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

Abstract

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 5 years, 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.

The evolution of the North Atlantic Meridional Overturning Circulation since 1980

“…In particular, the 2007-2015 cooling (and freshening) of the subpolar North Atlantic results from weak heat (and salt) transports across 45° N (ref. 145 ), consistent with the reduced strength of the AMOC at 45° N (ref. 12 ).…”

“…This pattern is consistent with Atlantic Multidecadal Variability 3 , which in models, is linked to an increase in the AMOC 51,78,80,125 . After 2007, these trends reversed, cooling the subpolar North Atlantic and warming the western subtropics 3,15,144,145 (fig. 4).…”

“…4). Since 2015, upper subpolar North Atlantic layers warmed by around 0.2-1 °C, with salinity also increasing 15,145 by 0.02-0.1 practical salinity units (PSU) (fig. 4).…”

Arrival of New Great Salinity Anomaly Weakens Convection in the Irminger Sea

“…In both the Iceland and Irminger basins, the ocean layers affected by the GSA after 2015 (Figure 1b) mainly consist of SPMW, which results from the transformation of subtropical-originated water masses in the Iceland Basin (Brambilla & Talley, 2008;Petit et al, 2020). The long term, basinwide, freshening since 2010 (Figure 1c)-consistent with the NAO-forced low inflow of subtropical waters to the SPNA (Lankhorst & Send, 2020;Tesdal & Haine, 2020)-and the apparent absence of SPMW in the central Irminger (Figure 3a) could indicate that the reduction in the inflow of subtropical water masses to the SPNA played a major role in forming the new GSA. Concurrently, the freshwater redistribution described by Holliday et al (2020) amplified the ongoing freshening.…”

Multidecadal Water Mass Dynamics on the West Greenland Shelf

“…The outline of the subpolar gyre (SPG) is shown in light red. Gray shading denotes bathymetry (the basis Figure is adapted from Tesdal & Haine, 2020, Figure 1, JGR‐Oceans). (b) Map of the Nuup Kangerlua system and the adjacent continental shelf and slope, showing locations of hydrographic stations as crosses.…”