ANTarctic COastal POLynyas as observed with passive satellite sensors: Total area, ice production, brine release, and thin-ice thickness and area for the period 1992-2006

Motivation and Introduction
Polynyas are irregular shaped meso-scale areas of open water and/or thin ice within the pack ice in the Polar Regions. During winter they can be regarded as windows to the atmosphere because of the large ocean-atmosphere heat flux which can exceed the one through the adjacent thick ice by two orders of magnitude. Cooling of the water masses exposed to the cold atmosphere in a polynya causes water mass modification by densification, typically in association with new ice formation. Therefore polynyas are known to be an important source of heat for the atmosphere during winter, a location of water mass densification and thus production of the Antarctic Bottom Water (AABW), and play an important role for the sea-ice mass balance; some polynyas are called “ice factories”.

Their location in the remote Polar Regions, their relatively small size (a few ten kilometres), and the fact that their role for ocean-atmosphere energy exchange is most pronounced during winter makes them a difficult topic to study, particularly those along the Antarctic coast line. Consequently the majority of the known approaches to obtain and monitor location, size, and productivity (in terms of ice formation and salt release to the ocean) of polynyas in the Polar Regions suffers either from a relatively coarse spatial resolution of the used data (satellite remote sensing and numerical models) or from the restriction to a limited geographical region and time period.

Aims
The current study therefore aims at answering the following questions:

a) Where do coastal polynyas occur around Antarctica?
b) How large is the wintertime circum-Antarctic polynya area?
c) Did the wintertime polynya area change significantly within 1992-2008?
d) How large is the associated ice production?

Methodology
We used the Polynya Signature Simulation Method (PSSM) applied to resolution-enhanced satellite Special Sensor Microwave / Imager (SSM/I) measurements of the brightness temperature at frequencies of 37 GHz and 85.5 GHz, vertical and horizontal polarization, to obtain (sub-)daily maps of the polynya distribution with 5 km grid cell size around Antarctica for the winters (basically April to Sep.) 1992 to 2008. The PSSM has been developed by Markus and Burns (see: J. Geophys. Res., 100(C3), 4473-4487, 1995). It uses an iterative classification approach that combines the finer spatial resolution at 85 GHz with the smaller sensitivity to the direct atmospheric weather influence at 37 GHz. Three surface types are classified: thick ice, thin ice, and open water, of which the two latter are considered the polynya area. We modified the PSSM so that its output is less sensitive for weather-induced variations in the radiometric surface properties of thick ice. We used a temporal filter to discard shelf ice, icebergs and fast ice from the obtained polynya maps; this turned out to be necessary because of a potentially similar radiometric signature of fast ice and thin ice. Finally, we developed and used a resolution-enhanced and corrected (by the use of Advanced Very High Resolution Radiometer data) land-mask for Antarctica (Kern et al., 2007). We evaluated the output of the modified PSSM with independent data. We examined the average location of coastal polynyas around Antarctica, derived their average area and investigated their persistence for every winter 1992-2008 (Kern, 2009).

We also applied the PSSM to SSM/I data of the Kara Sea, calculated daily wintertime polynya maps of the Kara Sea for 1995-2004 and investigated the variability of the total wintertime Kara Sea polynya area in comparison to the Kara Sea ice export (Kern et al., 2005; Kern, 2008).

We used the ice thickness estimation approach of Martin et al. (see: J. Geophys. Res., 108, doi: 10.1029/2004JC002428, 2004), to obtain an idea of the ice thickness distribution with the Antarctic (here: Ross Sea) polynya area for a limited period of time.

Meteorological data provided in the framework of the ERA40 Re-Analysis project of the European Centre for Medium Range Weather Forecast (ECMWF) are used to calculate the net total heat flux at the ocean surface, using standard bulk formulae for the turbulent heat fluxes, and assuming a “blue” ocean to have a fixed surface temperature. With the aid of high-resolution ice-concentration data obtained with the ARTIST Sea Ice (ASI) algorithm applied to satellite Advanced Microwave Scanning Radiometer (AMSR) measurements of the brightness temperature at a frequency of 89 GHz, vertical and horizontal polarization (Spreen et al., 2008), an estimate of the fractional open water area inside the polynya area is obtained and used as a mask to calculate the total ice and salt production associated with the respective polynya using the ERA40 data based net heat flux values. This is done for winters (here: June to Aug.) 1992-2002 (limited by the length of the ERA40 data set). The main reason for this short seasonal period is the late freeze-up in particularly the Western Indian Ocean sector and in the Bellingshausen Sea.

