85th AMS Annual Meeting

Tuesday, 11 January 2005: 8:30 AM
Are summer climate variations in north west Europe and West Africa linked?
Chris K. Folland, Hadley Centre, Met Office, Exeter, United Kingdom; and J. W. Hurrell, D. P. Rowell, and J. Knight
We show that the first mode of atmospheric circulation over the extratropical Atlantic region in July and August (high summer) is a dipole pattern that we call the Summer North Atlantic Oscillation (SNAO). The SNAO is shifted further north than the winter North Atlantic Oscillation, with a southern node centred over north western Europe. Not surprisingly, the SNAO strongly affects north west European climate in high summer through this southern node, related to changes in the storm tracks. The SNAO shows marked multidecadal variations like the winter NAO, though the time series of the two NAO modes are not well correlated. The SNAO also shows quite strong correlations with summer Sahel rainfall on ENSO and more particularly on decadal time scales, at least since the early twentieth century. The relationship is such that Sahel rainfall is positively correlated with the positive phase of the SNAO, defined to have anticyclonic anomalies in its southern node and cyclonic anomalies in its northern node over the North Atlantic Arctic. High summer drought periods in North west Europe associated with the anticyclonic node and Sahel droughts tend to be in phase on decadal time scales. On this time scale, both phenomena moved towards the drought phase in the 1960s.

Experiments with several atmospheric general circulation models forced with observed sea surface temperature (SST) show that the SNAO is partly forced by SST, like Sahel rainfall. We discuss some of the patterns of this apparent forcing by SST. Analyses of the HadAM3 model also show that the SNAO is an internal atmospheric mode with little differences between the internal and forced patterns. Finally, we discuss possible physical links between Sahel rainfall and the SNAO in high summer.

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