The physical oceanography in the northwest Indian Ocean is largely controlled by the seasonal monsoon. The seasonal variability in circulation is complex. Many studies have investigated processes in the Persian (Arabian) Gulf and Arabian Sea, but little is understood about the Sea of Oman. This thesis incorporated observations from Argo floats, surface drifters and satellite imagery to study the deep and surface circulation in the northwest Indian Ocean. An assessment of four independent moorings located in the Sea of Oman and Arabian Sea, as well as a model skill comparison of the Simple Ocean Data Assimilation (SODA) model, contributed to understanding the dynamics in this region. Spatial patterns of surface current velocity produced from surface drifter data from 1995-2009 agreed with previously known surface currents. The Somali Current, East Arabian Current, Equatorial Current, Northeast/Southwest Monsoon Current, Great Whirl and Ras al Hadd Jet were all identified. During the Southwest Monsoon, flow direction was to the east and southeast in the Arabian Sea. The Somali Current flowed northeast along the Somali Coast extending to the East Arabian Current along the Oman coast. During the Northeast Monsoon, evaporation increased over the Arabian Sea, which resulted in a salinity gradient. This imbalance caused low-salinity surface water from the northeast Indian Ocean to flow into the northwest Indian Ocean as the Northeast Monsoon Current. Current direction reversed with the change of wind direction from the Southwest Monsoon to the Northwest Monsoon. Many characteristics seen at the surface were also identified in the subsurface as deep as 1500m. The comparison of moored observations to the Argo observations co-located in space and time showed reasonable agreement with the largest salinity difference of 0.23 and largest temperature difference of 0.78?C. The Murray Ridge mooring had a temperature correlation of 0.97 when compared to Argo observations. Argo observations were compared with SODA model numerical output from 1992-2001, and, after Argo, were assimilated from 2002-2009. With assimilation of Argo data into the SODA model, the temperature and salinity from the model numerical output improved, with most differences between model numerical output and Argo observations falling within one standard deviation.

Atlantic Meridional Overturning Circulation (AMOC) is a vital process that transfers heat and nutrients throughout the world's oceans, helping to regulate global climate and support marine ecosystems. The timing and nature of the shift to modern AMOC, and especially to deep-water formation in the North Atlantic, has been a topic of ongoing study, with the Eocene-Oligocene Transition (EOT, ~34 Ma) as a potential focal point of this shift. However, the role played by abrupt EOT cooling and Antarctic glaciation in North Atlantic circulation remains unclear. Improved constraints on Paleogene circulation will provide insight into the sensitivity of AMOC to perturbations in global climate, which is particularly relevant in light of contemporary climate change. To examine deep North Atlantic circulation response to the EOT we obtained grain-size data from the terrigenous fraction of the mud-dominated sediments of the Southeast Newfoundland Ridge contourite drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current. We analyzed 195 samples that span 150 m of stratigraphy from 36-26 Ma. The main objective was to use the 'sortable silt' fraction (10-63 µm) to generate a record of relative change in bottom-current intensity. These data are complemented with a record of the abundance and size of lithogenic sand (>63 µm). Here we present the first physical proxy record of abyssal current intensity in the North Atlantic, from late Eocene to mid Oligocene. Invigoration of North Atlantic deep circulation occurred gradually (over Myr timescales), with no significant changes linked temporally to the EOT. We infer that deep circulation in the North Atlantic was not sensitive to the abrupt global cooling and Antarctic glaciation associated with the EOT. Rather, our data suggest that changes in North Atlantic circulation were likely governed by longer-term processes related to the opening of key tectonic gateways, such as the Greenland-Scotland Ridge in the North Atlantic, and the Drake and Tasman Passages in the Southern Ocean. Additionally, we identify a significant mid-Oligocene invigoration of North Atlantic abyssal circulation, which climaxes around 27.9 Ma, and is coeval with a decrease in atmospheric CO2. / Master of Science