Low-Frequency Anomalies of Atmospheric Flow and the Effects of Cyclone-Scale Eddies: A Canonical Correlation Analysis

Abstract

Canonical correlation analysis is applied to investigate the relationship between the low-pass filtered (barotropic) streamfunction and the effects of cyclone-scale eddies. The eddy effects are evaluated in terms of flux convergences using a spectral quasi-geostrophic twolevel formulation. The analysis is based on six winters of ECMWF observations. In particular only the intraseasonal low-frequency components of the streamfunction and the eddy effects having oscillation periods from 10 days to one season are considered. The canonical correlation analysis identifies canonical modes in terms of spatial streamfunction patterns that are optimally correlated with associated patterns of the eddy effects, together with amplitude time series that determine the instantaneous projections of analyzed variables onto the mode patterns. The analysis is carried out between the barotropic components of the streamfunction and the eddy effects and the robustness of the canonical modes is tested. Two robust canonical modes are obtained. The leading mode (CCA mode α) shows significant, regionally very confined, eddy effects only over the Pacific, while the second mode (CCA mode ?) is characterized by significant eddy effects over the Atlantic. The associated streamfunction patterns are larger-scale with the largest anomalies over the Pacific and Atlantic, respectively. In addition, the CCA mode α is also accompanied by significant patterns of the thermal streamfunction and baroclinic eddy effects. The temporal variability of the modes may be characterized by an integral time scale of about 20 days. Episodes of large positive amplitudes or the CCA mode α may be associated with (rare) cases of Pacific blocking, in which case the barotropic eddy effects tend to maintain the blocking high. On the other hand, for episodes of large negative amplitudes the mode is related to a highly baroclinic regional anomaly of the subtropical Pacific jet that occurs near the date line. Upstream of the center of this jet anomaly significant baroclinic eddy effects can be observed tending to dissipate its thermal component, while significant barotropic eddy effects are found downstream of the jet center that feed back onto the barotropic jet component. The streamfunction pattern for this phase of the mode is very reminiscent of the cases of the persistent Pacific anomalies of the low-pass filtered 500 mb height of Dole. The CCA mode ? is less pronounced than the leading mode. It may be related to a regional jet anomaly over the Atlantic, where the positive phase is characterized by a jet anomaly southwards and to the east of the climatological mean Atlantic jet while the negative phase shows the jet anomaly more northwards In both cases the barotropic eddy effects tend to feed back upon the barotropic jet anomaly. For this mode no significant patterns of the thermal flow and the baroclinic eddy effects were obtained.