Simulating interdecadal variation of the thermohaline circulation by assimilating time-dependent surface data into an ocean climate model

• Main Author: Li, Guoqing
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1994
• Subjects: North Atlantic
• Online Access: https://research.library.mun.ca/6708/; https://research.library.mun.ca/6708/1/GuoqingLi.pdf; https://research.library.mun.ca/6708/3/GuoqingLi.pdf

We explore the feasibility of simulating interdecadal variations of the temperature, salinity and thermohaline circulation in the North Atlantic using an ocean general circulation model (OGCM) driven by time-dependent surface data. The natural way to drive the ocean is to use the surface heat and freshwater fluxes. In this thesis, we investigate the alternative of using surface temperature and salinity data since compared to heat and freshwater flux data, they are more accurate and more readily available. -- We do the experiments using idealized North Atlantic sized box geometry. In order to obtain a set of interdecadally-varying data, we first reproduced the results described by Zhang, Greatbatch and Lin (1993). Temperature, salinity, surface heat and freshwater flux are output from this control run and serve as "observations" in the further experiments. -- We can apply either a restoring boundary condition or a flux boundary condition at the ocean surface. To simulate the interdecadal variations, there are four choices for the surface boundary conditions: i) flux conditions on both temperature and salinity; ii) restoring conditions on both temperature and salinity; iii) a restoring condition on temperature and a flux condition on salinity (mixed boundary conditions); and iv) a flux condition on temperature and a restoring condition on salinity ("reversed mixed" boundary conditions). The restoring boundary conditions are to be understood in a sense of data assimilation. -- The experiments show that all the choices work well except mixed boundary conditions. It is found that a correct simulation of the thermohaline circulation is necessary to obtain a correct distribution of the sub-surface variables. Under mixed boundary conditions, a positive feedback between the development of a freshwater cap and heat-loss reduction results in either a collapsed or violent overturning thermohaline circulation quite unlike what happens in the control run. So mixed boundary conditions are not suitable for interdecadal ...