CICS Researchers Link Ozone Hole to Changes in Southern Hemisphere
The midlatitude surface westerlies have been observed to shift polewards by several degrees of latitude in the past 20-30 years. This remarkable change in atmospheric circulation has numerous implications, ranging from Australian drought to carbon dioxide uptake in the Southern Ocean. While models of global warming predict a poleward shift, the observed shift is much larger than anticipated. It has recently emerged from climate models at several centers, including NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL), that the ozone hole in the Southern hemisphere stratosphere is likely responsible for a part of the observed displacement. How the ozone hole causes this tropospheric response has remained mysterious.
Background: Gang Chen, a graduate student in Princeton’s Atmospheric and Ocean Sciences Program working with Isaac Held at GFDL, has recently added an important new idea to this subject. Chen and Held (2007) have documented an increase in the eastward propagation speed of midlatitude cyclones in the Southern Hemisphere. In earlier theoretical work, they had described a mechanism through which an increase in this propagation speed would push the midlatitude storm track and westerly winds polewards.
Significance: This theory was developed to explain how changes in the roughness of the Earth’s surface can change the latitude of the surface westerlies (Chen, et al 2007), a seemingly unrelated problem that Chen and Held argue provides a clue to the ozone hole-tropospheric connection.. Based on these observations and this theory, they describe a mechanism whereby the temperature changes in the stratosphere caused by changes in ozone concentrations result in faster midlatitude eddies, thereby changing the latitude of the westerlies. Chen is currently pursing these ideas in a post-doctoral position at the Massachusetts Institute of Technology, supported by a NOAA Global Change Fellowship.
Chen, G., and I. M. Held, 2007: Phase speed spectra & the recent poleward shift of Southern Hemisphere surface westerlies. Geophysical Research Letters, 34, L21805, doi:10.1029/2007GE031200.
Chen, G., I.M. Held, and W.A. Robinson, 2007: Sensitivity of the Latitude of the Surface Westerlies to Surface Friction. J. Atmos. Sci., 64, 2899–2915.