Water mass transformation in the Iceland Sea

The water mass transformation that takes place in the Iceland Sea during winter is investigated using historical hydrographic data and atmospheric reanalysis fields. Surface densities exceeding σθ ¼ 27:8 kg=m3, and hence of sufficient density to contribute to the lower limb of the Atlantic Meridional Overturning Circulation via the overflows across the Greenland-Scotland Ridge, exist throughout the interior Iceland Sea east of the Kolbeinsey Ridge at the end of winter. The deepest and densest mixed layers are found in the northwest Iceland Sea on the outskirts of the basin׳s cyclonic gyre, largely determined by stronger atmospheric forcing near the ice edge. Much of the accumulated wintertime heat loss in that region takes place during a few extreme cold air outbreak events. Only a small number of hydrographic profiles (2%) recorded mixed layers sufficiently dense to supply the deepest part of the North Icelandic Jet, a current along the slope off northern Iceland that advects overflow water into the Denmark Strait. However, low values of potential vorticity at depth indicate that waters of this density class may be ventilated more regularly than the direct observations of dense mixed layers in the sparse data set indicate. A sudden increase in the depth of this deep isopycnal around 1995 suggests that the supply of dense water to the North Icelandic Jet, and hence to the densest component of the Atlantic Meridional Overturning Circulation, may have diminished over the past 20 years. Concurrent reductions in the turbulent heat fluxes and wind stress curl over the Iceland Sea are consistent with a decrease in convective activity and a weakening of the cyclonic gyre, both of which could have caused the increase in depth of these dense waters.acceptedVersio

Evolution and transformation of the North Icelandic Irminger Current along the North Iceland Shelf

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Semper, S., Våge, K., Pickart, R., Jónsson, S., & Valdimarsson, H. Evolution and transformation of the North Icelandic Irminger Current along the North Iceland Shelf. Journal of Geophysical Research: Oceans, 127(3), (2022): e2021JC017700, https://doi.org/10.1029/2021jc017700.The North Icelandic Irminger Current (NIIC) flowing northward through Denmark Strait is the main source of salt and heat to the north Iceland shelf. We quantify its along-stream evolution using the first high-resolution hydrographic/velocity survey north of Iceland that spans the entire shelf along with historical hydrographic measurements as well as data from satellites and surface drifters. The NIIC generally follows the shelf break. Portions of the flow recirculate near Denmark Strait and the Kolbeinsey Ridge. The current's volume transport diminishes northeast of Iceland before it merges with the Atlantic Water inflow east of Iceland. The hydrographic properties of the current are modified along its entire pathway, predominantly because of lateral mixing with cold, fresh offshore waters rather than air-sea interaction. Progressing eastward, the NIIC cools and freshens by approximately 0.3°C and 0.02–0.03 g kg−1 per 100 km, respectively, in both summer and winter. Dense-water formation on the shelf is limited, occurring only sporadically in the historical record. The hydrographic properties of this locally formed water match the lighter portion of the North Icelandic Jet (NIJ), which emerges northeast of Iceland and transports dense water toward Denmark Strait. In the region northeast of Iceland, the NIIC is prone to baroclinic instability. Enhanced eddy kinetic energy over the steep slope there suggests a dynamical link between eddies shed by the NIIC and the formation of the NIJ as previously hypothesized. Thus, while the NIIC rarely supplies the NIJ directly, it may be dynamically important for the overturning circulation in the Nordic Seas.This research was supported by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 101022251 (S. Semper), the Trond Mohn Foundation Grant BFS2016REK01 (S. Semper and K. Våge), and the U.S. National Science Foundation Grants OCE-1558742 and OCE-1259618 (R. S. Pickart)

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

Water mass transformation in the Iceland Sea: Contrasting two winters separated by four decades

Dense water masses formed in the Nordic Seas flow across the Greenland–Scotland Ridge and contribute substantially to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea gained renewed interest when the North Icelandic Jet — a current transporting dense water from the Iceland Sea into Denmark Strait — was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast the present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that the large-scale hydrographic structure of the central Iceland Sea has changed significantly over this period and that the locally transformed water has become less dense, in concert with a retreating sea-ice edge and diminished ocean-to-atmosphere heat fluxes. This has reduced the available supply of dense water to the North Icelandic Jet, but also permitted densification of the East Greenland Current during its transit through the presently ice-free western Iceland Sea in winter. Together, these changes have significantly altered the contribution from the Iceland Sea to the overturning in the Nordic Seas over the four decade period.publishedVersio

Structure and variability of the North Icelandic Jet from two years of mooring data

