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Despite their harmful effects in the upper atmosphere, anthropogenic chlorofluorocarbons dissolved in seawater are extremely useful for studying ocean circulation and ventilation, particularly in remote locations. Because they behave as a passive tracer in seawater, and their atmospheric concentrations are well-mixed, well-known, and have changed over time, they are ideal for gaining insight into the oceanographic characteristics of the isolated cavities found under Antarctic ice shelves, where direct observations are difficult to obtain. Here we present results from a modeling study of air–sea chlorofluorocarbon exchange and ocean circulation in the Ross Sea, Antarctica. We compare our model estimates of oceanic CFC-12 concentrations along an ice shelf edge transect to field data collected during three cruises spanning 16 yr. Our model produces chlorofluorocarbon concentrations that are quite similar to those measured in the field, both in magnitude and distribution, showing high values near the surface, decreasing with depth, and increasing over time. After validating modeled circulation and air–sea gas exchange through comparison of modeled temperature, salinity, and chlorofluorocarbons with field data, we estimate that the residence time of water in the Ross Ice Shelf cavity is approximately 2.2 yr and that basal melt rates for the ice shelf average 10 cm yr−1. The model predicts a seasonal signature to basal melting, with highest melt rates in the spring and also the fall.

[1] Three decades of satellite observations collected during spring and early summer have shown a recurring region of ice-free water forming in the sea-ice cover of the Ross Sea, Antarctica. This Ross Sea summer polynya plays an important role in heat exchange between the ocean and atmosphere, ventilation of deep water, and is characterized by high biological productivity. Despite its appearance each year, the relative importance of different physical processes to its formation and maintenance are not widely agreed upon. Here we use a three-dimensional coupled ice/ocean model to better understand processes controlling the dynamics of the Ross Sea polynya. Results from the model control run agree favorably with satellite microwave imagery of sea ice. Model sensitivity studies suggest that polynya dynamics are insensitive to the amount of snowfall, the presence of the Ross Ice Shelf cavity, tides, and solar radiation penetrating the ice. The model results also corroborate earlier findings that both the advection of sea ice and heat entrainment from warm Modified Circumpolar Deep Water play a role in Ross Sea polynya development. More importantly, the model further demonstrates that advection of sea ice due to wind stress plays the primary role in summer polynya formation. Additionally, we suggest that (1) heat entrainment reduces the rate of sea ice formation rather than melts existing sea ice, and (2) advection of sea ice due to synoptic wind events associated with variations in atmospheric pressure are the processes primarily responsible for the formation and expansion of the Ross Sea summer polynya.

International audience; The third primary production algorithm round robin (PPARR3) compares output from 24 models that estimate depth-integrated primary production from satellite measurements of ocean color, as well as seven general circulation models (GCMs) coupled with ecosystem or biogeochemical models. Here we compare the global primary production fields corresponding to eight months of 1998 and 1999 as estimated from common input fields of photosynthetically-available radiation (PAR), sea-surface temperature (SST), mixed-layer depth, and chlorophyll concentration. We also quantify the sensitivity of the ocean-color-based models to perturbations in their input variables. The pair-wise correlation between ocean-color models was used to cluster them into groups or related output, which reflect the regions and environmental conditions under which they respond differently. The groups do not follow model complexity with regards to wavelength or depth dependence, though they are related to the manner in which temperature is used to parameterize photosynthesis. Global average PP varies by a factor of two between models. The models diverged the most for the Southern Ocean, SST under 10 degrees C, and chlorophyll concentration exceeding 1 mg Chlm(-3). Based on the conditions under which the model results diverge most, we conclude that current ocean-color-based models are challenged by high-nutrient low-chlorophyll conditions, and extreme temperatures or chlorophyll concentrations. The GCM-based models predict comparable primary production to those based on ocean color: they estimate higher values in the Southern Ocean, at low SST, and in the equatorial band, while they estimate lower values in eutrophic regions (probably because the area of high chlorophyll concentrations is smaller in the GCMs). Further progress in primary production modeling requires improved understanding of the effect of temperature on photosynthesis and better parameterization of the maximum photosynthetic rate. (c) 2006 Elsevier Ltd. All rights reserved.

[1] An intense phytoplankton bloom forms over the continental shelf in the Ross Sea, Antarctica, every spring as the sea ice retreats during the expansion of the Ross Sea polynya. The extent of this bloom rarely covers the full area of the shelf for reasons that are not well understood. One clue may be found in satellite imagery spanning the last 25 years that shows frequently recurring, previously undescribed, large-scale plumes of high-chlorophyll water and jets of low-chlorophyll water demarking the northern boundary of the spring phytoplankton bloom. The plumes, with horizontal length scales of 100–200 km, appear at the same location each year. The spatial consistency of this pattern of jets and plumes suggests regulation by forcing that does not vary from year to year, such as the interaction between bathymetry and prevailing ocean currents. Here we present a modeling study indicating that this pattern in the spring phytoplankton bloom is caused by intrusions of offshore surface waters onto the continental shelf, guided by relatively deep polar shelf bathymetry. Our study suggests that currents from the Ross Sea gyre follow along the shelf break and prevent the phytoplankton bloom from extending farther toward the edge of the shelf, even when the waters there are ice-free, perhaps because the offshore waters have low iron concentrations. These results illustrate how phytoplankton distributions can be constrained by the interaction of deep bathymetry and surface geostrophic currents along the highly productive Antarctic continental shelf.