Ventilation age and remineralisation rates in polar and sub-polar regions as an indicator for climate change

In the Nordic Seas mean values of water mass ages for the 1990s and the 2000s clearly show increased ventilation in the upper 1000-1500 m, while the deep waters have got older. The latter is a combination ventilation processes not reaching deep enough, and that ocean circulation has redistributed water masses, changing their characteristics. Ventilation of the upper layers is largely connected to the vertical mixing in the Greenland Sea, resulting in an intermediate water mass with high density. The spreading of this water mass to other basins in the area have ventilated intermediate layers in large part of the region. In a published study from the project we find a clear link between changes in salinity and temperature in the Atlantic Water flowing into the Nordic Seas close to the Faroe Islands, and observed changes in the upper layers of the Greenland Sea, with a three-year time lag. Thus it takes three years for the Atlantic Water to enter and affect the Greenland Sea. In another published study we used the Earth system model NorESM to evaluate the method we use to estimate water mass ages and concentrations of man-made CO2 (C-ant). It was found that the largest uncertainties lie in the assumption of constant mixing behavior, constant full saturation of the transient tracers we use, and constant disequilibrium in CO2 between air and sea water. These assumptions together give a total uncertainty of 16% on the total amount C-ant taken up by the ocean. The analysis shows that this can be reduced to 9% if the assumptions are adjusted to more realistic values. The model gave a mixing behaviour in the Nordic Seas and Arctic Ocean that is somewhat lower than what is frequently used. We also conducted a thorough analysis of mixing behaviour in the Arctic Ocean based on observational data. We found regional differences, but also that the mixing generally is different at different depths. Focusing on year 2002 in the Nordic Seas and 2005 in the Arctic Ocean we evaluated remineralisation rates and connected carbon fluxes in the region. It was a clear regional difference among the different basins, where the highest fluxes were found in the Greenland Sea and the Lofoten Basin. These two basins show the greatest vertical mixing of all the basins in the Nordic Seas and Arctic Ocean, but have totally different driving processes. Seen from early 1980s to 2016 in the Greenland Sea it is a continues increase in the amount C-ant in the upper 2000 m, and that the magnitude of this amount is strongly linked to the ventilation depth. The '90s had a much weaker ventilation than during early '80s, but due to the atmospheric increase in C-ant the ocean uptake increased in the upper 1000 m. In early 2000s the ventilation strengthened, and reached much greater depths than during the '90s, and this generally continued throughout the 2000s. The largest increase in C-ant is observed deeper and deeper from early ?90s until 2009. In 2016 the largest increase is found at 1800-1900 m depth, indicating some ventilation of the upper deep waters. The other basins in the Nordic Seas have less data. In the Iceland Sea, with data until 2002, the clearest increase is seen from early '90s to 2002, for all depths. In the Norwegian Sea the evolution differ between the two basins. The Norwegian Basin show a clear increase in the upper 2000 m from 1982 to 1993, but only for the upper 1000 m from 1993 to 2002. The Lofoten Basin has a stronger connection to the Greenland Sea, and also show a similar evolution with the latter for the upper 1000 m from 1982 to 1993 and for the upper 2000 m between 1993 to 2002. For the Arctic Ocean the amount of C-ant has increased with 40% from the end of the '80s to 2015. However, the total increase between 2005 and 2015 is not considerable, which could indicate a variability in ventilation over time, but also regionally. A model simulation of the Nordic Seas for future low and high CO2 scenarios shows that the uptake of CO2 from the atmosphere increase steadily until year 2100, for both scenarios, while this uptake stabilises globally for the low scenario (i.e. for a reduced increase in total CO2 emissions). In the Nordic Seas, the uptake increase more than the amount stored in the region, especially for the low scenario, while globally there is a decrease for both scenarios. This indicate that the Nordic Seas is a region very sensitive to future changes in carbon emissions.

VENTILATE har bidratt til økt forståelse av - alder og endringer i ventilasjon i de Nordiske hav - intermediære vann i de Nordiske hav, noe som allerede blitt sitert et par ganger - usikkerhetene i den metode vi bruker for å estimere aldrer og menneskeskapt karbon - koblingen mellom innstrømmende Atlantervann og konveksjon i Grønlandshavet - endring i menneskeskapt karbon i de Nordiske hav og i den Arktiske Osean - remineralisering og koblede karbonflukser i de Nordiske og Arktiske havene - fremtida endringer i opptak og lagring av karbon i de Nordiske hav i ulike CO2-scenario Denne økte kunnskapen er et viktig bidrag til et brett fagområde relatert til hav og klima, noe som kan bidra til forbedringer av jordsystemmodeller, der spesielt ventilasjon og konveksjon ofte er dårlig beskrevet. Den nye kunnskapen av remineralisering fyller et kunnskapshull for de Nordiske hav, og er et viktig bidrag til alle som er opptatt av karbonsyklus og biologiske prosesser i de nordlige hav.

VENTILATE applies for funding for observational and model based multi-parameter studies of physical and biological processes of importance for the oceanic carbon fluxes in the Arctic Ocean and the Nordic Seas. The work is divided in three workpackages : WP1: Recent changes in the Nordic Seas and the Arctic Ocean ventilation WP2: Oceanic carbon fluxes inferred from tracers WP3: Present and future water mass evolution in the Nordic Seas The work will be interdisciplinary, utilizing observational data (tracer, biogeochemical, hydrographical) and model data outputs. We will utilize historical and recent data from large observational databases (CARINA, GLODAPv2), from recent cruises, and new data collected during the project. The full range of data used in the project, and management of these, are described in a separate data management attachment. The approaches in the project offer new results and significant increased understanding of the links between climate variability and variability in physical and biological processes relevant for the oceanic carbon cycle in the Nordic Seas and the Arctic Ocean.