The Effects of Varying Winter Surface Heat Loss on the Labrador Sea Water and Its Export

Abstract

As the environment is changing temperatures are changing, becoming more extreme. This is expected to affect the oceans 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. Deep convection is previously studied extensively as it is a unique and important process of the global ocean 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.

Previous studies have shown that the properties of the boundary current water are strongly dependent on the eddy exchange, and therefore on the surface heat loss. However, it is not known how consecutive strong winters impact the dynamics of the Labrador Sea on various timescales, which will therefore be the focus of this thesis. Data for this research will be obtained by using an idealised model configuration of the Labrador Sea, where the hydrostatic primitive equations of motion are solved by the MIT general circulation model (MITgcm). Different types of scenarios are defined 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 throughout the boundary current, per density class and per vertical layer change.

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 in the downstream direction. Additionally, more and denser interior water accumulates, depending on how many consecutive strong winters occur. This deep convected water in the interior partly remains near the bottom of the basin. In the next winter, it is mixed again due to deep convection, consequently a positive feedback loop occurs. Meaning, that the number of consecutive winters positively impacts the interactions in the basin, as the horizontal density gradient increases, and thus the velocity and eddy kinetic energy increase as well, in respect to the previous winter. 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 near the bottom out of the basin, due to a pressure difference. The flow near the bottom is a negative feedback loop, as the volume of dense convected water decreases and can therefore not be further cooled during consecutive strong winters. Finally, the properties and the transport of the boundary current water are directly related to the interior water and eddy exchange. As the MLD in the interior and eddies in the BC are still relatively large in the years after the additional surface heat loss, the export of boundary current water therefore also remains affected. In conclusion, the effect of wintertime surface heat loss on the Labrador Sea Water in the short term has the most influence on the MLD and EKE, however the influence of the MLD and EKE remains and therefore in the long term affects the export through the BC. These conclusions can help to better interpret the limited available measurements of the Labrador Sea Water.