Trends in the Southern Hemisphere Tropospheric Circulation

An analysis has been made of the mean height and wind fields throughout the troposphere of the Southern Hemisphere for May 1972–January 1978 based on daily synoptic hemispheric analyses of geopotential height produced by the Australian World Meteorological Center. While the means resemble those previously obtained in a series of atlases based primarily on the period 1957–66, significant differences do exist. The reality of these trends is investigated using station data. Some of the changes in the means are due to different analysis techniques, inhomogeneities in the data used by the atlases, and changes in the station network and data availability. Consequently, spurious trends are evident in the South Pacific near Rapa Island where the data network has improved, and near Easter Island and southern South America where the Australian analyses at 0000 GMT are less reliable than elsewhere. The geopotential height increases over Antarctica and height decreases over the subtropical continents appear to be mostly real, whereas the increased intensity of the oceanic subtropical highs cannot be confirmed although this probably reflects a real change in the circulation. Nevertheless, there is a distinctive overall pattern to the trends. Between the periods 1957–66 and 1972–78 the relatively warm atmosphere over the subtropical continents has become cooler, particularly in the western regions, while over the oceans the atmosphere has warmed slightly. In high latitudes, atmospheric temperature contrasts in the warm and cool sectors of Antarctica have become further enhanced. The close association between the trends and the distribution of continents is perhaps a hint that the trends may be related to changes in continentality.