Reconciling Theories for Human and Natural Attribution of Recent East Africa Drying

Two theories for observed East Africa drying trends during March-May 1979-2013 are reconciled.  Both hypothesize that variations in tropical sea surface temperatures (SSTs) caused East Africa drying.  The first invokes a mainly human cause resulting from sensitivity to secular warming of Indo-west Pacific SSTs.  The second invokes a mainly natural cause resulting from sensitivity to a strong articulation of ENSO-like Pacific decadal variability involving warming of the west Pacific and cooling of the central Pacific.

Historical atmospheric model simulations indicate that observed SST variations contributed significantly to the East Africa drying trend during March-May 1979-2013.  By contrast, historical coupled model simulations indicate that external radiative forcing alone, including the ocean’s response to that forcing, did not contribute significantly to East Africa drying.  Recognizing that the observed SST variations involved a comingling of natural and anthropogenic effects, we diagnosed how East African rainfall sensitivity was conditionally dependent on the interplay of those factors.  East African rainfall trends in historical coupled models were inter-compared between two composites of ENSO-like decadal variability, one operating in the early 20^th century before appreciable global warming and the other in the early 21^st century of strong global warming.  We find the co-action of global warming with ENSO-like decadal variability to significantly enhance 35-yr East Africa drying trends relative to when the natural mode of ocean variability acts alone.  A human-induced change and its interplay with natural variability is thus speculated to be a key aspect toward understanding recent Africa drying; however, our results are suggestive owing to differences among two independent suites of coupled model ensembles.