So I have enough understanding to give you one of two options on the weather. That large of a mountain range creates a water shed. It will block storm fronts and cause all the water to be dropped on one side as the moisture is forced up in elevation. (Colder air holds less moisture.) It depends upon witch way the strongest air currents travel in your world.
If they travel from North to south in this area then the weather shall often be cold but mild. Few storms and little rain fall. Farmers in the area depend upon the river for water. Moisture evaporating from the Mediterranean does not fall as rain in the area since there was so little moisture all ready. There is probably a desert on the southern shore for quite a distance. The northern border of the mountains have much precipitation, but most of this occurs as snowfall. Massive drifts may accumulate.
If they travel South to North there are likely many thunder storms and much rain especially in season on the Northern coast. They may experience near torrential downpours for part of the year. The rains will tend to be warm, but there shall often be sleet and hair mixed in. Storms in the rest shall be very similar to the current Mediterranean, just slightly cooler. They won’t be to frequent, but can come out of no wear. The river itself is wide enough body of water that it might help push some of the worst of the storms away. But only slightly.(4 miles at most) Flooding would still be an issue but the worst of the rain might fall a few miles away.
Major air currents flow from both directions. Merge the rainy portions of both answers. Add in frequent and freak tornadoes, especially along the mountain passes and valleys.
In general you shall have the frequency of storms of the Mediterranean, but with much of the temperature of the North Sea and Baltic. This is of course assuming depth of the Mediterranean. (rather shallow). If you chance the deapth storm frequency shall change.
I believe Altera to be wrong about the diverse or tropical life. Winters shall be to cold.