Southern Hemisphere Airstream Climatology

Abstract

Estimates of mean monthly surface streamlines for the Southern Hemisphere were prepared from gridded mean surface geostrophic winds, from ship and land-based observations collected over several decades. The streamlines were arbitrarily drawn 20° clockwise to the geostrophic direction over oceans, 35° over relatively flat land, and 65° over rough terrain. Except over southern Africa, these assumptions were supported by a comparison between geostrophic directions and those of nearby mean vector winds. Four annual airstream sources were identified within the Southern Hemisphere: one each over the Indian, Pacific, and Atlantic Oceans, and a continental source over Antarctica. A fifth source region was identified over Australia during seven months centered on the Southern winter. Airstreams originating in the Northern Hemisphere penetrate into the Southern Hemisphere during summer, following the Intertropical Zone. During a mean year, 46% of the Southern Hemisphere experiences air originating from only one of four sources the Pacific Ocean being the greatest contributor (20%), followed by the Indian Ocean (12%), Atlantic Ocean (9%), and Antarctica (5%). Model confluences were found in four areas 1) at about 65°S latitude, 2) a confluence from equatorial Africa southwest to the coast, continuing eastward into the Indian Ocean, 3) a north-south confluence parallel to the Andes and 4) an east-west confluence south of the Australian cost from April through October.