PATHWAY - Pathways, processes, and impacts of poleward ocean heat transport

The overall aim of PATHWAY is to understand the formation and propagation of warm and cold conditions with the Gulf Stream extension toward the Arctic, how these variations in ocean temperature interact with the atmosphere, and, hence, affect climate over land and its predictability. Results from the project show that temperature changes in the Gulf Stream propagate slowly northwards - from the east coast of the USA to the west coast of Norway and toward the Arctic - spending up to ten years on the journey (Årthun et al. 2017; Langehaug et al. 2019). Our research shows that warmer or colder conditions in the Gulf Stream therefore can predict the temperature in the Norwegian Sea several years in advance. We furthermore find that observed changes in ocean temperature affect both air temperature and rainfall over western and northern Europe (Årthun et al. 2017; Årthun et al. 2018a), as well as the size of the commercially valuable Barents Sea cod stock (Årthun et al. 2018b). Predicting ocean temperature several years in advance thus provides predictability of climate over land. Observations show that warmer and colder conditions along the Gulf Stream occur approximately every 15 years. In Årthun et al. (2020) we use a new method to show that these changes in ocean temperature originate from an interplay between ocean and atmosphere. The mechanism identified in Årthun et al. (2020) can also explain the reasons for the recent observed cooling in the subpolar North Atlantic. In order to better understand the mechanisms underlying ocean temperature anomalies in the Norwegian and Barents seas a heat budget analysis has been performed, with a special emphasis on quantifying the relative importance of oceanic and atmospheric heat fluxes (Asbjørnsen et al. 2019; 2020). Results show that heat advection by ocean circulation is the dominant driver along the Atlantic water pathway. Anomalous ocean heat transport furthermore depends on the strength of the Atlantic water inflow, which is related to large-scale circulation changes in the subpolar North Atlantic. Increased transport of warm, moist air has also contributed to a warming of waters flowing through the Barents Sea (Skagseth et al. 2020). This warming could have implications for the downstream Arctic Ocean, hence, for global ocean circulation. As ocean heat transport across the Greenland-Scotland ridge is a key component of climate variability and predictability in the Norwegian Sea, Heuzé and Årthun (2019) assessed the representation of the Atlantic inflow to the Nordic Seas in current generation climate models (CMIP5). We find that the mean poleward heat transport and its interannual variability are inconsistently represented across these models. The main source of error is the bathymetry of the Greenland-Scotland ridge, which is related to the model's horizontal resolution. We also know that the amount of heat flowing from the Atlantic Ocean to the Arctic strongly affects the sea ice cover in the Barents Sea, and that the winter ice cover has retreated over the past decades. By analyzing sea ice observations from the Barents Sea back to 1850, we find that the winter sea ice cover has never been smaller, and that the reduction has never happened faster than now (Onarheim and Årthun 2017). The future fate of the Arctic sea ice cover depends on whether greenhouse gas emissions continue to increase. If emissions continue like today, large parts of the Arctic will become seasonally ice-free by the end of this century. The Barents Sea will be the first region to become ice-free the whole year. If we manage to reduce our emissions, we can limit the sea ice loss, however the summer sea ice will disappear regardless of future emissions (Årthun et al. 2021). Although Arctic sea ice is gradually disappearing as a response to global warming, internal variability in poleward ocean heat transport can lead to intermittent recoveries of the sea ice cover (Årthun et al. 2019). Such periods with increased sea ice cover and a more southern ice edge will have consequences for offshore and marine industry in the Arctic and is therefore important to be able to predict. Our results show that ocean heat transport remains a good predictor of the Barents Sea ice cover also in the future. We therefore expect interannual to decadal sea ice changes to be predictable also in the future. Changes in Arctic sea ice and ocean temperature also influence climate in lower latitudes. Ocean temperature in the Barents Sea can therefore potentially be used for predictions of European climate (Kolstad and Årthun 2018). We find that the link between ocean temperature in the Barents Sea and European temperature is strongest when ocean temperature in autumn is used to predict land temperatures in winter. However, this link has not been robust throughout the last century and needs to be better understood.

Resultater fra prosjektet viser at vi kan forutsi deler av klimasystemet flere år på forskudd basert på endringer i havet. Denne kunnskapen er av interesse for alle som jobber med klimavarsling, det vil si arbeidet med å utvikle varsler for, for eksempel, temperatur og nedbør opp mot 10 år frem i tid. Muligheten for å varsle havtemperatur kan også brukes til å forutsi endringer i enkelte fiskearter. Slike varsel er av stor interesse for både næringsliv og offentlig sektor, og resultater fra prosjektet er brukt som direkte motivasjon til flere søknader om klimavarsling. For prosjektets deltagere har prosjektet ført til økt kompetanse, ledererfaring, og opprettelse av nye samarbeid med nasjonale og internasjonale forskere. Resultater fra prosjektet har ført til økt synlighet av prosjektleder, både nasjonalt og internasjonalt, og muligheter for å bidra til nye forskningsprosjekt.

An apparent northward progression of ocean anomalies - heat and salt - from the subpolar North Atlantic toward the Arctic is a robust finding in observations and in ocean and climate models. From this progression there can be, or at least there is a clear potential for, practical and useful predictions of the climate state of the North Atlantic Ocean, including fisheries, and of the adjacent continental climate and Arctic sea ice. There is, however, at present neither consensus nor any complete mechanistic understanding of the cause of thermohaline anomalies that travel the Arctic-Atlantic sector, nor for their eventual imprint on the atmosphere above. PATHWAY therefore aims to understand the formation and propagation of ocean heat anomalies with the Gulf Stream's extension toward the Arctic, its interaction with the atmosphere above, and eventual influence on continental climate variability. PATHWAY has a particular focus on ocean heat transport through the Nordic Seas with the Norwegian Atlantic Current (NwAC) and what can be resolved from the instrumental record to elucidate the relative roles of advection, lateral mixing and atmospheric fluxes for the ocean heat and freshwater budgets. NwAC and its extension into the Barents Sea offer maybe the most complete observational basis for a detailed assessment of such mechanisms, including some of the world's longest continuous current meter time series. PATHWAY will also assess to what extent and by which mechanisms heat and freshwater anomalies are communicated between the subpolar North Atlantic and Nordic Seas-Arctic. There is a growing scientific interest in climate prediction and in the potential important role played by the ocean, and especially by the northward ocean heat transport. PATHWAY results will thus be of great interest to the scientific community and benefit for society.