Occurrence of synoptic flaw leads of sea ice in the Gulf of Finland

In spite of their importance to the marine ecosystem and winter navigation, flaw leads have not received much attention. This study presents an analysis of the flaw lead occurrence frequency in the Gulf of Finland. We used ice charts of the Estonian Meteorological and Hydrological Institute covering the years 1971―2007 and the Helsinki multi-category sea-ice model. Flaw leads are formed in the Gulf of Finland almost everywhere along the fast-ice edge when moderate or strong winds are acting, but the winds from W, NW, N and S sectors generate a rather uniform lead pattern, thus facilitating navigation in the ice. On average, flaw leads are most common in the Estonian coastal region where their occurrence is typically 10%―30%. However, during severe winters when northerly winds are more frequent flaw leads are also common in the Finnish coastal region.