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

Abstract

Here, 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 littlemore » 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.« less

Authors:

Kay, Jennifer E. ^[1]; Wall, Casey ^[2]; Yettella, Vineel ^[1]; Medeiros, Brian ^[3]; Hannay, Cecile ^[3]; Caldwell, Peter ^[4]; Bitz, Cecilia ^[2]

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+ Show Author Affiliations

1. Univ. of Colorado, Boulder, CO (United States)
2. Univ. of Washington, Seattle, WA (United States)
3. National Center for Atmospheric Research, Boulder, CO (United States)
4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Publication Date:

2016-06-10

Research Org.:

Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE

OSTI Identifier:

1438736

Report Number(s):

LLNL-JRNL-736961
Journal ID: ISSN 0894-8755

Grant/Contract Number:  

AC52-07NA27344

Resource Type:

Journal Article: Accepted Manuscript

Journal Name:

Journal of Climate

Additional Journal Information:

Journal Volume: 29; Journal Issue: 12; Journal ID: ISSN 0894-8755

Publisher:

American Meteorological Society

Country of Publication:

United States

Language:

English

Subject:

58 GEOSCIENCES; Geographic location/entity; Southern Ocean; Circulation/ Dynamics; Atmosphere-ocean interaction; Atm/Ocean Structure/ Phenomena; Clouds; Models and modeling; Climate models