Energetic constraints on the pattern of changes to the hydrological
cycle under global warming
Abstract
The response of precipitation minus evaporation (P-E) to global warming
is investigated using a moist energy balance model (MEBM) with a simple
Hadley-Cell parameterization. The MEBM accurately emulates P-E changes
simulated by a suite of global climate models (GCMs) under
greenhouse-gas forcing. The MEBM also accounts for most of the
intermodel differences in GCM P-E changes and better emulates GCM P-E
changes when compared to the “wet-gets-wetter, dry-gets-drier”
thermodynamic mechanism. The intermodel spread in P-E changes are
attributed to intermodel differences in radiative feedbacks, which
account for 60-70% of the intermodel variance, with smaller
contributions from radiative forcing and ocean heat uptake. Isolating
the intermodel spread of feedbacks to specific regions shows that
tropical feedbacks are the primary source of intermodel spread in P-E
changes. The ability of the MEBM to emulate GCM P-E changes is further
investigated using idealized feedback patterns. A less negative and
narrowly peaked feedback pattern near the equator results in more
atmospheric heating, which strengthens the Hadley Cell circulation in
the deep tropics through an enhanced poleward heat flux. This pattern
also increases gross moist stability, which weakens the subtropical
Hadley Cell circulation. These two processes in unison increase P-E in
the deep tropics, decrease P-E in the subtropics, and narrow the
Intertropical Convergence Zone. Additionally, a feedback pattern that
produces polar-amplified warming reduces the poleward moisture flux by
weakening the meridional temperature gradient and the Clausius-Clapeyron
relation. It is shown that changes to the Hadley Cell circulation and
the poleward moisture flux are crucial for understanding the pattern of
GCM P-E changes under warming.