Centennial Southern Annular Mode Behavior in Observations and IPCC AR4 Models
Ryan Fogt, ESRL/PSD NRC Post Doctoral Fellow
Two techniques are used to reconstruct the long-term (1865-2005) variability of the leading mode of Southern Hemisphere atmospheric circulation, the Southern Hemisphere Annular Mode (SAM). Studies link upward trends in the SAM since the 1960s to climate changes across the Southern Hemisphere, specifically warming in the mid-latitudes (40o-60oS) and Antarctic Peninsula, and cooling across the Antarctic continent. A number of modeling studies attribute the recent upward trends in the SAM to anthropogenic forcing from stratospheric ozone depletion and/or greenhouse gas increases, both of which can cool the polar stratosphere and strengthen the polar vortex. Other studies recognize the role of natural forcing in modulating SAM variability. Given the climatic importance of the SAM, assessing whether the recent upward trends are outside the range of past variability is necessary.
The talk will demonstrate that the seasonally reconstructed SAM has considerable multi-decadal variability, and only during the austral summer and fall is the late-20th century positive SAM trend outside of the range of internal climate variability. The reconstructions are compared to the SAM inferred from 20th century global climate model simulations associated with the Intergovernmental Panel on Climate Change Fourth Assessment Report. Although the models do a reasonable job of predicting the recent trends, the model trends tend to start earlier than observed. Further, most models predict recent significant upward trends in the austral spring when observations indicate near-zero or negative trends, suggesting that the models may be too sensitive to external forcing in the spring. The results indicate that realistic stratospheric ozone forcing is needed to adequately simulate long-term SAM variability in the summer. Further work is needed to understand the mechanisms inducing historical SAM variability in the models and observations, so that the reliability of future SAM projections can be assessed.
SECURITY: If you are coming from outside the NOAA campus, please be advised that you will need an on-site sponsor. Please contact that person in advance of the seminar to be put on the list and allow 10 minutes extra on the day of the seminar. Please contact Joe Barsugli (303-497-6042) or Barbara Herrli (303-497-3876) at least a day before the seminar if you have any questions.