Impact of coastal polynyas on dense shelf water formation in the Weddell Sea

Dense shelf water is an essential ingredient to the formation of Antarctic Bottom Water (AABW). It is formed on the continental shelves surrounding Antarctica, when freezing rates are sufficiently high to push ocean salinity to values of 34.65 and higher. Coastal polynyas, where the ice is driven away from the coastline, maintain the highest freezing rates in Antarctic winter. Since theWeddell Sea is considered the most productive source region of AABW, we investigate the dense water formation on the continental shelves of the southwestern Weddell Sea, with a focus on the role of coastal polynyas, using the Finite Element Sea ice-Ocean Model (FESOM), a primitive-equation, hydrostatic ocean model coupled with a dynamic-thermodynamic sea ice model. The horizontal resolution of the global, unstructured mesh is up to 3 km at the southwestern Weddell Sea coastline; in vertical direction the mesh features 37 depth levels (resolution increases toward the surface). The model was initialized on 01/01/1980 with data from the Polar Hydrographic Climatology and forced with NCEP/NCAR Reanalysis data. The 20-year period 1990-2009 is used for analysis. Our results indicate that in an average year, the polynya freezing rates of 9 cm d--1 (corresponding to a salt input of 2.5 kg m--2d--1) cause a seasonal variation in salinity of 0.3 psu under the Ronne polynya and result in the production of 5.10-4 km-3 dense shelf water, which leaves the continental shelf (outlined by the 700 m isobath in this study) at a long-term mean volume flux of 5.2 Sv. Some of this water contributes to the formation of Weddell Sea Deep/BottomWater, but a large fraction is diluted by mixing with ambient water and leaves the Weddell Sea at intermediate levels

On the drivers of regime shifts in the Antarctic marginal seas, exemplified by the Weddell Sea

Recent studies have found evidence for a potential future tipping point, when the density of Antarctic continental shelf waters, specifically in the southern Weddell Sea, will allow for the onshore flow of warm waters of open ocean origin. A cold-to-warm regime shift in the adjacent ice shelf cavities entails a strong enhancement of ice shelf basal melt rates and could trigger instabilities in the ice sheet. From a suite of numerical experiments, aimed to force such a regime shift on the continental shelf, we identified the density balance between the shelf waters formed by sea ice production and the warmer water at the shelf break as the defining element of a tipping into a warm state. In our experiments, this process is reversible but there is evidence for hysteresis behaviour. Using HadCM3 20th-century output as atmospheric forcing, the resulting state of the Filchner-Ronne cavity depends on the initial state. In contrast, ERA Interim forcing pushes even a warm-initialized cavity into a cold state, i.e. it pushes the system back across the reversal threshold to the cold side. However, it turns out that for forcing data perturbations of a realistic magnitude, a unique and universal recipe for triggering a regime shift in Antarctic marginal seas was not found; instead, various ocean states can lead to an intrusion of off-shelf waters onto the continental shelf and into the cavities. Whether or not any given forcing or perturbation yields a density imbalance and thus allows for the inflow of warm water depends on the complex interplay between bottom topography, mean ocean state, sea ice processes, and atmospheric conditions. Copyright

Simulation of coastal polynyas in the western Weddell Sea

Coastal polynyas play a prominent role in the formation and modification of water masses in the polar oceans. A coastal polynya is usually kept open mechanically, primarily by winds, and the ocean surface is at freezing point. Thus a major fraction of the annual ice production of the high-latitude oceans occurs in polynyas and hence the duration and extent of their appearance has a substantial effect on bottom water formation. In the western Weddell Sea, recurring coastal polynyas are formed in front of the Filchner-Ronne Ice Shelf and in the area of the decayed Larsen A/B Ice Shelf. Simulations to study polynya formation and their impact on ice production and bottom water formation in the western Weddell Sea were performed with the Finite Element Sea ice-Ocean Model (FESOM) of Alfred-Wegener-Institute (AWI). FESOM is a fully coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. The simulations were conducted on a global grid with a resolution varying between roughly 300 km in tropical latitudes and <5 km along the coast of the southwestern Weddell Sea. In vertical direction, the grid uses terrain-following coordinates. The model results give insight into the mechanisms governing the formation of transient and persistent polynyas and their influence on ice production and deep water formation. Water mass formation and ice export rates are quantified and compared to observation-based estimates

