Summer upper-level vortex over the North Pacific

The midocean trough in the North Pacific may form a favorable environment f or the genesis of some synoptic disturbances. In contrast. the North Pacifi c anticyclone may hinder the downward penetration of these disturbances int o the lower troposphere and prevent the moisture supply to these disturbanc es from the lower troposphere. Because no thick clouds, rainfall, and destr uctive surface winds are associated with these disturbances to attract atte ntion, they have not been analyzed or documented. Actually, the upper-level wind speed within these disturbances is sometimes as strong as tropical cy clones and has the possibility of causing air traffic hazards in the wester n subtropic Pacific. With infrared images of the Japanese Geostationary Met eorological Satellite and the NCEP-NCAR reanalysis data, 25 North Pacific d isturbances were identified over six summers (1993-98). Two aspects of thes e disturbances were explored: spatial structure and basic dynamics. For the ir structure, the disturbances possess a well-organized vortex in the middl e to upper troposphere with a descending dry/cold core encircled by the moi st ascending air around the vortex periphery; the secondary circulation of the vortex is opposite to other types of synoptic disturbances. Since vorti city reaches maximum values along the midocean trough line, barotrophic ins tability is suggested as a likely genesis mechanism of the vortex. After th e vortex is formed, the horizontal advection of total vorticity results in its westward propagation, while the secondary circulation hinders this move ment. Along its westward moving course, close to East Asia, there is a redu ction in vortex size and a tangential speed increase inversely proportional to the vortex size. Diminishing its horizontal convergence/descending moti on by the upper-tropospheric East Asian high and the lower-tropospheric mon soon low, the vortex eventually dissipates along the East Asian coast.