Widening trend of the Hadley cell during the past 40 years

We investigate the interannual and decadal variability of the meridional extent of the Hadley cell by analyzing variations in the shape of the lapse-rate define tropopause. The poleward boundaries of the Hadley cell are determined from the steep gradient between the high tropical and low extratropical tropopause. This method is applied to global monthly mean temperature fields from radiosonde data, reanalysis data, and to historical coupled simulations with the GFDL climate model. All data sources indicate that the latitudinal extent of the Hadley cell has been gradually widening by a few degrees latitude over the past decades. The importance of the location of the Hadley cell for the general circulation raises the question of what may have caused this trend.