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Response of the CHIME coupled climate model to two climate forcing scenarios
Adam Blaker, Alex Megann, Adrian New, Bablu Sinha
NOCS
(Abstract received 05/06/2009 for session X)
ABSTRACT
CHIME (Coupled Hadley Isopycnic Model Experiment) is designed to explore the effects of the choice of vertical coordinate in the ocean component of a coupled climate model. It is based on the UK Met Office\'s model, HadCM3, but the ocean component is replaced by HYCOM (HYbrid Coordinate Ocean Model). Under the RAPID funded UK THCMIP (THermohaline Circulation Model Intercomparison Project) both HadCM3 and CHIME have completed a control integration and two climate forcing experiments. The first, in which atmospheric CO2 concentrations are increased from pre-industrial levels (280ppm) by 1% per year compound to achieve 4x initial levels after 140 years shows the MOC (Meridional Overturning Circulation) in both models responding similarly, with a reduction of around 25%, although it is not yet clear whether the mechanisms for the responses in the two models are the same. Global average surface air temperature in CHIME is 2 degC higher than the control climate after 70 years (2xCO2), and 4 degC by the end of the 140 year experiment. There is also a reduction in decadal mean precipitation over the Amazon and Carribean of up to 50 mm/month after 70 years. In the second experiment, a 0.1 Sverdrup (1Sv = 1 x 10e6 m3/s) hosing is applied over the North Atlantic between 50N and 70N for a period of 100 years, followed by a recovery period. The MOC in HadCM3 reduces by 25% over the 100 years, similar to the transient CO2 experiment. During the recovery phase it returns, and overshoots, its control overturning. In CHIME the MOC reduces initially, but recovers whilst still in the hosing phase of the experiment. The difference in response is not fully understood, but may be related to the differences in the control states of the models over the North Atlantic.
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2009 LOM Workshop, Miami, Florida Jume 1 - 3, 2009