Investigations of the Recent Polar Ice Variations Observed from the Space-Borne Optical and Microwave Sensors

By: Sandip R. Oza*, R. K. Kamaljit Singh* and Megha Maheshwari*
Polar ice regime plays an important role in the Earth's climate system. It consists of ocean, ice sheet, sea ice, ice shelf, atmosphere and polynyas. Because of the complex feedback mechanism amongst these components, improved knowledge of the physical processes is required for a better understanding of the potential changes in ice mass balance. The changes observed in Polar sea ice extent in the recent past are alarming. The present paper is focused on the changes observed over the Arctic and the Antarctic ice surface, sea-ice as well as ice-shelf, using the data derived from space-borne observations. The recent changes in the sea-ice concentration and ice-shelf melting are discussed in detail. The inter-relationship of sea-ice, sea surface temperature (SST) and ocean productivity and their year-to-year variations have also been discussed. The paper summarises the studies carried out at Space Applications Centre (SAC), Indian Space Research Organisation) for the assessment of Arctic and Antarctic sea ice extents and surface melting over east-Antarctic ice-shelves, using the space borne data.
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Introduction

Global climate change remains a topic of concern over the last few decades. The Arctic and the Antarctic polar regions are important components of the Earth’s climate system through their influence on the global atmospheric and oceanic circulations. Most of the global climate models predict the increase in Earth’s surface temperature with much larger increase over the polar regions. This warming phenomenon is also supported by the thinning as well as shrinking of Arctic sea ice cover. The Polar icy regions have the potential to influence global atmospheric and oceanic circulations and are thereby remotely connected with the climate of tropics. The present paper summarises the work carried out using space borne observations at the Space Applications Centre (ISRO), Ahmedabad during the last five years.

 

 

Assessment of sea ice cover

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Polar sea ice has an important climate regulating impact by limiting exchanges of momentum, heat and moisture between the ocean and atmosphere. It modulates the normal exchange of heat and mass between the atmosphere and ocean by isolating sea surface from atmosphere. It also affects the atmosphere through a strong ice albedo-feedback mechanism and the ocean through the release of brine/fresh water during its growth/melt cycle. Since the total time duration of the growth and the melting is of the order of one year, the ice cover effectively integrates the climate signal over this short period and acts as an indicator of climate (Ress, 2006).

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In the recent years a significant decline in the summer sea ice extent has been observed in the Arctic (Figure 1). Here, mid-month 0.2 degree resolution daily composite QuikSCAT Ku-band scatterometer data, from August 1999 to July 2009, was used. QuikSCAT sigma-0 data are obtained from the NASA sponsored Scatterometer Climate Record Pathfinder at Brigham Young University through the courtesy of Dr. David G. Long (http://www.scp.byu.edu). The sea ice cover was derived using the spatio-temporal coherence technique (Oza et al., 2011). The fraction sea ice pixels within 1×1 degree grid-cell were analysed for the investigation of sea ice trends.

As observed in Figure 1, in the Arctic, a significant negative trend in the Chukchi and East Siberian seas is visible in the summer minimum extent. A weak positive trend is also visible in the Laptev and East Greenland seas. However, negative trend in winter-maximum in Barents Sea is a distinct pattern. The positive trend in the Bering Sea and negative trend in Okhostsk Sea are also visible in the winter-maximum image.

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The grid-wise trends in the Antarctic during summer time show a mix of positive and negative trends. This is in contrast to the Arctic, where a dominant negative trend is visible. The significant negative trend in the Amundsen Sea to the Ross Sea is evident in Figure 1 and Figure 2. The results shown in Figure 2 are based on Oceansat-2 OSCAT scatterometer launched by India in September 2009, which is providing continuity to the QuikSCAT Ku-band Sigma-0 time series.

Further investigation was carried out using coastal altimeter products from PISTACH, Aviso (www.aviso.org). As seen from Figure 3, significant difference between the year-to-year variability is observed in the Antarctic sea ice extent. This could be due to the fact that sea ice in the Arctic is confined by the land mass, however in the Antarctic the boundary of entire sea ice cover is in direct interaction with the open ocean, which might have resulted in high year-to-year variability.

