Earth's Tropical dynamics

The Hadley circulation is commonly defined as the meridional circulation of the zonally averaged flow in the tropical region. In recent years, several studies looked at the longitudinal dependence of the meridional circulation via decomposition of the three-dimensional atmospheric flow into local meridional and zonal flows, thus allowing to quantitatively distinguish, for example, between the Hadley circulation in the west Pacific and the Walker circulation at the equatorial Pacific. These studies gave useful analysis on the regionality and variability of the meridional circulation, yet their spatial analysis remained essentially three-dimensional, and the temporal variability was strongly dependent on assumptions such as the connection to ENSO.

We derived a simplified definition of the local meridional circulation (LMC) and use hierarchical clustering to objectively study both the seasonality and the interannual variability of the LMC. Using the new definition we find that the effective meridional circulation is even more longitudinally restricted than was found in earlier studies, stressing the importance of treating the Hadley circulation as a regional phenomenon. Using hierarchical clustering we discuss first the seasonal cycle of the LMC, and then its interannual variability. We find that the seasonal cycle is defined with 3 clusters and the interannual variability with additional 5 clusters. The most prominent interannual variability of the LMC is the shifting in the east-west direction of the circulation, which is strongly related to other atmospheric variables such as the sea surface temperature, precipitation and air temperature. Using multiple linear regression we analyze these dependencies and discuss their implications for the climate system.

The interannual variability of the Longitudinally dependent Meridional Circulation