Formation and pathways of dense water in the Nordic Seas

Sammendrag De nordiske hav er viktig for dannelsen av kalde, tette og dype vannmasser som strømmer sørover på tvers av Grønland-Skottland-ryggen og forsyner den dype grenen av omveltningssirkulasjonen i Atlanterhavet. På tross av at størrelsen på dypvannstransporten over ryggen er godt kjent, gjenstår det mange åpne spørsmål angående hvor og hvordan de dype vannmassene dannes og transporteres til ryggen. Det er også stor usikkerhet rundt variasjonene i dypvannsdannelse og hvilke implikasjoner dette har for omveltningssirkulasjonen. I denne oppgaven bruker vi observasjonsdata til å kvantifisere hvor dype vannmasser dannes, hvordan de strømmer mot Grønland-Skottland-ryggen, og hvordan dette har endret seg de siste 70 årene. Oppgaven retter et spesielt fokus mot Grønlandshavet, som er en viktig kilde til dypvannet i de nordiske hav. I Artikkel I benyttet vi hydrografiske observasjoner fra 1986 til 2016, sammen med en endimensjonal blandalagsmodell, til å undersøke mellomårlig variabilitet og langtidsendringer i dypvannsdannelsen i Grønlandshavet. Vi fant at perioden før midten av 1990-tallet var spesielt fersk og sterkt stratifisert, noe som resulterte i grunn konveksjon (<300 m), til tross for sterkt atmosfærisk pådriv. Saltinnholdet i Grønlandshavet økte etter midten av 1990-tallet på grunn av høyere saltholdighet i Atlanterhavsvannet som strømmer nordover inn i de nordiske hav. Dette førte til svekket stratifisering, dypere konveksjon (500–1500 m), og dannelse av en ny klasse dypvann som har vært hovedproduktet av konveksjonen i Grønlandshavet frem til i dag. Denne nye vannmassen er mindre tett enn dypvannet som ble produsert i Grønlandshavet før 1980-tallet. Den vertikale utstrekningen av den nye vannmassen er derfor begrenset til den øvre halvdelen av vannsøylen. Store mengder varme ekstraheres fra de nordiske hav til atmosfæren om vinteren. Omtrent 60–80% av varmen frigjøres under intense, kortvarige kaldluftsutbrudd (heretter omtalt som utbrudd). I Artikkel II brukte vi et unikt 10-årig (1999–2009) hydrografisk datasett fra profilerende instrumenter med 1–2 dagers tidsoppløsning til å kvantifisere, for aller første gang, den direkte påvirkningen av slike utbrudd på blandalaget i Grønlandshavet. Dette viste at responsen i blandalagsegenskapene var avhengig av styrken på utbruddene og når de inntraff. Kaldluftsutbrudd som inntraff tidlig på vinteren (november–januar) førte i hovedsak til en nedkjøling av blandalaget, mens utbrudd som inntraff senere på vinteren (februar–april) førte til en økning i blandalagsdyp. Idealiserte simuleringer med en endimensjonal blandalagsmodell antyder at tidspunktet når dyp konveksjon inntreffer avhenger av fordelingen av utbrudd, mens blandalagsegenskapene mot slutten av vinteren er mer avhengig av styrken og det totale antallet utbrudd gjennom vinteren. Responsen i blandalagsegenskapene var også avhengig av laterale varme og salt flukser. Disse ble kvantifisert og inkludert i blandalagsmodellen. Resultatene viste at deres kombinerte effekt er en reduksjon i blandalagsdybden på opptil flere hundre meter. I Artikkel III utviklet vi en inversjonsmodell med høy romlig oppløsning for vannmassene i de nordiske hav. Denne ble brukt til å identifisere opprinnelsen til de to største dypvannsstrømmene som passerer Grønland-Skottland-ryggen i Danmarkstredet og Færøybankkanalen. Inversjonsmodellen er basert på hydrografiske og geokjemiske vannegenskaper observert i perioden 2000–2019 og viser hvor dypvann dannes og hvordan de strømmer mot ryggen. Dypvannsstrømmen i Danmarkstredet består hovedsakelig av vannmasser fra Grønlandshavet (39±2%), Islandshavet (20±3%) og fra Norskehavet (19±2% ). Dypvann dannet i Grønlandshavet beveger seg sørover langs to distinkte strømningsveier: en ytre kjerne av Østgrønlandsstrømmen og en tidligere ukjent strømningsvei som krysser Jan Mayen-ryggen inn mot Islandshavet sør for Jan Mayen. Begge disse strømningsveiene forsyner Nordislandsjeten som består av 82±2% dypvann dannet i Grønlandshavet. Det meste av dypvannsstrømmen i Færøbankkanalen har sin opprinnelse i Grønlandshavet (46±8%) og Polhavet (25±9%). Disse vannmassene strømmer sørover mot kanalen med Island–Færøy-jeten og langs den østlige delen av Jan Mayenryggen. Den sistnevnte strømningsveien