 

 

Assessment of the surface melting over
Antarctic ice-sheet

The Polar ice sheets play an important role in the climate system (Convey et al., 2009). The mass balances of the Greenland and Antarctic ice sheets are of interest because of their complex linkage to climate variability and their direct effects on sea-level change. In recent decades, the estimation of mass input and output fluxes have greatly improved due to the advances in remote sensing techniques and dynamic modelling (Zwally et al., 2005). The concept of slow changes, in ice sheet mass balance, predicted by climate models have been challenged by recent space-borne observations (Rignot, 2006). Understanding of the mass balance and surface dynamics of the Earth’s major ice sheets in Greenland and Antarctic is of fundamental importance for accurate prediction of sea-level rise (Quincey and Lackman, 2009).

Profound regional warming in the Antarctic Peninsula triggered ice-shelf collapse that led to 10 fold increase in glacier flow and rapid ice sheet retreat (Rignot, 2006). Similar to the western part of Antarctic, eastern part is also not immune to change (Oza et al., 2011). The penetration of melt water, which has increased due to increasing temperature, could contribute to break up of the large size icebergs from the ice shelves (Fricker et al. 2009).

A study has been carried out for the Amery Ice Shelf (AIS) region of east Antarctica. The AIS is the third largest embayed shelf in Antarctica. Moreover, it is the largest ice shelf within east Antarctica and is near to the third Indian Antarctic station (http://www.ncaor.gov.in). It drains the grounded ice from the interior of the Lambert Glacier drainage basin, which covers an area of 16 per cent of the east Antarctic ice sheet and is the world’s largest glacier by volume. The mid-month QuikSCAT (2000-01 to 2008-09) and OSCAT (2009-10) dataset obtained for the sea ice study was utilised for the assessment of the change in the surface characteristics of the AIS. The melting index (MI), which is an accumulated deviation of decline in summer backscattering from the backscatter value observed during the preceding winter for the same location. This decline in the backscattering coefficient is due to the increase of moisture content either within or above the snow pack. Hence, this MI is an indicator of the surface melting observed over the ice-shelves (Figure 4).

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As observed from the Figure 4, high degree of year-to-year variations in MI exists in the east Antarctic ice shelves. Significant melting was observed over the Shackleton ice-shelf compared to the other two ice shelves. However, during 2003-04 and 2004-05 majority of the grid cells with higher MI values has been observed. These results indicate that the east Antarctica ice shelves are also not free from the warming effect. It will be noted that subsequent to the launch of Oceansat-2 OSCAT scatterometer, the Indian Space Research Organisation has already launched the C-band RISAT SAR and about to launch the Ka-band SARAL-ALtika altimeter in the space. The data from these sensors are expected to provide valuable information for polar ice applications.

 

 

Conclusion

A number of polar ice studies have been carried out at SAC, Ahmedabad. The 1×1 degree grid cell wise analysis based on scatterometer data supports the large scale decline in the summer minimum sea ice extent in the Arctic, as also observed by other researchers. Significant decline in some parts of the Arctic observed in winter maximum, demands further research. The high year-to-year variability in the sea ice extent observed in the Antarctic, however numbers of patches observed showing statistically significant trend and required further investigation to understand the cause. It is also observed that the ice shelves of east Antarctica are also not free from the warming effect.

 

 

Acknowledgements

Authors gratefully acknowledge the suggestions and encouragement provided by Dr. R. R. Navalgund, former Director, Space Applications Centre (SAC). The guidance provided by Shri A. S. Kiran Kumar for the utilisation of OSCAT data is gratefully acknowledged. Kind suggestions received from Dr. J. S. Parihar, Dr. Pradip K. Pal and Dr. Rajkumar (SAC), are thankfully acknowledged. QuikSCAT sigma-0 data are obtained from the NASA sponsored Scatterometer Climate Record Pathfinder at Brigham Young University through the courtesy of David G. Long (http://www.scp.byu.edu). Sea ice climatology and near real time data were obtained from NSIDC website http://nsidc.org.

 

 

References

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