The cold winters of the middle 1990s in the northern lower stratosphere


  • Steven Pawson,

  • Barbara Naujokat


Lower stratospheric temperatures in the northern winters of 1994/1995, 1995/1996, and 1996/1997 were low enough to support polar stratospheric cloud (PSC) formation for prolonged periods. While the seasonal evolution of each winter was quite different, there are some common characteristics: notably, the occurrence of extremely cold periods of long duration and the coldness of the late winter in each year. Comparison with observations over more than three decades indicate the stratosphere was atypically cold in these three years, with the largest anomalies occurring in the late winter and spring. In January and February the coldness seems to be determined by the interannual variability of the circulation, while in March the persistence of the polar vortex dominated the circulation in these three years. This may be related to the lack of major midwinter warmings in those years. Comparison with other winters shows that although the persistence of the polar vortex well into the spring is not unprecedented, this did not occur frequently in the previous two decades. Further, there is a general temperature decrease in the northern lower stratosphere which contributed to the coldness of the three winters. Comparison of the late winter and spring of 1997 with 1967, both of which were forced only weakly by dynamics, supports the idea that this is due to a change in the radiative balance (with equilibrium at a lower temperature), although there are many caveats to this conclusion.