svinger østover til den norske kontinentalskråningen, som den deretter følger sørover til Færøy-Shetland-kanalen. Denne strømningsveien kan bidra med 24±3% av dypvannet i Færøybankkanalen, mens Island–Færøyjeten forsyner 58±3%. Disse resultatene øker vår forståelse av hvor de dype vannmassene dannes og hvordan de transporteres til Grønland-Skottland-ryggen. Fokuset i Artikkel IV var langtidsendringer i dypvannet i de nordiske hav. Til å undersøke dette ble observasjonsdata over en 70-års periode (1950–2019) benyttet, sammen med inversjonsmodellen for periodene 1950–1979 og 2000–2019. Resultatene avslørte at dypvannsreservene i de nordiske hav har blitt varmere og mindre tett på grunn av økt temperatur i det innstrømmende Atlanterhavsvannet og opphør av konveksjon til bunnen av Grønlandshavet etter 1980-tallet. Dette har påvirket hele tetthetsstrukturen i de nordiske hav. Den reduserte konveksjonen har ført til en nedgang i tettheten og bidraget fra Grønlandshavet til dypvannsstrømmen gjennom Færøybankkanalen. Derimot har bidraget til Danmarkstredet fra den nye, mindre tette vannmassen i Grønlandshavet økt. Våre analyser av egenskapene og sammensetningen av dybvannsstrømmene på tvers av Grønland-Skottland-ryggen demonstrerer at det er viktig å ta hensyn til både romlige og tidsmessige variasjoner i dypvannsdannelse for å forstå langtidsendringene. Dersom trenden mot varmere og mindre tette dypvannsreserver fortsetter i fremtiden, forventes en tetthetsreduksjon i omveltningssirkulasjonen i de nordiske hav. Til sammen har de fire artiklene i denne oppgaven økt vår kunnskap om dannelsen, strømingsveiene, og variabiliteten til dypvannet i de nordiske hav i betydelig grad. Denne kunnskapen er kritisk for å kunne bedre forstå dypvannstrømmene på tvers av Grønnland-Skottland-ryggen, deres bidrag til omveltningssirkulasjonen i Atlanterhavet og hva vi kan forvente av disse i et framtidig varmere klima.Abstract Dense water formed in the Nordic Seas flows southward across the Greenland-Scotland Ridge and sinks to great depths in the North Atlantic to supply the lower limb of the Atlantic Meridional Overturning Circulation. While the exchange flows across the ridge have been monitored for several decades, gaps in our knowledge remain regarding where and how the dense overflow waters are formed and transported to the ridge. Questions also remain regarding the variability in dense-water formation and its implications for the dense-water reservoir and overflows from the Nordic Seas, which are critical to understand the overturning in the Nordic Seas. Based on observational data, this thesis quantifies the origin and upstream pathways of the overflow waters, as well as how and why they have changed over the past 70 years. A particular focus was on the variability in dense-water formation in the Greenland Sea, where a major portion of the overflow waters originate. In Paper I, we focused on the interannual and long-term changes in dense-water formation in the Greenland Sea based on hydrographic observations from 1986 to 2016 and a one-dimensional mixed-layer model. We found that the period prior to the mid-1990s was particularly fresh and strongly stratified, resulting in predominantly shallow convection (<300 m), despite strong atmospheric forcing. Increased salinity, linked to higher salinity in the Atlantic Water inflow into the Nordic Seas, weakened the water column stability after the mid-1990s. This transition led to increased convection depths (500–1500 m) and the formation of a new, less dense class of intermediate water that has been the main product of convection in the Greenland Sea until present. Although the volume of the new water mass increased from the 1990s to the 2000s, its vertical extent has been constrained to the upper half of the Greenland Sea water column, above the remnants of the denser Greenland Sea deep water that was the main product of convection prior to the 1980s. Approximately 60–80% of the heat lost to the atmosphere during winter is related to intense, short-lived events called cold-air outbreaks (CAOs). In Paper II, we utilized a unique 10-year (1999–2009) hydrographic record from moored profilers with 1–2 days temporal resolution to examine, for the first time, the direct impact of CAOs on the mixed-layer development in the Greenland Sea. This revealed