Graduation of Freek Kollaard

The effects of varying winter surface heat loss on the Labrador Sea Water and its export

• Professor of graduation: Prof. dr. Caroline Katsman

• Supervisors: Prof. dr. Julie Pietrzak (TU Delft), Dr. Stephan de Roode (TU Delft)

   

As the environment is changing temperatures are changing, becoming more extreme. This affects the ocean and its transport, specifically the Atlantic Meridional Overturning Circulation (AMOC). The Labrador Sea is a part of the AMOC, where overturning in depth and density space occurs, due to deep convection. Deep convection is the process of seawater losing its heat to the atmosphere, due to atmospheric cooling during the winter. This causes the seawater to become colder and denser, and it therefore sinks towards the bottom of the basin. As extremities are expected to occur more often due to climate change this process is expected to happen more often and with more intensity. Deep convection is previously studied extensively as it is a unique and important process of the global water circulation system. The key process that causes the AMOC water to overturn, is due to buoyant eddies shedding from the boundary current into the interior. The buoyant eddies exchange their buoyant boundary current water with the dense interior water, causing the boundary current (and in extension the AMOC water) to cool down.

However, consecutive strong winters have not been widely researched. Therefore, this thesis will focus on how consecutive strong winters affect the dynamics of the Labrador Sea. Data for this research will be gathered by using an idealised configuration of the Labrador Sea, where the hydrostatic primitive equations of motion are solved on the MIT general circulation model (MITgcm). Different types of scenarios are made to analyse different effects on the dynamics. These scenarios are analysed by looking into how the mean basin temperature changes, how the eddy kinetic energy (EKE) and mixed layer depth (MLD) develop, and how the properties through a transect of the basin change. The effects of these interactions are then studied by looking at how the transport of water per density interval, per vertical layer change throughout the boundary current.

The thesis mainly shows that the mixed layer depth in the interior increases during a strong winter. As a result, the eddy kinetic energy increases significantly in the boundary current, as the horizontal density gradient increases, thus causing an increase in boundary current velocity. Additionally, more and denser interior water accumulates, depending on how many consecutive strong winters occur. This deep convected water in the interior partly remains at the bottom of the basin. It is then mixed again due to deep convection in the next winter, consequently causing a positive feedback loop. Therefore, the number of consecutive winters directly impacts the interactions in the basin, as the horizontal density gradient increases, and thus the velocity and eddy kinetic energy increase as well. The effect of the strong winters persists in the years afterwards, as the interior remains relatively cold. Additionally, a part of the accumulated convected interior water resides too deep in the basin to be exchanged by the eddy exchange and therefore flows out over the bottom of the basin, due to a pressure difference. Finally, the properties and the transport of the boundary current water are directly related to the interior water and eddy exchange. As the interior and eddies remain affected in the years after the additional heat loss, the export of boundary current water also remains affected. In conclusion, the effect of wintertime heat loss on the Labrador Sea Water has both short term and long term effects on the dynamics and interactions.