An intercomparison between low-frequency variability indices

Abstract

Possible connections between spatial patterns, of limited regional extent and identified in teleconnectionpatterns and in blocking climatology studies, with hemispheric planetary-wave activitymodes defined by the wave amplitude index (WAI) are investigated. The WAI probabilitydensity function (PDF) for the northern extratropics winter fields is estimated and the sensitivityof the WAI distribution to the presence of low-frequency variability modes is evaluated bystratifying 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 thewave amplitude index. Both the positive phase of the North Atlantic Oscillation (NAO) andthe negative phase of the Pacific North American pattern (PNA) are associated with anenhanced frequency of very large amplitude planetary waves. Furthermore, distributions characterisedby a maximum corresponding to high WAI values also exhibit a large variance. NegativeNAO and positive PNA influence the mean and the variance of WAI PDF in the oppositesense. Similar results are found when the blocking index is considered. WAI PDFs relative tohighly blocked months are broader with a secondary maximum corresponding to very highWAI values.