Sensitivity of ocean circulation and marine biogeochemical processes to variations in surface wind stress and diapycnal mixing in the surface ocean

摘要

This thesis investigates the sensitivity of the biogeochemical cycling of dissolved inorganic carbon and oxygen to variations in global ocean circulation and transport in response to perturbations in (i) the prevailing surface wind stress conditions in a global climate model, and (ii) the diapycnal mixing background parameterization scheme in the upper 1000 m of a numerical ocean model. The global climate model is a fully coupled earth system model of intermediate complexity (UVic ESCM) using a relatively coarse resolution to facilitate the assessment of equilibrated climate conditions. The global ocean model is a one-degree ocean general circulation model (MOM4p1) coupled to a biogeochemical model of intermediate complexity (TOPAZ2). Special attention is given to the comparison between the impacts of low- and mid-latitude perturbations and the identification of low-latitude mechanisms with possible implications for the global climate. The three main findings of this thesis are: (i) low-latitude surface wind stress changes can significantly affect the biogeochemical cycling of oxygen and carbon and could thus have an important contribution to the overall control of the global climate. (ii) The commonly accepted Drake Passage Effect does not dominate the link between Southern Hemisphere westerly wind stress and the formation of North Atlantic Deep Water or the Atlantic outflow if the impact of wind-driven changes on Antarctic sea-ice is considered. (iii) Variations in thermocline thickness and biogeochemical processes, in particular remineralization, can offset and even reverse the influence of alterations in equatorial ocean stratification and vertical mixing on ocean oxygenation and the extent of low-oxygen regions. This will be shown in the discussion of two cases, in which ocean stratification is increased (decreased) due to variations in low-latitude surface wind stress and diapycnal mixing, while ocean oxygenation increases (decreases) and low-oxygen regions contract (extend). Additionally, this study identifies a dominance of Southern Hemisphere westerly wind stress compared to tropical winds in the determination of the global overturning circulation. Changes in the nutrient supply to one of the key marine ecosystems in the south east Pacific due to tropical and mid-latitude wind stress variations are also examined.