Long-term mean, interannual and seasonal circulation in the Gulf of Finland — The wide salt wedge estuary or gulf type ROFI

Abstract

Abstract Circulation of the wide gulf type ROFI or salt wedge estuary, the Gulf of Finland (GoF), was studied using a numerical 3D hydrodynamic model with 1 nmi horizontal resolution and 40 vertically adaptive layers for a period of 40 years. The results show an extensive down estuary or westward coastal current (WCC) on the left hand flank of the GoF. The WCC is more vigorous during the spring and summer months when longitudinal positive estuarine circulation with salt wedge up estuary transport and less saline upper layer water (LSULW) down estuary transport prevails. In the beginning of summer, the coastal current forms an extensive northward cross-shore current in the center of the gulf, giving rise to two basin-wide anticyclonic circulation gyres. The intensity of the WCC, anticyclonic gyres and estuarine transport vary interannually. The zonal wind component averaged over the period of the dominance of a particular feature — April–June for the WCC, July–September for the anticyclonic gyres and annual for estuarine exchange flow — modulates the strength of these circulation patterns. A negative/positive zonal wind supports/destroys the WCC and increase/decrease the intensity of the salt wedge and LSULW transport. The anticyclonic gyres are well developed during weak positive zonal wind, mainly.