Author: Lilian Carolina Garcia Oliva —

Advisors: Anne Marie Treguier and Guillaume Maze —

Abstract:

The mixed layer is the region in the surface of the ocean that is in direct contact with the atmo- sphere. Because of this, it plays a role as intermediary in the exchange of momentum, heat, gases and fresh water between the atmosphere and the deep ocean. Therefore it is important from, both climatic and biological points of view. It is formed by turbulent mixing of the surface layer of the ocean, driven by heat loss, wind stress and the local circulation. The mixed layer depth is an ocean feature with large spatial and temporal variability.

Therefore, a qualitative description of the spatio-temporal variability of the mixed layer depth is done in this report. This is achieved using the new dataset ARMOR 3D, a high spatial (0.25°) and temporal (weekly) database of mixed layer depth. We analyzed the period from 2005 to 2018. The study is focused on the seasonal cycle variability, regarding three regions of the North Atlantic Ocean. These regions are: Gulf Stream, Labrador and Irminger Seas; and North East Atlantic. Each one of these regions has different surface circulation, net heat flux and mixed layer dynamics.

This report covers individual features in each one of the regions, for example the spatial dis- tribution of the temporal variability, for which we find that the areas of large temporal variability are located over areas of deep mixed layers. Regarding the deep mixed layers, we found that they concentrate in specific clusters, i.e., the deep mixed layers of the Labrador Sea have an absolute maximum in the center of the basin. In addition, such deep mixed layer clusters are related to specific features of each one of the regions: to the Gulf Stream, in the Gulf Stream region; to the localized cyclonic circulation, in the Labrador Sea; and to the continental slope, in the case of the North East Atlantic region. For all the three regions, shallow mixed layers were found along the seasonal cycle. The Labrador and Irminger Seas are the region in which the presence of these shallow mixed layers was more evident. Finally, the duration of the processes present during the seasonal cycle: deepening, shoaling and a constant phase of the mixed layer was measured. The maximum MLD is reached, in general, in the final weeks of February and beginning of March. The deepening takes between 6 to 8 months. The shallowing takes around 3 to 4 months and the constant phase, reaching the minimum MLD, between 1 and 3 months.