Iceberg melting substantially modifies oceanic heat flux towards a major Greenlandic tidewater glacier

Fjord dynamics influence oceanic heat flux to the Greenland ice sheet. Submarine iceberg melting releases large volumes of freshwater within Greenland’s fjords, yet its impact on fjord dynamics remains unclear. We modify an ocean model to simulate submarine iceberg melting in Sermilik Fjord, east Greenland. Here we find that submarine iceberg melting cools and freshens the fjord by up to ~5 °C and 0.7 psu in the upper 100-200 m. The release of freshwater from icebergs drives an overturning circulation, resulting in a ~10% increase in net up-fjord heat flux. In addition, we find that submarine iceberg melting accounts for over 95% of heat used for ice melt in Sermilik Fjord. Our results highlight the substantial impact that icebergs have on the dynamics of a major Greenlandic fjord, demonstrating the importance of including related processes in studies that seek to quantify interactions between the ice sheet and the ocean.

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.

Roles of Sea Ice–Surface Wind Feedback in Maintaining the Glacial Atlantic Meridional Overturning Circulation and Climate

During glacial periods, climate varies between two contrasting modes, the interstadials and stadials. These climate changes are often associated with drastic reorganizations of the Atlantic meridional overturning circulation (AMOC). Previous studies highlight the important role of sea ice in maintaining contrasting modes of the AMOC through its insulating effect on the oceanic heat flux and the buoyancy flux (sea ice–buoyancy flux feedback); however, the effect of feedback from the atmosphere remains unclear. Here, the effect of sea ice–surface wind interactions over the North Atlantic Ocean on the AMOC is explored. For this purpose, results from comprehensive atmosphere–ocean coupled general circulation models (AOGCMs) are analyzed. Then, sensitivity experiments are conducted with the atmospheric component of the AOGCM. Last, to explore the impact of modifications in surface winds induced by sea ice on the maintenance of the AMOC, partially coupled experiments are conducted with the AOGCMs. Experiments show that the expansion of sea ice associated with a weakening of the AMOC reduces surface winds by suppressing the oceanic heat flux and increasing the atmospheric static stability. This wind anomaly then causes a weakening of the wind-driven ocean salt transport to the northern North Atlantic and maintains the weak AMOC, therefore working as a positive feedback. It is shown that, together with the sea ice–buoyancy flux and sea ice–surface wind feedback, changes in sea ice and oceanic heat flux sustain the contrasting modes of the AMOC. These results may provide useful information for interpreting the differences in the self-sustained internal oscillations of the AMOC produced by recent AOGCMs.

Ice front blocking of ocean heat transport to an Antarctic ice shelf

<p>Shoreward oceanic heat flux in deep channels on the continental shelf typically far exceeds that required to match observed ice shelf melt rates, suggesting other critical controls.  IN the present study we study the depth-independent (barotropic) and the density-driven (baroclinic) components of the flow of warm ocean water towards an ice shelf. Using observations from the Getz Ice Shelf system as well as geophysical laboratory experiments on a rotating platform, it is shown that the dramatic step shape of the ice front blocks the barotropic component, and that only the baroclinic component, typically much smaller, can enter the sub-ice cavity.  A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf. Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates.</p>

Lessons learnt from the former bed of Thwaites Glacier: a new multibeam-bathymetric dataset

<p>The coastal bathymetry of Thwaites Glacier (TG) is poorly known yet nearshore sea-floor highs have the potential to act as pinning points for floating ice shelves, or to block warm water incursions to the grounding line. In contrast, deeper areas control warm water routing. Here, we present more than 2000 km<sup>2</sup> of new multibeam echo-sounder data (MBES) acquired offshore TG during the first cruise of the <em>International Thwaites Glacier Collaboration</em> (<em>ITGC</em>) project on the RV/IB <em>Nathaniel B. Palmer</em> (NBP19-02) in February-March 2019. Beyond TG, the bathymetry is dominated by a >1200 m deep, structurally-controlled trough and discontinuous ridge, on which the Eastern Ice Shelf is pinned. The geometry and composition of the ridge varies spatially with some sea-floor highs having distinctive flat-topped morphologies produced as their tops were planed-off by erosion at the base of the seaward-moving Thwaites Ice Shelf. In addition, submarine landform evidence indicates at least some unconsolidated sediment cover on the highs, as well as in the troughs that separate them. Knowing that this offshore area of ridges and troughs is a former bed for TG, we also used a novel spectral approach and existing ice-flow theory to investigate bed roughness and basal drag over the newly-revealed offshore topography. We show that the sea-floor bathymetry is a good analogue for extant bed areas of TG and that ice-sheet retreat over the sea-floor troughs and ridges would have been affected by high basal drag similar to that acting in the grounding zone today.</p><p>Comparisons of the new MBES data with existing regional compilations show that high-frequency (finer than 5 km) bathymetric variability beyond Antarctic ice shelves can only be resolved by observations such as MBES and that without these data calculations of the oceanic heat flux may be significantly underestimated. This work supports the findings of recent numerical ice-sheet and ocean modelling studies that recognise the need for accurate and high-resolution bathymetry to determine warm water routing to the grounding zone and, ultimately, for predicting glacier retreat behaviour.</p>

