26 December 2014

The Tropical Inversion will Strengthen in a Warming Climate


The lowest few kilometers of the tropical marine atmosphere are frequently capped by an inversion layer. Under this inversion lie several types of boundary layer clouds. A key factor controlling the amount of the clouds is the strength of the inversion. A stronger inversion suppresses the mixing of boundary layer air with warmer and drier air in the free-troposphere, leading to a shallower, moister and cloudier boundary layer. In this study, we examine the realism of the tropical inversion and the response of inversion strength to anthropogenic forcing simulated in 18 CMIP5 models.


Idealized perturbation experiments (amip4xCO2) reveal that anthropogenic forcing leads directly to EIS increases, independent of "temperature-mediated" EIS increases associated with long-term oceanic warming. This fast EIS response to anthropogenic forcing is strongly impacted by nearly instantaneous continental warming. The amip4K and amipFuture simulations reveal that the temperature-mediated EIS change has contributions from both uniform and non-uniform oceanic warming. The substantial EIS increases in uniform oceanic warming simulations are due to warming with height exceeding the moist adiabatic lapse rate in tropical warm pools. EIS also increases in fully-coupled ocean-atmosphere simulations where CO2 concentration is instantaneously quadrupled, due to both fast and temperature-mediated changes. The temperature-mediated EIS change varies with tropical warming in a nonlinear fashion: The EIS change per degree tropical warming is much larger in the early stage of the simulations than in the late stage, due to delayed warming in the eastern parts of the subtropical oceans. 


Given the strong link between low-cloud cover (LCC) and EIS, LCC ought to increase, due to a EIS increase. One study by Caldwell et al. (2012) supports this view. In their study, imposing CMIP3-model-simulated large-scale changes on a mixed layer model yields an increase in LCC. They attributed this increase primarily to the EIS increase. Furthermore, increases in EIS cause LCC to increase (albeit less strongly than observed) in many fully coupled atmosphere-ocean models (CMIP3 and CMIP5, see Qu et al. 2014). Nevertheless, LCC changes in those models are less impacted by the EIS increase than they are by other cloud controlling factors (e.g., an increase in SST). A recent study using Large-Eddy Simulations also concludes that the EIS rise may be less important to the overall LCC change than other factors, namely the increases in moisture transport through the boundary layer which are fundamentally tied to the warmer temperature in the boundary layer (Bretherton and Blossey 2014). Even so, the EIS change may still play a significant role in coupled model simulations at least in two ways. First, it may contribute to the fast cloud response and thus modulate effective GHG radiative forcing (see Webb et al. 2012). Second, due to its intrinsic nonlinearity, the EIS change may contribute to the nonlinearity of the tropical low-cloud feedback (see Williams et al. 2008 and Andrews et al. 2012). 

Xin Qu
University of California at Los Angeles (UCLA)

All authors are supported by DOE's Regional and Global Climate Modeling Program under the project "Identifying Robust Cloud Feedbacks in Observations and Model". The work of LLNL authors was performed under the auspices of the United States Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. We acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP's Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP5 multi-model datasets. Support of these datasets is provided by the Office of Science, U.S. Department of Energy.