Graduate Thesis Or Dissertation
 

Role of oceanic heat transport processes in CO₂-induced warming : analysis of simulations by the OSU coupled atmosphere-ocean general circulation model

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  • The OSU global coupled atmosphere/ocean general circulation model (A/O GCM) has been used to simulate the present (lxCO₂) climate and to investigate a CO₂-induced (2xCO₂) climate change. Previous analysis of the lxCO₂ simulation showed distinct errors in the simulated sea surface temperature (SST) and sea ice which were attributed primarily to the atmospheric GM (AGCM). Analysis of the 2xCO₂ simulation showed that the CO₂-induced warming penetrated into the ocean; this caused a delay in the equilibration of the climate system with an estimated e-folding time of 50-75 years. The present study has two objectives. The principal objective is to answer the question: By what pathways and through which physical processes does the simulated ocean general circulation produce the penetration of the CO₂-induced warming into the ocean? The secondary objective is to evaluate the performance of the oceanic GCM (OGCM) in the lxCO₂ simulation. The comparison of the simulated lxCO₂ internal oceanic fields with the corresponding observations shows that although they are basically similar, there are distinct errors. Further analysis shows that these errors were generated by the OGCM during its spin-up integration prior to its coupling with the AGCM. This study thus shows that it is not sufficient to compare the simulated SST with the observed SST to evaluate the performance of the OGCM. It is also necessary to compare the simulated internal oceanic quantities with the corresponding observed quantities. The global mean analysis of the CO₂-induced climate changes shows that the ocean gains heat at a rate of 3 W/m² due to the CO₂ doubling. This heat penetrates downward into the ocean predominantly through the reduction in the convective overturning. The zonal mean analysis shows that the surface warming increased from the tropics toward the mid-latitudes of both hemispheres and penetrated gradually to the deeper ocean. The oceanic warming penetrated to a greater depth in the subtropics and mid-latitudes than in the equatorial region. A zonal mean heat budget analysis shows that the CO₂-induced warming of the ocean occurs predominantly through the downward transport of heat, with the meridional heat flux being only of secondary importance. In the tropics the penetration of the CO₂-induced heating is minimized by the upwelling of cold water. In the subtropics the heating is transported downward more readily by the downwelling existing there. In the high latitudes the suppressed convection plays the dominant role in the downward penetration of the CO₂-induced heating.
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