Abstract
Abstract. In contrast to the Arctic, where total sea ice extent (SIE) has been decreasing for the last three decades, Antarctic SIE has shown a small, but significant, increase during the same time period. However, in 2016, an unusually early onset of the melt season was observed; the maximum Antarctic SIE was already reached as early as August rather than the end of September, and was followed by a rapid decrease. The decay was particularly strong in November, when Antarctic SIE exhibited a negative anomaly (compared to the 1979–2015 average) of approximately 2 million km2. ECMWF Interim reanalysis data showed that the early onset of the melt and the rapid decrease in sea ice area (SIA) and SIE were associated with atmospheric flow patterns related to a positive zonal wave number three (ZW3) index, i.e., synoptic situations leading to strong meridional flow and anomalously strong southward heat advection in the regions of strongest sea ice decline. A persistently positive ZW3 index from May to August suggests that SIE decrease was preconditioned by SIA decrease. In particular, in the first third of November northerly flow conditions in the Weddell Sea and the Western Pacific triggered accelerated sea ice decay, which was continued in the following weeks due to positive feedback effects, leading to the unusually low November SIE. In 2016, the monthly mean Southern Annular Mode (SAM) index reached its second lowest November value since the beginning of the satellite observations. A better spatial and temporal coverage of reliable ice thickness data is needed to assess the change in ice mass rather than ice area.
Highlights
Introduction and previous workSea ice plays a critically important role in the cryosphere as well as the global climate system
The Eastern Weddell Sea (EWS), western Pacific/western Ross Sea (WP), and Western Weddell Sea (WWS) regions contributed to the negative sea ice extent (SIE) and sea ice area (SIA) anomalies through strong events associated with the corresponding atmospheric flow discussed above, whereas the Western Indian Ocean (WIO) and Amundsen–Bellingshausen seas (ABS) exhibited more or less continuous negative deviations from the longterm mean
Other components of the climate system, such as the ocean, certainly contributed to this anomalous melt, our analysis suggests that increased meridional heat exchange with middle latitudes due to strongly meandering westerlies played a significant role
Summary
Sea ice plays a critically important role in the cryosphere as well as the global climate system. Holland and Kwok (2012) investigated the influence of surface winds on Antarctic sea ice changes over the period 1992 to 2010 They state that while in most parts of West Antarctica mainly wind-driven changes in ice advection are responsible for the observed sea ice concentration (SIC) trends, winddriven thermodynamic changes dominate in all other parts of the Southern Ocean. Stuecker et al (2017) hypothesize that the anomalously low SIE in November–December 2016 was largely related to positive sea surface temperature anomalies due to the extreme El Niño event peaking in December–February 2015–2016 that persisted throughout the winter and the concurrent negative phase of the SAM Changes in both oceanic and atmospheric processes could have been responsible for the observed 2016 anomaly. We compared those changes to ice drift data and related it to the general atmospheric flow conditions derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data and considered the relationship to SAM and ZW3 indices and patterns
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