Northern Hemisphere Airstream Regions

Abstract

Near-surface airstream source regions of the Northern Hemisphere have been identified using 16-year mean resultant winds from 3° latitude by 3° longitude grids. Tracing the airstreams back to their divergent centers reveals 19 different sources during various seasons of the year. Five of these sources(air originating over the North and South Pacific and Atlantic Oceans and air over Turkey) are resident in the Northern Hemisphere 12 months of the year. Another three (central Asian, Arctic and central East Asian air) exist for at least 11 months per year. The remaining 11 source regions are present from 1?9 months per year and their area of influence is much less than that of the 5 year-long sources. In the mean, there are several favored locations for frontal zones, e.g., a north?south band in Mexico (dividing Atlantic from Pacific air), a north?south band in northern South America, and two northeast?southwest trending bands over the cast coasts of Asia and North America, representing the mean leading edges of continental airmasses. Mean dew points demonstrate the character of the moisture discontinuity across several mean frontal boundaries. Gradients in moisture are apparent as one progresses from one airstream to another. These gradients are sharpest along confluences, coasts and mountain barriers, particularly when a confluence is near-coincident with a topographic boundary.