We carried out an uncertainty estimation for both the polynya area and the associated ice production. The proposed method to obtain an estimate of the ice production allows an accuracy of about 30%. However, the use of ERA40 data might cause, in addition, an under-estimation of the obtained ice- and salt production values of a so far unknown amount, because in particular surface wind speeds are severely underestimated by ERA40 compared to in-situ observations at manned stations and Automatic Weather Stations (AWS). We therefore recommend to carefully interpret the obtained ice- and salt production (see Fig. 2), and to consider them as mean minimum values, particular for those polynyas which are known to be strongly influenced by catabatic winds, like the Terra Nova Bay polynya and basically all polynyas along the East Antarctic coast.

Results


Fig. 1: a) Overview over the regions of interest. b) Wintertime (June to Sep.) circum-Antarctic polynya area given as number of polynya days for 2008. c) to h) Time series of the total average wintertime polynya area of the six polynya regions indicated by colored boxes in b) for 1992-2008. Vertical bars denote plus/minus one standard deviation.

Table 1: Total mean wintertime (June to August) polynya area (km²) of every region together with the mean absolute (middle row) and relative (bottom row) error of the mean total polynya area. Note that sub-region Balleny Islands (region EPS) has been excluded because it is not a coastal polynya. Note also that sub-regions Rothera and Cape Byrd (region EBS) have been excluded because of the too large influence of the marginal ice zone.





Fig. 2: a) Average maximum number of polynya days, 1992-2008. b) Average area with a least 90 polynya days, 1992-2008. Bars (width) are scaled with the average wintertime polynya area of each sub-region (see Fig. 1) which is largest for the Ross Ice Shelf: 22.3±5.4 × 10³ km². Barplots: Cumulative June-Aug. ice- and salt-production estimates for the polynya area of selected sub-regions (frame color = box color in b) and Fig. 1 for orientation)

• The average wintertime (June-Aug.) total circum-Antarctic polynya area amounts about (235 000 ± 12 400) km²; the largest contribution to this area is made by polynyas along East Antarctica and the Ross Sea.

• The most frequent polynyas are the Mertz Glacier polynya, the Cape Darnley polynya and the Terra Nova Bay polynya; these occurred on average on at least 80% of all days during winter.

• The average winter-time polynya area of Antarctic coastal polynyas seems to have been constant during 1992-2008 in almost all investigated regions.

• The average (1992-2002) wintertime (June-Aug.) total circum-Antarctic ice production is about 860 km³ with an uncertainty of about 30 %; for the period May-Sep. this production is about 1600 km³. These two values translate into an average salt production of about 19 x 1012 kg and 36 x 1012 kg, respectively.

• The average winter-time (Jan.-Apr.) Kara Sea polynya area is (25 000 ± 7 200) km² for the period 1995-2004; an extension of this time series to the period 1979-2004 reveals an increase in the total Kara Sea polynya area by 2400 km² / decade.

Scientists

Jens Meincke
University of Hamburg
Institute of Oceanography

Detlef Stammer
University of Hamburg
Institute of Oceanography

Georg Heygster
University of Bremen
Institute of Environmental Physics

Stefan Kern
University of Hamburg
Institute of Oceanography

Sara de la Rosa
Geophysical Institute Bergen, Norway
Bergen Polar Ice Group

Gunnar Spreen
California Institute of Technology, Pasadena, USA
Jet Propulsion Laboratory

Research areas

Circum Antractica

Publications

2009:
Kern, S., Wintertime Antarctic coastal polynya area: 1992-2008, Geophys. Res. Lett., 36, L14501, doi: 10.1029/2009GL038062, 2009.

2008:
Kern, S., Polynya Area in the Kara Sea, Arctic, obtained with Microwave Radiometry for 1979-2003, Geosci. Rem. Sens. Lett., 5(2), 171-175, doi:10.1109/LGRS.2008.916831, 2008.

Spreen, G., L. Kaleschke and G. Heygster, Sea ice remote sensing using AMSR-E 89 GHz channels. J. Geophys. Res, 113, C02S03, doi:10.1029/2005JC003384, 2008.

Wiebe, H., G. Heygster, and L. Meyer-Lerbs, Geolocation of AMSR-E Data. IEEE Trans. Geosci. Rem. Sens., in press, 2008.

2007:
Kern, S., G. Spreen, L. Kaleschke, S. de la Rosa, and G. Heygster, Polynya Signature Simulation Method polynya area in comparison to AMSR-E 89 GHz sea-ice concentrations in the Ross Sea and off Adelie Coast, Antarctica, for 2002-2005: First Results, Ann. Glac., 46(1), 409-418, doi:10.3189/172756407782871585, 2007.

2005:
Kern, S., I. Harms, S. Bakan, and Y. Chen, A Comprehensive View of Kara Sea Polynya Dynamics, Sea-Ice Compactness and Export from Model and Remote Sensing Data, Geophys. Res. Lett., 32(15), L15501, 10.1029/2005GL023532, 2005.

Homepage

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Research funding organisation

German Research Foundation

Project number: ME-487/40-1,2, STA-410/6-3, HE-1746/10-1,2,3
Funding period: 01-2004 to 12-2007