The changing Arctic sea ice from in-situ and remote sensing approaches

The purpose of the study is to assess the rapid changes of sea ice in Arctic in recent years under a climate changing scenario, and discuss association climatic and environmental processes, based on data and information obtained with in-situ (field) investigations and remote sensing techniques. The changing of Arctic sea ice has a significant potential in affecting local and regional climate, environment, and the global ocean circulation. There are several processes associated with the Arctic sea ice, like large scale fall/winter forming and summer melting of sea ice, insulation of oceanic/atmospheric heat exchange, and various related dynamic and thermodynamic processes, like wind forcing, ocean current forcing albedo feedback, and thermohaline circulation.

To assess and understand these processes, monitoring of several large-scale physical characteristics, such as sea ice concentration (fraction of area covered by sea ice in a unit area), sea ice extent (areas that covered by sea ice with sea ice concentration higher than a certain threshold value), sea ice albedo (the fraction between energy in reflected electromagnetic wave and incident solar energy), and sea ice thickness, is crucial. With the implementation of satellite remote sensing technology, large scale monitoring of sea ice in Arctic region became available, revealing a rapid decline in Arctic sea ice, which would mean an ice-free Arctic in 50 years (with the projection of climate model) and an ongoing changing in climate in Arctic and in the future.

Other than the background introduction of chapter one and summaries of chapter six, this study discusses the monitoring and associated processes with four chapters. The first of them is focused on the areal changes of sea ice, by using in-situ and passive microwave remote sensing to monitor the sea ice extent dynamic during 1979--2012; the second is focused on in-situ measurement of the Arctic sea ice albedo in Pacific sector of Arctic in summer; the third assesse differences three different retrackers in Cryosat-2 sea ice freeboard estimation by comparison with NASA Operation IceBridge airborne altimeter data; and the fourth of them is focused on Cryosat-2 altimetry for the estimation of sea ice thickness changes in Arctic during the 2011--2014. The results show a rapid declining Arctic sea ice extent and frequent occasions of recorded minimum extent in recent years, the ice edge is retreating by 0.022--0.056 degree latitude per year in average, or a 6.26--10.00 km areal decline per year, and the trend is statistically significant (0.05 significance level). The sea ice albedo change in Arctic is rapidly fluctuating and significantly affected by weather events, which can be reduced from ~0.75--0.80 (high albedo new snow from snowfall) to ~0.60 (surface melting due to rainfall) within one day. The sea ice thickness shows a slightly declining in 2012 and 2013 and correspond to the recorded low minimum sea ice extent in these years.

These results show an Arctic of rapid sea ice changes, with a long-term declining trend, in both extent and thickness that corresponded with each other. It is most likely that the sea ice in Arctic is in a positive feedback, that the decline in sea ice makes it much more prone to climate changes.