On the modification of the high- and low-frequency eddies associated with the PNA anomaly: an observational study

A 24-year NMC data set was used to study the interannual fluctuations in the extratropical atmospheric flow and the interactions between transients of different frequencies. Significant differences were found in the transient activities between winters with positive and negative Pacific/North American (PNA) seasonal anomalies. During the winters with an enhanced positive PNA pattern, the eddy activity is reduced over the North Pacific. The reduction of eddy activity is dominated by the low-frequency transients (periods from 10 days to a season). The occurrence of persistent anomalies in this region is also less frequent. The high-frequency baroclinic waves are shifted south-eastward of their normal position in the Pacific. The weak low-frequency activity over the North Pacific during positive PNA winters is linked to two processes. Firstly, the large-scale seasonal mean flow makes a smaller contribution to the low-frequency height variance. Secondly, the strong seasonal-mean Aleutian low tends to keep the baroclinic synoptic-scale eddies moving along its southern side, causing only a weak interaction with the low-frequency eddies over the North Pacific, and thus a smaller synoptic-scale eddy forcing of the low-frequency flow. The low-frequency activity is closely associated with the atmospheric natural variability which causes error growth in medium- and extended-range numerical predictions. Our result suggests that during positive PNA winters a better predictive skill for medium- and extended-range numerical predictions may be possible.