Land and sea-ice prediction

Sea ice cover seasonally insulates the atmosphere from the ocean preventing the exchange of heat and gases. The formation of sea ice during winter allows atmospheric temperatures to fall, due to a lack of sunlight, to colder than -30 °C. In addition, its formation extracts fresh water from the ocean, producing the cold saline bottom water which influences ocean circulation. As sunlight returns to the poles in summer, the ice melts back allowing the atmosphere to be warmed by the ocean and releasing fresh water to stratify the regional oceans. Sea ice, in both the Arctic and Antarctic, is expected to decline with global warming, the Arctic becoming ice-free in summer in the latter half of this century. Transport of heat to the Arctic from mid-latitudes, by ocean currents and winds is variable year by year resulting in different degrees of ice melt each year. Some climate models, including that of the Met Office Hadley Centre, successfully depict the observed variability and decline of the Arctic sea ice.

Sea ice decline does not significantly influence sea level as the ice is already floating. However, the melting of land based ice, ice sheets and mountain glaciers, has an important contribution to global sea level rise. The melt from mountain glaciers also contributes to river flow and impacts the availability of summer water for agriculture. The Greenland ice sheet loses ice through surface melting in summer and through drainage of ice from the interior by large glaciers. Many glaciers, at the sea-ward fringes of both Greenland and Antarctica, have been speeding up. It is suggested that the speed-up, and the resulting increased ice flow to the ocean and associated sea level rise, are caused by a warming of the adjacent oceans.

Key aims

• Quantify confidence in predictions of the cryosphere.  
• Assess missing ice processes in the climate models.  
• Improve model processes and predictive capability.

Current activities

The current focus of the sea ice work over the next two years is to assess the risk of rapid loss of sea ice in the Arctic. Specific themes are:

•  Demonstrate the ability of Met Office climate prediction model: HadGEM1 to reproduce the observed trends in sea ice extent and understand why Met Office climate prediction model: HadGEM1 and a few other models are able to do so through the use of process-based model evaluation. This work increases our confidence in the future model predictions. 
• Understand the mechanisms for low ice events in Met Office climate prediction model: HadGEM1 and compare with observations. This work enables us to gauge the ability of Met Office climate prediction model: HadGEM1 to model the correct processes and to identify if there are processes missing from the model. 
• Analyse the mechanisms for projected declines in Arctic sea ice to understand the relative roles of atmospheric forcing, oceanic forcing and preconditioning. This work will enable us to quantify the causes of sea ice changes and to compare with observations as they become available.
• Carry out further work to understand the reversibility of sea ice loss. This work will feed into Climate change mitigation advice. 
• Implement multilayer thermodynamics in Met Office climate prediction model: HadGEM3 family to help increase the robustness of future model predictions. In-situ observations of ice thickness suggest that models should include this process. We have identified this as the highest priority missing process.
• Developing a seasonal sea ice forecast capability. There is some evidence to suggest that processes on seasonal timescales are important on climate change timescales. This work will enable us to provide early warning of low sea ice extents and to identify any important missing processes in the model on seasonal timescales.

The objectives for the land ice work is to develop a basic capacity to represent glaciers and ice sheets within the climate model, including:

• Development of a climate model including the coupling of the ocean to Antarctic ice shelves, in collaboration with Bristol University
• Coupling ice sheets to regional climate models to improve predictions of sea level rise as part of an EU project Ice2sea
• Development of a mountain glacier model to improve sea level predictions and assess water resource impacts

Recent results

Simulations of the Arctic sea ice cover, generated by the Met Office climate prediction model: HadGEM1 coupled climate model, are able to capture the observed long term decline in mean September ice extent. Met Office climate prediction model: HadGEM1 is also capable of producing an episode of low September ice extent of similar magnitude to the anomalously low extent observed in 2007. The episode of low summer ice extent is largely driven by the synoptic conditions over the summer moving the ice across and out of the Arctic basin, and also due to pre-conditioning of the snow and ice which is thinner than usual in the Eastern Arctic at the start of the melt season.

Published in:  Keen, A. B., H. T. Hewitt and J. K. Ridley, A case study of a modelled episode of low Arctic sea ice, Climate Dynamics, 41, 1229-1244, 2013

The Met Office climate prediction model: HadGEM1 climate model has projected Arctic ice loss with reasonable accuracy up to the present day. The model projects that sea ice decline will continue over the 21st Century; however it also shows the possibility of a period of up to 20 years when the ice loss is temporarily slowed. This period is caused by reductions in ice export from north of Greenland, and by a weakening of the North Atlantic overturning circulation (which results in a reduction of warm water transported into the Arctic).

Published in: West, A. E., A. B. Keen, and H. T. Hewitt, Mechanisms causing reduced Arctic sea ice loss in a coupled climate model, The Cryosphere, 7, 555-567, 2013