On the effect of climate trends in coastal density on deep water renewal frequency in sill fjords—A statistical approach

The basin water of a sill fjord is renewed intermittently, when the density of the water at sill level in the ocean outside the fjord is high enough for it to sink down to the bottom of the fjord. How often the basin water is renewed, the renewal frequency, depends on how fast diffusion and mixing cause the density in the basin to decrease and on the variability of the density in the ambient ocean. Here, we suggest a statistical approach to investigate how a trend – e.g. imposed by global warming – in the ambient ocean density will affect the renewal frequency of a sill fjord. Negative ambient trends that are large compared to the rate at which density decreases the fjord will have large impact on the renewal frequency. It is shown that the observed negative trend in the annual maximum density at a hydrographical station near the Norwegian fjord Masfjorden, is very likely to have reduced the renewal frequency and increased the length of the stagnation period compared to pre-trend conditions. Negative trends in the annual maximum density since 1990 are observed at six out of eight longterm hydrographical stations along the Atlantic Norwegian coast, suggesting that the deep water renewal frequency in many Norwegian fjords has been reduced during the last 30 years.publishedVersio

Faroe Bank Channel Overflow: Mesoscale Variability

The Faroe Bank Channel is the deepest connection through the Greenland–Scotland Ridge, where dense water formed north of the ridge flows southward over the sill crest, contributing to the formation of North Atlantic Deep Water. The overflow region is characterized by high mesoscale variability and energetic os- cillations, accompanied by a high degree of sea surface level variability. Here, 2-month-long time series of velocity and temperature from 12 moorings deployed in May 2008 are analyzed to describe the oscillations and explore their generation and propagation. The observed 2.5–5-day oscillations in velocity and temper- ature are highly coherent both horizontally and vertically, and they are associated with 100–200-m-thick boluses of cold plume water flowing along the slope. A positive correlation between temperature and relative vorticity and the distribution of clockwise/counterclockwise rotation across the slope suggest a train of al- ternating warm cyclonic and cold anticyclonic eddies, where the maximum plume thickness is located downslope of the eddy center. The along-slope phase velocity is found to be 25–60 cm s-1, corresponding to a wavelength of 75–180 km, while the vertical phase propagation is downward. The oscillations are present already in the sill region. The observations do not match predictions for eddies generated either by vortex stretching or baroclinic instability but agree broadly with properties of topographic Rossby waves.publishedVersio

Observations of barotropic oscillations and their influence on mixing in the Faroe bank channel overflow region

Observations of hydrography, currents, and microstructure are presented together with sea surface height (SSH) patterns from concurrent satellite tracks to describe the subinertial oscillations in the region downstream of the Faroe Bank Channel overflow. Energetic oscillations with a dominant 3–5-day period have previously been observed in the dense bottom layer and found to be consistent with topographic Rossby waves. Here, the authors present evidence that the oscillations extend over the whole water column and are connected to a wave-like pattern in SSH along the continental slope. The waves are observed in two satellite tracks running parallel to the slope and indicate a wavelength of 50–75 km, an amplitude of about 5 cm, and a phase speed of 15–20 cm s−1. The pattern extends at least 450 km along the slope. Repeat occupations of a section through a 4-day period show a barotropic velocity anomaly that is associated with an increase in plume transport [from 0.5 to 2.5 Sv (1 Sv ≡ 106 m3 s−1)] and interface height (from 100 to 200 m) as well as changes in dissipation rates and mixing. Estimates of entrainment velocity wE vary with a factor of 102 over the oscillation period, and there is an inverse relation between wE and plume thickness, that is, mixing is most intense when the dense bottom layer is thin. High values of wE coincide with a large percentage of critical Richardson numbers in the interfacial layer. The rotational motion, or the horizontal “stirring,” is observed to bring water from the south, traceable because of its low oxygen concentrations, into the plume.publishedVersio

Intermittent Reduction in Ocean Heat Transport Into the Getz Ice Shelf Cavity During Strong Wind Events

The flow of warm water toward the western Getz Ice Shelf along the Siple Trough, West Antarctica, is intermittently disrupted during short events of Winter Water deepening. Here we show, using mooring records, that these 5–10 days-long events reduced the heat transport toward the ice shelf cavity by 25% in the winter of 2016. The events coincide with strong easterly winds and polynya opening in the region, but the Winter Water deepening is controlled by non-local coastal Ekman downwelling rather than polynya-related surface fluxes. The thermocline depth anomalies are forced by Ekman downwelling at the northern coast of Siple Island and propagate to the ice front as a coastal trapped wave. During the events, the flow at depth does no longer continue along isobaths into the ice shelf cavity but aligns with the ice front.publishedVersio

