The Role of the Cloud Radiative Effect in the Sensitivity of the Intertropical Convergence Zone to Convective Mixing

Abstract

Studies have shown that the location and structure of the simulated intertropical convergence zone (ITCZ) is sensitive to the treatment of sub-gridscale convection and cloud–radiation interactions. This sensitivity remains in idealized aquaplanet experiments with fixed surface temperatures. However, studies have not considered the role of cloud-radiative effects (CRE; atmospheric heating due to cloud–radiation interactions) in the sensitivity of the ITCZ to the treatment of convection. We use an atmospheric energy input (AEI) framework to explore how the CRE modulates the sensitivity of the ITCZ to convective mixing in aquaplanet simulations. Simulations show a sensitivity of the ITCZ to convective mixing, with stronger convective mixing favoring a single ITCZ. For simulations with a single ITCZ, the CRE maintains the positive equatorial AEI. To explore the role of the CRE further, we prescribe the CRE as either zero or a meridionally and diurnally varying climatology. Removing the CRE is associated with a reduced equatorial AEI and an increase in the range of convective mixing rates that produce a double ITCZ. Prescribing the CRE reduces the sensitivity of the ITCZ to convective mixing by 50%. In prescribed-CRE simulations, other AEI components, in particular the surface latent heat flux, modulate the sensitivity of the AEI to convective mixing. Analysis of the meridional moist static energy transport shows that a shallower Hadley circulation can produce an equatorward energy transport at low latitudes even with equatorial ascent.

Advances in understanding large‐scale responses of the water cycle to climate change

“…161,181 This leads to a drying tendency on the equatorward edges of the ITCZ 177 and a moistening tendency in the ITCZ core: stronger ascent in the ITCZ core amplifies the "wet get wetter" response, while reduced moisture inflow near the ITCZ edges reduces the "wet gets wetter" response relative to the thermodynamic increase in moisture transport. Overall, ITCZ responses have been linked with hemispheric asymmetry in radiative forcing from greenhouse gases and aerosols, 168,182,183 feedbacks involving clouds, [184][185][186] and vertical energy stratification. 179,187 Changes in the regional tropical rain belt are larger than for the global ITCZ and involve more complex dynamical mechanisms, 188,189 including monsoons.…”

Large-scale controls of propagation of the Madden-Julian Oscillation

“…Additionally, weak equatorial mean westerlies in the mid-upper troposphere between 10°S and 10°N in QOBS are replaced by equatorial easterlies throughout the entire troposphere in FLAT. Note that for a given SST profile, the corresponding model mean state can differ from model to model due to their different sensitivities of model convective mixing and cloud-radiative feedbacks to the SST distribution [29][30][31][32] .…”

Relationships Between Tropical Ascent and High Cloud Fraction Changes With Warming Revealed by Perturbation Physics Experiments in CAM5

“…The ITCZ has been shown to be sensitive to convective parameterization (Frierson, ; Liu et al, ; Mobis & Stevens, ; Talib et al, ; Webb et al, ) and differences in cloud radiative effects (Dixit et al, ; Fermepin & Bony, ; Fläschner et al, ; Harrop & Hartmann, ; Popp & Silvers, ; Randall et al, ; Slingo & Slingo, ; Voigt & Shaw, ). This work specifically aims to identify model physics that could contribute significantly to the CMIP5 intermodel spread, and thus uncertainty, in projections of tropical ascent area ( A a ) and high cloud fraction (HCF) changes with warming.…”

“…The ITCZ has been shown to be sensitive to convective parameterization (Frierson, 2007;Liu et al, 2010;Mobis & Stevens, 2012;Talib et al, 2018;Webb et al, 2015) and differences in cloud radiative effects ©2019. American Geophysical Union.…”