Abstract：
In this study, sea ice concentration (SIC) budgets were calculated for five ocean-sea ice reanalyses (CFSR, C-GLORSv7, GLORYS12v1, NEMO-EnKF and ORAS5), in the Southern Ocean and compared with observations. When applying the SIC budget diagnostics to decompose the changes in SIC into contributions from advection, divergence, thermodynamics, deformation and data assimilation, we find that the driving atmospheric and oceanic forcings are the most significant contributors on the budget differences. For the CFSR, the primary source of deviation compared to other reanalyses is the stronger northward component of surface-ocean current, which results in stronger sea ice advection and divergence. These deviations are related to excessive freezing that results in an average CFSR sea ice thickness being twice that of other reanalyses. The secondary source, because Coriolis force is proportional to sea ice thickness, is the more northward velocity of free-drifting sea ice from the predominantly westerly winds and currents in the Antarctic Circumpolar Current. In summary, this could be interpreted as a positive feedback between sea ice thickness and northward sea ice transport. The other three reanalyses, C-GLORSv7, GLORYS12V1 and ORAS5, in contrast to the CFSR, underestimate the contribution of dynamics to changes in SIC with respect to observations. The NEMO-EnKF, using NCEP as an atmospheric forcing, has the largest zonal sea ice velocity among all reanalyses, but compared to observations, the effect of the dynamics on sea ice change is also underestimated because the northward velocity and north-south velocity gradient are not large enough. Another effect of excessively large wind speeds on the NEMO-EnKF is that they produce more sea ice convergence than other systems.
short-bio：
Yafei Nie is a PhD student from the Ocean University of China (supervisor: Xianqing Lv), now visiting in University of Helsinki (supervisor: Petteri Uotila). His research topics include ocean dynamics, sea-ice geophysics, data assimilation and ocean reanalysis.
