Mean Distribution of 500-mb Topography and Sea-Level Pressure in Middle and High Latitudes of the Northern Hemisphere during the 1950–59 Decade, January and July

Abstract

Circumpolar decadal 500-mb mean maps and data for middle and high latitudes are presented for January and July, 1950?59. These maps are based largely on recent Northern Hemispheric upper-air observations and are superior to previous normals. The 500-mb January map is the first long-period mean for that month to show a splitting of the jet stream over the North Pacific with a secondary branch passing to the north of Alaska. In July, the polar vortex is rather symmetrically centered over the North Pole at 500 mb. The tendency during the past decade for the Aleutian and Icelandic lows to form winter auxiliary cells to the cast of the parent cells is supported by the sea-level-pressure data for January. Relatively low sea-level mean pressures in the vicinity of the North Pole are confirmed for July. It is implied that July mean virtual temperatures over the Polar Basin previously have bun underestimated by approximately 3K.