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We have presented a combined analysis of the relationship of air temperature and wind speed with the area of the TNBP at specific temperatures at polar night during 2005−15. The results from both the reanalysis and observations indicate that air temperature gradually shows an increasing positive correlation with the polynya area as temperature declines, while the correlation of the eastward and northward wind speed with the polynya area decreases. The negative or very weak correlations between the heat flux and the air temperature suggest that the heating of the air by the polynya is not an important factor, which means the positive correlation between the air temperature and the polynya area is unlikely to be from the response of the temperature to the polynya. From the aspect of rapid ice formation in lower air temperature, the positive correlation is possibly due to the response of the polynya to air temperature (note: here, we only propose a potential hypothesis; model simulations are needed for further conclusions). The relationship between the air temperature and the polynya area can be divided into three categories as temperature declines. First, the positive correlations between air temperature and the polynya area appears near the coast and the significant areas gradually retreat as temperature declines. Second, air temperature shows weak and negative correlation with the polynya area, which might be owing to the stronger correlation of the air temperature with the northward wind speed in this category. Third, the positive correlations of the even lower air temperature with the polynya area distribute widely over TNB, which is the major result in this study, i.e., that the polynya area has a significantly closer relationship with the low air temperature.
The pressure gradient between the ice sheet and the ocean surface is also affected by air turbulence, which influences the output of katabatic winds. Strong pressure gradients are more likely to disrupt the production of cold air in winter as a result of mixing between warm maritime air and cold continental air (Bromwich, 1989). However, considering the very weak correlation between wind speed and lower air temperature (Fig. 10b), we suggest that the polynya area is more directly related to the low air temperature and not the synergistic relation with winds. The surface conductive heat flux of thin ice at −20°C ± 1°C is about twice that at −10°C ± 1°C (Lei et al., 2010). It has been observed that open water rapidly freezes when the air temperature is very low. The Canadian Ice Service observed that thin ice quickly thickens to 100 mm within 24 hours at a steady air temperature of −25°C (Shokr and Sinha, 2015). Though the effect of lower air temperature on the polynya area from the aspect of the rapid ice formation is a hypothesis in this study, it needs to be seriously considered in further studies of the polynya, i.e., the TNBP. It is essential to examine the relationship between the specific temperatures and the polynya, for the objective of obtaining detailed polynya variations. Though the TNBP is a smaller polynya in the Antarctic, the high rate of sea ice production and high-salinity shelf water in the polynya will directly affect the Antarctic bottom water in the Ross Sea and, in turn, the circumpolar deep water currents. Our study shows a changing relationship between the air temperature and the polynya area at specific temperature intervals. Further studies will apply a regional model to Antarctic coastal polynyas to examine the underlying mechanisms.
The polynya area estimated in the study was based on the microwave products of SIC. Previous studies have proposed another method to retrieve the polynya area by using the MODIS IST data (Ciappa et al., 2012; Aulicino et al., 2018). The MODIS IST data derived from the thermal infrared MODIS bands provide new polynya observations of high horizontal resolution (1 km) and seem to have higher accuracy in area estimation than the microwave data. Table 1 shows the TNBP area estimated in different research. The area of ~0.9 × 103 km2 estimated from the MODIS IST data is smaller (Ciappa et al., 2012), which might be due to the smaller size of the subregion of TNB and the finer resolution of the MODIS data. The polynya area estimated in Kern (2009) and Martin et al. (2007) are both from the microwave data but based on different methods. Kern used the difference of the brightness temperature, while Martin et al. used ice thickness to determine the polynya area. The average area estimated in this study is about 1.5 × 103 km2 larger than that from the MODIS IST. However, the results show that the polynya areas estimated from the microwave data based on the different methods [4.2 × 103 km2 in Kern (2009) and 3.0 × 103 km2 in Martin et al. (2007)] are both greater than that from the MODIS IST. The difference is highly likely due to the different datasets. In general, the area estimated in this study is smaller than that from Kern (2009) and Martin et al. (2007), which also used the microwave data for area estimation, but our results range in the middle of the three given studies. The difference is highly likely due to the different study periods and the methods used for the area estimation [note: the area in Kern (2009) and Martin et al. (2007) was estimated during 1992−2002].
Year TNBP area (×103 km2) estimated from This study Ciappa et al. (2012) Kern. (2009) Martin. et al. (2007) 2005 2.4 ~0.97 2006 1.8 ~0.60 2007 3.1 ~0.98 2008 3.0 ~0.90 2009 2.7 ~0.93 2010 3.1 ~0.85 2011 1.8 2012 2.3 2013 2.4 2014 2.3 2015 1.6 Average 2.4 ± 0.5 (2005−15) ~0.87 ± 0.14 (2005−10) 4.2 ± 0.8 (1992−2002) 3.0 ± 0.8 (1992−2002) Note: The polynya area from this study, Ciappa et al. (2012) and Martin et al. (2007) was estimated in the period of April to October. The polynya area from Kern (2009) was estimated in the period of June to September. Table 1. Averaged polynya area estimated from this study and previous research.