Ocean-Atmosphere positive feedback in the Barents Sea region with reanalyses data

<p>This study investigates the mechanism of positive feedback in the Barents Sea region, using the results of reanalyses from 1993 to 2014. Vertical heat fluxes, wind and pressure fields are obtained from OAFlux and ERA-Interim databases, the water temperature and currents from the ARMOR-3D database.</p><p>Oceanic heat transport was computed through three sections-at the entrance to the Barents Sea (BSO), in the southern part of the Norwegian sea and in the west of Spitsbergen. The results show that, during the study period, the oceanic heat flux through BSO was rapidly increasing, significantly faster than in the northwards heat transport in the Norwegian Sea. west of Spitsbergen, a negative linear trend was observed, indicating a redistribution of the increasing transport of the Atlantic Water into the Nordic Seas.</p><p>Based on reanalyses data, we show the tight relationship between the current velocities through the BSO and the change in the gradient of the zonal component of wind velocity. The variability of the atmospheric circulation and the variability of the convergence of atmospheric heat fluxes for the studied region was also assessed.</p><p>The results also show that, in winter, with increasing oceanic heat flux through the BSO, the turbulent heat fluxes in the southwestern part of the sea decreased, and the northern part of the sea and west of Novaya Zemlya increased. In the annual means, the increasing heat flux from the ocean to the atmosphere is due to a retreat of the ice edge and an increase in the ice-free area of the sea. The sea-surface atmospheric pressure also increased over the water area, with a maximum changes in the south-east of the sea.</p><p>For the years with the maximum oceanic winter heat fluxes into the Barents Sea, the atmospheric heat flux across the southern boundary increased, while it across the northern border weakened. The convergence of the atmospheric heat fluxes increased only at the sea surface (1000-975 hPa), whereas above (975-100 hPa) the convergence decreased, and the total atmospheric heat convergence varies out of phase with that of the ocean.</p><div> <div> <div> <p>This study was supported by the Russian Science Foun- dation, project no. 17-17-01151.</p> </div> </div> </div>

On the mechanism of a positive feedback in long-term variations of the convergence of oceanic and atmospheric heat fluxes, and the ice cover in the Barents sea

This paper presents a study the interannual variability of the convergence oceanic and atmospheric advective heat fluxes in the Barents Sea region for 19932014, using combined in situ, satellite and numerical model-based oceanic and atmospheric data-sets: ARMOR-3D and ERA-Interim. On inter-decadal scales, the leading role of convergence of the oceanic heat flux, and on interannual scale of atmospheric heat flux are demonstrated to play the leading role in variations of the sea-ice area of the Barents Sea. The inter-decadal and the interannual variations of the oceanic heat flux are found to be mainly shaped by variations of the current velocity. In the long-term tendencies the current velocity is responsible for about 70% of the increase in the oceanic heat flux, mainly due to a higher transport in the North Cape Current. Variations in transport of the North Cape current and of the Return current are governed by variations in the meridional gradients of the zonal wind speed, in turn, caused by the stronger oceanic heat transport into the Barents sea and by the consequent melting of the sea-ice. The in situ observations supports the effectiveness of the previously suggested positive feedback between variations in the oceanic heat flux into the Barents Sea, and changes of the sea-ice area and of the atmospheric circulation in the Barents Sea region on the decadal time scales.

Observations and modelling of first-year ice growth and simultaneous second-year ice ablation in the Prydz Bay, East Antarctica

ABSTRACT The seasonal cycle of fast ice thickness in Prydz Bay, East Antarctica, was observed between March and December 2012. In March, we observed a 0.16 m thickness gain of 0.22 m-thick first-year ice (FYI), while 1.16 m-thick second-year ice (SYI) nearby simultaneously ablated by 0.59 m. A 1-D thermodynamic sea-ice model was applied to identify the factors that led to the simultaneous growth of FYI and melt of SYI. The different evolutions were explained by the difference in the conductive heat flux between the FYI and SYI. As the FYI was thin, there was a large temperature gradient between the ice base and the colder ice surface. This generated an upward conductive heat flux, which was larger than the heat flux from the ocean to the ice base, yielding basal growth of ice. In the case of the thicker SYI the temperature gradient and, hence, the conductive heat flux were smaller, and not sufficient to balance the oceanic heat flux at the ice base, yielding basal ablation. Penetration of solar radiation affected the conductive heat flux in both cases, and the model results were sensitive to the initial ice temperature profile and the uncertainty of the oceanic heat flux.