Abstract
Abstract. An observation product for thin sea ice thickness (SMOS-Ice) is derived from the brightness temperature data of the European Space Agency's (ESA) Soil Moisture and Ocean Salinity (SMOS) mission. This product is available in near-real time, at daily frequency, during the cold season. In this study, we investigate the benefit of assimilating SMOS-Ice into the TOPAZ coupled ocean and sea ice forecasting system, which is the Arctic component of the Copernicus marine environment monitoring services. The TOPAZ system assimilates sea surface temperature (SST), altimetry data, temperature and salinity profiles, ice concentration, and ice drift with the ensemble Kalman filter (EnKF). The conditions for assimilation of sea ice thickness thinner than 0.4 m are favorable, as observations are reliable below this threshold and their probability distribution is comparable to that of the model. Two parallel Observing System Experiments (OSE) have been performed in March and November 2014, in which the thicknesses from SMOS-Ice (thinner than 0.4 m) are assimilated in addition to the standard observational data sets. It is found that the root mean square difference (RMSD) of thin sea ice thickness is reduced by 11 % in March and 22 % in November compared to the daily thin ice thicknesses of SMOS-Ice, which suggests that SMOS-Ice has a larger impact during the beginning of the cold season. Validation against independent observations of ice thickness from buoys and ice draft from moorings indicates that there are no degradations in the pack ice but there are some improvements near the ice edge close to where the SMOS-Ice has been assimilated. Assimilation of SMOS-Ice yields a slight improvement for ice concentration and degrades neither SST nor sea level anomaly. Analysis of the degrees of freedom for signal (DFS) indicates that the SMOS-Ice has a comparatively small impact but it has a significant contribution in constraining the system (> 20 % of the impact of all ice and ocean observations) near the ice edge. The areas of largest impact are the Kara Sea, Canadian Archipelago, Baffin Bay, Beaufort Sea and Greenland Sea. This study suggests that the SMOS-Ice is a good complementary data set that can be safely included in the TOPAZ system.
Highlights
The Arctic climate system has undergone large changes during the last 20 years: increase of temperature (Chapman and Walsh, 1993; Serreze et al, 2000; Karl et al, 2015; Roemmich et al, 2015), decrease of sea ice extent (Johannessen et al, 1999; Comiso et al, 2008; Stroeve et al, 2012), sea ice thinning, and loss of sea ice volume (Rothrock et al, 1999; Kwok and Rothrock, 2009; Laxon et al, 2013)
The study in this paper investigates the impact of assimilating this data set within the TOPAZ system, which is the Arctic component of the Copernicus Marine Services
It is shown that for thin ice, the TOPAZ reanalysis and the Soil Moisture and Ocean Salinity (SMOS)-Ice have comparable distributions and that conditions are favorable for assimilating this data set
Summary
The Arctic climate system has undergone large changes during the last 20 years: increase of temperature (Chapman and Walsh, 1993; Serreze et al, 2000; Karl et al, 2015; Roemmich et al, 2015), decrease of sea ice extent (Johannessen et al, 1999; Comiso et al, 2008; Stroeve et al, 2012), sea ice thinning, and loss of sea ice volume (Rothrock et al, 1999; Kwok and Rothrock, 2009; Laxon et al, 2013). The first demonstration of assimilating SMOS-Ice has been presented by Yang et al (2014) for the period from November 2011 to January 2012 The system assimilates both SIT (thinner than 1 m) from SMOS-Ice and SIC from Special Sensor Microwave Imager/Sounder (SSMIS) in a nested Arctic configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). It uses the Localized Singular Evolutive Interpolated Kalman (LSEIK; Nerger et al, 2005) data assimilation method with a 15member ensemble.
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