Contrasting Precipitation Response in the Ascending and Descending Branches of the Atmospheric Circulation

Theoretical considerations, backed up by modeling studies, suggest that not only will mean precipitation increase in a warmer, moister world but that wet regions will become wetter and, counter-intuitively, dry regions drier. Observational evidence from GPCP and SSM/I appears to back up this prediction over the period 1979-2006 with ascending regions of the tropical circulation displaying rapid rises in precipitation (+0.18 mm/day per decade) with regions of mean descent showing declining rainfall trends (-0.10 mm/day per decade). However, the rises and falls in precipitation appear substantially larger than model simulations over the same period. Here we analyze data in the tropics and mid-latitudes to investigate whether this discrepancy can be explained by inadequacies in the observing system or whether current models may be underestimating the present day variability in the atmospheric hydrological cycle.