A dichotomy between model responses of tropical ascent and descent to surface warming

摘要

Simulations of tropical atmospheric circulation response to surface warming vary substantially across models, causing large uncertainties in projections of regional precipitation change. Understanding the physical processes that drive the model spread in tropical circulation changes is critically needed. Here we employ the basic mass balance and energetic constraints on tropical circulation to identify the dominant factors that determine multidecadal circulation strength and area changes in climate models. We show that the models produce a robust weakening of descent rate under warming regardless of surface warming patterns; however, ascent rate change exhibits inter-model spread twice as large as descent rate because of diverse model responses in the radiative effects of clouds, water vapor, and aerosols. As ascent area change is dictated by the disparate descent and ascent rate changes due to the mass budget and the inter-model spread in descent rate change is small, the model spread in ascent area change is dominated by that of ascent rate change, resulting in a strong anti-correlation of -0.85 between the fractional changes of ascent strength and area across 77 climate model simulations. This anti-correlation leads to a corresponding inverse relationship between the rates of precipitation intensifying and narrowing of the inter-tropical convergence zone (ITCZ), suggesting tropical ascent area change can be potentially used to constrain the ITCZ precipitation change. Longwave cloud radiative effect at the top-of-atmosphere (TOA) in the convective region is identified to be a major source of uncertainties for tropical ascent rate change and thus for regional precipitation change. Tropical precipitation in the future will be influenced by the balance between changes in the area and rate of atmospheric ascent and descent. Hui Su from the Jet Propulsion Laboratory, together with a multidisciplinary team, probe several climate modeling experiments and show that models consistently simulate a lower descent rate in a warming climate. But changes in ascent rate range from strongly positive to strongly negative, a diversity that is dominated by variations in both the simulated longwave energy fluxes at the top of the atmosphere and the absorption of shortwave energy. Because mass must be conserved and the descent rate change is approximately constant, ascent rate change is anticorrelated with the ascending area change. Consequently, models with a major increase in ascent rate also show a narrowing of the ascending area, leading to a "wetter get narrower" environment. Robust observations are critically needed to constrain the simulated ascent rates and controlling processes.