The variability of surface pCO2 and nutrients in the North Atlantic Ocean

This PhD thesis was part of the EU-funded project CAVASSOO (Carbon Variability Studies by Ships of Opportunity). The installation of a autonomously working pCO2 unit onboard the carcarrier M/V Falstaff was completed in January, 2002. Measurements started a month later with the first transatlantic crossing. The data of one year showed that the correlatin between pCO2 and nitrate was promising in this context and it could also be shown that nitrate correlated well with the mixed layer depth. Parameters such as temperature and chlorophyll on the other hand did not reveal a unique correlation with the pCO2. Within this thesis it was also shown that the seawater pCO2 in the eastern basin (10°W-35°W) showed smaller seasonal changes than in the western basin (36°W-70°W) in the North Atlantic. This was explained by the fact that in the eastern basin the temperature effect on the seawater pCO2 was counteracted by the biological effect yielding a damped seasonal pCO2 cycle. In the western basin, however, temperature was the major force on the pCO2 which was not reduced by a counteracting biology effect thus yielding a pronounced seasonal pCO2 cycle. The CO2 flux calculation showed that this region of the North Atlantic was a sink for atmospheric CO2 in 2002. When comparing the CO2 flux to a well-cited pCO2 climatology the difference was small (4%). The seasonal cycles of nutrients within different watermasses showed distinct patterns. The C:N ratio of the seasonal new production were similar to the Redfield ratio for all watermasses excecpt for the Gulfstream watermass. In the latter a carbon overconsumption with respect to Redfield could be shown which pointed at N2 fixation

CO2 fluxes in the sub-tropical and sub-arctic North Atlantic based on measurements from a Volunteer Observing Ship

Surface seawater pCO2 and related parameters were measured at high frequency onboard the volunteer observing ship M/V Falstaff in the North Atlantic Ocean between 36° and 52°N. Over 90,000 data points were used to produce monthly CO2 fluxes for 2002/2003. The air-sea CO2 fluxes calculated by two different averaging schemes were compared. The first approach used gas transfer velocity determined from wind speed retrieved at the location of the ship and called colocated winds, while for the second approach a monthly averaged gas transfer velocity was calculated from the wind for each grid pixel including the variability in wind. The colocated wind speeds determined during the time of passage do not capture the monthly wind speed variability of the grid resulting in fluxes that were 47% lower than fluxes using the monthly averaged wind products. The Falstaff CO2 fluxes were in good agreement with a climatology using averaged winds. Over the entire region they differed by 2–5%, depending on the time-dependent correction scheme to account for the atmospheric in increase in pCO2. However, locally the flux differences between the ship measurements and the climatology were greater, especially in regions north of 45°N, like the eastern sector. A comparison of two wind speed products showed that the annual CO2 sink is 4% less when using 6 hourly NCEP/NCAR wind speeds compared to the QuikSCAT wind speed data

The pCO₂ variability in the midlatitude North Atlantic Ocean during a full annual cycle

The results of 1 year of automated pCO2 measurements in 2002/2003 onboard the car carrier M/V Falstaff are presented and analyzed with regard to the driving forces that change the seawater pCO2 in the midlatitude North Atlantic Ocean. The pCO2 in surface seawater is controlled by thermodynamics, biology, air-sea gas exchange, and physical mixing. Here we estimate the effects on the annual cycle of pCO2 and relate this property to parameters like SST, nitrate, and chlorophyll. On the basis of the amplitude in seawater pCO2 for all 4° × 5° grid boxes, this region can be separated into an eastern and western basin. The annual pCO2 cycle in the eastern basin (10°W–35°W) is less variable, which can be related to the two counteracting effects of temperature and biology; air-sea gas exchange plays a minor role when using climatological MLD. In the western basin (36°W–70°W) the pCO2 amplitude is more variable and strongly follows the thermodynamic forcing, since the biological forcing (as derived from nitrate concentrations) is decreased. Biology and air-sea exchange strongly depend on the MLD and therefore also include physical mixing effects. The pCO2 data of the analyzed region between 34°N and 52°N compare well to the Takahashi et al. [2002] climatology except for regions north of 45°N during the wintertime where the bias is significant