Catalogus

The calculations suggest that changes in the balance of the atmospheric forcing on a daily scale could lead to less pronounced fluctuations of the mixed layer in winter as during most of the day the heat input by the sun exceeds or compensates the heat loss by nonsolar heat flux components (i.e. transient or moderate conditions). Furthermore, the model results show that the springtime shallowing of the mixed layer is not typically a smooth transition, but is interrupted by mixing events stimulated by passage of weather systems. The frequency and intensity of these synoptic events varies from year to year due to shift in storm tracks and modes of atmospheric circulation. The simulations suggest that such synoptic events (storms) associated with time scales of several days, and the period of the year when the heat flux changes sign, are important for the development of the phytoplankton population. Enhanced or reduced storm frequency associated with changes in vertical mixing intensity during the late winter/spring period leads to an increase/decrease in amplitude and duration of the bloom caused by enhanced/reduced nutrient supply or retard it due to light limitation. Finally, supported by the model results and observations, the role of interannual variability in physical forcing on timing, amplitude and structure of the phytoplankton bloom for the period 1989–1997 is examined.