Local Changes in the Hadley and Walker Circulations in the last 30 Years

In this study we present a new method to investigate the Hadley and Walker circulations. Our analysis is based on four reanalyses (ERA-Interim, NCEP2, MERRA and JRA). Among the important quantities calculated is the psi vector, which is a generalized vector streamfunction defined in such a way as to objectively decompose three-dimensional divergent circulations into two circulations projected onto zonal and meridional vertical planes. The psi vector provides a way of distinguishing between meridional and zonal circulations, which might be thought of as local Hadley and local Walker circulations. A great advantage of this method is that the overturning circulation in the meridional direction and the overturning circulation in the zonal direction are not defined by zonal averaging, and that the pair of two-dimensional overturning circulations can be added to give the original three-dimensional circulation. We present a climatology of the zonally and meridionally overturning circulations based on the psi-vector and show how these circulations change during El Nino and La Nina years.

The zonally overturning circulation in the tropical Pacific, which can be thought of as the psi-Walker circulation, weakens during El Nino and strengthens during La Nina. Importantly, the meridionally overturning circulation, which can be thought of as the psi-Hadley circulation, responds more strongly to an El Nino than the zonally overturning circulation. For instance, in DJF the meridionally overturning circulation over the Maritime Continent is stronger and the region of ascent is wider during La Nina than during El Nino. In JJA, the meridionally overturning circulation over the Maritime Continent is stronger during El Nino. Additionally, in DJF the meridionally overturning circulation has a double cell structure and in JJA a single cell structure. During an El Nino, the zonally overturning circulation in the Pacific is characterized by strong upward mass flux in the central Pacific and downward mass flux in the eastern Pacific. During La Nina the region of upward mass flux is shifted westwards but the magnitude remains unchanged. The downward mass flux intensifies in the eastern Pacific. Hence, during La Nina the zonally overturning circulation in the Pacific becomes wider. The zonally overturning circulation in the Indian Ocean is relatively weak in DJF. However, in JJA during La Nina it becomes much more pronounced and is the strongest zonally overturning circulation in the three ocean basins. The zonally overturning circulation in the Atlantic is relatively narrow, intensifies during La Nina, but it is of similar intensity to that in the Pacific.

Trends are calculated for the zonally and meridionally overturning circulations to determine how they have changed between 1979 and 2009. We find that there are large regional differences in the trends and also large differences between the seasons. The zonally overturning circulations weaken, mainly in the Indian Ocean and the Atlantic. In all ocean basins they are shifted slightly to the west. Further, the meridional overturning circulation is shifted poleward in some regions (e.g. over Africa, the Indian Ocean and the Maritime Continent).