Associations between Twice-Yearly Oscillations of the North Pacific Cyclone Track and Upper-Tropospheric Circulations over the Eastern Hemisphere

Abstract

A reexamination is made of the seasonal course of extratropical cyclone activity over the North Pacific based mainly on monthly cyclone statistics by Whittaker and Horn and upper-tropospheric wind analyses by Sadler, with support by other investigations. In the Asiatic coastal region, the prevailing cyclone track is south of Japan from midwinter through spring, shifting abruptly northward following a collapse of Kuroshio cyclogenesis from June to July. Farther east over the central Pacific this sequence gives way to a twice-yearly oscillation. Tracks are farthest north in April and August, and farthest south in January?February and secondarily in June. Eastward from Japan, the oscillations of cyclone activity take place in unison with fluctuations in latitude of the monthly mean jet stream. It is related to the intensifying and weakening phases of the subtropical jet during the winter monsoon regime and migrations of the mean jet between positions south and north of the Tibetan Plateau during most of the summer monsoon regime. The double cycle of baroclinic wave amplitude, with a ?midwinter suppression? as well as midsummer minimum, is synchronous with prominent variations of mean zonal wind over the midlatitude Pacific, with maxima at the monsoon transitions. A double cycle of cyclone frequency on the arctic track across Siberia is due to attrition of Atlantic?European cyclones by static stability in winter. Throughout the year there is a trilogy of cyclone tracks: the arctic track, a track in the lee of the Altai?Sayan (A?S) Mountains, and the Pacific track originating in the coastal sector. A synthesis is presented showing their interconnection by the midtropospheric waveguide, which is enhanced by trough genesis in the A?S lee that in turn induces coastal cyclogenesis.