Comparing the Evolution of the Hadley Circulation in the ECMWF ERA-20C Centennial Reanalysis and in the Atmospheric Model Ensemble ERA-20CM

Abstract:

This study compares the evolution of the Hadley Circulation (HC) during the 20^th Century in two datasets: ERA-20CM and ERA-20C. ERA-20CM is an ensemble of Atmospheric Model simulations forced by an ensemble of 10 different realizations of prescribed SSTs and by radiative forcing that follow CMIP5 protocol. ERA-20C is similarly forced, but, additionally, it assimilates surface pressure and marine winds. Therefore, this comparison allows to evidence the effect of the assimilation of surface observations on HC characteristics and trends. In fact differences are relevant. The HCs are weaker and the whole Northern Hemisphere HC is located more southward in ERA-20C than in ERA-20CM. Further, magnitude of trends is larger and more statistically significant in ERA-20C than in ERA-20CM and only ERA-20C supports centennial changes of the HC strength. In general, both datasets show the presence of large multi-decadal variability, which make uncertain the interpretation of all features of the recent behavior of the HC as the onset of sustained long term trends. However, the southward shift of the Southern Edge of the Southern Hemisphere HC and widening of the Southern Hemisphere HC appear as a robust feature in both datasets. In fact, though actual expansion rates remain uncertain, both ERA-20C and ERA-20CM support the impact of global warming on the widening of the HCs and show that it is larger for the Southern Hemisphere than for the northern Hemisphere HC.