Interaction of the Faroe Bank Channel overflow with Iceland Basin intermediate waters

Author Posting. © American Geophysical Union, 2014. 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 119 (2014): 228-240, doi:10.1002/2013JC009437.The narrow and deep Faroe Bank Channel (FBC) is an important pathway for cold, dense waters from the Nordic Seas to flow across the Iceland-Scotland ridge into the North Atlantic. The swift, turbulent FBC overflow is associated with strong vertical mixing. Hydrographic profiles from a shipboard survey and two Slocum electric gliders deployed during a cruise in May–June 2012 show an intermediate water mass characterized by low salinity and low oxygen concentration between the upper waters of Atlantic origin and the dense overflow water. A weak low-salinity signal originating north-east of Iceland is discernible at the exit of the FBC, but smeared out by intense mixing. Further west (downstream) marked salinity and oxygen minima are found, which we hypothesize are indicators of a mixture of Labrador Sea Water and Intermediate Water from the Iceland Basin. Water mass characteristics vary strongly on short time scales. Low-salinity, low-oxygen water in the stratified interface above the overflow plume is shown to move along isopycnals toward the Iceland-Faroe Front as a result of eddy stirring and a secondary, transverse circulation in the plume interface. The interaction of low-salinity, low-oxygen intermediate waters with the overflow plume already at a short distance downstream of the sill, here reported for the first time, affects the final properties of the overflow waters through entrainment and mixing.This work was funded by the Research Council of Norway, through the FRINAT program, under the project 204867/V30, ‘‘Faroe Bank Channel Overflow: Dynamics and Mixing.’’2014-07-1

Mitigation of oxygen decline in fjords by freshwater injection

The exchange of water masses between deep fjords and the open ocean is commonly constrained by a topographical barrier called the sill. While fjord water above the sill depth communicates relatively freely with the open ocean, water below the sill depth is caught inside the fjord basin. This basin water may remain stagnant in deep fjords for many successive years. During these periods, the biological consumption of dissolved oxygen is larger than the supply of new oxygen, and the fjord basin might experience hypoxia and even anoxia. Such deoxygenation is natural but can be amplified by warming and human activities involving supplies of organic matter and other nutrients. Here, we use a general circulation model to explore how deoxygenation can be mitigated by injecting fresh water into the fjord basin. The freshwater injection causes density reduction of the basin water with subsequent water exchange and oxygenation. Our results suggest that the basin water of Masfjorden, a 480 m deep fjord with a basin volume of 4 × 109 m3, can avoid deoxygenation with a continuous freshwater injection of 0.05 m3 s−1. We conclude that freshwater injection might be an efficient tool to mitigate the deoxygenation of fjord basins.publishedVersio

Sudden, local temperature increase above the continental slope in the southern Weddell Sea, Antarctica

Around most of Antarctica, the Circumpolar Deep Water (CDW) shows a warming trend. At the same time, the thermocline is shoaling, thereby increasing the potential for CDW to enter the shallow continental shelves and ultimately increase basal melt in the ice shelf cavities that line the coast. Similar trends, on the order of 0.05 ∘C and 3 m per decade, have been observed in the Warm Deep Water (WDW), the slightly cooled CDW derivative found at depth in the Weddell Sea. Here, we report on a sudden, local increase in the temperature maximum of the WDW above the continental slope north of the Filchner Trough (74∘ S, 25–40∘ W), a region identified as a hotspot for both Antarctic Bottom Water formation (AABW) and potential changes in the flow of WDW towards the large Filchner–Ronne Ice Shelf. New conductivity–temperature–depth profiles, obtained in summer 2021, and recent (2017–2021) mooring records show that the temperature of the warm-water core increased by about 0.1 ∘C over the upper part of the slope (700–2750 m depth) compared with historical (1973–2018) measurements. The temperature increase occurred relatively suddenly in late 2019 and was accompanied by an unprecedented (in observations) freshening of the overlying winter water. The AABW descending down the continental slope from Filchner Trough is sourced by dense ice shelf water and consists to a large degree (60 %) of entrained WDW. The observed temperature increase can hence be expected to imprint directly on deep-water properties, increasing the temperature of newly produced bottom water (by up to 0.06 ∘C) and reducing its density.publishedVersio