An intercomparison between low-frequency variability indices

Possible connections between spatial patterns, of limited regional extent a nd identified in teleconnection patterns and in blocking climatology studie s, with hemispheric planetary-wave activity modes defined by the wave ampli tude index (WAI) are investigated. The WAI probability density function (PD F) for the northern extratropics winter fields is estimated and the sensiti vity of the WAI distribution to the presence of low-frequency variability m odes is evaluated by stratifying the available dataset according to the sig n of blocking and teleconnection indices. It is found that low-frequency va riability 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 associa ted with an enhanced frequency of very large amplitude planetary waves. Fur thermore, distributions characterised by a maximum corresponding to high WA I values also exhibit a large variance. Negative NAO and positive PNA influ ence the mean and the variance of WAI PDF in the opposite sense. Similar re sults are found when the blocking index is considered. WAI PDFs relative to highly blocked months are broader with a secondary maximum corresponding t o very high WAI values.