Seasonal And Interannual Variability Of The Meridional Overturning Circulation In The Subpolar North Atlantic Diagnosed From A High Resolution Reanalysis Data Set

Subpolar North Atlantic is a vital region to modulate the variation of Atlantic Meridional Overturning Circulation (MOC). Recently, trans-basin mooring arrays were deployed to monitor the meridional volume and heat transport in this region. Motivated by these new observations, this study investigates the structure and variability of MOC using a high resolution data assimilative reanalysis product. Our results indicate that the reanalysis product successfully captures the vertical structure and variability for the basinwide integrated stream function. In accord with observations, the meridional volume transport across the section from Greenland to Scotland (i.e., the East Section) is much stronger than that in the Labrador Sea (i.e., the West Section). While the boundary currents in the Labrador Sea display dramatic seasonal variations, their seasonal cycles are largely compensated, leading to very weak seasonality in the MOC. On the other hand, the MOC across the East section has an apparent seasonal cycle with the density changes in the Irminger Basin playing a dominant role. On the interannual time scale, MOC variations across both the West and East sections have comparable amplitudes. In addition, we further split the MOC fluctuations into components induced by barotropic and baroclinic processes. The decomposed fields point out that the isopycnal layer thickness at the west of Greenland is the primary factor to modulate the MOC interannual variability across the West section. MOC interannual variability across the East section is mostly determined by barotropic velocity changes near the pathway of the North Atlantic Current.Plain Language Summary The Meridional Overturning Circulation (MOC) in the Atlantic is a critical player to modulate the global climate change. To record and understand the MOC variation, a trans-basin mooring array was deployed in summer 2014 along the sections from Canadian east coast to Greenland and to Scotland. The release of first few years' data set provides an unprecedented time series for MOC strength and the associated heat and freshwater fluxes. Here we combine these new data and a high resolution data assimilative reanalysis data set to understand the governing processes for the MOC in the subpolar North Atlantic. Both observations and reanalysis data set show that the mean strength of MOC is carried through the section between Greenland and Scotland. The longer time series of reanalysis data set enables examination of MOC variability on different time scales. Our results reveal that a remarkable annual cycle only exists the section between Greenland and Scotland, which is mostly produced by the density changes in the Irminger Basin. The MOC interannual variation in the Labrador Sea is linked to the upper ocean volume transport west of Greenland. For the section from Greenland to Scotland, surface wind forcing is found to be important to drive the interannual variation.