Variability of pCO2 on diel to seasonal timescales in the Sargasso Sea near Bermuda
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Continuous underway measurements of atmospheric and surface seawater pCO2 were collected on numerous cruises in the Sargasso Sea (32°N, 64°W) near Bermuda from June 1994 to November 1995. We observed that seawater pCO2 was highly variable on different timescales, ranging from diel to seasonal. On diel timescales, pCO2 changes of 5-25 μatm occurred in response to diurnal warming and cooling associated with solar heat fluxes. Over longer timescales, pCO2 was influenced by atmospheric forcing and tropical cyclones. For example, a surface cooling of 3°C and decrease in pCO2 of 45-50 μatm occurred after Hurricane Felix passed near Bermuda in August 1995. The decrease in pCO2 was significant considering the annual change was 90-100 μatm. Over all timescales, temperature was the dominant control on pCO2 variability. We found that surface pCO2 conditions were accurately predicted from temperatures with small errors (4-9 μatm) if seasonal pCO2-temperature relationships were established. In future synthesis of regional pCO2 data it should be feasible to use surface temperature, remotely sensed from space, as a tool for extrapolation over wider spatial scales in the North Atlantic subtropical gyre. Net annual fluxes of CO2 for 1994 and 1995 (-0.25 to -0.6 mol CO2 m-2 yr-1) were directed from atmosphere to ocean and were similar to values reported for 1989-1993 by Bates et al. [1996b]. We found that short-term variability of pCO2 (diel warming and cooling or atmospheric forcing), frequency of sampling (every 3-4 days or monthly), or use of temperature-derived pCO2 did not affect estimates of net yearly CO2 fluxes by more than 10-20%. However, strong winds associated with hurricanes decreased the net annual flux of CO2 into the ocean by 19-28% in 1995. The major sources of error for air-sea gas exchange was uncertainty associated with gas transfer-wind speed relationships and differences in the types of wind speed data used (daily averaged versus climatological). Such uncertainties make it difficult to quantify the contribution of gas exchange to the carbon cycle and the balance of carbon import and export terms in the upper ocean of the Sargasso Sea.
author list (cited authors)
Bates, N. R., Takahashi, T., Chipman, D. W., & Knap, A. H.