Abstract
Abstract. Accurately forecasting the sea-ice thickness (SIT) in the Arctic is a major challenge. The new SIT product (referred to as CS2SMOS) merges measurements from the CryoSat-2 and SMOS satellites on a weekly basis during the winter. The impact of assimilating CS2SMOS data is tested for the TOPAZ4 system – the Arctic component of the Copernicus Marine Environment Monitoring Services (CMEMS). TOPAZ4 currently assimilates a large set of ocean and sea-ice observations with the Deterministic Ensemble Kalman Filter (DEnKF). Two parallel reanalyses are conducted without (Official run) and with (Test run) assimilation of CS2SMOS data from 19 March 2014 to 31 March 2015. Since only mapping errors were provided in the CS2SMOS observation, an arbitrary term was added to compensate for the missing errors, but was found a posteriori too large. The SIT bias (too thin) is reduced from 16 to 5 cm and the standard errors decrease from 53 to 38 cm (by 28 %) when compared to the assimilated SIT. When compared to independent SIT observations, the error reduction is 24 % against the ice mass balance (IMB) buoy 2013F and by 12.5 % against SIT data from the IceBridge campaigns. The improvement of sea-ice volume persists through the summer months in the absence of CS2SMOS data. Comparisons to sea-ice drift from the satellites show that dynamical adjustments reduce the drift errors around the North Pole by about 8 %–9 % in December 2014 and February 2015. Finally, using the degrees of freedom for signal (DFS), we find that CS2SMOS makes the prime source of information in the central Arctic and in the Kara Sea. We therefore recommend the assimilation of C2SMOS for Arctic reanalyses in order to improve the ice thickness and the ice drift.
Highlights
Sea ice plays an important role in the Arctic climate system because it prevents the rapid exchange of heat flux between the ocean and atmosphere
The CS2SMOS mapping error is low for sea ice below 0.5 m: about 4 times lower than the uncertainties obtained by error propagation in the Soil Moisture and Ocean Salinity (SMOS) processing chain, which would make the assimilation of SMOS seaice thickness (SIT) too strong
4.2 Impact on the sea-ice extent and volume in the Central Arctic In Fig. 3, we show that the Arctic SIT has been improved everywhere, the assessment of the sea-ice drift is less conclusive but tends to suggest a slight improvement localized in the Central Arctic
Summary
Sea ice plays an important role in the Arctic climate system because it prevents the rapid exchange of heat flux between the ocean and atmosphere. The resulting SIT estimates are still very uncertain because of uncertainties in the snow depth (using climatology), snow penetration and sea-ice density (Kern et al, 2015; Khvorostovsky and Rampal, 2016). The CS2SMOS will be assimilated into the TOPAZ4 forecast system, which is a coupled ocean–sea-ice data assimilation system using the Deterministic Ensemble Kalman Filter (DEnKF; Sakov and Oke, 2008). It publishes a 10-day forecast of the ocean physics and biogeochemistry in the Arctic through the CMEMS portal It provides a long reanalysis from 1990 to the present – currently 2016 – that is extended every year.
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