The CSIRO Mk3L climate system model version 1.0 - Part 2: Response to external forcings

• Published in: Geoscientific Model Development
• Main Authors: Phipps, SJ, Rotstayn, LD, Gordon, HB, Roberts, JL, Hirst, AC, Budd, WF
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications on behalf of the European Geosciences Union (EGU) 2012
• Subjects: Earth Sciences; Other Earth Sciences; Earth Sciences not elsewhere classified; Antarc*; North Atlantic; Sea ice
• Online Access: https://doi.org/10.5194/gmd-5-649-2012; http://ecite.utas.edu.au/77633

The CSIRO Mk3L climate system model is acoupled general circulation model, designed primarily formillennial-scale climate simulation and palaeoclimate research.Mk3L includes components which describe the atmosphere,ocean, sea ice and land surface, and combinescomputational efficiency with a stable and realistic controlclimatology. It is freely available to the research community.This paper evaluates the response of the model to externalforcings which correspond to past and future changes in theclimate system. A simulation of the mid-Holocene climate is performed,in which changes in the seasonal and meridional distributionof incoming solar radiation are imposed. Mk3L correctlysimulates increased summer temperatures at northernmid-latitudes and cooling in the tropics. However, it is unableto capture some of the regional-scale features of themid-Holocene climate, with the precipitation over NorthernAfrica being deficient. The model simulates a reductionof between 7 and 15% in the amplitude of El Nio-Southern Oscillation, a smaller decrease than that implied bythe palaeoclimate record. However, the realism of the simulatedENSO is limited by the models relatively coarse spatialresolution. Transient simulations of the late Holocene climate are thenperformed. The evolving distribution of insolation is imposed,and an acceleration technique is applied and assessed.The model successfully captures the temperature changes in each hemisphere and the upward trend in ENSO variability. However, the lack of a dynamic vegetation scheme does not allow it to simulate an abrupt desertification of the Sahara. To assess the response ofMk3L to other forcings, transientsimulations of the last millennium are performed. Changesin solar irradiance, atmospheric greenhouse gas concentrationsand volcanic emissions are applied to the model. Themodel is again broadly successful at simulating larger-scalechanges in the climate system. Both the magnitude and thespatial pattern of the simulated 20th century warming areconsistent with observations. However, the model underestimatesthe magnitude of the relative warmth associated withthe Mediaeval Climate Anomaly. Finally, three transient simulations are performed, inwhich the atmospheric CO 2 concentration is stabilised attwo, three and four times the pre-industrial value. All threesimulations exhibit ongoing surface warming, reduced seaice cover, and a reduction in the rate of North Atlantic DeepWater formation followed by its gradual recovery. AntarcticBottom Water formation ceases, with the shutdown beingpermanent for a trebling and quadrupling of the CO 2 concentration.The transient and equilibrium climate sensitivities ofthe model are determined. The short-term transient responseto a doubling of the CO 2 concentration at 1% per year is awarming of 1.590.08 K, while the long-term equilibriumresponse is a warming of at least 3.850.02 K.