Question

average level of the surface of the sea

Answer 1

HELLO MY DEAR FRIEND

THE AVERAGE LEVEL OF THE SURFACE OF THE SEA IS 

CALLED "MEAN SEA LEVEL".

I  HOPE THIS WILL HELPFUL TO U.....*-*

Answer 2

It's true that the sea-surface is not "flat" with respect to the geoid; local high-level surfaces stand just to the equatorward side of an eastward current, or just to the east side of a poleward current. Is that what you're getting at? The sea-level difference across the Gulf Stream is about 1 meter.