Ed Blockley (ed.blockley[@]metoffice.gov.uk), Met Office Hadley Centre

Arctic summer sea ice cover has declined at a rate of 13% per decade since satellite observation began, and there is much interest in how this decline will continue in the future. Global coupled models are arguably the best tool we have for making future projections of Arctic sea ice, but generate a wide spread of projections of future decline.

Several previous CMIP analyses/studies have attempted to decrease the spread of plausible future projections by sub-selecting models based on their ability to simulate current day sea ice and/or past observed changes. However such an approach ignores the considerable influence of internal variability in the Arctic climate system as well as the existence of cancelling errors in models and the potential for these to change in climate simulations as the system warms.

Comparing frequently analysed integrated quantities – such as ice extent and volume – alone is not sufficient to understand the reasons for differences in model projections. It is becoming increasingly clear that it is also necessary to consider, compare and evaluate the underlying processes causing ice growth and decline, and how they are likely to change in a warming world.

Areas of opportunity
* Improved knowledge of the sea ice volume budget and Surface Energy Budget (SEB) in climate models will help understand the spread in climate simulations
* Understanding differences between climate model simulations will help us better understand the causes/drivers of sea ice change/decline
* Extra sea ice budget diagnostics will be available for CMIP6 models, allowing easier inter-model comparison of underlying processes. Models will also be more sophisticated and run at higher resolution
* Multi-sensor observation platforms and campaigns in the Arctic can be exploited to assess how realistic are the processes simulated by climate models
Proposed work
* A sea ice volume budget comparison using CMIP6 model simulations
    – Performed in alignment with the APPLICATE project (EU H2020)
    – Including both inter-model and intra-model (i.e., within ensembles) comparisons
* Model processes assessed against available in-situ and satellite observations. In particular using multi-sensor sites (MOSAiC, SHEBA, IMBs, MIDO, IAOOS)
* Particular focus areas will be:
    – Surface Energy Budget (SEB)
       — SIMIP surface energy balance comparison and evaluation with reference to satellite and multi-sensor observations
       — Analysis of links between surface energy balance and ice volume budget in the ensemble
       — Examination of possible model drivers of SEB anomalies
    – Whole sea ice volume/energy budget decomposition (including the relative impact of dynamic vs. thermodynamic processes)
    – Initially focused on the Arctic but with extension to Antarctic

Open questions
* How to determine which model is more realistic?
* What variables are necessary to do this well?
* Looking for contacts to help with analysis of the various climate models


•    SIMIP sea ice & snow mass budget inter-comparison activity. This activity is progressing well and things are on track for submission before the AR6 deadline in December.

The Met Office Hadley Centre contribution to & leadership of this activity is funded under the EU-APPLICATE project. More information is available on the EU-APPLICATE project website at https://applicate.eu/simip