that the mixed-layer response depended on when the CAO events occurred and on their intensity. Early in winter (November–January) the response was primarily a cooling of the mixed layer, while later in winter (February–April) the mixed layer mainly deepened. Idealized simulations with a one-dimensional mixed-layer model suggest that the temporal distribution of CAOs impacts the timing of the onset of the deepening phase, while the end-of-winter mixed-layer depth and hydrographic properties are more sensitive to the integrated heat loss over the winter, which is determined by the total number and intensity of CAOs. Considerable variability was observed in the mixed-layer response to CAOs, highlighting the importance of lateral heat and salt fluxes. These were quantified and included in the mixed-layer model, which suggests that their combined effect is a reduction in the end-of-winter mixed-layer depth of up to several hundred meters. In Paper III we developed a regional high-resolution water-mass inversion for the Nordic Seas to determine the origin and upstream pathways of the two main overflow plumes passing the Greenland-Scotland Ridge in Denmark Strait and the Faroe Bank Channel. The inversion is based on the geographical distribution of hydrographic and geochemical water properties from observations covering the period 2000–2019 and resolves the pathways that connect the overflow plumes to their origins. The Denmark Strait overflow is mainly composed of water originating in the Greenland Sea (39±2%), the Iceland Sea (20±3%), and in the Atlantic Domain (19±2%) of the Nordic Seas. Dense water from the Greenland Sea propagates southward along two distinct pathways: an outer core of the East Greenland Current and along a previously unknown pathway that crosses the Jan Mayen Ridge into the Iceland Sea just south of Jan Mayen. Both of these pathways feed the North Icelandic Jet that consists of 82±2% dense-water formed in the Greenland Sea. Most of the Faroe Bank Channel overflow originates in the Greenland Sea (46±8%) and the Arctic Ocean (25±9%) and propagates toward the channel with the Iceland-Faroe Slope Jet and along the eastern margin of the Jan Mayen Ridge. The latter pathway turns eastward over to the Norwegian continental slope, which it then follows southward to the Faroe-Shetland Channel. This pathway can account for 24±3% of the Faroe Bank Channel overflow, while the Iceland-Faroe Slope Jet supplies 58±3%. These results improve our understanding on the origin and upstream pathways of the overflows, in particular regarding the dense-water pathways from the Greenland Sea and how the overflow water approaches the Faroe-Shetland Channel. The focus in Paper IV was long-term variability in the Nordic Seas reservoir and overflows using a 70-year long (1950–2019) observational record and the regional water-mass inversion for the two periods 1950–1979 and 2000–2019. The results revealed that the Nordic Seas reservoir has warmed and become less dense due to changes in the Atlantic Water inflow and the cessation of bottom-reaching convection in the Greenland Sea. This has, in turn, impacted the entire density structure in the Nordic Seas. The transition from bottom to intermediate-depth convection has reduced the density and supply from the Greenland Sea to the Faroe Bank Channel overflow, while the contribution of the less dense intermediate water to the overflow through Denmark Strait has increased. Our analyses of the overflow water composition and properties demonstrate that it is important to take both the spatial and temporal variability in dense-water formation into account when examining the long-term changes in the overflows. The Atlantic Water has warmed and become less dense over the past 2-3 decades. If this trend continues in the future, it is expected to further decrease the density of the overturning the Nordic Seas. Collectively, the four papers in this thesis have significantly advanced our knowledge about the formation and pathways of dense water in the Nordic Seas, their variability, and the contributions to the overflow waters across the Greenland-Scotland Ridge from an observational point of view. As such, the thesis provides an important step forward to understand the overturning in the Nordic Seas and its variability.Doktorgradsavhandlin

