A Mechanisms-Based Approach to Detecting Recent Anthropogenic Hydroclimate Change

Abstract

oth naturally occurring La Niña events and model-projected anthropogenic-driven global warming are associated with widespread drying in the subtropics to midlatitudes. Models suggest anthropogenic drying should already be underway but climate variability on interannual to multidecadal time scales can easily obscure any emerging trend, making it hard to assess the validity of the simulated forced change. Here, the authors address this problem by using model simulations and the twentieth-century reanalysis to distinguish between natural variability of, and radiatively forced change in, hydroclimate on the basis of the mechanisms of variations in the three-dimensional moisture budget that drive variations in precipitation minus evaporation (P ? E). Natural variability of P ? E is dominated by the El Niño?Southern Oscillation (ENSO) cycle and is ?dynamics dominated? in that the associated global P ? E anomalies are primarily driven by changes in circulation. This is quite well reproduced in the multimodel mean of 15 models used in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Coupled Model Intercomparison Project 3 (CMIP3). In contrast, radiatively forced P ? E change is ?thermodynamics mediated? in that the rise in specific humidity leads to intensified patterns of moisture transport and P ? E. But, as for ENSO, the poleward shift of the storm tracks and mean meridional circulation cells also contribute to changes in P ? E. However, La Niña and radiatively forced changes in the zonal mean flow are distinct in the tropics. These distinctions are applied to the post-1979 record of P ? E in the twentieth-century reanalysis. ENSO-related variations strongly influence the observed P ? E trend since 1979, but removal of this influence leaves an emerging pattern of P ? E change consistent with the predictions of the IPCC AR4/CMIP3 models over this period together with, to some extent, consistent contributions from dynamical and thermodynamical mechanisms and consistent changes in the zonal mean circulation. The forced trends are currently weak compared to those caused by internal variability.