Much of the water mass transformation that drives the Atlantic Meridional Overturning Circulation (AMOC) occurs in the high latitude North Atlantic and Arctic Oceans. The warm, salty North Atlantic Current in the upper layer of the ocean flows north, becomes cooler and fresher through interaction with the atmosphere and through mixing with Arctic waters, and then the products of these transformations return south again at depth. A moored array deployed in 2014 for the Overturning in the Subpolar North Atlantic Program (OSNAP) has been making continuous observations along two sections which span across the subpolar North Atlantic, allowing the AMOC at these latitudes to be quantified. To complete the picture provided by the OSNAP observations, we seek to understand what processes have been responsible for the densification of waters that we observe at the array; in other words what happens in between whiles to the light waters flowing north that turns them into the denser waters we see returning south.
To answer this question, I will present the application of a recently developed inverse method to observations in the region. The method analyses the ocean in thermohaline coordinates, which simplifies the complex spatial structure of the circulation and enables us to diagnose the impact of surface forcing and interior mixing on individual water masses which we identify by their temperature and salinity. I will focus particularly on the Labrador Sea Water, a water mass whose role in the AMOC is uncertain; and will argue that interior mixing plays a significant part in its formation.