The Ozone Hole and Global Warming Patterns: A New Interpretation
Over the last several decades, large temperature increases in winter have been observed over Siberia and Alaska. Part of this effect has been associated with the prevailing west wind bringing in warmer air off the ocean. The increased west winds are related to lower sea level pressure at high latitudes, with greater sea level pressure in mid-latitudes.
Such a pressure variation is known as the "positive phase of the Northern Annular Mode" (NAM). (The negative phase is the reverse situation, higher pressure near the pole and lower pressure at mid-latitudes.) The NAM (and its North Atlantic relation, the North Atlantic Oscillation) represents the leading mode of variability of sea level pressure in the Northern Hemisphere.
As implied by the noted temperature variation, this positive phase was prevalent from the late 1970s through the 1990s. Recently the situation has been more mixed, but nevertheless, eight of the last 13 winters have still had a positive phase. Various explanations have been offered for why the positive phase has been predominant, including natural variability and greenhouse warming. Indeed, the majority of climate models suggest global warming will produce a more positive phase of the NAM during this century, although the models, and the modeling community, are not in complete agreement.
Also from the late 1970s through the 1990s, in the Southern Hemisphere the Antarctic ozone hole developed and deepened. This was caused by the release of chemicals into the atmosphere, primarily chlorine associated with CFCs. The Montreal Protocol and follow-up treaties have since succeeded in controlling the emission of the primary ozone-destroying gases, so the ozone hole is no longer growing deeper. Nevertheless, given the long residence time of chlorine in the atmosphere, the ozone hole is still with us.
These two phenomena would appear to be completely unrelated but a recent study suggests otherwise. Our modeling experiments have shown that the S.H. ozone hole appears to produce a more positive phase of the NAM. The process works like this: the ozone hole reduces the vertical stability of the S.H. troposphere, by making the air colder aloft (ozone normally absorbs radiation, and the ambient air is colder when ozone in the lower stratosphere is absent). The reduced atmospheric stability allows more storms — atmospheric waves — to develop. This wave energy propagates up into the S.H. stratosphere and intensifies the stratospheric circulation. The circulation change extends into the Northern Hemisphere, producing cooling at high northern latitudes. The N.H. cooling leads to stronger west winds that alter wave energy propagation in that hemisphere, further amplifying the effect. The result: a more positive NAM. In the model, the influence is substantial, accounting for significant portions of the observed trend.
As discussed in our paper, tropospheric and stratospheric observations, as well as the cotemporaneous nature of the two phenomena, are consistent with this interpretation. Nevertheless, there are numerous caveats. The observations are somewhat suspect, with the introduction of satellites having changed observing techniques. Various other factors likely influence the NAM, as discussed in the opening paragraph. Our own experiments show that the magnitude of the effect may very well vary with model physics. The concurrent trends may therefore still be coincidental.
However, to the extent that the relationship is as important as the modeling study suggests, it has various implications. Since the connecting link occurs in the stratosphere, it emphasizes the importance of accurate modeling of the stratosphere when attempting to predict future climate; this conclusion has been offered previously, but from different perspectives. It furthermore suggests that as chlorine and bromine in the stratosphere decrease during this century, due to their restricted emissions, the NAM may become less positive in the future; this would change the geographical pattern of warming relative to what has been observed. And it emphasizes once again that the planet we live on is interconnected; our actions in one locale or arena can well have unexpected consequences far removed from their source.
Rind, D., J. Jonas, S. Stammerjohn, and P. Lonergan, 2009: The Antarctic ozone hole and the Northern Annular Mode: A stratospheric interhemispheric connection. Geophys. Res. Lett., 36, L09818, doi:10.1029/2009GL037866.
Please address all inquiries about this research to Dr. David Rind.