Trends in the intensity of dense water cascading from the Arctic shelves due to ice cover reduction in the Arctic seas

The article discusses the possible relationship between changes in the ice cover area of the shelf seas of the Arctic Ocean and the intensity of dense water cascading, based on calculation data obtained with the NEMO model for the period 1986–2010, with the findings issued at 5-day intervals and a spatial resolution of 1/10°. The cascading cases were calculated using an innovative method developed by the author. The work is based on the assumption that as the ice cover in the seas retreats, the formation of cooled dense water masses is intensified, which submerge and flow down the slope from the shelf to great depths. Thus, in the Arctic shelf seas, the mechanism of water densification due to cooling is added to the mechanism of water densification during ice formation, or, replaces it for certain regions. It was found that in the Barents Sea, the Laptev Sea and the Beaufort Sea, a decrease in the ice cover area causes an increase in the number of cases of cascading. However, in most of the Arctic seas, as the area of ice cover decreases, the number of cases of cascading also decreases. As a consequence, for the whole Arctic shelf area, the number of cases of cascading also decreases with decreasing ice cover. It is shown that in the Beaufort Sea the maximum number of cascading cases was observed in the winter period of 2007–2008, which was preceded by the summer minimum of the ice cover area in the Arctic Ocean. In the Barents Sea after 2000, a situation has been observed where the ice area has been decreasing to zero values, whereas the number of cascading cases has for some time (1 month approximately) remained close to high winter values. This possibly means that the cooling and densification of the waters in ice-free areas occurs due to thermal convection. Based on the calculation of the number of cases of cascading, it can be argued that the intensification of cascading due to a reduction in the ice cover is a feature of individual seas of the Arctic Ocean, those in which there is no excessive freshening of the upper water layer due to ice melting.

Impact of dense-water flow over a sloping bottom on open-sea circulation: laboratory experiments and an Ionian Sea (Mediterranean) example

The North Ionian Gyre (NIG) displays prominent inversions on decadal scales. We investigate the role of internal forcing induced by changes in the horizontal pressure gradient due to the varying density of Adriatic Deep Water (AdDW), which spreads into the deep layers of the northern Ionian Sea. In turn, the AdDW density fluctuates according to the circulation of the NIG through a feedback mechanism known as the bimodal oscillating system. We set up laboratory experiments with a two-layer ambient fluid in a circular rotating tank, where densities of 1000 and 1015 kg m−3 characterize the upper and lower layers, respectively. From the potential vorticity evolution during the dense-water outflow from a marginal sea, we analyze the response of the open-sea circulation to the along-slope dense-water flow. In addition, we show some features of the cyclonic and anticyclonic eddies that form in the upper layer over the slope area. We illustrate the outcome of the experiments of varying density and varying discharge rates associated with dense-water injection. When the density is high (1020 kg m−3) and the discharge is large, the kinetic energy of the mean flow is stronger than the eddy kinetic energy. Conversely, when the density is lower (1010 kg m−3) and the discharge is reduced, vortices are more energetic than the mean flow – that is, the eddy kinetic energy is larger than the kinetic energy of the mean flow. In general, over the slope, following the onset of dense-water injection, the cyclonic vorticity associated with current shear develops in the upper layer. The vorticity behaves in a two-layer fashion, thereby becoming anticyclonic in the lower layer of the slope area. Concurrently, over the deep flat-bottom portion of the basin, a large-scale anticyclonic gyre forms in the upper layer extending partly toward a sloping rim. The density record shows the rise of the pycnocline due to the dense-water sinking toward the flat-bottom portion of the tank. We show that the rate of increase in the anticyclonic potential vorticity is proportional to the rate of the rise of the interface, namely to the rate of decrease in the upper-layer thickness (i.e., the upper-layer squeezing). The comparison of laboratory experiments with the Ionian Sea is made for a situation when the sudden switch from cyclonic to anticyclonic basin-wide circulation took place following extremely dense Adriatic water overflow after the harsh winter in 2012. We show how similar the temporal evolution and the vertical structure are in both laboratory and oceanic conditions. The demonstrated similarity further supports the assertion that the wind-stress curl over the Ionian Sea is not of paramount importance in generating basin-wide circulation inversions compared with the internal forcing.

Navigating the challenges of explicitly including ocean-ice shelf interactions in a global ocean model using an adapted ISOMIP+ configuration as a fit-for-purpose tool

