ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-7-2631-2007 A study on natural and manmade global interannual fluctuations of cirrus cloud cover for the period 1984–2004 Eleftheratos K. 1 2 Zerefos C. S. 1 2 3 Zanis P. 4 Balis D. S. 5 Tselioudis G. 6 Gierens K. 7 Sausen R. 7 Laboratory of Climatology & Atmospheric Environment, University of Athens, Greece Foundation for Biomedical Research, Academy of Athens, Greece National Observatory of Athens, Greece Department of Meteorology and Climatology, Aristotle University of Thessaloniki, Greece Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Greece Goddard Institute for Space Studies, National Aeronautics and Space Administration, USA Institute of Atmospheric Physics, DLR, Germany 21 05 2007 7 10 2631 2642 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/7/2631/2007/acp-7-2631-2007.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/7/2631/2007/acp-7-2631-2007.pdf

The seasonal variability and the interannual variance explained by ENSO and NAO to cirrus cloud cover (CCC) are examined during the twenty-year period 1984–2004. CCC was found to be significantly correlated with vertical velocities and relative humidity from ECMWF/ERA40 in the tropics (correlations up to −0.7 and +0.7 at some locations, respectively) suggesting that variations in large-scale vertical winds and relative humidity fields can be the origin of up to half of the local variability in CCC over these regions. These correlations reflect mostly the seasonal cycle. Although the annual cycle is dominant in all latitudes and longitudes, peaking over the tropics and subtropics, its amplitude can be exceeded during strong El Nino/La Nina events. Over the eastern tropical Pacific Ocean the interannual variance of CCC which can be explained by ENSO is about 6.8% and it is ~2.3 times larger than the amplitude of the annual cycle. Natural long-term trends in the tropics are generally small (about −0.3% cloud cover per decade) and possible manmade trends in those regions are also small. The contributions of NAO and QBO to the variance of CCC in the tropics are also small. In the northern mid-latitudes, on the other hand, the effect of NAO is more significant and can be very important regionally. Over northern Europe and the eastern part of the North Atlantic Flight Corridor (NAFC) there is a small positive correlation between CCC and NAO index during the wintertime of about 0.3. In this region, the interannual variance of CCC explained by NAO is 2.6% and the amplitude of the annual cycle is 3.1%. Long-term trends over this region are about +1.6% cloud cover per decade and compare well with the observed manmade trends over congested air traffic regions in Europe and the North Atlantic as have been evidenced from earlier findings.

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