WORLD CLIMATE RESEARCH PROGRAMME JSC/CLIVAR Workshop on Decadal Predictability

The low-frequency climate variability simulated in the North Atlantic by a coupled ocean-at-mosphere model is diagnosed and compared to available observations. A variety of statisticalmethods is used to study the quasi-decadal (QD) oscillations. They tend to show that local di-rect interactions between the atmosphere and the ocean may help maintaining them. 1. Observed low-frequency variability The observed power spectrum of climate records shows significant variability at different timescales. Some well defined periods, as those of the diurnal or annual cycle, are governed by as-tronomy. At the other extreme of the frequency record, variability at the scale of thousandyears has been documented by paleoclimatic indicators. Following Milankovitch’s theory(1941), periodic variations of the orbital parameters can serve to trigger the glacial/inter-gla-cial cycles over the last 3 millions years. Between those extremes, we may define for ourpurpose a low frequency variability (LFV), at the scale of inter-annual to inter-decadal varia-bility. We are interested in these time scales because they are those of our life time, althoughthey are not dominant in the power-spectrum. The spatial pattern associated with this LFV is well characterized. In the North Atlantic, theatmospheric variability at any time scale is dominated by a large-scale fluctuation, between theAzores Anticyclone and the Iceland Low: this is the North Atlantic Oscillation (NAO). A pos-itive phase of this oscillation corresponds to both a strengthening of the high and lowpressures, intensifying western winds (figure 1, left). This is clearer in winter. To represent thisfeature, Hurrell (1995) defined a NAO index as the difference of sea-level pressure (SLP) anom-alies between Azores and Iceland, respectively normalized by their standard deviation. From1864 to 1995, this index shows a strong high-frequency variability, modulated by “low-fre-quency” fluctuations. A positive trend since 1960, which implies an increase of the positivephase of NAO, is also apparent in the record. The sea surface temperature (SST) anomalies as-sociated to this zonal SLP pattern (figure 1, right) are warmer in the west side of the basin,south of New-Foundland and near Europe, and colder in the subpolar gyre and trade-windszone. This association of atmospheric (SLP) and oceanic (SST) modes is prevailing at both in-ter-annual and intraseasonal time scales. NAO potential importance comes from its clear correlation with other climatic fields over Eu-rope, like precipitation. In a positive phase of the NAO, there is a deficit of precipitation abovethe Mediterranean Sea and in the South of Europe, with an opposite excess in the North ofEurope.