Formation and pathways of dense water in the Nordic Seas based on a regional inversion

Dense waters formed in the Nordic Seas spill across gaps in the Greenland-Scotland Ridge into the abyss of the North Atlantic to feed the lower limb of the Atlantic Meridional Overturning Circulation. The overflow water transport is well known, but open questions remain regarding where and how the dense overflow waters are formed and transported to the ridge. Here we develop a regional high-resolution version of an inverse method called Total Matrix Intercomparison, which combines hydrographic and geochemical tracer observations between 2000 and 2019 to resolve the pathways that connect the overflows to their origins. Consistent with previous studies we find two main pathways feeding the Denmark Strait Overflow Water (DSOW): the East Greenland Current and the North Icelandic Jet. Most of the water supplied by the North Icelandic Jet originates in the Greenland Sea (82 ± 2%) and flows southward along an outer core of the East Greenland Current, as well as along a previously unknown pathway crossing the Jan Mayen Ridge into the Iceland Sea. In total, 39 ± 2% of the DSOW originates in the Greenland Sea, while the Iceland Sea and the Atlantic Domain of the Nordic Seas account for 20 ± 3% and 19 ± 2%, respectively. The majority of the Faroe Bank Channel Overflow Water originates in the Greenland Sea (46 ± 8%) and the Arctic Ocean (25 ± 9%). These dense waters approach the sill in the Iceland-Faroe Slope Jet and along the eastern side of the Jan Mayen Ridge. The inversion reveals unprecedented details on the upstream sources and pathways of the overflows, which have not previously been obtained using observations.publishedVersio

The Impact of Cold-Air Outbreaks and Oceanic Lateral Fluxes on Dense-Water Formation in the Greenland Sea from a 10-Year Moored Record (1999–2009)

The Greenland Sea produces a significant portion of the dense water from the Nordic seas that supplies the lower limb of the Atlantic meridional overturning circulation. Here, we use a continuous 10-yr hydrographic record from moored profilers to examine dense-water formation in the central Greenland Sea between 1999 and 2009. Of primary importance for dense-water formation is air–sea heat exchange, and 60%–80% of the heat lost to the atmosphere during winter occurs during intense, short-lived events called cold-air outbreaks (CAOs). The long duration and high temporal resolution of the moored record has for the first time facilitated a statistical quantification of the direct impact of CAOs on the wintertime mixed layer in the Greenland Sea. The mixed layer development can be divided into two phases: a cooling phase and a deepening phase. During the cooling phase (typically between November and January), CAOs cooled the mixed layer by up to 0.08 K day−1, depending on the intensity of the events, while the mixed layer depth remained nearly constant. Later in winter (February–April), heat fluxes during CAOs primarily led to mixed layer deepening of up to 38 m day−1. Considerable variability was observed in the mixed layer response, indicating that lateral fluxes of heat and salt were also important. The magnitude and vertical distributions of these fluxes were quantified, and idealized mixed layer simulations suggest that their combined effect is a reduction in the mixed layer depth at the end of winter of up to several hundred meters.publishedVersio

Sources and upstream pathways of the densest overflow water in the Nordic Seas

Overflow water from the Nordic Seas comprises the deepest limb of the Atlantic Meridional Overturning Circulation, yet questions remain as to where it is ventilated and how it reaches the Greenland-Scotland Ridge. Here we use historical hydrographic data from 2005-2015, together with satellite altimeter data, to elucidate the source regions of the Denmark Strait and Faroe Bank Channel overflows and the pathways feeding these respective sills. A recently-developed metric is used to calculate how similar two water parcels are, based on potential density and potential spicity. This reveals that the interior of the Greenland Sea gyre is the primary wintertime source of the densest portion of both overflows. After subducting, the water progresses southward along several ridge systems towards the Greenland-Scotland Ridge. Kinematic evidence supports the inferred pathways. Extending the calculation back to the 1980s reveals that the ventilation occurred previously along the periphery of the Greenland Sea gyre.publishedVersio

How is the ocean anthropogenic carbon reservoir filled?

