Eastern Arctic Ocean Diapycnal Heat Fluxes through Large Double-Diffusive Steps

The diffusive layering (DL) form of double-diffusive convection cools the Atlantic Water (AW) as it circulates around the Arctic Ocean. Large DL steps, with heights of homogeneous layers often greater than 10 m, have been found above the AW core in the Eurasian Basin (EB) of the eastern Arctic. Within these DL staircases, heat and salt fluxes are determined by the mechanisms for vertical transport through the high-gradient regions (HGRs) between the homogeneous layers. These HGRs can be thick (up to 5 m and more) and are frequently complex, being composed of multiple small steps or continuous stratification. Microstructure data collected in the EB in 2007 and 2008 are used to estimate heat fluxes through large steps in three ways: using the measured dissipation rate in the large homogeneous layers; utilizing empirical flux laws based on the density ratio and temperature step across HGRs after scaling to account for the presence of multiple small DL interfaces within each HGR; and averaging estimates of heat fluxes computed separately for individual small interfaces (as laminar conductive fluxes), small convective layers (via dissipation rates within small DL layers), and turbulent patches (using dissipation rate and buoyancy) within each HGR. Diapycnal heat fluxes through HGRs evaluated by each method agree with each other and range from ~2 to ~8 W m−2, with an average flux of ~3–4 W m−2. These large fluxes confirm a critical role for the DL instability in cooling and thickening the AW layer as it circulates around the eastern Arctic Ocean

Ocean stratification and sea-ice cover in Barents and Kara seas modulate sea-air methane flux: satellite data

The diverse range of mechanisms driving the Arctic amplification and global climate are not completely understood and, in particular, the role of the greenhouse gas methane (CH4) in the Arctic warming remains unclear. Strong sources of methane at the ocean seabed in the Barents Sea and other polar regions are well documented. Nevertheless, some of those publications suggest that negligible amounts of methane fluxed from the seabed enter the atmosphere, with roughly 90% of the methane consumed by bacteria. Most in situ observations are taken during summer, which is favorable for collecting data but also characterized by a stratified water column. We present perennial observations of three Thermal IR space-borne spectrometers in the Arctic between 2002 and 2020. According to estimates derived from the data synthesis ECCO (Estimating the Circulation and Climate of the Ocean), in the ice-free Barents Sea the stratification in winter weakens after the summer strong stability. The convection, storms, and turbulent diffusion mix the full-depth water column. CH4 excess over a control area in North Atlantic, measured by three sounders, and the oceanic Mixed Layer Depth (MLD) both maximize in winter. A significant seasonal increase of sea-air exchange in ice-free seas is assumed. The amplitude of the seasonal methane cycle for the Kara Sea significantly increased since the beginning of the century. This may be explained by a decline of ice concentration there. The annual CH4 emission from the Arctic seas is estimated as 2/3 of land emission. The Barents/Kara seas contribute between 1/3 and 1/2 into the Arctic seas annual emission

Intensification of Near-Surface Currents and Shear in the Eastern Arctic Ocean:A More Dynamic Eastern Arctic Ocean

A 15-year (2004–2018) record of mooring observations from the upper 50 m of the ocean in the eastern Eurasian Basin reveals increased current speeds and vertical shear, associated with an increasing coupling between wind, ice, and the upper ocean over 2004–2018, particularly in summer. Substantial increases in current speeds and shears in the upper 50 m are dominated by a two times amplification of currents in the semidiurnal band, which includes tides and wind-forced near-inertial oscillations. For the first time the strengthened upper ocean currents and shear are observed to coincide with weakening stratification. This coupling links the Atlantic Water heat to the sea ice, a consequence of which would be reducing regional sea ice volume. These results point to a new positive feedback mechanism in which reduced sea ice extent facilitates more energetic inertial oscillations and associated upper-ocean shear, thus leading to enhanced ventilation of the Atlantic Water

