Thermodynamic and dynamic responses of the hydrological cycle to solar dimming

Abstract

Abstract. The fundamental role of the hydrological cycle in the global climate system motivates a thorough evaluation of its responses to climate change and mitigation. The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinated international effort to assess the climate impacts of solar geoengineering, a proposal to counteract global warming with a reduction in incoming solar radiation. We assess the mechanisms underlying the rainfall response to a simplified simulation of such solar dimming (G1) in the suite of GeoMIP models and identify robust features. While solar geoengineering nearly restores preindustrial temperatures, the global hydrology is altered. Tropical precipitation changes dominate the response across the model suite, and these are driven primarily by shifts of the Hadley circulation cells. We report a damping of the seasonal migration of the Intertropical Convergence Zone (ITCZ) in G1, associated with preferential cooling of the summer hemisphere, and annual mean ITCZ shifts in some models that are correlated with the warming of one hemisphere relative to the other. Dynamical changes better explain the varying tropical rainfall anomalies between models than changes in relative humidity or the Clausius-Clapeyron scaling of precipitation minus evaporation (P − E), given that the relative humidity and temperature responses are robust across the suite. Strong reductions in relative humidity over vegetated land regions are likely related to the CO 2 physiological response in plants. The uncertainty in the spatial distribution of tropical P − E changes highlights the need for cautious consideration and continued study before any implementation of solar geoengineering.

Energy transport, polar amplification, and ITCZ shifts in the GeoMIP G1 ensemble

“…This is similar to Fig. 7 of Smyth et al (2017) but for the specific set of models included in this study. Here, there is a positive correlation (r = 0.61), slightly weaker than that for cross-equatorial MSE transport.…”

“…4b imply that solar reductions applied equally in both hemispheres could still cause regional shifts in precipitation based on factors like the base state albedo and local radiative feedbacks that might warm one hemisphere relative to the other. This, along with reductions in the seasonal ITCZ migration due to preferential cooling of the summer hemisphere, is discussed elsewhere (Smyth et al, 2017). Our focus here is on the reasons for the intermodel spread in the ITCZ shifts.…”

“…In the context of solar geoengineering, Haywood et al (2013) demonstrated that preferentially injecting reflective aerosols into one hemisphere shifts the ITCZ towards the other hemisphere, causing drought in some tropical areas. Even under a hemispherically symmetric solar geoengineering deployment, such as space-based mirrors (represented by a solar constant reduction), ITCZ shifts can still occur (Smyth et al, 2017).…”

The Regional Hydroclimate Response to Stratospheric Sulfate Geoengineering and the Role of Stratospheric Heating

“…A number of studies have considered the effects of solar dimming on regional hydroclimate in multiple models within the Geoengineering Model Intercomparison Project framwork (Kravitz et al, ). Tilmes et al () note the substantial reduction of precipitation over land areas in many of the monsoon regions, and Smyth et al () further relate this to altered interhemispheric temperature gradients and changes to the seasonal migration of the intertropical convergence zone. Jones et al () point out that the overall impact of solar dimming on global mean precipitation is dependent on the fraction of global warming that is being compensated, and Irvine et al () recently demonstrated that using solar dimming to only halve the amount of global warming, successfully moderates the hydroclimate impacts of rising GHGs over the majority of land regions.…”

Sensitivity of the ITCZ Location to Ocean Forcing Via Q‐Flux Green's Function Experiments

“…Additionally, it has been shown that forcing exerted in the extratropics invokes a larger shift of the ITCZ than similar forcing applied within the tropics, owing at least in part to water vapor and cloud feedbacks (Kang et al, 2009;Seo et al, 2014). Other factors have also been shown to be important for understanding the sensitivity of ITCZ shifts to forcing, including (i) subtropical wind-driven ocean circulation (Green & Marshall, 2017), which tends to dampen the shifts in the ITCZ; (ii) changes in solar constant (Smyth et al, 2017), which reduce seasonal migration of the ITCZ because less cross-equatorial energy transport is needed to balance the interhemispheric difference in energy input; and (iii) gross moist stability (Kang et al, 2009;Seo et al, 2017), which relates fluxes of energy to fluxes of mass in the tropical overturning circulation and allows for a decoupling of the relationship between ITCZ position and AHT eq . In short, the shift in the ITCZ in response to changing AHT eq can be modulated by both oceanic responses as well as gross moist stability changes in the atmosphere.…”