The Diversity of Cloud Responses to Twentieth Century Sea Surface Temperatures

Abstract

Low‐level clouds are shown to be the conduit between the observed sea surface temperatures (SST) and large decadal fluctuations of the top of the atmosphere radiative imbalance. The influence of low‐level clouds on the climate feedback is shown for global mean time series as well as particular geographic regions. The changes of clouds are found to be important for a midcentury period of high sensitivity and a late century period of low sensitivity. These conclusions are drawn from analysis of amip‐piForcing simulations using three atmospheric general circulation models (AM2.1, AM3, and AM4.0). All three models confirm the importance of the relationship between the global climate sensitivity and the eastern Pacific trends of SST and low‐level clouds. However, this work argues that the variability of the climate feedback parameter is not driven by stratocumulus‐dominated regions in the eastern ocean basins, but rather by the cloudy response in the rest of the tropics.

Accounting for Changing Temperature Patterns Increases Historical Estimates of Climate Sensitivity

“…This mostly comes from SW cloud feedback processes, with historical LW cloud feedback processes generally being representative of that seen in abrupt-4xCO 2 (Figure 2e). These findings are consistent with process-orientated studies that suggest lapserate (which affect LW clear sky) and low-cloud (which affect SW, NET, and CRE) feedbacks vary the most with SST patterns, especially in the Pacific (see below and Andrews et al, 2015;Andrews & Webb, 2018;Ceppi & Gregory, 2017;Rose et al, 2014;Silvers et al, 2018;Zhou et al, 2016Zhou et al, , 2017.…”

“…With constant forcings the variation in radiative fluxes comes about solely from the changing SST and sea ice boundary conditions, allowing radiative feedbacks to be accurately diagnosed directly from top‐of‐atmosphere (TOA) radiation fields (e.g., Haugstad et al, ). For details of individual simulations see Gregory and Andrews () for HadGEM2 and HadAM3; Silvers et al () for GFDL‐AM2.1, GFDL‐AM3, and GFDL‐AM4.0; and Zhou et al () for CAM4 and CAM5.3. We additionally include simulations from ECHAM6.3, which is closely related to the atmospheric component of the MPI‐ESM 1.2 model to be used in CMIP6.…”

“…The relationship between global surface temperature change and the Earth's radiative response—a measure of the radiative feedbacks in the system and a key determinant of the Earth's climate sensitivity—can vary on timescales of decades to millennia. Thus, feedbacks governing warming over the observed historical record may be different from those acting on the Earth's long‐term climate sensitivity to rising greenhouse gas concentrations (e.g., Armour, ; Gregory & Andrews, ; Marvel et al, ; Proistosescu & Huybers, ; Silvers et al, ; Zhou et al, ). This is in contrast to decades of studies that explicitly or implicitly assume that the relationship between historical temperature change and energy budget variations provides a direct constraint on long‐term climate sensitivity (e.g., Gregory et al, ; Otto et al, ).…”

“…The primary reason why radiative feedback and sensitivity is not constant is because climate feedback depends on the spatial structure of surface temperature change (Andrews et al, 2015;Andrews & Webb, 2018;Armour et al, 2013;Ceppi & Gregory, 2017;Haugstad et al, 2017;Rose et al, 2014;Silvers et al, 2018;Zhou et al, 2016Zhou et al, , 2017. This evolves on annual to decadal timescales with modes of unforced coupled atmosphere-ocean variability (e.g., Xie et al, 2016) and spatiotemporal variations in anthropogenic or natural forcings (e.g., Smith et al, 2016;Takahashi & Watanabe, 2016).…”

Observational Evidence for Two Modes of Coupling Between Sea Surface Temperatures, Tropospheric Temperature Profile, and Shortwave Cloud Radiative Effect in the Tropics

“…However, it is increasingly recognized that a range of observations suggest that additional degrees freedom need to be considered. Here, we argue that open questions regarding the cloud impact on climate sensitivity (the "pattern effect", Andrews & Webb, 2018;Ceppi & Gregory, 2017;Silvers et al, 2018;Stevens et al, 2016;Zhou et al, 2016) are related to the controversy surrounding the question whether the tropical tropospheric temperature trend profile follows moist adiabatic scaling (Flannaghan et al, 2014;Po-Chedley & Fu, 2012;Santer et al, 2005). That is, both depend on the question whether the temperature difference between the warmest regions with atmospheric deep convection and the tropical average remains constant under climate change.…”