Observations and simulations of physical and biological processes at ocean weather station P, 1951–1980

Abstract

Physical and biological processes in the mixed layer at ocean weather station (OWS) P (50°N, 145°W) over a 30‐year period (1951–1980) were investigated using observations and model simulations. The observations include 30 years of surface meteorological and sea surface temperature data collected at OWS P and Ekman upwelling velocities derived from the Comprehensive Ocean‐Atmosphere Data Set, 14 years (1953–1966) of daily temperature profiles, nearly 150 chlorophyll a profiles spanning all months of the year, monthly climatological solar irradiances, and 0‐ to 50‐m integrated nitrate concentrations. The simulations incorporated models for the estimation of surface solar downwelling irradiance, surface heat fluxes, subsurface diffuse attenuation, mixed layer dynamics, and biological processes. The time‐dependent model inputs were the surface observations of cloud cover, air temperature, dew point temperature, and wind speed. The atmospheric irradiance, marine diffuse attenuation, and mixed layer models were adapted from existing models developed by others. The biological model, developed by the authors, has four components (nitrate, ammonium, phytoplankton nitrogen, and zooplankton nitrogen) and computes a variety of additional quantities, including chlorophyll a concentration and gross and new production. Model comparisons with in situ time series showed that predictions of sea surface temperature and mixed layer depth were reasonably accurate. Climatological monthly profiles of chlorophyll a and temperature were within 1 standard deviation of the observed values at nearly all depths. Also, the climatological annual cycles of solar irradiance and 0‐ to 50‐m integrated nitrate accurately reproduced observed values. Annual primary production was estimated to be ∼190 g C m−2 yr−1 and varied by no more than ±5% in any year. This estimate is consistent with recent observations but is much greater than earlier estimates, indicating that carbon cycling in the North Pacific is much more important to the global carbon budget than previously thought. Significant interannual variability in sea surface temperature, Ekman upwelling, mixed layer depth, and surface nitrate concentration had little impact on productivity. The model also indicates that the nitrate supply to the euphotic zone is very sensitive to Ekman upwelling and that amplification of the wind stress curl can result in complete nitrate depletion when the winds are persistently downwelling favorable.