Energetic constraints on the magnitude and pattern of changes in the hydrologic cycle under global warming.

Friday, 18 December 2015
Poster Hall (Moscone South)
Nicholas Siler, Gerard Roe and Kyle Armour, University of Washington Seattle Campus, Seattle, WA, United States
Recent studies have shown that changes in poleward energy transport under global warming can be understood as the down-gradient flux of near-surface moist static energy (MSE), modified by the spatial pattern of radiative feedbacks. The MSE flux is a constraint on the vertically integrated moisture flux, and thus on the pattern of precipitation minus evaporation (P-E). We explore the implications of this link for our understanding of changes to the Earth's hydrologic cycle under anthropogenic warming.

Beginning with the Penman-Montieth equation, we derive a simple expression for the scaling of E with changes in temperature and net surface radiation. This equation suggests that, in the global mean, E (and thus P) should increase by about 2\% per degree of surface warming, consistent with comprehensive global climate model (GCM) simulations. This scaling is relatively insensitive to the magnitude and pattern of temperature change.

Further, we show that this constraint on E, together with the down-gradient transport of near-surface MSE anomalies, requires changes in the hydrologic cycle that closely mirror those found in GCMs: E must increase everywhere; P must increase in the deep tropics, decrease in the subtropics, and increase in mid- and high-latitudes; fractional changes in P must increase towards high latitudes, and are sensitive to the degree of polar amplification. Reasonable variations in the spatial pattern of radiative feedbacks modify, rather than overturn, this basic picture. Finally, global conservation of MSE divergence requires both a poleward expansion of the subtropics and a poleward migration of the latitude of maximum P, implying energetic constraints on tropical and midlatitude circulation responses.

Thus, the principle that climate adjusts by down-gradient transport of MSE anomalies is a single and simple explanation for the basic structure of hydrologic cycle changes under global warming, which is borne out both qualitatively and quantitatively in comprehensive GCMs.