Verification of modelled locations of coastal areas exposed to current-driven pollution in the Gulf of Finland by using surface drifters

Statistical properties of the drift of floating items from the major fairway to the coast and numerically simulated transport of pollution by surface currents to the nearshore are compared for the Gulf of Finland. The comparison is based on tracks of 23 surface drifters that crossed the fairway in the central part of the gulf in 2011-2014 and 17 280 simulated trajectories of passive virtual parcels with starting points in the same section of the fairway and evaluated using velocity fields from the Rossby Centre Ocean (RCO) model in 2000-2004. More than 25% of the drifters that crossed the major fairway in the area north and north-west of Tallinn reached either the southern (Estonian) or northern (Finnish) coast. This probability matches similar estimates for single water parcels that are locked in the surface layer and exclusively carried by simulated currents. The probability of reaching the Estonian and Finnish nearshore by simulated parcels or the coast by drifters is roughly equal. Both surface drifters and virtual parcels generally drifted to the west before they reached the coast or nearshore, except for surface drifters that arrived on the Estonian coast.