Planetary-Scale Atmospheric Phenomena Associated with the Southern Oscillation

Abstract

Atmospheric phenomena associated with the Southern Oscillation are examined, with emphasis on vertical structure and teleconnections to middle latitudes. This paper is specifically concerned with the interannual variability of seasonal means for the Northern Hemisphere winter during the period 1951–78. Among the variables considered are sea surface temperature in the equatorial Pacific, precipitation at selected equatorial Pacific stations, a “Southern Oscillation Index” of sea level pressure, 200 mb height and tropospheric mean temperature at stations throughout the tropics, and Northern Hemisphere geopotential height fields. Selected statistics derived from surface data also are examined for the period 1910–45. Results are presented in the form of time series and correlation statistics for the variables listed above. Results concerning the relationships between sea surface temperature, sea level pressure and rainfall are consistent with the major conclusions of previous studies by J. Bjerknes and others. Fluctuations in mean tropospheric temperature and 200 mb height are shown to vary simultaneously with equatorial Pacific sea surface temperature fluctuations, not only in the Pacific sector, but at stations throughout the tropics. The zonally symmetric component of these 200 mb height fluctuations is considerably larger than the Southern Oscillation in 1000 mb height, and the corresponding fluctuations in the mean temperature of the tropical troposphere are on the order of nearly 1 K. The correlations between the tropical time series and Northern Hemisphere geopotential height fields exhibit well-defined teleconnection patterns. Warm episodes in equatorial Pacific sea surface temperature tend to be accompanied by below-normal heights in the North Pacific and the south–eastern United States and above-normal heights over western Canada. Recent theoretical work by Opsteegh and Van den Dool (1980), Hoskins and Karoly (1981) and Webster (1981) on Rossby wave propagation on a sphere provides a basis for understanding the teleconnection in terms of the distribution of sea surface temperature and rainfall in the equatorial Pacific. The theory successfully explains several characteristics of the observed teleconnection patterns, including their horizontal scale and shape, their vertical structure and their seasonal dependence.