Sources of the Intermodel Spread in Projected Global Monsoon Hydrological Sensitivity

Abstract

The projected monsoon hydrological sensitivity, namely, the precipitation change rate per kelvin of global warming, shows substantial intermodel spread among 40 Coupled Model Intercomparison Project phase 5 models. The hydrological sensitivity of the Northern Hemisphere summer monsoon is negatively correlated with that of the Southern Hemisphere summer monsoon. The intermodel spread of the Northern Hemisphere summer monsoon hydrological sensitivity is mainly attributed to the projected interhemispheric temperature gradients and the associated low-level cross-equatorial flows. The intermodel spread of the Afro-Asia summer monsoon sensitivity is rooted in the projected continent-ocean thermal gradients, while the spread of the North American monsoon sensitivity is related to the projected sea surface temperature pattern in the tropical eastern Pacific and Atlantic. These findings suggest that further constraining monsoon hydrological sensitivity requires a better projection of the warming rate between the Northern and Southern Hemispheres and between the land and ocean, and the sea surface warming pattern in the tropical eastern Pacific and Atlantic. Plain Language Summary Global monsoon precipitation has far-reaching impacts on agriculture planning and social-economic sustainable development. Previous studies suggested that the increasing of global monsoon precipitation is sensitive to global warming rate, but the precipitation sensitivities, i.e., the precipitation change rate per kelvin of global warming, are also different among state-of-the-art coupled models. The causes of uncertainty in the monsoon precipitation sensitivity and the associated dominant physical mechanism remain elusive. Based on 40 state-of-the-art coupled climate models, we reveal that the greenhouse gas-induced interhemispheric temperature gradient causes the uncertainty of global monsoon precipitation sensitivity by changing the strengths of monsoon circulation and Hadley circulation and altering the Intertropical Convergence Zone position. Our results highlight the importance of a reliable projection of interhemispheric differential warming to the future change of global monsoon precipitation.

Uncertainty in Projected Changes in Precipitation Minus Evaporation: Dominant Role of Dynamic Circulation Changes and Weak Role for Thermodynamic Changes

“…These dynamic changes can be due to poleward shifts of the midlatitude jets or changes in stationary wave patterns, among other possibilities (Shaw, 2019;Shaw et al, 2016;Wills & Schneider, 2016). Furthermore, intermodel variability in projected drying/wettening is associated with intermodel variability in circulation changes especially on regional scales (Cao et al, 2020;Garfinkel et al, 2020;Shaw et al, 2016;Simpson et al, 2018Simpson et al, , 2016Zappa et al, 2015).…”

Higher Sensitivity of Northern Hemisphere Monsoon to Anthropogenic Aerosol Than Greenhouse Gases

“…It indicates that the CMIP6 model can well reproduce the negative monsoon precipitation sensitivity during the historical warming period (1850–2014). This contradicts the future projection result that gives a positive value of ∼2% K −1 (Cao et al., 2020; Wang et al., 2020). The individual forcing experiment shows that the historical GHG forced a rate of ∼2.1% K −1 ; however, the AA causes a rate of ∼11% K −1 .…”

“…Note that all indices are scaled by the GMST change from hist‐aer and hist‐GHG experiments, respectively. As suggested by previous studies, two thermal indices are used to represent the heterogeneous global warming that has dominant effects on monsoon circulation changes (Cao et al., 2020; Wang et al., 2013, 2020). The ITD index is defined as the surface temperature difference between the NH [0–40°N, 0°–360°] and SH [0°–60°S, 0°–360°].…”

“…In common sense, global warming would increase monsoon precipitation. Studies show that the NH monsoon precipitation increases under future global warming (e.g., Cao et al., 2020; Cao & Zhao, 2020; Chadwick et al., 2013; Kitoh et al., 2013; Lee & Wang, 2014; Moon & Ha, 2020; Wang et al., 2020). The increase in global mean surface temperature (GMST) enhances atmospheric moisture, contributing to the increased precipitation (Jin et al., 2020; Kitoh et al., 2013; Monerie et al., 2022).…”

Uncertainty in projected changes in precipitation minus evaporation: dominant role of dynamic circulation changes and weak role for thermodynamic changes

“…These thermodynamic and dynamic factors are important not just for the multimodel mean response, but also have been associated with intermodel variability in projected drying/wettening. Specifically, a larger increase in mean temperature due to a larger climate sensitivity in a given model would imply a stronger thermodynamic effect, while intermodel variability in circulation changes are associated with uncertainty in regional precipitation (Zappa et al, 2015;Simpson et al, 2016Simpson et al, , 2018Garfinkel et al, 2020;Cao et al, 2020). An example of a region with a wide spread in model projections is the Eastern Mediterannean: CMIP models project a decrease of 20-30% of Mediterranean precipitation by the end of the 21st century as compared to present-day averages if the multi-model mean is computed (Giorgi & Lionello, 2008;Kelley et al, 2012;Polade et al, 2017;Tuel & Eltahir, 2020;Garfinkel et al, 2020).…”