About a quarter of the total anthropogenic CO2 emissions during the industrial era has been absorbed by the ocean. The rate limiting step for this uptake is the transport of the anthropogenic carbon (Cant) from the ocean mixed layer where it is absorbed to the interior ocean where it is stored. While it is generally known that deep water formation sites are important for vertical carbon transport, the exact magnitude of the fluxes across the base of the mixed layer in different regions is uncertain. Here, we determine where, when, and how much Cant has been injected across the mixed-layer base and into the interior ocean since the start of the industrialized era. We do this by combining a transport matrix derived from observations with a time-evolving boundary condition obtained from already published estimates of ocean Cant. Our results show that most of the Cant stored below the mixed layer are injected in the subtropics (40.1%) and the Southern Ocean (36.0%), while the Subpolar North Atlantic has the largest fluxes. The Subpolar North Atlantic is also the most important region for injecting Cant into the deep ocean with 81.6% of the Cant reaching depths greater than 1,000 m. The subtropics, on the other hand, have been the most efficient in transporting Cant across the mixed-layer base per volume of water ventilated. This study shows how the oceanic Cant uptake relies on vertical transports in a few oceanic regions and sheds light on the pathways that fill the ocean Cant reservoir

Nordic Seas Heat Loss, Atlantic Inflow, and Arctic Sea Ice cover over the last century

Poleward ocean heat transport is a key process in the earth system. We detail and review the northward Atlantic Water (AW) flow, Arctic Ocean heat transport, and heat loss to the atmosphere since 1900 in relation to sea ice cover. Our synthesis is largely based on a sea ice-ocean model forced by a reanalysis atmosphere (1900-2018) corroborated by a comprehensive hydrographic database (1950-), AW inflow observations (1996-), and other long-term time series of sea ice extent (1900-), glacier retreat (1984-) and Barents Sea hydrography (1900-). The Arctic Ocean, including the Nordic and Barents Seas, has warmed since the 1970s. This warming is congruent with increased ocean heat transport and sea ice loss and has contributed to the retreat of marine-terminating glaciers on Greenland. Heat loss to the atmosphere is largest in the Nordic Seas (60% of total) with large variability linked to the frequency of Cold Air Outbreaks and cyclones in the region, but there is no long-term statistically significant trend. Heat loss from the Barents Sea (∼30%) and Arctic seas farther north (∼10%) is overall smaller, but exhibit large positive trends. The AW inflow, total heat loss to the atmosphere, and dense outflow have all increased since 1900. These are consistently related through theoretical scaling, but the AW inflow increase is also wind-driven. The Arctic Ocean CO2 uptake has increased by ∼30% over the last century - consistent with Arctic sea ice loss allowing stronger air-sea interaction and is ∼8% of the global uptake

Water mass transformation in the Greenland Sea during the period 1986-2016

Hydrographic measurements from ships, autonomous profiling floats, and instrumented seals over the period 1986–2016 are used to examine the temporal variability in open-ocean convection in the Greenland Sea during winter. This process replenishes the deep ocean with oxygen and is central to maintaining its thermohaline properties. The deepest and densest mixed layers in the Greenland Sea were located within its cyclonic gyre and exhibited large interannual variability. Beginning in winter 1994, a transition to deeper (>500 m) mixed layers took place. This resulted in the formation of a new, less dense class of intermediate water that has since become the main product of convection in the Greenland Sea. In the preceding winters, convection was limited to <300-m depth, despite strong atmospheric forcing. Sensitivity studies, performed with a one-dimensional mixed layer model, suggest that the deeper convection was primarily the result of reduced water-column stability. While anomalously fresh conditions that increased the stability of the upper part of the water column had previously inhibited convection, the transition to deeper mixed layers was associated with increased near-surface salinities. Our analysis further suggests that the volume of the new class of intermediate water has expanded in line with generally increased depths of convection over the past 10–15 years. The mean export of this water mass from the Greenland Sea gyre from 1994 to present was estimated to be 0.9 ± 0.7 Sv (1 Sv ≡ 10^6 m^3 s^−1), although rates in excess of 1.5 Sv occurred in summers following winters with deep convection