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Comparing the CHIME coupled model with HadCM3
Alex Megann; Adam Blaker; Adrian New; Bablu Sinha
National Oceanography Centre, Southampton
(Abstract received 05/05/2009 for session X)
ABSTRACT
The control climates of two IPCC-class coupled climate models are intercompared. The first is the HadCM3 model of the UK Meteorological Office’s Hadley Centre, while the second, the Coupled Hadley-Isopycnic Model (CHIME), is identical to the first except for the replacement of its ocean model by the Hybrid-Coordinate Ocean Model, HYCOM. CHIME has a generally realistic climate, close in many respects to that of HadCM3, with similar ocean heat transports and overturning circulations. However, substantial differences in the mean state of the two models are observed, some of which are directly attributable to the different vertical coordinate in the ocean. North Atlantic Deep Water is shown to lie at a deeper and more realistic depth in CHIME, and Antarctic Intermediate Water and Subantarctic Mode Water are also shown to be better represented and preserved in CHIME than in HadCM3, where their signatures are progressively diffused into the surrounding water masses. The thermocline in the northern hemisphere subtropical gyres retains its sharpness in CHIME, but becomes increasingly diffuse in HadCM3. The North Pacific cold error present in HadCM3 and similar models is absent in CHIME, and is related to changes in separation position of the Kuroshio. CHIME has a warm error in the Southern Ocean and excessively deep wintertime mixing in the Arctic and subpolar North Atlantic, which are attributed partially to biases in the KPP mixing scheme. The North Atlantic subpolar gyre becomes saltier and warmer in both models, but a shallow fresh surface layer in HadCM3 reduces the surface signature of this drift, in contrast to CHIME, which has a warm and salty surface error.
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2009 LOM Workshop, Miami, Florida Jume 1 - 3, 2009