On the Changing Upper-Ocean Stratification : Trends and Variability of the Upper-Ocean Structure during the Argo Observation Period

The warming climate is causing an increasing ocean stratification as a consequence of intensified ocean surface warming. Changes in ocean stratification affect e.g., ocean ventilation, mixed layer entrainment, heat exchange and air-sea interactions. In turn, these changes play a role for carbon uptake, oxygen distribution and nutrient fluxes that are crucial for marine ecosystems. How exactly these physical and biological processes will be affected remains uncertain and the effects of stratification changes probably vary among regions. Taking advantage of the excessive Argo observation dataset from 2006-2022, this thesis examines the decadal variability and recent changes of the upper-ocean stratification and surface mixed layer for the period of the most intense warming to-date. In particular, the southeastern tropical Atlantic Ocean (10°S-20°S, 5°W-15°E) shows continuous warming and freshening of the mixed layer for the period of 2006-2020. The resulting surface density reduction affects the upper-ocean stratification that strengthened by 30%. A procedure is developed to determine the vertical stratification maximum from the Argo array on global scales. Globally, the summer and winter vertical stratification maxima intensify by 7-8% from 2006-2021 while the mixed layer deepens by 4 m, proposing a de-coupling of the upper-ocean from the ocean interior. Changes in the upper-ocean structure impact the potential vorticity (PV) of the water column. This thesis establishes a relation between the decadal variability of North Atlantic Oscillation (NAO) and a zone of high PV in the northeastern tropical Atlantic, which plays a role for the subtropical-tropical pathways. During positive NAO phases, intensified trade winds result in isopycnal heave and squeezing of density layers that strengthen the PV. Analyses of the Sverdrup streamfunction and geostrophic velocities indicate enhanced thermocline flow via the western boundary toward the equator.