DOI 10.1393/ncc/i2007-10271-5 IL NUOVO CIMENTO Online First Multiple jets observed in the summer Northern Hemisphere troposphere

wind during July show an intermittent formation of multiple tropospheric jet streams. In particular, a tropospheric westerly, or easterly, jet occurs at latitudes greater than 75 ◦N: it co-exists with the mid-latitude jet and is characterized by variability on synoptic time scale. Two sample years are here considered, July 1996 and July 1985, when prevailing westerly and easterly jets occur at high latitudes, respectively. Analysis is consistent with a picture where the physical mechanism, which creates and maintains the polar jet in the summer troposphere, is the baro-clinic instability process acting on a westerly, or easterly, background zonal flow. Due to the synoptic variability, monthly mean maps of the zonal mean zonal wind for July may show different jet patterns as a function of the year, depending on the occurrences (number and duration) of westerly or easterly polar jets within each month considered. The study of the inter-annual variability of the zonal mean zonal wind through the principal component analysis indicates, in fact, that three tropo-spheric jet stream patterns can be distinguished. Consistency of observations with the available theories on the double-jet formation is provided and the impact of polar jet occurrences on surface temperature field is evaluated. PACS 92.60.Bh – General circulation. 1