The North Atlantic Oscillation influence on the Odra river estuary hydrological conditions

The study focused on the relationships between the North Atlantic Oscillation index and water temperature, number of days with ice, and water level in the Odra river estuaries. Statistically significant relationships occur mainly in the winter period (December–February, December–March, January–March), even up to α = 0.01. The strongest relationships with the NAO index are observed for the water temperature and number of days with ice, mainly in areas with minor water motion (Szczecin Lagoon). The correlation coefficients for water temperature are even >0.80. Only slightly weaker correlation coefficients apply to the NAO relationships to the number of days with ice, especially in the Odra river (in Widuchowa) and in the Pomeranian Bay (in Międzyzdroje). The relationships are deteriorated there by the relatively intensive water motion (ice inflow and outflow). During extreme winters (very harsh or very mild) in the Odra river estuary, local factors, mainly the local southern Baltic Sea atmospheric circulation, negatively affect the NAO relationships to the thermal and ice parameters. The weakest relationships to the NAO index apply to the water level; they are characterized by correlation coefficients <0.60 and weaken inland in the Odra river estuary. During some winters the NAO index relationships to the water level may be distorted by the wind and extreme inflows from the upstream sections of the Odra river.