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Convection Parameterization, Tropical Pacific Double ITCZ, and Upper-Ocean Biases in the NCAR CCSM3. Part II: Coupled Feedback and the Role of Ocean Heat Transport

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  • 1 Scripps Institution of Oceanography, La Jolla, California
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Abstract

This study investigates the coupled atmosphere–ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR Community Climate System Model, version 3 (CCSM3) and its alleviation when a revised Zhang–McFarlane (ZM) convection scheme is used. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the Community Atmosphere Model, version 3 (CAM3) to investigate the local feedback between the atmosphere and the ocean. It is shown that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme seen in the stand-alone CAM3 simulations can cool the SST by up to 1.5°C. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport required in the slab ocean model. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when the revised ZM scheme is used is largely responsible for the reduction of SST bias in the southern ITCZ region, although surface energy flux also helps to cool the SST in the first few months of the year in seasonal variation.

The results, together with those from , suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double-ITCZ bias through complex coupled interactions between atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper-ocean circulations. The mitigation of the double-ITCZ bias using the revised ZM scheme is achieved by altering this chain of interactions.

Corresponding author address: Guang J. Zhang, Scripps Institution of Oceanography, La Jolla, CA 92093-0221. Email: gzhang@ucsd.edu

Abstract

This study investigates the coupled atmosphere–ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR Community Climate System Model, version 3 (CCSM3) and its alleviation when a revised Zhang–McFarlane (ZM) convection scheme is used. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the Community Atmosphere Model, version 3 (CAM3) to investigate the local feedback between the atmosphere and the ocean. It is shown that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme seen in the stand-alone CAM3 simulations can cool the SST by up to 1.5°C. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport required in the slab ocean model. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when the revised ZM scheme is used is largely responsible for the reduction of SST bias in the southern ITCZ region, although surface energy flux also helps to cool the SST in the first few months of the year in seasonal variation.

The results, together with those from , suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double-ITCZ bias through complex coupled interactions between atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper-ocean circulations. The mitigation of the double-ITCZ bias using the revised ZM scheme is achieved by altering this chain of interactions.

Corresponding author address: Guang J. Zhang, Scripps Institution of Oceanography, La Jolla, CA 92093-0221. Email: gzhang@ucsd.edu

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