Dominant Terms in the Freshwater and Heat Budgets of the Subpolar North Atlantic Ocean and Nordic Seas From 1992 to 2015

The Arctic and subarctic oceans exhibit distinct decadal variations in freshwater and heat content. We describe freshwater and heat budgets with the ECCOv4 reanalysis product and compare budget variability and mechanisms within the subpolar North Atlantic Ocean, Nordic Seas, and Labrador Sea from 1992 to 2015. For all regions, changes in freshwater content are largely anticorrelated with changes in heat content. Since 1995, the subpolar North Atlantic Ocean has undergone a decade of warming and salinification followed by ongoing cooling and freshening. The recent increase in freshwater content and the reduction in heat in the subpolar North Atlantic can largely be attributed to anomalous circulation of mean salinity and temperature, respectively. Interannual variability in heat and freshwater mostly corresponds to boundary fluxes from the subtropics. Meanwhile, the Nordic Seas have undergone an overall warming and salinification from the mid-1990s to 2015. Salinification is primarily driven by reduced sea ice flux through Fram Strait, while warming is due to changes in both sea surface heating and advective flux. In the last 5years, Labrador Sea freshwater convergence remained unchanged, as increased inflow via the Baffin Island Current is balanced by increased outflow via the Labrador Current. Hence, the observed freshening of the Arctic Ocean is expected to be an increasingly important source of future freshwater increases in the subpolar North Atlantic. This stands in contrast to variability in freshwater flux from the subtropical North Atlantic, which is associated with variability in the Atlantic Meridional Overturning Circulation. Plain Language Summary The subpolar North Atlantic Ocean has cycled between decades of becoming warmer and saltier and decades of becoming colder and fresher. The Nordic Seas have mostly become saltier in the last few decades. We used an ocean model to investigate processes behind these changes over the period 1992 to 2015. The model accurately matches real-world measurements. The warming and salinification patterns were broken into individual components, such as atmospheric exchanges and transport processes within the ocean. Ocean circulation in the North Atlantic mainly controls freshwater and heat content in the subpolar North Atlantic, mostly by changing the movement of water masses from the subtropical Atlantic. The decline in freshwater content in the Nordic Seas comes mostly from a drop in sea ice export from the Arctic Ocean. These findings help us to better understand what drives the year-to-year and longer-term variations in freshwater and heat content in the northern North Atlantic. This is important because changes in freshwater and heat in the upper layers of that region can affect the global climate by influencing the amount of atmospheric heat and carbon stored in the ocean. Key Points An ocean state estimate identified mechanisms governing freshwater and heat content in the northern North Atlantic over 1992-2015 Decadal variation in the subpolar North Atlantic is due to advective convergence while sea ice melt is more relevant in the Nordic Seas Freshwater variability is due to southern boundary transport in subpolar North Atlantic and Fram Strait sea ice export in Nordic Seas