Strong inertial currents and marginal internal wave stability in the central North Sea

Solar insolation stabilizes the water column and suppresses vertical exchan ge. Observations from the central North Sea clearly show that increased hea ting in summer is accompanied by enhanced de-stabilizing vertical current d ifferences (shear), surprisingly to such extent that the equilibrium state is marginally stable. Under calm weather conditions, the shear is predomina ntly generated by near-inertial motions while the internal wave spectrum pr imarily results from nonlinear interaction between the dominating tidal and near-inertial motions. In terms of the associated enhanced vertical mixing across the largest vertical temperature gradients, shelf seas are not diff erent from the abyssal ocean, despite the proximity to energy sources near boundaries in the former. By the lack of sufficiently strong wind- and tida l mixing this internal mixing is considered to be responsible for the diapy cnal transport of nutrients leading to the observed increase in near-surfac e values and triggering the late-summer phytoplankton bloom.