Regional, seasonal, and predictable Arctic sea ice change

Abstract

The loss of Arctic sea ice is one of the most prominent and best quantified indicators of ongoing global climate change. Satellite passive microwave observations since 1979 indicate significant negative sea ice extent trends in all months, accompanied by pronounced interannual variability. The recent changes in the Arctic sea ice cover have profound environmental, societal, and ecological impacts, and have led to an increased demand for reliable sea ice predictions. This thesis considers observed regional, seasonal, and predictable Arctic sea ice variability and trends, and particularly assesses the impact of Atlantic water on the recent winter sea ice area variability and loss. Updated through 2016, the recent Arctic sea ice extent decline is most pronounced in summer along the Russian and North American shelves. Winter trends are currently smaller and generally further south. The largest winter sea ice extent loss has occurred in the Atlantic sector, particularly in the Barents Sea and Nansen Basin, where the sea ice extent is currently less than half of the pre-satellite mean (1850–1978). The recent winter sea ice extent trend and interannual variability in the Arctic Ocean, carried by the Barents Sea and Nansen Basin, largely depend on variations in the advection of Atlantic heat. Based on recent winter observations, it is shown that the warm Atlantic water in the Nansen Basin typically melts sea ice advected from the Arctic Ocean. In contrast, most sea ice forms locally in the Barents Sea and the inflow of Atlantic water mainly inhibits sea ice freezing there. Rooted in observations, the thesis presents and evaluates prognostic frameworks to predict winter sea ice variability in the Barents Sea and Nansen Basin. It is shown that the Barents Sea ice area may be skillfully predicted, both qualitatively and quantitatively, up to two years in advance based on observed ocean heat transport and regional sea ice area. By using observed hydrography upstream, the Nansen Basin sea ice area appears skillfully predictable up to three years in advance. Anomalously strong meridional winds also impact interannual sea ice variability, and partly explain model imperfection. The recent loss of Arctic sea ice is unprecedented in all seasons considering the available historical record since 1850. Both climate models and extrapolation of present trends project the Arctic sea ice extent loss to continue toward 2100. In particular, the Barents Sea is expected to be ice-free year-round by the end of the century. If the current sea ice extent trends persist, the Arctic Ocean will become ice-free in summer, and trends gradually have to increase toward the winter season and in regions presently fully ice covered in winter. In summary, this thesis demonstrates that the recent observed Arctic winter sea ice extent variability and trends mainly reflect variations in the Atlantic inflow to the Arctic, and can skillfully be predicted one to three years in advance. More generally, the thesis documents from observations how the ongoing pan-Arctic shift toward a seasonal sea ice cover is increasingly concerned with wintertime change.