NUMERICAL HINDCASTING OF SEA-SURFACE PCO2 AT WEATHERSHIP-STATION-PAPA

A one-dimensional model of temperature, salinity, nutrients, oxygen, carbon, and argon chemistry is used to hindcast the annual cycle of sea surface pCO2 at weathership Station Papa in the subarctic Pacific (50-degrees-N, 145-degrees-W), based on recent biological and chemical measurements made in conjunction with the SUPER program. Heat fluxes are calculated from the meteorological time series data from the Canadian weathership program. The PRICE, WELLER and PINKLE (1986) model is used for predicting mixed layer dynamics. The rate of new production in the model is based on sediment trap data (WELSCHMEYER, personal communication) and a comparison of model oxygen and argon concentrations with the recent data of EMERSON, QUAY, STUMP, WILBUR and KNOX (1991). The balances of nutrients and oxygen in the permanent halocline require isopycnal ventilation on a time frame of approximately 10 years; this estimate is consistent with the estimate of VAN SCOY, FINE and OSTLUND (1991) based in tritium data from Geosecs and Long Lines programs. The model results are compared with the 5 year time series data presented by WONG and CHANG (1990). The model appears to capture the mean sea surface PCO2 and the magnitude and timing of the seasonal cycle. The data, however, contain much greater high frequency variation than the model results, which we believe is caused by patchiness in the horizontal distribution of NO3. The model pCO2 sensitivity to the chemistry of the upwelling water and the rate of biological new production are presented. Although the model simulation of pCO2 is satisfactory, this study illustrates the limitations of modelling the chemistry of the upper ocean in one dimension. The slow currents and horizontally homogeneous sea surface in the subarctic Pacific make Papa one of the best available candidates for modelling in 1-D. In spite of this, a 1-D formulation is inadequate to address the chemistry of the halocline (a crucial lower boundary condition to the mixed layer) and does not constrain the transport of the nutrients by wind-driven currents in the mixed layer. We conclude that further progress in modelling the upper ocean nutrient and carbon cycles will require simulation in three dimensions.