Freshwater Variability in the Northern North Atlantic: Does the Hydrological Cycle Play an Important Role?

Abstract:

The Northern North Atlantic (NNA) is one of the most important regions of the World Ocean as it is one of just a few locations where surface waters can sink to deep depths. This, in turn, helps drive the main limb of the global thermohaline circulation — the Atlantic Meridional Overturning Circulation (AMOC). The sinking of water parcels in the NNA depends on the vertical density stratification in the upper ocean, with weak (strong) stratification allowing (forbidding) the sinking of water. Density, particularly in the cold NNA, is very sensitive to changes in salinity and therefore the increase (decrease) of freshwater can act to decrease (increase) salinity and stabilize (destabilize) the water column which ultimately determines the amount of water able to sink to deeper depths. In our previous study on this subject (Reagan et al, 2018), we revealed that changes in the salinity and hydrological cycle over the NNA significantly correlated with moisture fluxes from the subtropical North Atlantic. Here, we take one step further and analyze whether the hydrological cycle (namely evaporation minus precipitation) over various regions in the NNA can provide sufficient freshwater to significantly alter salinity or do other factors in the mixed layer salinity budget (e.g., advection and eddy fluxes) overwhelm the hydrological cycle forcing. We perform this analysis over three decadal time periods: 1985-1994, 1995-2004, and 2005-2017. We use the World Ocean Atlas 2018 Salinity Climatology, the newly released World Ocean Atlas 2018 Mixed Layer Depth Climatology, SODA Reanalysis, a high-resolution ocean model, Evaporation (OAFlux), and Precipitation (GPCP) products to address this problem. We found that the hydrological cycle in the NNA does play a critical role in inputting freshwater into multiple NNA regions over multiple decades and that these freshwater fluxes can ultimately impact salinity more than advection and eddy fluxes. These results and their long-term implications will be discussed in our presentation.