Sunday, 11 January 2009
Influence of the El Niņo Southern Oscillation on Seasonal Global Clouds
Phoenix Convention Center
Seasonal averages of global cloudiness distributions from the International Satellite Cloud Climatology Project (ISCCP) at high, middle, and low levels during 1983-2006 were explored to examine the influence of the El Niņo Southern Oscillation (ENSO). Difference fields for El Niņo minus La Niņa cloudiness patterns were assessed for statistical significance with a one-tailed Students t-test, using a threshold normalized ENSO index exceeding an absolute value of 0.4. Each year is assumed to be statistically independent, yielding 24 degrees of freedom. As expected, the tropical Eastern Pacific has more high level clouds during El Niņo than during La Niņa, with differences significant at the 99% level, validating the use of the one-tailed test. The seasonal dependence of this signal in the tropics is discussed. Interesting coherent regions of ENSO differences in high level clouds are also found in the extratropics. Some of these exhibit plume-like signals of poleward-eastward extensions from tropical positive cloud anomalies. ENSO differences for middle and low level clouds also exhibit significant coherent regional changes. Some of these occur on the fringes of seasonal mean cloud maxima. Seasonal cloud patterns and their ENSO differences are interpreted in terms of seasonal mean and ENSO differences in large-scale wind and height fields.