Sea ice kinematics around the Ronne polynia derived from satellite images and model simulations – first results of a comparison

A coastal polynia occurs when sea ice is moved away from the coast by strong offshore winds. Polynias are of great importance for the ice production within the region as well as for the heat balance and the deep bottom water formation. To estimate the ice production and export from the polynia region, it is necessary to know the motion of the sea ice. Two time series of satellite images from the Ronne polynia in the Weddell Sea, Antarctica, and drift vector fields calculated from these time series were analysed. The drift fields were compared with high resolution model results from the Finite Element Sea Ice Ocean Model (FESOM).Differences between the model and observation data will be shown and possible reasons will be discussed. The objective of the project in cooperation with the developers of FESOM is to identify improvements of the model including parametrizations and input parameters

Dense shelf water formation at coastal polynyas in the Weddell Sea

For any climate signal to leave an imprint on the Antarctic Bottom Water (AABW) that fills the World Ocean abyss, it has to pass through the process of bottom water formation in the marginal seas of the Southern Ocean. An indispensable component of AABW is the dense shelf waters created on the continental shelves around Antarctica, particularly in the Ross and Weddell Seas. At coastal polynyas we find strong atmospheric cooling and high freezing rates that lead to a strong salinification of the water column. Here the bulk of High Salinity Shelf Water (HSSW) is formed. The impact of coastal polynyas on ice production and water mass formation in the southwestern Weddell Sea was studied employing the Finite Element Sea ice-Ocean Model (FESOM) of Alfred Wegener Institute, Bremerhaven. FESOM is a coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. Simulations were conducted on a global unstructured mesh with a strong focus on the southwestern Weddell Sea coastline (up to 3 km resolution). The model runs were initialised in 1980 and forced with NCEP reanalysis data (daily resolution). For 2008 also higher-resolution GME data and results from the regional COSMO atmosphere model of University Trier were applied as atmospheric forcing data. The period 1990-2009 is used for data analysis. Our simulations indicate that mean winter sea ice production within the coastal polynyas exceeds the surrounding area’s ice production by a factor of 7, giving a polynya contribution to total sea ice formation of 3 %. This small percentage is due to their even smaller areal percentage (0.4 %), and also the existence of leads and small polynyas in the ‘ice-covered’ ocean. The latter contribute substantially to sea ice production, but not to bottom water formation since they are transient elements that open, move and close dependent on the ice drift, whereas coastal polynyas are fixed in space and often open for days, enabling the salinification necessary for HSSW formation. From our simulations we derive a mean HSSW-formation of 4.2∙10^5 km^3/winter, but only 0.5 Sv thereof are exported over the shelf break, the rest stays on the shelf and is warmed and diluted during the following summer. The WSBW formation rate for the southwestern Weddell Sea continental shelf in our simulation is about 6.3∙10^4 km^3/yr (2 Sv), which is on the low side but still reasonable compared to independent estimates

Coastal polynyas in the southwestern Weddell Sea: Surface fluxes, sea ice production and water mass modification

Coastal polynyas in the southwestern Weddell Sea were simulated using the Finite Element Sea Ice-Ocean Model forced with NCEP/NCAR Reanalysis data. The period 1990-2009 was used in analysis. Also, shorter model runs forced with GME and COSMO model data were branched off and compared. Depending on the region, the 20-year mean winter heat flux to the atmosphere is 310-510 W/m^2, whereof 50-60 W/m^2 are supplied by cooling the ocean and the remainder induces a sea ice production of 7-13 cm/(d m^2). The coastal polynyas (0.6% of the area) contribute 11% to the southwestern Weddell Sea sea ice production. In the mean 2 Sv High Salinity Shelf Water are exported from the continental shelves. Different forcing data sets can cause substantial disparities in the regional results

