Bimodality of Fast Ice Extent Distribution in the northeastern Kara Sea

The temporal-spatial variation of shore-fast ice extent in the northeast part of the Kara Sea during 1953 - 1999 and its sensitivity to interannual variability of regional climate are investigated. Regular observations by the Arctic and Antarctic Research Institute (Russia) during 1953 - 1990 and SSM/I surface brightness temperatures during 1987 - 1999 provide necessary data. Monthly mean surface layer air temperature, snow depth and precipitation rate from meteorological polar stations located in the study area are used to assess an influence of atmospheric conditions on fast ice formation. The data about wind velocity and direction from the Dikson Island are used to estimate impact of wind stress on fast ice extent. Monthly mean data of the Ob and Yenisei rivers discharges in 1950-1987 are used for estimation of possible influence on fast ice area.

Data analysis shows a significant correlation (about -50%) between mean winter air temperature anomalies and fast ice area in May for all meteorological stations located in the region under study. Analysis of the sensitivity of fast ice extent to the wind direction points to significant role of strong eastward and southeast winds in fast ice formation during winter. It is indicated by correlation of the same order (-50%) as for average winter temperature anomalies. During June-July winds from northeast direction increase fast ice destruction while winds from northwest tends to restrain it. Any significant correlations between fast ice extent and cumulative winter precipitation, average snow depth, and preceding summer river outflow rate are not found.

It is revealed that the relative frequency of fast ice area has bimodal distribution. It indicates the existence of two different regimes of fast ice formation, driven by the system of prevailing winds. The favourable conditions for expansion of fast ice seawards in the northeast Kara Sea correspond to atmospheric circulation controlled by the Arctic high. Such circulation determines offshore winds from the second quarter and significant decrease of the surface air temperature in the whole Kara Sea. In opposite cyclones coming from the west are responsible for the partial break-up and decrease of fast ice extent.

In total the shore fast ice area in the Kara Sea decreased from 50th to 90th. This decrease was most pronounced in May. Simultaneously break up of fast ice in the northeast part of the Kara Sea had taken place more early in 1990-1999 compared to 1953-1960.