From environmentaresearchweb.org, a new paper examines the contribution of natural cycles the AMO, AO and AMOC to the reduction in Arctic sea ice. A Japanese and UK collaboration used five CMIP3 coupled general circulation models with pre-industrial forcing to investigate the impact of the AO, AMO and AMOC on March and September sea ice extent.
Negative correlations of sea ice extent and the AMO and AMOC, but not the AO, were significant at the 90% level indicating that the increasingly positive values of the AMO over the period of satellite ice observations from 1979 up to ~2001 may contribute, at least in part, to the decline of sea ice. The predictive capability and correlation of the models with observation differed depending on their handling of the AMO and the northward transport of ocean heat, affecting the flow of Atlantic water into the Arctic.
The group then used a simple linear model to predict the sea ice extent over 1979-2010 and an extended period 1953-2010, using the five models and the AMO index to estimate the AMO driven component. Depending on the model used, 5-30% of the sea ice loss during the satellite era (1979-2010) is attributable to the natural cycle of the AMO. Over the longer period from 1953 the proportion was smaller and the authors suggest that, despite increased observational uncertainty in the pre-satellite era, this “is more likely to be representative of the anthropogenically forced component.” This says even more about the lack of predictive capability of the models.
Sources of multi-decadal variability in Arctic sea ice extent
J J Day et al 2012 Environ. Res. Lett. 7 034011
The observed dramatic decrease in September sea ice extent (SIE) has been widely discussed in the scientific literature. Though there is qualitative agreement between observations and ensemble members of the Third Coupled Model Intercomparison Project (CMIP3), it is concerning that the observed trend (1979–2010) is not captured by any ensemble member. The potential sources of this discrepancy include: observational uncertainty, physical model limitations and vigorous natural climate variability. The latter has received less attention and is difficult to assess using the relatively short observational sea ice records. In this study multi-centennial pre-industrial control simulations with five CMIP3 climate models are used to investigate the role that the Arctic oscillation (AO), the Atlantic multi-decadal oscillation (AMO) and the Atlantic meridional overturning circulation (AMOC) play in decadal sea ice variability. Further, we use the models to determine the impact that these sources of variability have had on SIE over both the era of satellite observation (1979–2010) and an extended observational record (1953–2010). There is little evidence of a relationship between the AO and SIE in the models. However, we find that both the AMO and AMOC indices are significantly correlated with SIE in all the models considered. Using sensitivity statistics derived from the models, assuming a linear relationship, we attribute 0.5–3.1%/decade of the 10.1%/decade decline in September SIE (1979–2010) to AMO driven variability.
It’s Open Access, so read the rest here: doi:10.1088/1748-9326/7/3/034011