Küstenpolynjas im südwestlichen Weddellmeer: Oberflächenflüsse, Meereisbildung und Wassermassenmodifikation

Coastal polynyas in the southwestern Weddell Sea were simulated using the Finite Element Sea Ice-Ocean Model forced with NCEP/NCAR Reanalysis data. The period 1990-2009 was used in analysis. Also, shorter model runs forced with GME and COSMO model data were branched off and compared. Depending on the region, the 20-year mean winter heat flux to the atmosphere is 310-510 W/m^2, whereof 50-60 W/m^2 are supplied by cooling the ocean and the remainder induces a sea ice production of 7-13 cm/(d m^2). The coastal polynyas (0.6% of the area) contribute 11% to the southwestern Weddell Sea sea ice production. In the mean 2 Sv High Salinity Shelf Water are exported from the continental shelves. Different forcing data sets can cause substantial disparities in the regional results

Coastal polynyas in the western Weddell Sea

Coastal polynyas play a prominent role in the formation and modification of water masses in the polar oceans. A coastal polynya is usually kept open mechanically, primarily by winds, and the ocean surface is at freezing point. Thus a major fraction of the annual ice production of the high-latitude oceans occurs in polynyas and hence the duration and extent of their appearance has a substantial effect on bottom water formation. In the western Weddell Sea, recurring coastal polynyas are formed in front of the Filchner-Ronne Ice Shelf and in the area of the decayed Larsen A/B Ice Shelf. Simulations to study polynya formation and their impact on ice production and bottom water formation in the western Weddell Sea were performed with the Finite Element Sea ice-Ocean Model (FESOM) of Alfred-Wegener-Institute (AWI). FESOM is a fully coupled system of a primitive-equation, hydrostatic ocean model and a dynamic-thermodynamic sea ice model. The simulations were conducted on a global grid with a resolution varying between roughly 300 km in tropical latitudes and <5 km along the coast of the southwestern Weddell Sea. In vertical direction, the grid uses terrain-following coordinates. The model results give insight into the mechanisms governing the formation of transient and persistent polynyas and their influence on ice production and deep water formation. Water mass formation and ice export rates are quantified and compared to observation-based estimates

Simulated heat flux and sea ice production at coastal polynyas in the southwestern Weddell Sea

Coastal polynyas are areas in an ice-covered ocean where the ice cover is exported, mostly by off-shore winds. The resulting reduction of sea ice enables an enhanced ocean-atmosphere heat transfer. Once the water temperatures are at the freezing point, further heat loss induces sea ice production. The heat exchange and ice production in coastal polynyas in the southwestern Weddell Sea is addressed using the Finite-Element Sea-ice Ocean Model, a primitive-equation, hydrostatic ocean circulation model coupled with a dynamic-thermodynamic sea-ice model, which allows to quantify the amount of heat associated with cooling of the water column. Three important polynya regions are identified: at Brunt Ice Shelf, at Ronne Ice Shelf and along the southern part of the Antarctic Peninsula. Multiyear winter means (May–September 1990–2009) give an upward heat flux to the atmosphere of 311 W/m^2 in the Brunt polynyas, 511 W/m^2 in Ronne Polynya and 364 W/m^2 in the Antarctic Peninsula polynyas, whereof 57 W/m^2, 49 W/m^2 and 48 W/m^2, respectively, are supplied as oceanic heat flux from deeper layers. The mean winter sea ice production is 7.2 cm/d in the Brunt polynyas corresponding to an ice volume of 1.3x10^10 m^3/winter, 13.2 cm/d at Ronne polynya (4.4x10^10 m^3/winter), and 9.2 cm/d in the Antarctic Peninsula polynyas (2.1x10^10 m^3/winter). The heat flux to the atmosphere inside polynyas is 7 to 9 times higher than the heat flux in the adjacent area; polynya ice production per unit area exceeds adjacent values by a factor of 9 to 14