Investigation of polynya dynamics in the northern Bering Sea using greyscale morphology image-processing techniques

Abstract

Arctic coastal polynyas, defined here as persistent openings in the winter sea ice pack, are significant areas for ocean–atmosphere heat exchange, high winter sea ice production with resulting brine rejection and the dependence of local ecosystems. It is therefore critical to accurately quantify polynya dynamics to understand their spatial and temporal variability, particularly in the context of recent dramatic Arctic sea ice declines. In this study, the so-called erosion algorithm (a greyscale morphology image-processing technique) is applied to satellite-derived sea ice concentrations in the northern Bering Sea to investigate polynya dynamics throughout this region. Greyscale morphology allows for the estimation of sea ice extent with a defined error tolerance through the removal of regions with low sea ice concentrations in the marginal ice zone. Furthermore, since polynyas are the primary source of water within the sea ice pack, the presence of water here can therefore be utilized to define the areal extent of polynyas. We utilized AMSR-E (Advanced Microwave Scanning Radiometer-EOS) sea ice concentrations during January–April from 2003 to 2008 in the northern Bering Sea to extract daily time series of water area within polynyas by a water integration method after data are preprocessed using greyscale morphology techniques. These results compare well with those calculated by more traditional methods utilizing sea ice concentration thresholds and show the great utility of greyscale morphology techniques as a preprocessing method (which eliminates artificial determination of polynya areal extents and enables automation of the overall image-processing routine). In addition, based on the results of our algorithms we investigated the potential driving forces (e.g. offshore wind velocity) of polynya development in the northern Bering Sea as well as calculate the spatial and interannual variability of heat fluxes across these water surfaces owing to polynya formation.

Acknowledgement

This research was supported by Global Change Research Programme of China (Grant # 2010CB951403 and 2008AA121701) and the US National Science Foundation Arctic Natural Sciences Program (Grant # 0713939).

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