Development of Coupled Data Assimilation With the BCC Climate System Model: Highlighting the Role of Sea-Ice Assimilation for Global Analysis

The coupled data assimilation (CDA) system consisting of ocean, sea-ice, and atmosphere data assimilation components with the Beijing Climate Center (BCC) Climate System Model has been developed to provide reliable analyses of the atmosphere, ocean, and sea-ice states. It incorporates ocean temperature/salinity profiles, sea surface temperature, sea level height, and sea-ice concentration observations at a daily frequency, and atmosphere reanalysis at a 6-hourly frequency. Results show that the system is capable of realistically reproducing the climatology and variability of ocean, sea-ice, and atmosphere. The performances in analyzing ocean component are comparable to those of well-known ocean reanalyses that were once used to initialize the BCC model for climate predictions. A series of experiments with and without sea-ice observations in the CDA framework are designed to explore the role of sea-ice data assimilation (DA). The addition of sea-ice DA exerts very small influence to the analysis of upper ocean temperature over the Arctic area, but leads to an evident reduction of temperature error in the upper 1000 m of ocean south of 60 degrees S. Particularly, only the inclusion of sea-ice DA can make the ocean/ocean-atmosphere DA effective in providing skillful analysis in the high-latitude Southern Ocean. Furthermore, we find that only on the basis of ocean DA, can the addition of sea-ice concentration assimilation improve the analysis of tropical tropospheric atmosphere, along with a better analysis of mid- and high-latitude stratospheric atmosphere. These results address the importance of coordination of sea-ice observations and ocean observations in CDA.

Automated summary

The development of a coupled data assimilation system has led to reliable analysis of the atmosphere, ocean, and sea-ice states, with a focus on the importance of coordinating sea-ice observations and ocean observations. The system realistically reproduces the climatology and variability of ocean, sea-ice, and atmosphere, showing significant improvements in temperature error reduction in the upper ocean and effective analysis in the high-latitude Southern Ocean through sea-ice data assimilation.