Interannual variability in summer precipitation and surface cyclone activity in northern Eurasia

This study examines the interannual variability in summer precipitation, surface cyclone activity in northern Eurasia, and their possible connections to other climate signals. An east--west seesaw alternation of relatively dry and wet extremes is the primary mode of interannual variability in summer precipitation in Siberia. This signal occurs on time scales of about 6- to 8- years, which has been pronounced during the last 30 years. Our previous works have focused on the large-scale atmospheric circulation associated with this precipitation seesaw and their generation and maintenance processes. We extend those to diagnose northern Eurasian summertime storm track activity from a Lagrangian approach. The purpose of this study is to identify occurrence locations and primary paths of surface cyclones associated with the summer precipitation seesaw events, and to confirm this seesaw signal is linked to year-to-year variation of high-latitude cyclone activity. Spatial characteristics of cyclone frequency and cyclone tracks involved in two opposite phases of the precipitation seesaw (eastern Siberia (ES)-wet--western Siberia (WS)-dry and WS-wet--ES-dry) are examined, and temporal characteristics of regional cyclone frequency are compared with the basin-scale precipitation variability. The number of cyclones increases (decreases) in the above (below) normal precipitation region in both two phases. The difference field of cyclone frequency anomalies exhibits quite similar east-west pattern in Siberia with that of precipitation. Most of cyclones moving into ES for the ES-wet--WS-dry summers originate in western Siberia and the Kara Sea, and pass through central and eastern Siberia. On the other hand, those enter WS for the WS-wet--ES-dry summers cross western Siberia. These tracks start in northeast Europe, European Russia, and the Barents Sea and extend into western and central Siberia. Summer cyclone occurrences are counted for two boxes located over the cyclone frequency dipole. Time series of cyclone counts for these boxes tend to be out-of-phase with each other. The interannual variability of the cyclone counts in box E (W) for ES (WS) significantly correlates with that of the area-averaged precipitation for the ES (WS) domain. These results suggest that the interannual variability in the cyclone activity is capable of producing the precipitation seesaw. We further explore connections of the regional precipitation and cyclone activity with various climate signals. The result shows that statistical relationships between the cyclone counts and precipitation series for the WS region and the winter (DJF) North Atlantic Oscillation (NAO) index. The correlation coefficient between the summer (JJA) counts (precipitation) in box W (the WS domain) and the winter (DJF) NAO index is -0.39 (-0.47) for 1973--2002, which exceeds the 95% confidence level. However, dynamical linkage between the summer cyclone occurrences over WS and preceding climate conditions associated with the winter NAO is still unclear.