abstracts
P. Bongioannini Cerlini, S. Corti and S. Tibaldi, 1999 "An intercomparison between low-frequency variability indices". Tellus 51A, 773-789Possible connections between spatial patterns, of limited regional extent and identified in teleconnection patterns and in blocking climatology studies, with hemispheric planetary-wave activity modes defined by the wave amplitude index (WAI) are investigated. The WAI probability density function (PDF) for the northern extratropics winter fields is estimated and the sensitivity of the WAI distribution to the presence of low-frequency variability modes is evaluated by stratifying the available dataset according to the sign of blocking and teleconnection indices. It is found that low-frequency variability modes affect both the mean and the variance of the wave amplitude index. Both the positive phase of the North Atlantic Oscillation (NAO) and the negative phase of the Pacific North American pattern (PNA) are associated with an enhanced frequency of very large amplitude planetary waves. Furthermore, distributions characterised by a maximum corresponding to high WAI values exhibit also a large variance. Negative NAO and positive PNA influence the mean and the variance of WAI PDF in the opposite sense. Similar results are found when the blocking index is considered. WAI PDFs relative to highly blocked months are broader with a secondary maximum corresponding to very high WAI values.