Observed Changes in the Arctic Freshwater Outflow in Fram Strait

We have updated time series of liquid fresh water transport (FWT) in the East Greenland Current (EGC) in the western Fram Strait with mooring observations since 2015. Novel data have been used to correct earlier estimates when instrument coverage was lower. The updated FWT (reference salinity 34.9) shows that the increased export between 2010 and 2015 has not continued, but FWT has decreased to pre-2009 levels. Salt transport independent of a reference salinity is shown not to be sensitive to salinity changes. Between 2015 and 2019, the FWT in the Polar Water (PW) decreased to an average of 59.9 (±4.5) mSV, 15% less than the 2003–2019 long-term mean, however, high FWT events occurred in 2017. The overall decrease is related to a slowdown of the EGC, partly attributed to a decrease of the zonal density gradient, due to stronger salinification of the halocline waters (26.5 < σθ < 27.7 kg/m3) over the shelf. This salinification counterbalances the freshening of the surface layer (σθ < 26.5 kg/m3) and the fresh water content decreases. Our results show changes in the PW between 2003 and 2019: Salinity stratification increased as the salinity difference between 155 and 55 m increased by 0.63 psu, the PW layer became thinner by 40–50 m and the Polar-Atlantic front moved ∼10 km west in June 2015. All processes point to an “Atlantification” of the western Fram Strait and a reduced Polar outflow. Including the novel data sets reduced the uncertainty of the FWT to an average of 8% after 2015, as opposed to 17% in earlier estimates.publishedVersio

Recent changes in the freshwater composition east of Greenland

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 2326–2332, doi:10.1002/2014GL062759.Results from three hydrographic surveys across the East Greenland Current between 2011 and 2013 are presented with focus on the freshwater sources. End-member analysis using salinity, δ18O, and nutrient data shows that while meteoric water dominated the freshwater content, a significant amount of Pacific freshwater was present near Denmark Strait with a maximum in August 2013. While in 2011 and 2012 the net sea ice melt was dominated by brine, in 2013 it became close to zero. The amount of Pacific freshwater observed near Denmark Strait in 2013 is as large as the previous maximum in 1998. This, together with the decrease in meteoric water and brine, suggests a larger contribution from the Canadian Basin. We hypothesize that the increase of Pacific freshwater is the result of enhanced flux through Bering Strait and a shorter pathway of Pacific water through the interior Arctic to Fram Strait.The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7 2007–2013) under grant agreement 308299, NACLIM Project, and from the U.S. National Science Foundation under grant OCE-085041.2015-10-0

Deep convection in the Irminger Sea observed with a dense mooring array

Deep convection is a key process in the Atlantic Meridional Overturning Circulation, but because it acts at small scales, it remains poorly resolved by climate models. The occurrence of deep convection depends on weak initial stratification and strong surface buoyancy forcing, conditions that are satisfied in only a few ocean basins. In 2014, one of the Ocean Observatories Initiative (OOI) global arrays was installed close to the Central Irminger Sea (CIS) and the Long-term Ocean Circulation Observations (LOCO) moorings in the central Irminger Sea. These programs’ six moorings are located in the center of an area of deep convection and are distributed within a 50 km radius, thus offering detailed insight into spatial differences during the strong convection events that occurred during the winters of 2014/2015 and 2015/2016. Deep mixed layers, down to approximately 1,600 m, formed during both winters. The properties of the convectively renewed water mass at each mooring converge to a common temperature and salinity before restratification sets in at the end of winter. The largest differences in onset (or timing) of convection and restratification are seen between the northernmost and southernmost moorings. High-resolution atmospheric reanalysis data show there is higher atmospheric forcing at the northernmost mooring due to a more favorable position with respect to the Greenland tip jet. Nevertheless, earlier onset, and more continuous cooling and deepening of mixed layers, occurs at the southernmost mooring, while convection at the northern mooring is frequently interrupted by warm events. We propose that these warm events are associated with eddies and filaments originating from the Irminger Current off the coast of Greenland and that convection further south benefits from cold inflow from the southwest

Warming beneath an East Antarctic ice shelf due to increased subpolar westerlies and reduced sea ice

Understanding how climate change influences ocean-driven melting of the Antarctic ice shelves is one of the greatest challenges for projecting future sea level rise. The East Antarctic ice shelf cavities host cold water masses that limit melting, and only a few short-term observational studies exist on what drives warm water intrusions into these cavities. We analyse nine years of continuous oceanographic records from below Fimbulisen and relate them to oceanic and atmospheric forcing. On monthly time scales, warm inflow events are associated with weakened coastal easterlies reducing downwelling in front of the ice shelf. Since 2016, however, we observe sustained warming, with inflowing Warm Deep Water temperatures reaching above 0 °C. This is concurrent with an increase in satellite-derived basal melt rates of 0.62 m/yr, which nearly doubles the basal mass loss at this relatively cold ice shelf cavity. We find that this transition is linked to a reduction in coastal sea ice cover through an increase in atmosphere–ocean momentum transfer and to a strengthening of remote subpolar westerlies. These results imply that East Antarctic ice shelves may become more exposed to warmer waters with a projected increase of circum-Antarctic westerlies, increasing this region’s relevance for sea level rise projections.publishedVersio

Vertical redistribution of principle water masses on the Northeast Greenland Shelf

The Northeast Greenland shelf (NEGS) is a recipient of Polar Water (PW) from the Arctic Ocean, Greenland Ice Sheet melt, and Atlantic Water (AW). Here, we compile hydrographical measurements to quantify long-term changes in fjords and coastal waters. We find a profound change in the vertical distribution of water masses, with AW shoaling >60 m and PW thinning >50 m since early 2000’s. The properties of these waters have also changed. AW is now 1 °C warmer and the salinity of surface waters and PW are 1.8 and 0.68 lower, respectively. The AW changes have substantially weakened stratification south of ~74°N, indicating increased accessibility of heat and potentially nutrients associated with AW. The Atlantification earlier reported for the eastern Fram Strait and Barents Sea region has also propagated to the NEGS. The increased presence of AW, is an important driver for regional change leading to a likely shift in ecosystem structure and function