Subtropical Droughts and Cross-Equatorial Energy Transports

Abstract

The spatial coherence of subtropical rainfall anomalies is documented by variance analysis. Major droughts repeatedly were felt at the same time around the globe along the arid margins of the tropical rainfall belt. The persistence of anomalies becomes apparent in precipitation time series which combine data from relatively large areas and in streamflow records. These can be used to demonstrate autocorrelations and unexpectedly long runs of wet or dry years. The latest drought episode culminated in 1972 not only in the Sahel and the Sudan, but also along the borders of the Indian desert and in Central America. It is shown to have been accompanied by relatively low temperatures in the southern subtropics and by abnormally high temperatures in the antarctic. The meridional temperature gradient and the meridional slope of the 500 mb surface were correspondingly reduced. It is suggested that this was associated with a reduced demand for energy (and zonal momentum) exports from the tropics and therefore relatively weak direct tropical circulations. As a result, these circulations–which tend to straddle the equator–did not deliver the normal amount of precipitation along their northernmost borders in the monsoonal fringe area.