Abstract
Abstract. Recent studies have investigated the potential link between the freshwater input derived from the melting of the Antarctic ice sheet and the observed recent increase in sea ice extent in the Southern Ocean. In this study, we assess the impact of an additional freshwater flux on the trend in sea ice extent and concentration in simulations with data assimilation, spanning the period 1850–2009, as well as in retrospective forecasts (hindcasts) initialised in 1980. In the simulations with data assimilation, the inclusion of an additional freshwater flux that follows an autoregressive process improves the reconstruction of the trend in ice extent and concentration between 1980 and 2009. This is linked to a better efficiency of the data assimilation procedure but can also be due to a better representation of the freshwater cycle in the Southern Ocean. The results of the hindcast simulations show that an adequate initial state, reconstructed thanks to the data assimilation procedure including an additional freshwater flux, can lead to an increase in the sea ice extent spanning several decades that is in agreement with satellite observations. In our hindcast simulations, an increase in sea ice extent is obtained even in the absence of any major change in the freshwater input over the last decades. Therefore, while the additional freshwater flux appears to play a key role in the reconstruction of the evolution of the sea ice in the simulation with data assimilation, it does not seem to be required in the hindcast simulations. The present work thus provides encouraging results for sea ice predictions in the Southern Ocean, as in our simulation the positive trend in ice extent over the last 30 years is largely determined by the state of the system in the late 1970s.
Highlights
The sea ice extent in the Southern Ocean has been increasing at a rate estimated to be between 0.13 and 0.2 million km2 per decade between November 1978 and December 2012 (Vaughan et al, 2013)
The recent work of Eisenman et al (2014) suggests that the positive trend in Antarctic sea ice extent may be in reality smaller than the value given in Vaughan et al (2013)
When no data assimilation is included in the model simulation (NODA), the ensemble mean displays a decreasing trend in sea ice extent in response to the external forcing (Fig. 2a and b), similar to the one found in other climate models (e.g. Zunz et al, 2013)
Summary
The sea ice extent in the Southern Ocean has been increasing at a rate estimated to be between 0.13 and 0.2 million km per decade between November 1978 and December 2012 (Vaughan et al, 2013). Other studies have underlined the fact that the positive trend in sea ice extent could be attributed to the internal variability of the system (e.g. Mahlstein et al, 2013; Zunz et al, 2013; Polvani and Smith, 2013; Swart and Fyfe, 2013). This explanation cannot be confirmed by present-day general circulation models (GCMs) involved in the Fifth Coupled Model Intercomparison Project
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