Fluctuating Atlantic inflows modulate Arctic atlantification

Enhanced warm, salty subarctic inflows drive high-latitude atlantification, which weakens oceanic stratification, amplifies heat fluxes, and reduces sea ice. In this work, we show that the atmospheric Arctic Dipole (AD) associated with anticyclonic winds over North America and cyclonic winds over Eurasia modulates inflows from the North Atlantic across the Nordic Seas. The alternating AD phases create a “switchgear mechanism.” From 2007 to 2021, this switchgear mechanism weakened northward inflows and enhanced sea-ice export across Fram Strait and increased inflows throughout the Barents Sea. By favoring stronger Arctic Ocean circulation, transferring freshwater into the Amerasian Basin, boosting stratification, and lowering oceanic heat fluxes there after 2007, AD+ contributed to slowing sea-ice loss. A transition to an AD− phase may accelerate the Arctic sea-ice decline, which would further change the Arctic climate system.acceptedVersio

Weakening of cold halocline layer exposes sea ice to oceanic heat in the eastern Arctic Ocean

A 15-yr duration record of mooring observations from the eastern (>70°E) Eurasian Basin (EB) of the Arctic Ocean is used to show and quantify the recently increased oceanic heat flux from intermediate-depth (~150–900 m) warm Atlantic Water (AW) to the surface mixed layer and sea ice. The upward release of AW heat is regulated by the stability of the overlying halocline, which we show has weakened substantially in recent years. Shoaling of the AW has also contributed, with observations in winter 2017–18 showing AW at only 80 m depth, just below the wintertime surface mixed layer, the shallowest in our mooring records. The weakening of the halocline for several months at this time implies that AW heat was linked to winter convection associated with brine rejection during sea ice formation. This resulted in a substantial increase of upward oceanic heat flux during the winter season, from an average of 3–4 W m−2 in 2007–08 to >10 W m−2 in 2016–18. This seasonal AW heat loss in the eastern EB is equivalent to a more than a twofold reduction of winter ice growth. These changes imply a positive feedback as reduced sea ice cover permits increased mixing, augmenting the summer-dominated ice-albedo feedback

On the seasonal cycles observed at the continental slope of the Eastern Eurasian Basin of the Arctic Ocean

The Eurasian Basin (EB) of the Arctic Ocean is subject to substantial seasonality. We here use data collected between 2013 and 2015 from six moorings across the continental slope in the eastern EB and identify three domains, each with its own unique seasonal cycle: 1) The upper ocean (<100 m), with seasonal temperature and salinity differences of Δθ = 0.16°C and ΔS = 0.17, is chiefly driven by the seasonal sea ice cycle. 2) The upper-slope domain is characterized by the influence of a hydrographic front that spans the water column around the ~750-m isobath. The domain features a strong temperature and moderate salinity seasonality (Δθ = 1.4°C; ΔS = 0.06), which is traceable down to ~600-m depth. Probable cause of this signal is a combination of along-slope advection of signals by the Arctic Circumpolar Boundary Current, local wind-driven upwelling, and a cross-slope shift of the front. 3) The lower-slope domain, located offshore of the front, with seasonality in temperature and salinity mainly confined to the halocline (Δθ = 0.83°C; ΔS = 0.11; ~100–200 m). This seasonal cycle can be explained by a vertical isopycnal displacement (ΔZ ~ 36 m), arguably as a baroclinic response to sea level changes. Available long-term oceanographic records indicate a recent amplification of the seasonal cycle within the halocline layer, possibly associated with the erosion of the halocline. This reduces the halocline’s ability to isolate the ocean surface layer and sea ice from the underlying Atlantic Water heat with direct implications for the evolution of Arctic sea ice cover and climate

