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

Abstract

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 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 CO 2 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. Plain Language SummaryThe major flow to and from the Arctic Ocean occurs across the Greenland-Scotland Ridge. The inflow is mostly warm Atlantic Water (AW) flowing northwards and cooling gradually. After completing different loops within the Arctic Ocean, portions of this water eventually flows south as cold freshened polar water at the surface and cold, dense overflow water at depth. We review and synthesize how the AW cooling evolved over the last century in relation to the Arctic sea ice cover. In the mean 60% of the heat loss occurred in the Nordic Seas, 30% in the Barents Sea, and only 10% in the Arctic seas further north. Arctic sea ice decrease the last century created more open water and permitted stronger ocean heat loss. The ocean volume and heat transport also increased, consistently with increased heat loss, and increased wind forcing. Ocean temperatures have generally increased in many areas during the last 50 years, and on Greenland this drove the retreat of marine-terminating glaciers. Variability in ocean heat loss to the atmosphere was primarily driven by Cold Air Outbreaks and cyclones in the Nordic and Barents Seas, and explain variability in Arctic Ocean CO 2 uptake, being ∼8% of the global uptake.

Separating Annual, Interannual and Regional Change in Sea Surface Temperature in the Northeastern Atlantic and Nordic Seas

“…These findings are aligned with a suite of recent studies on the heat budget of the Nordic Seas recently reviewed by Smedsrud et al. (2022) and provide a separation of the response in SST into independent underlying factors. Increases in AW inflow in part contradict evidence for decreases in Atlantic meridional overturning circulation (AMOC) (e.g., Smeed et al., 2018), which are based on deepwater moorings further south.…”

“…Strong seasonal variability is also evident in the annual build‐up and melt of sea‐ice. Summertime sea‐ice melt along the east Greenland shelf absorbs considerable amounts of latent heat, and releases cold, fresh waters initially lowering summer SST in the western part of the Nordic Seas (Smedsrud et al., 2022). This is reflected in the high frequency signal captured in EOF3 (Figure 6).…”

Observed Changes in the Arctic Freshwater Outflow in Fram Strait

“…The “Atlantification” in our observations is demonstrated by the shoaling of the AW and the westward shift of the Polar‐Atlantic front (Figures 6g and 6h). In general, the “Atlantification” of the Arctic Ocean, which has been occurring over the last century, denotes a larger presence of AW in the water column, and an increase in heat transport toward the Arctic Ocean (Smedsrud et al., 2021; Steele & Boyd, 1998). At present, it is not clear if the “Atlantification” of the western Fram Strait described here is due to increased AW recirculation north of the mooring array or due to advection from the Eurasian basin.…”

Subinertial Variability in Four Southeast Greenland Fjords in Realistic Numerical Simulations

“…All three model simulations reproduce the water masses found in the observational casts well, and the pycnocline is located at the right depth. The IGP run diverges somewhat from the ASR and ERA runs; this is likely due to interannual to decadal variability, linked to for example, variability in the warm water inflow to the Nordic Seas (Østerhus et al., 2019), variability in heat loss over the Nordic Seas (Smedsrud et al., 2022), and variability in the amount of retroflection of the West Spitsbergen Current in Fram Strait (Hattermann et al., 2016) which redirects some of the warm and saline water into the East Greenland Current. The salinity profile in the IGP run is an excellent match to the observations, while the ASR and ERA runs have a fresh bias between 50 and 250 m depth; this too can be due to interannual variability.…”