Leads as natural fairways in the Gulf of Finland

The monitoring and prediction of sea ice conditions are vital for safe and economical winter navigation in the Gulf of Finland (GoF), where ice conditions are very variable and dynamic. Flaw leads and ice ridges are common features in the GoF. In this study the occurrence frequency of the leads and ridges during 1971-2009 is analyzed. For the historical period, the ice charts of the Estonian Meteorological and Hydrological Institute (EMHI) were used for determining their occurrence and location in the Gulf of Finland. A case study was performed for a severe winter in 2002/2003. MODIS satellite imagery was used in addition to ice charts for determining the location of the leads in the Gulf of Finland. Also, the HELMI ice model was used to examine ice characteristics during the winter of 2002/2003 and the relationship between deformed ice growth rates and the speed and direction of wind. The results of the analysis of historical data show that on the average the leads are most common in the Estonian coastal region where their occurrence is typically 10-30%. During severe winters, leads occur quite often also in the Finnish coastal region, but the ridges are more common in the Estonian coast.