Transformation of the warm waters of the North Atlantic from a geostrophic stream function perspective

In order to obtain a description of the hydrographic state of the North Atlantic Current-Subpolar Front (NAC-SPF) system, historical hydrographic data from the subpolar North Atlantic are projected into a baroclinic stream function space, resulting in three dimensional gravest empirical mode (GEM) fields for temperature and specific volume anomaly, parameterized by pressure, dynamic height, and day of the year. From the specific volume anomaly GEM, the corresponding potential vorticity field is calculated. These fields are constructed for 12 subregions, chosen to follow the mean path of the North Atlantic CurrentSubpolar Front system. Analysis of the seasonal potential vorticity cycle of the GEM fields shows that the main mechanism for the formation of Subpolar Mode Water is winter convection. The GEM fields are also used to obtain the approximate location of formation sites for the different Subpolar Mode Water classes. The evolution of the mean fields for the waters is studied along baroclinic streamlines of the NAC-SPF system. This shows that cross-frontal mixing, between the cold and fresh subpolar waters, and the salty and warm waters coming north from the subtropics via the Gulf Stream, is the dominant mechanism for the light-to-dense transformation process of the NAC-SPF waters that enter the western subpolar region. On the other hand, a combination of atmospheric cooling, vertical mixing during winter time convection, and en1 trainment of the saltier waters found on the northeastern subtropical gyre, are the main factors transforming the NAC-SPF waters that enter the eastern subpolar gyre. This suggests that an influx along the eastern margin of salty water from the European Basin plays a significant role in the transformation of the NAC-SPF waters that continue their way towards the Nordic Seas.