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Climate sensitivity of coupled models with differing ocean components
Alex Megann, Adam Blaker and Adrian New
National Oceanography Centre, Southampton
(Abstract received 01/06/2011 for session X)
ABSTRACT
The responses of two IPCC-class coupled climate models to climate perturbations are compared. 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. The models are first forced by atmospheric CO2 increasing at 1% per year until the concentration is four times that in the control experiment: the Atlantic meridional overturning circulation (AMOC) reduces by ~20% in both of the models. Although similar overall warming results in both models, there are significant regional differences: in particular, the large reduction in precipitation over the Amazon region seen in HadCM3 does not occur in CHIME, and we will show that this is related to differences in the models’ representation of ENSO and to changes in the SST and the ITCZ in the tropical Atlantic. The models are then forced for 100 years with an additional surface freshwater “hosing” flux of 0.1 Sv into the North Atlantic between 50° and 70°N to assess their stability with respect to the enhanced hydrological cycle that is expected under global warming. The AMOC decreases by a similar amount in HadCM3 as with increasing CO2, but in CHIME after an initial decrease the AMOC recovers before the hosing is removed. We find that variations in AMOC match closely changes in the upper ocean salinity in the hosing region; by analysing the salt budget in the subpolar region we show that the increased surface freshwater input is balanced by horizontal advection, and the differences in advection in the models therefore explain the different sensitivities observed.
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2011 LOM Workshop, Miami, Florida February 7 - 9, 2011