An objective study of 500-hPa moving troughs in the Southern Hemisphere

Abstract

The location and movement of 500-hPa troughs using an automated method are studied with data from a 24-yr period with the objective of determining the trough formation and dissipation regions in the Southern Hemisphere. To identify the 500-hPa mobile troughs, an objective method that uses the Eulerian centripetal acceleration (ECA) is developed. On average, 868 troughs per year were identified by the method, with an increase in trend during the period studied. The troughs have an average lifetime of 4.3 days, being longer (shorter) in subtropical (high) latitudes. The average calculated phase velocity was 13.6 ms-1, being higher (lower) in middle (high) latitudes. The troughs are normally found in the 60°-40°S latitudinal band, with maximum occurrence at 50°S. The longitudinal distribution of trough genesis has three maximum regions: over the Drake Strait and the South Atlantic Ocean, over the Indian Ocean around 50°S, and over the southwestern Pacific Ocean between 150°E and 150°W. The trough dissipation regions are less concentrated than the genesis regions and also show three maxima: over the west of the Andes, south of the African continent, and south of Australia. The seasonal variation in the trough dissipation shows that the 30°-40°S band is more active during winter than in summer. The difference between the genesis and dissipation location is that formation occurs more in middle and high latitudes, while dissipation is more common in the 40°-30°S latitude belt. © 2008 American Meteorological Society.