Causes of recent changes in the strength of the Hadley cell.

Strengthening trends in the tropical mean meridional circulation (Hadley cell) have been recently documented in both NCEP and ERA40 reanalyses. This trend coincides with trends in the high latitude annular modes. Here we examine the extent to which these trends in low and high latitude circulations are related to each other. We use both reanalyses data and an optimal interpolation of COADS data to calculate the trends in sea-level pressure which shows both a clear annular mode signal in the extra-tropics, and a Hadley cell signal in the tropics. Possible mechanisms of extratropical influence to the Hadley cell circulation by eddy heat and momentum fluxes in the subtropics are investigated using reanalyses data.