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(6), (2019): 3987-4002, doi:10.1029/2019JC015134.Mooring data from September 2011 to July 2013 on the Iceland slope north of Denmark Strait are analyzed to better understand the structure and variability of the North Icelandic Jet (NIJ). Three basic configurations of the flow were identified: (1) a strong separated East Greenland Current (EGC) on the mid‐Iceland slope coincident with a weak NIJ on the upper slope, (2) a merged separated EGC and NIJ, and (3) a strong NIJ located at its climatological mean position, coincident with a weak signature of the separated EGC at the base of the Iceland slope. Our study reveals that the NIJ‐dominant scenario was present during different times of the year for the two successive mooring deployments—appearing mainly from September to February the first year and from January to July the second year. Furthermore, when this scenario was active it varied on short timescales. An energetics analysis demonstrates that the high‐frequency variability is driven by mean‐to‐eddy baroclinic conversion at the shoreward edge of the NIJ, consistent with previous modeling work. The seasonal timing of the NIJ dominant scenario is investigated in relation to the atmospheric forcing upstream of Denmark Strait. The resulting lagged correlations imply that strong turbulent heat fluxes in a localized region on the continental slope of Iceland, south of the Spar Fracture zone, lead to a stronger NIJ dominant state with a two‐month lag. This can be explained dynamically in terms of previous modeling work addressing the circulation response to dense water formation near an island.The authors thank the crew members of the R/V Knorr, RRS James Clark Ross, and R/V Bjarni Sæmundsson for the deployment and recovery of the moorings. D. Torres and F. Bahr processed the second year of mooring data. We thank K. Våge, B. Harden, Z. Song, J. Li, and M. Li for helpful discussions regarding the work. Funding was provided by the National Science Foundation under grants OCE‐1558742 (J. H., R. P., P. L., and M. S.) and OCE‐1534618 (M. S.). The mooring data are available at http://kogur.whoi.edu/php/index.php.2019-12-0

A numerical study of interannual variability in the North Icelandic Irminger Current

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 123(12), (2018): 8994-9009, doi:10.1029/2018JC013800.The North Icelandic Irminger Current (NIIC) is an important component of the Atlantic Water (AW) inflow to the Nordic Seas. In this study, both observations and a high‐resolution (1/12°) numerical model are used to investigate the seasonal to interannual variability of the NIIC and its forcing mechanisms. The model‐simulated velocity and hydrographic fields compare well with the available observations. The water mass over the entire north Icelandic shelf exhibits strong seasonal variations in both temperature and salinity, and such variations are closely tied to the AW seasonality in the NIIC. In addition to seasonal variability, there is considerable variation on interannual time scales, including a prominent event in 2003 when the AW volume transport increased by about 0.5 Sv. To identify and examine key forcing mechanisms for this event, we analyzed outputs from two additional numerical experiments: using only the seasonal climatology for buoyancy flux (the momentum case) and using only the seasonal climatology for wind stress (the buoyancy case). It is found that changes in the wind stress are predominantly responsible for the interannual variations in the AW volume transport, AW fraction in the NIIC water, and salinity. Temperature changes on the shelf, however, are equally attributable to the buoyancy flux and wind forcing. Correlational analyses indicate that the AW volume transport is most sensitive to the wind stress southwest of Iceland.This work is supported by the U.S. National Science Foundation (NSF) under grants OCE‐1634886 (J. Zhao and J. Yang) and OCE‐1558742 (R. Pickart), and by the Bergen Research Foundation grant BFS2016REK01 (K. Våge and S. Semper). We thank Xiaobiao Xu at Florida State University for providing the initial model configuration. Comments from anonymous reviewers help to improve the manuscript. The altimeter products are produced and distributed by the Copernicus Marine and Environment Monitoring Service (CMEMS, http://www.marine.copernicus.eu). The hydrographic maps along the Hornbanki section are available at http://www.hafro.is/Sjora/.2019-04-1

On the hydrography of Denmark Strait

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 306–321, doi:10.1002/2016JC012007.Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990 and 2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait, the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified “boluses” of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22 year record, which can be explained by changes in end-member properties and their relative contributions to bolus composition.US National Science Foundation (RP and DM) Grant Number: OCE-0959381; ;Norwegian Research Council Grant Number: 231647 (KV)2017-07-2

North Atlantic extratropical and subpolar gyre variability during the last 120 years: a gridded dataset of surface temperature, salinity, and density. Part 1: dataset validation and RMS variability

We present a binned annual product (BINS) of sea surface temperature (SST), sea surface salinity (SSS), and sea surface density (SSD) observations for 1896–2015 of the subpolar North Atlantic between 40° N and 70° N, mostly excluding the shelf areas. The product of bin averages over spatial scales on the order of 200 to 500 km, reproducing most of the interannual variability in different time series covering at least the last three decades or of the along-track ship monitoring. Comparisons with other SSS and SST gridded products available since 1950 suggest that BINS captures the large decadal to multidecadal variability. Comparison with the HadSST3 SST product since 1896 also indicates that the decadal and multidecadal variability is usually well-reproduced, with small differences in long-term trends or in areas with marginal data coverage in either of the two products. Outside of the Labrador Sea and Greenland margins, interannual variability is rather similar in different seasons. Variability at periods longer than 15 years is a large part of the total interannual variability, both for SST and SSS, except possibly in the south-western part of the domain. Variability in SST and SSS increases towards the west, with the contribution of salinity variability to density dominating that of temperature in the western Atlantic, except close to the Gulf Stream and North Atlantic Current in the southwest area. Weaker variability and larger relative temperature contributions to density changes are found in the eastern part of the gyre and south of Iceland