Seasonal variability of carbon dioxide, nutrients and oxygen in the northern North Atlantic surface water: observations and a model*

The seasonal variation of various surface water properties has been monitored at a station located at about 120 miles south of the Iceland-Greenland sill during the two-year period, March 1983 through May 1985. These properties include the temperature, salinity, mixedlayer depth, partial pressure of CO₂ in seawater and the concentrations of dissolved total CO₂, oxygen and nutrients. It was observed that during the summer, the C02 partial pressure and the concentrations of CO₂ and nutrients in surface water were lowest, while the oxygen concentration was highest. This situation was reversed during the winter. The seasonal variation is attributed mainly to the high photosynthetic utilization rate of carbon and nutrients in a strongly stratified and shallow surface mixed layer during the summer. The winter observations are attributed to the upward transport of deep waters rich in total CO₂ and nutrients by deep convective mixing. In order to account for the observed seasonal variation, a vertically one-dimensional, two-box Ocean model has been constructed. The vertical mixing between the surface mixed layer and deep water is characterized in terms of changes in the mixed layer thickness with time, and the biological productivity is related to the solar insolation and nutrient concentration in the mixed layer. Gas exchange of oxygen and CO₂ between the mixed layer and the atmosphere is taken into consideration. When this model is calibrated using the observed phosphate concentration in surface water, it yields seasonal variations of carbon and oxygen values consistent with the observations, with an exception of a large excursion of spring time values resulting from phytoplankton blooms. It is shown that the spring bloom effect can be simulated by a short-term reduction of the phosphate residence time and of the gas exchange rate.