Abstract
Abstract. The large natural variability of the Antarctic sea ice is a key characteristic of the system that might be responsible for the small positive trend in sea ice extent observed since 1979. In order to gain insight of the processes responsible for this variability, we have analysed in a control simulation performed with a coupled climate model a positive ice–ocean feedback that amplifies sea ice variations. When sea ice concentration increases in a region, in particular close to the ice edge, the mixed layer depth tends to decrease. This can be caused by a net inflow of ice, and thus of freshwater, that stabilizes the water column. A second stabilizing mechanism at interannual timescales is associated with the downward salt transport due to the seasonal cycle of ice formation: brine is released in winter and mixed over a deep layer while the freshwater flux caused by ice melting is included in a shallow layer, resulting in a net vertical transport of salt. Because of this stronger stratification due to the presence of sea ice, more heat is stored at depth in the ocean and the vertical oceanic heat flux is reduced, which contributes to maintaining a higher ice extent. This positive feedback is not associated with a particular spatial pattern. Consequently, the spatial distribution of the trend in ice concentration is largely imposed by the wind changes that can provide the initial perturbation. A positive freshwater flux could alternatively be the initial trigger but the amplitude of the final response of the sea ice extent is finally set up by the amplification related to the ice–ocean feedback. Initial conditions also have an influence as the chance to have a large increase in ice extent is higher if starting from a state characterized by a low value.
Highlights
In contrast to the Arctic where a strong decrease has been observed, in summer, sea ice extent has slightly increased over the last decades in the Southern Ocean (Comiso and Nishio, 2008; Cavalieri and Parkinson, 2012; Parkinson and Cavalieri, 2012)
Modifications in the hydrological cycle of the Southern Ocean could contribute to the observed positive trend (e.g. Liu and Curry, 2010; Bintanja et al, 2013): higher snowfalls induce a higher albedo as well as snow ice formation leading to an increase in ice thickness; larger precipitation rates lead to a lower sea surface salinity, a stronger oceanic stratification, and a reduction in the vertical oceanic heat transport that favours ice formation (e.g. Martinson, 1990; Fichefet and Morales Maqueda, 1999; Marsland and Wolff, 2001)
Each 30 year period characterized by a large increase in sea ice extent in a control simulation performed with LOVECLIM displays positive trend in ice concentration in the majority of the sectors of the Southern Ocean and negative ones in regions that differ among those periods
Summary
In contrast to the Arctic where a strong decrease has been observed, in summer, sea ice extent has slightly increased over the last decades in the Southern Ocean (Comiso and Nishio, 2008; Cavalieri and Parkinson, 2012; Parkinson and Cavalieri, 2012). A positive trend is noticed for every month, with a minimum in February at a value a bit lower than 105 km per decade (Cavalieri and Parkinson, 2012). The causes of this positive trend in a generally warming world are still debated. Enhanced melting of the Antarctic ice sheet, in particular of the ice shelves, induces a freshwater transport towards the ocean, which
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