It is widely accepted that the volume of Arctic sea ice has on average been declining over the last thirty years. However, the extent of the sea ice cover in the Arctic remains variable as synoptic weather conditions play a role in determining the growth and melt of sea ice and its movement within the Arctic basin. Arctic sea ice reaches its minimum extent in September and we are currently participating in the Study of Environmental Arctic Change (SEARCH) international effort to provide a community-wide summary of the expected September Arctic sea ice minimum ( SEARCH Sea Ice Outlook ).
The Met Office contribution should be considered very much an experimental forecast as further work is required to improve its credibility. We currently only provide the total extent of sea ice and not the absolute distribution or estimates of ice volume.
The SEARCH Sea Ice Outlook includes estimates of September sea ice extent from dynamical, statistical and heuristic methods which are combined to provide the mean of the estimates. Since 2010, the Met Office has provided an estimate based on our seasonal forecast system. The estimated sea ice extent from the SEARCH Sea Ice Outlook for 2010-2013 is shown in the table below:
|Year||Outlook mean||Outlook range||Observed||Met Office contribution||Link to Outlook|
|2010||5.0 × 106 km2||(4.2-5.7) × 106 km2||4.9 × 106 km2||5.5 × 106 km2||Sea ice outlook 2010 (PDF, 193 kB)|
|2011||4.7 × 106 km2||(4.0-5.6) × 106 km2||4.6 × 106 km2||(4.0 ± 1.2) × 106 km2||Sea ice outlook 2011 (PDF, 916 kB)|
|2012||4.4 × 106 km2||(4.1-4.9) × 106 km2||3.6 × 106 km2||(4.4 ± 0.9) × 106 km2||Sea ice outlook 2012 (PDF, 227 kB)|
|2013||(3.4 ± 1.5) × 106 km2||Sea ice outlook 2013 (PDF, 637 kB)|
Given current capabilities for seasonal forecasting of sea ice, we consider our contribution to the SEARCH outlook as an experimental forecast. A prediction of regional-scale ice coverage is beyond the capabilities of current long-range forecasting. Therefore our estimates are restricted to the extent of the Arctic sea ice. This is defined as the area of the Arctic Ocean with at least 15% ice coverage. In the long-term average, this area varies from 14.5-16.5 million km2 in March to from 3.5-8 million km2 in September (based on data for 1979-2010). The all time minimum extent for September during the satellite period was 3.6 million km2 in 2012, almost 20% below the next lowest ice extent of 4.3 million km2 in 2007.
Over the 1996-2009 verification period, our forecast September Arctic ice extents have a correlation with the observed ice extents of 0.63. This is significantly different from zero at the 95% confidence level. Shown below are our forecast ice extents (blue diamonds) compared to the observed ice extents during this period (black squares). The red diamond is the 2013 forecast.
September sea ice extent in forecasts (blue) and observations (black)