Medium-range predictability of early summer sea ice thickness distribution in the East Siberian Sea: Importance of dynamical and thermodynamic melting processes

Abstract. Accelerated retreat of Arctic Ocean summertime sea ice has focused attention on the potential use of the Northern Sea Route (NSR), for which sea ice thickness (SIT) information is crucial for safe maritime navigation. This study evaluated the medium-range (lead time below 10 days) forecast skill of SIT distribution in the East Siberian Sea (ESS) in early summer (June–July) based on the TOPAZ4 ice ocean data assimilation system. Comparison of the operational model SIT data to all available observations (in situ and satellite) showed that the TOPAZ4 reanalysis reproduces the observed seasonal cycle and the rates of advance and melting of SIT in the ESS, with average bias of approximately ±20 cm. Pattern correlation analysis of the SIT forecast data over 4 years (2013–2016) reveals that the early summer SIT distribution is skillfully predicted for a lead time of up to 3 days, but that the prediction skill drops abruptly after the 4th day, which is related to dynamical process controlled by synoptic-scale atmospheric fluctuations. For longer lead times (> 4 days), the thermodynamic melting process takes over, which makes most of the remaining prediction skill. In July 2014, during which an ice-blocking incident occurred, relatively thick SIT (approximately 150 cm) was simulated over the ESS, which is consistent with the reduction of vessel speed. These results suggest that TOPAZ4 sea ice information has a great potential for practical applications in summertime maritime navigation via the NSR.