an atmospheric component model with a well-resolved stratosphere (in addition to fully coupled land, ocean, and sea ice models). Two additional sets of integrations will be performed, one with GHGs held fixed at year 2000 levels, and the other with fixed year 2000 levels for ozone depleting substances (ODSs). These integrations will allow an assessment of the level of cancellation between the effects of GHG warming and ozone recovery. The work on stratospheric variability will examine sudden stratospheric warmings (SSWs) in long WACCM simulations. Work on SSWs has been hampered by the small sample size of observed SSWs (about 30), but WACCM is able to produce comparable events, and the sample size in the WACCM simulations (about 500) is large enough to produce robust statistical analyses. The work has broader impacts due to the societal value of improved understanding of the effects of stratosphere variability and change on surface climate. One portion of the work seeks to determine the effects of ozone recovery in the Antarctic ozone hole, and its possible opposition to the effects produced by GHG increases, which would allow better projections of future climate change. Another portion of the work seeks a better understanding of the effects of SSWs, which would be of value for long-term weather forecasts.