Mixing and Geometry in the North Atlantic Meridional Overturning Circulation

Vertical motions across the ocean are central to processes, like CO2 fixation, heat removal or pollutant transport, which are essential to the Earth's climate. This work explores 3D conveyor routes associated with the Atlantic Meridional Overturning Circulation (AMOC). Our findings show the geometry of mixing structures in the upper and deep ocean layers by means of Lagrangian Coherent Structures. This tool identifies among others, zones linked to vertical transport and characterizes vertical transport time scales. We focus the study in two regions. The first one is the Flemish Cap region, a zone of interaction between the major AMOC components, where our analysis identifies a domain of deep waters that ascend very rapidly to the ocean surface. The second one is the Irminger Sea, where our analysis confirms the existence of a downwelling zone, and reveals a previously unreported upwelling connection between very deep waters and the ocean surface. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a complex oceanic convective system involved in the tridimensional distribution of heat, carbon or nutrients. The horizontal transport across the AMOC is well studied, however, there are not many studies exploring mixing and transport processes across the vertical water column, that is, from the surface to the deep ocean and viceversa. To fully understand this 3D system, this paper follows a methodology that links apparently unrelated elements such as mixing and geometry. This approach is very promising to better characterize transport across the vertical column of major ocean currents and determining their impact on the global climate system. Key Points Lagrangian Coherent Structures reveal the complex mixing geometry of 3D circulating waters Supported by this tool we are able to highlight upwelling and downwelling routes across the Atlantic Meridional Overturning Circulation This approach seems promising to determine the role of vertical mixing on major ocean currents in the Earth's climate system