A Simple Moisture Advection Model of Specific Humidity Change over Land in Response to SST Warming

Abstract

A simple conceptual model of surface specific humidity change over land is described, based on the effect of increased moisture advection from the oceans in response to sea surface temperature (SST) warming. In this model, future q over land is determined by scaling the present-day pattern of land q by the fractional increase in the oceanic moisture source. Simple model estimates agree well with climate model projections of future (mean spatial correlation coefficient 0.87), so over both land and ocean can be viewed primarily as a thermodynamic process controlled by SST warming. Precipitation change is also affected by , and the new simple model can be included in a decomposition of tropical precipitation change, where it provides increased physical understanding of the processes that drive over land. Confidence in the thermodynamic part of extreme precipitation change over land is increased by this improved understanding, and this should scale approximately with Clausius–Clapeyron oceanic q increases under SST warming. Residuals of actual climate model from simple model estimates are often associated with regions of large circulation change, and can be thought of as the “dynamical” part of specific humidity change. The simple model is used to explore intermodel uncertainty in , and there are substantial contributions to uncertainty from both the thermodynamic (simple model) and dynamical (residual) terms. The largest cause of intermodel uncertainty within the thermodynamic term is uncertainty in the magnitude of global mean SST warming.

Comparison of land surface humidity between observations and CMIP5 models

“…The relative humidity over land would decrease accordingly. This is because the amount of water vapour required for saturation will have increased with rising temperature but disproportionately to the actual increase in water vapour (Joshi et al, 2008;O'Gorman and Muller, 2010;Simmons et al, 2010;Sherwood and Fu, 2014;Chadwick et al, 2016). This is broadly what has been observed over the period from the end of the twentieth century.…”

Effect of the Atlantic Multidecadal Variability on the Global Monsoon

“…We decompose precipitation change (ΔP) into its dynamical (ΔP dyn ), thermodynamical (ΔP therm ), and its nonlinear cross components (ΔP cross ) following Chadwick et al (2016). This method relies on the fact that tropical precipitation is dominated by convection (Chadwick et al, 2016;Rowell & Chadwick, 2018).…”

Separating the Influences of Land Warming, the Direct CO2 Effect, the Plant Physiological Effect, and SST Warming on Regional Precipitation Changes

“…Because surface moisture is generally more limited over land than over the ocean, the land surface warms more than the ocean in response to a given increase in free tropospheric temperature (Byrne & O'Gorman, 2013;Joshi et al, 2008). This response is closely coupled to the hydrological cycle, including reductions in cloud (Joshi et al, 2008), decreased surface relative humidity (Byrne & O'Gorman, 2016;Chadwick et al, 2016;O'Gorman & Muller, 2010), and soil moisture feedbacks (Berg et al, 2016). A second driver of land warming is the direct radiative effect of higher CO 2 concentrations (Figure 1b).…”