Variability of Large-scale Ocean Circulation and Meridional Heat Transport in the Atlantic Ocean

The research described in the dissertation addresses what controls the variability of ocean circulation and meridional heat transport (MHT) in the Atlantic Ocean on different timescales. Chapter 2 focuses on the contribution of surface heating and wind forcing with/without topography to the seasonal and interannual-to-decadal variations of large-scale sea surface height (SSH) using simplified models. On the seasonal timescale thermosteric height explains most of the SSH variance north of 18°N and first mode linear long Rossby wave explains the SSH between 10°N--15°N and east of Greenland. On interannual-to-decadal timescales, a topographic Sverdrup response explains interannual-to-decadal SSH between 53°N and 63°N east of Greenland, suggesting the important role of topography in the subpolar region. Farther south, the linear Rossby wave model explains SSH variations on interannual-to-decadal timescales between 30°N and 50°N from mid-basin to the eastern boundary. In Chapter 3, perturbation experiments and a 1000-year control simulation in the GFDL coupled model CM2.1 are used to investigate the evolution of the Atlantic meridional overturning circulation (AMOC) and its related upper ocean heat content (UOHC) on the decadal timescale. A slow southward propagation of positive AMOC anomaly in northern high latitudes leads to a convergence (divergence) of the Atlantic MHT anomaly in the subpolar (Gulf Stream) region, thus warming (cooling) in the subpolar (Gulf Stream) region after several years. The study presented in Chapter 4 examines the coherence structure of the interannual MHT variability in the Atlantic tropics and subtropics using seven simulations in the CMIP5 (Coupled Model Intercomparison Project Phase 5) archive as well as a hindcast simulation in the isopycnal ocean model GOLD (Generalized Ocean Layered Dynamics) from 1971 to 2009. The spatial pattern for the leading mode of the interannual MHT anomaly from all the model simulations has the same sign from 20°S--30°N, with a peak near the equator. Ekman heat transport anomalies between 7°S--20°N and the geostrophic transport beneath the Ekman layer from 13°S--27°N (except the equator) contribute to this MHT leading mode, while the contribution of the deep ocean is negligible. The connection between the hemispheres results from diapycnal transport of the northward geostrophic transport beneath the Ekman layer in the southern tropics; after this water reaches the upper ocean, it then moves northward where Ekman transport takes over. The wind is the main external forcing for the MHT coherence structure. The work in this thesis enhances the understanding of the contributions of heating, winds and topography on sea level changes on seasonal and interannual-to-decadal timescales, as well as the decadal and interannual variability of the AMOC and MHT in the Atlantic Ocean. It could be used to understand the effect of topography on the ocean circulation in the high latitude, improve the decadal prediction of the UOHC in the North Atlantic Ocean, and advance the knowledge of the tropical Atlantic heat transport and ocean-atmosphere coupling system.