Global Climate Impacts of Fixing the Southern Ocean Shortwave Radiation Bias in the Community Earth System Model (CESM)

Abstract

A large, long-standing, and pervasive climate model bias is excessive absorbed shortwave radiation (ASR) over the midlatitude oceans, especially the Southern Ocean. This study investigates both the underlying mechanisms for and climate impacts of this bias within the Community Earth System Model, version 1, with the Community Atmosphere Model, version 5 [CESM1(CAM5)]. Excessive Southern Ocean ASR in CESM1(CAM5) results in part because low-level clouds contain insufficient amounts of supercooled liquid. In a present-day atmosphere-only run, an observationally motivated modification to the shallow convection detrainment increases supercooled cloud liquid, brightens low-level clouds, and substantially reduces the Southern Ocean ASR bias. Tuning to maintain global energy balance enables reduction of a compensating tropical ASR bias. In the resulting preindustrial fully coupled run with a brighter Southern Ocean and dimmer tropics, the Southern Ocean cools and the tropics warm. As a result of the enhanced meridional temperature gradient, poleward heat transport increases in both hemispheres (especially the Southern Hemisphere), and the Southern Hemisphere atmospheric jet strengthens. Because northward cross-equatorial heat transport reductions occur primarily in the ocean (80%), not the atmosphere (20%), a proposed atmospheric teleconnection linking Southern Ocean ASR bias reduction and cooling with northward shifts in tropical precipitation has little impact. In summary, observationally motivated supercooled liquid water increases in shallow convective clouds enable large reductions in long-standing climate model shortwave radiation biases. Of relevance to both model bias reduction and climate dynamics, quantifying the influence of Southern Ocean cooling on tropical precipitation requires a model with dynamic ocean heat transport.

Southern Ocean albedo, inter-hemispheric energy transports and the double ITCZ: global impacts of biases in a coupled model

“…As discussed in Sects. 3 and 4, the results of this study bear remarkable similarity to those of Kay et al (2016), where cloud properties were adjusted, lending further support to this idealised approach. Zonally uniform perturbations to direct and diffuse ocean albedo are applied to counterbalance global annual mean biases in TOA albedo when compared to CERES observations, as further discussed in Sect.…”

“…The conclusions of the work were not affected by the longer integration period and the circulation changes analysed here are observed within the first few years of the simulations (not shown). Kay et al (2016) found a similar, fast response in their simulations with no material change in the model response after 200 years of simulation time. Given the timescale on which the Atlantic meridional overturning circulation has been observed to adjust to perturbations in other studies using HadGEM2-ES (Menary et al 2013;Menary and Scaife 2014), 100 years is more than sufficient to confirm the full dynamical response of the model to forcing has been achieved in the first two decades.…”

“…The recent work of Kay et al (2016) is of particular relevance to the present study. The aim of Kay et al was to assess the global climate impact of reducing shortwave biases in the Southern Ocean in the CESM-CAM5 model, with adjustments made in the tropics to maintain the global TOA energy balance.…”

“…Bodas-Salcedo et al 2014), but to examine the potential for an improved representation of the climate of the model if these biases are addressed. In addition to building upon Haywood et al (2016), this study is highly complementary to that of Kay et al (2016), since the present work investigates the impact of reducing the same, Southern Ocean, albedo biases that were a focus of Kay et al in an entirely independent climate model with a different experimental design. The present study then goes on to consider the further impact of reducing longwave biases, which neither Haywood et al (2016) or Kay et al (2016) considered.…”

Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model

“…0.2 W m −2 ), which requires an ITCZ shift towards the NH in order to induce an anomalous cross-equatorial atmospheric heat transport towards the SH (Kang et al, 2008(Kang et al, , 2009). It should be noted, however, that the use of a slab ocean model precludes changes in cross-equatorial ocean heat transport, which may exaggerate the ITCZ shift (Kay et al, 2016).…”