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 Sea Ice Prediction Network (SIPN) international effort to provide a community-wide summary of the expected September mean Arctic sea ice.
The SIPN sea ice outlook includes estimates of September mean sea ice extent from dynamical, statistical and heuristic methods which are combined to provide the median of the estimates. Since 2010, the Met Office has provided an estimate based on our seasonal forecast system. The estimated sea ice extent (for the forecast started in June, unless otherwise specified) from the SIPN sea ice outlook for 2010 onwards is shown in the table:
Outlook median (× 106 km2)
Outlook range (× 106 km2)
Observed (× 106 km2)
Met Office contribution (× 106 km2)
|2010 SEARCH report|
|2011 SEARCH report|
|2012 SEARCH report|
|2013 SEARCH report|
2014 SIPN report
2015 SIPN report
Given current capabilities for seasonal forecasting of sea ice, we consider our contribution to the SIPN 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 3.5-8.0 million km2 in September (based on data for 1979-2010). The minimum extent for September since 1979 (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.
Using retrospective forecasts for the period 1996-2009, we have assessed the skill of the current prediction system. Over this verification period, the correlation between our August forecast of September Arctic ice extents and the observed ice extents is 0.94. This represents improved skill over both simply predicting the linear trend, and persisting July Sea Ice anomalies forward into September. Shown below are our forecast ice extents (green diamonds) compared to the observed ice extents during this period (black squares). The dashed lines are the trends in the ice extent over the 1996-2009 period (black for observations, green for the forecast system), which has been extended to 2015. The August outlook for September 2015 sea ice extent, based on the ensemble mean, is 3.7 million km2 (red diamond). Forecasts for 2013 and 2014 from a previous version of GloSea are also included on the plot.
September sea ice extent forecasts (green) and observations (black)
Last updated: 1 September 2015