Reconstructing atmospheric circulation and sea-ice extent in the West Antarctic over the past 200 years using data assimilation

Quentin Dalaiden,Jeanne Rezsöhazy,Hugues Goosse,Elizabeth R Thomas

doi:10.1007/s00382-021-05879-6

Abstract

The West Antarctic climate has witnessed large changes during the second half of the twentieth century including a strong and widespread continental warming, important regional changes in sea-ice extent and snow accumulation, as well as a major mass loss from the melting of some ice shelves. However, the potential links between those observed changes are still unclear and instrumental data do not allow determination of whether they are part of a long-term evolution or specific to the recent decades. In this study, we analyze the climate variability of the past two centuries in the West Antarctic sector by reconstructing the key atmospheric variables (atmospheric circulation, near-surface air temperature and snow accumulation) as well as the sea-ice extent at the annual timescale using a data assimilation approach. To this end, information from Antarctic ice core records (snow accumulation and $$\delta ^{{18}}\hbox{O}$$ ) and tree-ring width records situated in the mid-latitudes of the Southern Hemisphere are combined with the physics of climate models using a data assimilation method. This ultimately provides a complete spatial reconstruction over the West Antarctic region. Our reconstruction reproduces well the main characteristics of the observed changes over the instrumental period. We show that the observed sea-ice reduction in the Bellingshausen-Amundsen Sea sector over the satellite era is part of a long-term trend, starting at around 1850 CE, while the sea-ice expansion in the Ross Sea sector has only started around 1950 CE. Furthermore, according to our reconstruction, the Amundsen Sea Low pressure (ASL) displays no significant linear trend in its strength or position over 1850–1950 CE but becomes stronger and shifts eastward afterwards. The year-to-year sea-ice variations in the Ross Sea sector are strongly related to the ASL variability over the past two centuries, including the recent trends. By contrast, the link between ASL and sea-ice in the Bellingshausen-Amundsen Sea sector changes with time, being stronger in recent decades than before. Our reconstruction also suggests that the continental response to the variability of the ASL may not be stationary over time, being significantly affected by modification of the mean atmospheric circulation. Finally, we show that the widespread warming since 1958 CE in West Antarctica is unusual in the context of past 200 years and is explained by both the deeper ASL and the positive phase of the Southern Annular Mode.

Highlights

The Antarctic Ice Sheet is the biggest reservoir of fresh water on Earth that would potentially raise the global sea-level by 58m if the entire ice sheet melted (Shepherd et al, 2018)
Before analyzing the long-term changes using the paleo-reconstruction over the past centuries, we first need to ensure that our data assimilation method works
We have investigated the West Antarctic climate variability over the past two centuries by providing a complete reconstruction for the main atmospheric variables, as well as the sea-ice concentration

Summary

Introduction

The Antarctic Ice Sheet is the biggest reservoir of fresh water on Earth that would potentially raise the global sea-level by 58m if the entire ice sheet melted (Shepherd et al, 2018). The Antarctic and the Southern Ocean have experienced major climate changes over the past decades (e.g., Jones et al, 2019; Bromwich et al, 2013; Medley and Thomas, 2019; Pritchard et al, 2012), demonstrating large variability and their vulnerability to the global climate change. The largest changes have been found over the Antarctic Peninsula and the West Antarctic Ice Sheet (hereafter WAIS) (e.g., Jones et al, 2019), together forming West Antarctica. The warming is pronounced over the Peninsula, the warming over the central WAIS since the 1950s is one of the fastest recorded on Earth (Steig et al, 2009; Bromwich et al, 2013). In the last decades, glaciers from West Antarctica

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