<p>Currently, none of the global 1° ocean-climate coupled models used for the Coupled Model Intercomparison Project (CMIP) explicitly simulate sub-ice shelf cavity circulation. This circulation plays a critical role in global ocean overturning as it transforms salty water formed at the surface in Antarctica into the parent waters of Antarctic Bottom Water (AABW). A challenge that the ocean-climate modelling community faces is the inclusion of these ocean-ice shelf interactions in global ocean 1° resolution models, so as to explicitly simulate dense water production and export. Choices regarding various numerical schemes and parameterizations need to be made, but in testing sensitivity to these choices and feedback effects of biases, large super-computing costs associated with running a global configuration are incurred. To address this we present an adapted configuration of the Ice Shelf-Ocean Model Intercomparison Project (ISOMIP), named ISOMIP+K, as the default idealised ISOMIP+ setup is not appropriate for modelling the deep, cold Antarctic cavities responsible for forming the dense parent waters of AABW. ISOMIP+K is currently adapted for the NEMO ocean model, motivated by the fact that this model is used for 6 of the climate groups participating in CMIP. We present results from ISOMIP+K configurations for Filchner-Ronne, Larsen-C and Ross ice shelves, which are important for dense water formation and large enough to be resolved, albeit coarsely, in a global 1° Earth System Model. This adapted ISOMIP+K test case, which is now far from idealized, is used to test the effect of initial conditions, the choice of values for lateral diffusion of momentum, mixing, drag coefficients and bathymetry on key indicators describing melt, sub-ice shelf circulation and dense water export. As opposed to regional high resolution Southern Ocean configurations, the ISOMIP+K configurations are designed so that the lessons learnt are directly transferable to a global ocean configuration where each choice made is backed-up by extensive, yet affordable, testing.</p>

Formation of dense water dome over the Central Bank under conditions of reduced ice cover in the Barents Sea

<p>On the basis of various data sets we traced formation of a ‘dome’- shaped density structure over the Central Bank - an important morphological element of the Barents Sea bottom topography. The major conclusion, which follows from our analysis, based on direct winter measurements in 2019, is that under reduced ice cover, transformation of thermohaline structure during the cold season principally differs from that under the ‘normal’ climate conditions in the 20th century. Transition from the stratified vertical structure (in summer) to the homogeneous one (in winter) is governed by thermal convection. Additional input of warm and salty water with inflowing AW is crucial to allow reaching the seabed vertical mixing before the temperature drops to the freezing point. Cascading of dense water from the bank commences as soon as convection has spread to the seabed. The influence of cascading on the Barents Sea hydrographic structure extends far away from the bank. In the absence of advective influx of salt and warm water vertical convection can also reach the seabed. However, under this condition formation of sea ice and haline convection is required. In this case water temperature in the homogeneous water column over the bank is close to the freezing point. Obtained results suggest that in the warmer climate the role of sea ice in winter transformation of thermohaline conditions over the bank is opposite to what it was in the ‘normal’ climate: imported sea ice blocks convection, thus making the water in the dense ‘dome’ warmer than it typically was throughout the 20th century.</p>

Role of the density structure and air-sea fluxes on subpolar transformation

<p>Convection in the North Atlantic Ocean is a key component of the global overturning circulation (MOC) as it produces dense water at high latitudes. Recent work has highlighted the dominant role of the Irminger and Iceland basins in the production of the North Atlantic deep waters. Dense water formation in these basins is mainly explained by buoyancy forcing that transforms surface waters to the deep waters of the MOC lower limb. Air-sea fluxes and the surface density field are both key determinants of the buoyancy-driven transformation. To better understand the connection between atmospheric forcing and the Atlantic overturning circulation, we analyze the contributions of the air-sea fluxes and of the density structure to the transformation of surface water over the eastern subpolar gyre. More precisely, we consider the densification of subpolar mode water (SPMW) in the Iceland Basin that ‘pre-conditions’ the dense water formation downstream. Analyses using 40 years of observations (1980–2019) reveal that variability in transformation is only weakly sensitive to changes in the heat and freshwater fluxes. Instead, changes in SPMW transformation are largely driven by the variance in the surface density structure, as expressed by the outcropping area for those isopycnals that define SPMW.This large influence of the surface density on the SPMW transformation partly explains the unusually large SPMW transformation in winter 2014–15 over the Iceland Basin.  </p>

Coastal Trapped Waves along the Southeast Greenland Coast in a realistic numerical simulation

<p>Ocean currents along the Southeast Greenland Coast play an important role in North Atlantic circulation and the global climate system. They carry dense water over the Denmark Strait sill, fresh water from the Arctic and the Greenland Ice Sheet into the subpolar ocean, and warm Atlantic water into Greenland’s fjords, where it can interact with outlet glaciers. Observational evidence from the OSNAP array and other mooring records shows that the circulation in this region displays substantial subinertial variability, typically with periods of several days. For the dense water flowing over the Denmark Strait sill, this variability augments the time-mean transport; on the shelf, the variability is large enough to occasionally reverse the mean transport direction of the coastal current, highlighting the importance of characterizing this variability when interpreting synoptic surveys. In this study, we used the output of a high-resolution realistic simulation to diagnose and characterize subinertial variability in sea surface height and velocity along the coast. The results show that the subinertial signals on the shelf and along the shelf break are coherent over hundreds of kilometers, and consistent with Coastal Trapped Waves in two subinertial frequency bands—at periods of 1–3 days and 5–18 days—portraying a combination of Mode I and higher modes waves. Furthermore, we find that northeasterly barrier winds may trigger the 5–18 day shelf waves, whereas the 1–3 day variability is linked to high wind speeds over Sermilik Deep.</p>