Stability of the arctic halocline: a new indicator of arctic climate change

In this study, we propose a new Arctic climate change indicator based on the strength of the Arctic halocline, a porous barrier between the cold and fresh upper ocean and ice and the warm intermediate Atlantic Water of the Arctic Ocean. This indicator provides a measure of the vulnerability of sea ice to upward heat fluxes from the ocean interior, as well as the efficiency of mixing affecting carbon and nutrient exchanges. It utilizes the well-accepted calculation of available potential energy (APE), which integrates anomalies of potential density from the surface downwards through the surface mixed layer to the base of the halocline. Regional APE contrasts are striking and show a strengthening of stratification in the Amerasian Basin (AB) and an overall weakening in the Eurasian Basin (EB). In contrast, Arctic-wide time series of APE is not reflective of these inter-basin contrasts. The use of two time series of APE—AB and EB—as an indicator of Arctic Ocean climate change provides a powerful tool for detecting and monitoring transition of the Arctic Ocean towards a seasonally ice-free Arctic Ocean. This new, straightforward climate indicator can be used to inform both the scientific community and the broader public about changes occurring in the Arctic Ocean interior and their potential impacts on the state of the ice cover, the productivity of marine ecosystems and mid-latitude weather

Recent oceanic changes in the Arctic in the context of long-term observations

This synthesis study assesses recent changes of Arctic Ocean physical parameters using a unique collection of observations from the 2000s and places them in the context of long-term climate trends and variability. Our analysis demonstrates that the 2000s were an exceptional decade with extraordinary upper Arctic Ocean freshening and intermediate Atlantic Water warming. We note that the Arctic Ocean is characterized by large amplitude multi-decadal variability in addition to a long-term trend, making the link of observed changes to climate drivers problematic. However, the exceptional magnitude of recent high-latitude changes (not only oceanic, but also ice and atmospheric) strongly suggests that these recent changes signify a potentially irreversible shift of the Arctic Ocean to a new climate state. These changes have important implications for the Arctic Ocean's marine ecosystem, especially those components that are dependent on sea ice or that have temperature-dependent sensitivities or thresholds. Addressing these and other questions requires a carefully orchestrated combination of sustained multidisciplinary observations and advanced modeling

Structure and variability of the boundary current in the Eurasian Basin of the Arctic Ocean

The Arctic Circumpolar Boundary Current (ACBC) transports a vast amount of mass and heat around cyclonic gyres of the deep basins, acting as a narrow, topographically-controlled flow, confined to the continental margins. Current observations during 2002–2011 at seven moorings along the major Atlantic Water (AW) pathway, complemented by an extensive collection of measured temperatures and salinities as well as results of state-of-the-art numerical modeling, have been used to examine the spatial structure and temporal variability of the ACBC within the Eurasian Basin (EB). These observations and modeling results suggest a gradual, six-fold decrease of boundary current speed (from 24 to 4 cm/s) on the route between Fram Strait and the Lomonosov Ridge, accompanied by a transformation of the vertical flow structure from mainly barotropic in Fram Strait to baroclinic between the area north of Spitsbergen and the central Laptev Sea continental slope. The relative role of density-driven currents in maintaining AW circulation increases with the progression of the ACBC eastward from Fram Strait, so that baroclinic ACBC forcing dominates over the barotropic in the eastern EB. Mooring records have revealed that waters within the AW and the cold halocline layers circulate in roughly the same direction in the eastern EB. The seasonal signal, meanwhile, is the most powerful mode of variability in the EB, contributing up to ~70% of the total variability in currents (resolved by moorings records) within the eastern EB. Seasonal signal amplitudes for current speed and AW temperature both decrease with the eastward progression of AW flow from source regions, and demonstrate strong interannual modulation. In the 2000s, the state of the EB (e.g., circulation pattern, thermohaline conditions, and freshwater balance) experienced remarkable changes. Results showing anomalous circulation patterns for an extended period of 30 months in 2008–2010 for the eastern EB, and a two-core AW temperature structure that emerged in this region of the Arctic Ocean in the most recent decade, suggest a shift of the EB toward a new, more dynamic state. This also likely suggests that the EB interior will become more susceptible to future climate change. Evaluating properties of the ACBC, its temporal variability at time scales from a season to several years, and possible governing mechanisms, this study contributes to a better understanding of Arctic Ocean circulation