Antarctic sea ice increase consistent with intrinsic variability of the Amundsen Sea Low

Clim Dyn DOI 10.1007/s00382-015-2708-9 Antarctic sea ice increase consistent with intrinsic variability of the Amundsen Sea Low John Turner1 · J. Scott Hosking1 · Gareth J. Marshall1 · Tony Phillips1 · Thomas J. Bracegirdle1 Received: 15 October 2014 / Accepted: 8 June 2015 © The Author(s) 2015. This article is published with open access at Springerlink.com Abstract We investigate the relationship between atmospheric circulation variability and the recent trends in Antarctic sea ice extent (SIE) using Coupled Model Intercomparison Project Phase 5 (CMIP5) atmospheric data, ECMWF Interim reanalysis fields and passive microwave satellite data processed with the Bootstrap version 2 algorithm. Over 1979–2013 the annual mean total Antarctic SIE increased at a rate of 195 × 103 km2 dec−1 (1.6 % dec−1), p < 0.01. The largest regional positive trend of annual mean SIE of 119 × 103 km2 dec−1 (4.0 % dec−1) has been in the Ross Sea sector. Off West Antarctica there is a high correlation between trends in SIE and trends in the nearsurface winds. The Ross Sea SIE seasonal trends are positive throughout the year, but largest in spring. The stronger meridional flow over the Ross Sea has been driven by a deepening of the Amundsen Sea Low (ASL). Pre-industrial control and historical simulations from CMIP5 indicate that the observed deepening of the ASL and stronger southerly flow over the Ross Sea are within the bounds of modeled intrinsic variability. The spring trend would need to continue for another 11 years for it to fall outside the 2 standard deviation range seen in 90 % of the simulations. Keywords Sea ice · Southern Ocean · Climate change · Ross Sea · Amundsen Sea Low * John Turner 1 British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 0ET, UK 1 Introduction A major question in global change studies is why has Antarctic sea ice extent (SIE) increased over recent decades when sea ice over the Arctic has been decreasing rapidly? Greenhouse gas (GHG) concentrations are now higher than at any time in the last one million years, and with the sensitivity of sea ice to increases in air and ocean temperature, it would be assumed intuitively that SIE in both polar regions would be declining. However, while Arctic sea ice reached a new record minimum extent in September 2012 (Parkinson and Comiso 2013), during the same month sea ice over the Southern Ocean attained a new maximum extent (Turner et al. 2013a), suggesting that the differences in SIE between the two polar regions are getting larger. A further problem is that the majority of coupled climate models when run over recent decades with observed forcings have Antarctic sea ice decreasing in a manner similar to Arctic Sea ice (Eisenman et al. 2011). This may indicate that some process is not included in the current generation of climate models or that the observed trend over the admittedly short period since 1979 is at the extreme limit of the simulations produced by the climate models. The record of reliable passive microwave satellite estimates of SIE begins in late 1978 with the availability of data from the Special Sensor Microwave Imager. During the first decade of the satellite record there was a slight decrease in Antarctic SIE, but subsequently a number of studies noted the statistically significant increase in extent (Cavalieri et al. 1997; Comiso and Nishio 2008; Zwally et al. 2002). However, it was often pointed out that this overall increase in SIE masked large regional variations and in particular a decrease in the Bellingshausen Sea (Parkinson and Cavalieri 2012; Stammerjohn et al. 2012) and 13 J. Turner et al. Fig. 1  The trend in annual mean sea ice concentration for 1979– 2013 (% dec−1). Areas where the trend is significant at p < 0.05 are enclosed by a bold line. Sectors discussed in this study are indicated—Weddell Sea (WS), Indian Ocean (IO), Western Pacific Ocean (WPO), Ross Sea (RS), Amundsen Sea (AS) and Bellingshausen Sea (BS) increase in the Ross Sea (Comiso et al. 2011) (for places referred to in the text see Fig. 1). A number of theories have been put forward to explain why the SIE is increasing. The West Antarctic Ice Sheet has been losing mass over recent decades, particularly from the Amundsen Sea Embayment area (Wingham et al. 2009), resulting in a freshening of the waters off the coast (Jacobs et al. 2002). Bintanja et al. (2013) suggested that the fresher water will impede the upward flux of heat from deeper levels and so contribute to the greater extent of ice in the Ross Sea. However, Swart and Fyfe (2013) carried out model experiments that injected freshwater into the Amundsen Sea and found that the resultant impact of such changes on the sea ice was small compared to internal variability. The role of such ice–ocean feedback processes was also examined by Zhang (2007), however, the process suggested involved an increase in near-surface air temperature, and this has not been observed at coastal stations beyond the Antarctic Peninsula (Turner et al. 2005). The advection of sea ice is strongly influenced by the near-surface wind field and therefore the broadscale atmospheric circulation. Holland and Kwok (2012) used a dataset of satellite-tracked sea ice motion and atmospheric fields to investigate the relationship between the atmospheric circulation and sea ice anomalies. They found that wind-driven changes in ice advection were the dominant driver of ice trends around much of West 13 Antarctica, with wind-driven thermodynamic changes dominant elsewhere. The contrasting SIE trends of increasing (decreasing) sea ice in the Ross (Bellingshausen) Sea and the known association between ice anomalies and the wind field suggests a link with the Amundsen Sea Low (ASL), which is the dominant climatological feature in this area (Fogt et al. 2012; Hosking et al. 2013; Turner et al. 2012a). Model results presented by Turner et al. (2009) suggested that the loss of stratospheric ozone had deepened the ASL, increasing the strength of the southerly winds over the Ross Sea and contributing to the increase of SIE in this area. However, not all models have ozone loss giving this deepening of the ASL. The ASL is the deepest climatological mean sea level pressure (MSLP) centre within the circumpolar trough that rings Antarctica between 60° and 70°S, and its presence has been linked to the interaction between the orography of the Antarctic continent and the strength of the westerly winds over the Southern Ocean (Baines and Fraedrich 1989). Inter-annual variability of MSLP in the area of the ASL is larger than at any other location in the Southern Hemisphere (SH) (Connolley 1997; Lachlan-Cope et al. 2001), with surface pressure here influenced by tropical climate variability in the Pacific (Hoskins and Karoly 1981; Yuan and Martinson 2000) and Atlantic (Li et al. 2014; Simpkins et al. 2013) Oceans. The ASL has deepened in recent decades (Turner et al. 20 (...truncated)