The Response of Precipitation Minus Evapotranspiration to Climate Warming: Why the “Wet-Get-Wetter, Dry-Get-Drier” Scaling Does Not Hold over Land
Journal of Climate American Meteorological Society 28:20 (2015) 8078-8092
Abstract:
Simulations with climate models show a land–ocean contrast in the response of P − E (precipitation minus evaporation or evapotranspiration) to global warming, with larger changes over ocean than over land. The changes over ocean broadly follow a simple thermodynamic scaling of the atmospheric moisture convergence: the so-called “wet-get-wetter, dry-get-drier” mechanism. Over land, however, the simple scaling fails to give any regions with decreases in P − E, and it overestimates increases in P − E compared to the simulations. Changes in circulation cause deviations from the simple scaling, but they are not sufficient to explain this systematic moist bias. It is shown here that horizontal gradients of changes in temperature and fractional changes in relative humidity, not accounted for in the simple scaling, are important over land and high-latitude oceans. An extended scaling that incorporates these gradients is shown to better capture the response of P − E over land, including a smaller increase in global-mean runoff and several regions with decreases in P − E. In the zonal mean over land, the gradient terms lead to a robust drying tendency at almost all latitudes. This drying tendency is shown to relate, in part, to the polar amplification of warming in the Northern Hemisphere, and to the amplified warming over continental interiors and on the eastern side of midlatitude continents.Link between land‐ocean warming contrast and surface relative humidities in simulations with coupled climate models
Geophysical Research Letters American Geophysical Union 40:19 (2013) 5223-5227
Abstract:
Simulations of warming climates with coupled climate models exhibit strong land‐ocean contrasts in changes in surface temperature and relative humidity, but little land‐ocean contrast in changes in equivalent potential temperature. A theory that assumes equal changes in equivalent potential temperature over land and ocean captures the simulated land‐ocean warming contrast in the tropics if changes in relative humidity and ocean temperature are taken as given. According to the theory, land relative humidity changes and the land‐ocean contrast in the control climate contribute equally to the tropical warming contrast, while ocean relative humidity changes make a smaller (but also positive) contribution. Intermodel scatter in the tropical warming contrast is primarily linked to land relative humidity changes. These results emphasize the need to better constrain land relative humidity changes in model simulations, and they are also relevant for changes in heat stress over land.Land–ocean warming contrast over a wide range of climates: convective quasi-equilibrium theory and idealized simulations
Journal of Climate American Meteorological Society 26:12 (2013) 4000-4016
Abstract:
Surface temperatures increase at a greater rate over land than ocean in simulations and observations of global warming. It has previously been proposed that this land–ocean warming contrast is related to different changes in lapse rates over land and ocean because of limited moisture availability over land. A simple theory of the land–ocean warming contrast is developed here in which lapse rates are determined by an assumption of convective quasi-equilibrium. The theory predicts that the difference between land and ocean temperatures increases monotonically as the climate warms or as the land becomes more arid. However, the ratio of differential warming over land and ocean varies nonmonotonically with temperature for constant relative humidities and reaches a maximum at roughly 290 K. The theory is applied to simulations with an idealized general circulation model in which the continental configuration and climate are varied systematically. The simulated warming contrast is confined to latitudes below 50° when climate is varied by changes in longwave optical thickness. The warming contrast depends on land aridity and is larger for zonal land bands than for continents with finite zonal extent. A land–ocean temperature contrast may be induced at higher latitudes by enforcing an arid land surface, but its magnitude is relatively small. The warming contrast is generally well described by the theory, although inclusion of a land–ocean albedo contrast causes the theory to overestimate the land temperatures. Extensions of the theory are discussed to include the effect of large-scale eddies on the extratropical thermal stratification and to account for warming contrasts in both surface air and surface skin temperatures.Energetic constraints on precipitation under climate change
Surveys in Geophysics Springer Verlag 33:3-4 (2011) 585-608