Arctic sea ice
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.
International sea ice outlook
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
|2016 (June)||4.3||3.4-5.2||TBA||3.6 ± 1.0||2016 SIPN report|
Met Office contribution to the SIPN outlook
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 1993-2015, we have assessed the skill of the current prediction system. Over this verification period, the correlation between our June forecast of September Arctic ice extents and the observed ice extents is 0.89. This represents improved skill over both simply predicting the linear trend, and persisting May Sea Ice anomalies forward into September. Shown below are our forecast ice extents (blue diamonds) compared to the observed ice extents during this period (black squares). The dashed lines are the trends in the ice extent over the 1993-2015 period (black for observations, blue for the forecast system). The June outlook for September 2016 sea ice extent, based on the ensemble mean, is 3.6 million km2 (red diamond).
Regional predictions of ice cover are still very experimental. In a collaborative effort to improve these regional predictions, the Met Office provides maps of sea ice probability - the probability that ice fraction is over 15% in a given grid cell - as part of a SIPN multi-model ensemble (see Figure 4 of the SIPN June Report). The forecast from June of sea ice probability in September 2016 as provided by the Met Office is shown in the figure below. The black line is the ice edge of our September ensemble mean sea ice concentration, along with the climatological ice edge for the period 1993-2015 from the model (green) and observations (cyan).