ON THE MODIFICATION OF THE HIGH-FREQUENCY 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 tra nsients of different frequencies. Significant differences were found i n the transient activities between winters with positive and negative Pacific/North American (PNA) seasonal anomalies. During the winters wi th 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 occu rrence of persistent anomalies in this region is also less frequent. T he 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 process es. Firstly, the large-scale seasonal mean flow makes a smaller contri bution to the low-frequency height variance. Secondly, the strong seas onal-mean Aleutian low tends to keep the baroclinic synoptic-scale edd ies moving along its southern side, causing only a weak interaction wi th the low-frequency eddies over the North Pacific, and thus a smaller synoptic-scale eddy forcing of the low-frequency flow. The low-freque ncy activity is closely associated with the atmospheric natural variab ility which causes error growth in medium- and extended range numerica l predictions. Our result suggests that during positive PNA winters a better predictive skill for medium- and extended-range numerical predi ctions may be possible.