Giannetta Fusco

A study of surface heat fluxes in the Ross Sea (Antarctica)

In the polar regions, dynamical and thermodynamical interactions between atmosphere and ocean are strongly influenced by the presence or absence of the ice cover, which forms an insulating layer over the ocean, hindering sensible heat fluxes and forming an effective barrier to evaporation and thus preventing latent heat loss. In the framework of the CLIMA (Climatic Long-term Interactions for the Mass-balance in Antarctica) project of the Italian PNRA (National Program for Antarctic Research) we focused our attention on the evaluation of the heat fluxes between the ocean and the atmosphere in the Ross Sea, where the ice covers the sea for many months of the year. Wherever the ice cover is absent all year round, such as in leads or polynyas, the air-sea fluxes can be very large, especially in winter when the air-sea temperature differences are strong. In this work heat exchanges between sea and atmosphere, whether ice cover was present or not, were calculated from climatological data obtained from the European Centre for Medium Range Weather Forecasts, while sea ice data were collected from the US National Ice Center and National Climatic Data Center. Each of the terms in the sea surface heat budget were computed for 1994 with a temporal resolution of six hours and a spatial resolution of 0.5° using bulk formulae and obtaining monthly averaged horizontal distributions. The surface heat budget is dominated in November, December, January and February by shortwave radiation, while for the other months the turbulent and conductive heat fluxes dominate the heat exchange between the atmosphere and the sea surface. The annual total heat loss at the surface in 1994 has been estimated at about −90 W m−2 with the highest heat loss occurring close to the coast; the maximum heat loss occurred in May (−217 W m−2) while in January the heat gain by the ocean was 196 W m−2. In addition, weekly averaged values over the whole Ross Sea from 1994 to 1997 were calculated with the same parameterisation in order to study the temporal variability in this basin of each individual component and of the total surface heat budget. For this purpose only the data inside the continental shelf of the Ross Sea were considered in calculating the averaged fluxes. The 1994–97 total heat budget ranges from −87 to −107 W m−2with an average of −96 W m−2; this amount of heat loss was supposed to be compensated for by the heat advected by the Circumpolar Deep Water and its transport was estimated at about 2.9 Sv.

Estimation of Sea-Ice Thickness in Ross and Weddell Seas from SSM/I Brightness Temperatures

In polar regions, ocean-atmosphere interactions are strongly influenced by sea ice and its thickness. Since satellite passive microwave observations became available in the 1970s, significant progress has been made in the study of snow depth and sea ice concentration and extent in these regions. Estimating sea-ice thickness (SIT), instead, turned out to be considerably more difficult. We present a new empirical algorithm to estimate SIT in the Ross and Weddell Seas from Special Sensor Microwave/Imager brightness temperatures. This algorithm combines brightness temperature polarization difference and ratio values to obtain SIT for seasonal ice up to a thickness of about 90 cm during freezing conditions. A series of filters accounts for open water, new ice, and snow on sea ice. Our SIT estimates are consistent with colocated visual ship-based SIT observations made according to the Antarctic Sea Ice Processes and Climate project, showing linear correlation values between 0.73 and 0.96 and root-mean-square-error values between 14 and 24 cm. The seasonal development of the region average SIT derived with our approach agrees with the corresponding values derived from U.S. National Ice Center ice charts. Comparison with colocated polynya distribution maps suggests that the algorithm could be optimized for its performance with regard to SIT values around 50 cm and that a closer investigation of the snow impact on the SIT retrieval is required.

The Antarctic Circumpolar Wave: Its Presence and Interdecadal Changes during the Last 142 Years

AbstractThe Southern Ocean (SO) is the region of the World Ocean bordering on Antarctica over which significant exchanges between the atmosphere, the ocean, and the sea ice take place. Here, the strong and nearly unhindered eastward flow of the Antarctic Circumpolar Current plays an important role in mean global climate as it transmits climate anomalies around the hemisphere. Features of interannual variability have been observed to propagate eastward around the SO with the circumpolar flow in the form of a system of coupled anomalies, known as the Antarctic circumpolar wave (ACW). In the present study, the 142-yr series of the Twentieth Century Reanalysis, version 2, dataset (850-hPa geopotential height, sea level pressure, sea surface temperature, surface meridional wind, and surface air temperature) spanning from 1871 to 2012 is used to investigate the presence and variability of ACWs. This examination shows, for the first time, the presence of the ACW before the mid-1950s and interdecadal changes in i...

A New Approach for Monitoring the Terra Nova Bay Polynya through MODIS Ice Surface Temperature Imagery and Its Validation during 2010 and 2011 Winter Seasons

Polynyas are dynamic stretches of open water surrounded by ice. They typically occur in remote regions of the Arctic and Antarctic, thus remote sensing is essential for monitoring their dynamics. On regional scales, daily passive microwave radiometers provide useful information about their extent because of their independence from cloud coverage and daylight; nonetheless, their coarse resolution often does not allow an accurate discrimination between sea ice and open water. Despite its sensitivity to the presence of clouds, thermal infrared (TIR) Moderate Resolution Imaging Spectroradiometer (MODIS) provides higher-resolution information (typically 1 km) at large swath widths, several times per day, proving to be useful for the retrieval of the size of polynyas. In this study, we deal with Aqua satellite MODIS observations of a frequently occurring coastal polynya in the Terra Nova Bay (TNB), Ross Sea (Antarctica). The potential of a new methodology for estimating the variability of this polynya through MODIS TIR during the 2010 and 2011 freezing season (April to October) is presented and discussed. The polynya is observed in more than 1600 radiance scenes, after a preliminary filter evaluates and discards cloudy and fog-contaminated scenes. This reduces the useful MODIS swaths to about 50% of the available acquisitions, but a revisit time of less than 24 h is kept for about 90% of the study period. As expected, results show a high interannual variability with an opening/closing fluctuation clearly depending on the regime of the katabatic winds recorded by the automatic weather stations Rita and Eneide along the TNB coast. Retrievals are also validated through a comparison with a set of 196 co-located high-resolution ENVISAT ASAR images. Although our estimations slightly underestimate the ASAR derived extents, a good agreement is found, the linear correlation reaching 0.75 and the average relative error being about 6%. Finally, a sensitivity test on the applied thermal thresholds supports the effectiveness of our setting.

Mesoscale variability related to iron speciation in a coastal Ross Sea area (Antarctica) during summer 2014

ABSTRACT Dissolved iron (Fe) distribution and speciation was determined in water samples (0–200 m) collected in a coastal area near Terra Nova Bay during the austral summer of 2014. Nutrients, dissolved oxygen, chlorophyll-a, phytoplankton composition and prokaryotic biomass distribution were investigated in combination with measurements of the physical properties of the water columns and its dynamics. The dFe value was above the limiting growth concentration, ranging from 0.52 to 4.51 nM, and it showed a spatial variability with a horizontal length scale of about 10 km, according to the variability of the water column physical properties and to iron sources. The organic ligands (L) maintained the concentrations of dFe at levels much higher than the inorganic solubility of Fe, keeping it available for phytoplankton and the log K’FeL values found (from 22.1 to 23.6) highlighted the presence of complexes of differing stabilities.