Long-term variability and methods of forecasting dates of ice break-up in the mouth area of the Ob and Yenisei rivers

Long-term characteristics of the onset dates of spring ice phases at hydrological stations in the mouth areas of the Ob and Yenisei rivers for 1936–2006 are obtained. The correlations are analyzed between these dates and the frequency of different synoptic processes over the Atlantic and Eurasia in the fall-winter months, the dates when the accumulated temperatures at meteorological stations in the middle parts of the river basins reach 5, 10, 20, 30, and 40°C, and the dates of the start of ice drift at the upstream sections of the Yenisei. Prognostic relations are suggested and test forecasts of the onset dates of ice phases are verified. The percentage of correct forecasts was 67–86%, which makes it possible to recommend the relations for using in prognostic practice. The forecast lead time ranges from 3 to 110 days.