Trends of upper‐air circulation and water vapour over equatorial South America and adjacent oceans

Abstract

A novel 40-year upper-air dataset along with satellite measurements have been analyzed to explore the vertical circulation and water vapour transport and their long-term evolution over equatorial South America and the adjacent oceans. Lower-tropospheric convergence, mid-tropospheric ascending motion, upper-tropospheric divergence, as well as precipitable water, are concentrated in the realm of the Pacific and Atlantic Inter-tropical Convergence Zone (ITCZ) throughout the year and the Amazon basin during austral summer. Water vapour is imported into the Amazon basin from the equatorial and tropical South Atlantic and during austral summer also from the tropical North Atlantic. The recognition of long-term developments in the tropical climate system hinges on the homogeneity of the upper-air dataset. With this qualification, statistically significant increasing trends of lower-tropospheric convergence, upward motion, upper-tropospheric divergence, convergence of atmospheric water vapour transport, and precipitable water are found over the Amazon basin throughout the year. In all seasons, the pattern of atmospheric water vapour transport shows a statistically significant trend towards the development of a clockwise turning vortex over eastern Brazil, concordant with a centre of falling tendency in 1000 mb height and associated vortex in the wind field. The trends towards reduced/enhanced water vapour import at the northern/southern side of the vortex largely offset each other, so that the trend towards greater moisture convergence for the basin as a whole results from the combination of increasing vapour import through the northern and decreasing export through the western and southern boundaries of the Amazon watershed. Copyright © 1999 Royal Meteorological Society