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The seasonal variability and the interannual variance explained by ENSO andNAO to cirrus cloud cover (CCC) are examined during the twenty-year period1984–2004. CCC was found to be significantly correlated with verticalvelocities 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 relativehumidity fields can be the origin of up to half of the local variability inCCC over these regions. These correlations reflect mostly the seasonalcycle. Although the annual cycle is dominant in all latitudes andlongitudes, peaking over the tropics and subtropics, its amplitude can beexceeded during strong El Nino/La Nina events. Over the eastern tropicalPacific Ocean the interannual variance of CCC which can be explained by ENSOis about 6.8% and it is ~2.3 times larger than the amplitude of theannual cycle. Natural long-term trends in the tropics are generally small(about −0.3% cloud cover per decade) and possible manmade trends inthose regions are also small. The contributions of NAO and QBO to thevariance of CCC in the tropics are also small. In the northernmid-latitudes, on the other hand, the effect of NAO is more significant andcan be very important regionally. Over northern Europe and the eastern partof the North Atlantic Flight Corridor (NAFC) there is a small positivecorrelation between CCC and NAO index during the wintertime of about 0.3. Inthis region, the interannual variance of CCC explained by NAO is 2.6% andthe amplitude of the annual cycle is 3.1%. Long-term trends over thisregion are about +1.6% cloud cover per decade and compare well with theobserved manmade trends over congested air traffic regions in Europe and theNorth Atlantic as have been evidenced from earlier findings

A study on natural and manmade global interannual fluctuations of cirrus cloud cover for the period 1984–2004