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Midlatitude Fronts and Variability in the Southern Hemisphere Tropical Width

This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.