Remote ENSO influence on Mediterranean sky conditions during late summer and autumn: Evidence for a slowly evolving atmospheric bridge

Several previous studies have reported significant El Nino Southern Oscillation (ENSO) signals in precipitation fields over the Mediterranean region during boreal summer and autumn. However, responsible physical mechanisms are unclear. In this study, we report significant simultaneous ENSO signals in ship-observed visible-sky conditions over the north-eastern Atlantic and western Mediterranean Sea (WM) during late summer and autumn in 1956–95, consistent with previous studies of precipitation. This remote correlation was stronger in 1956–75 than in 1976–95 and 30-year moving average correlations were stronger throughout the 1956–95 epoch than at any other time over the last 120 years. Previous epochs of relatively weak positive and negative correlation occurred between 1890 and 1920 and between 1920 and 1950, respectively. Analysis of atmospheric flows during 1956–95 indicates that the strong ENSO–WM correlation in the August–October season arises from a previously unreported quasi-stationary Rossby wave propagating eastwards from the western equatorial Pacific. This component weakens between 1956–75 and 1976–95, consistent with the reduced correlation of the ENSO–WM sky conditions between the same periods. In order to fully understand the observed long-term variations of remote ENSO correlation over the WM, further studies are required, including the potential roles of the Pacific Decadal Oscillation, Sahelian rainfall and Atlantic sea surface temperature. Copyright © 2004 Royal Meteorological Society