North Atlantic open ocean water column physical and biogeochemical properties including productivity and carbon dioxide-fluxes at the PAP site
The Porcupine Abyssal Plain sustained observatory (PAP-SO) is in the inter-gyre northeast Atlantic (49◦ N, 16.5◦ W; water depth of 4850 m). The site is shown in Figure 1.
Multidisciplinary sensors are fitted to a mooring there and autonomous biogeochemical measurements have been made since 2002. Upper ocean variables measured include temperature, salinity, chlorophyll-a, dissolved inorganic nitrate, carbon dioxide, pH and dissolved oxygen.
Many of these datasets are available in near real-time delivered by satellite in high resolution and allow scientific users to evaluate the natural variation, episodic processes and long-term trends occurring in the North Atlantic. Non-QC data are visually displayed in near real-time on the FixO3 and PAP (noc.ac.uk/pap) websites. An example of the real time data is shown in Figure2
In situ data can be compared with regional time-series data from a ship of opportunity and mixed layer depth measurements from profiling Argo floats. The 2010–2012 period shows an overall increase in carbon dioxide values when compared to the 2003–2005 period as would be expected from increases due to anthropogenic carbon dioxide emissions. The surface temperature, wind speed and mixed layer depth measurements are similar for both periods of time.
Calibration coefficients are applied to the raw data and the data are further quality controlled. Then monthly means (& standard deviation) are calculated. An example is shown in Figure 3.
In addition to the multidisciplinary variables two main derived products are provided:
CARBON DIOXIDE FLUX: Calculated from surface ocean carbon dioxide together with the corresponding in situ atmospheric (e.g., wind) datasets from the same installation (provided by the Met Office UK) to assess the changing capacity for the North Atlantic as a carbon sink.
PRODUCTIVITY: Temperature, Salinity, oxygen, nutrients and chlorophyll-a will be used to assess the trends in the timing and quantity of open ocean productivity.
CARBON DIOXIDE FLUX:
The air-sea carbon dioxide flux can be calculated from the air-sea carbon dioxide difference, temperature and salinity (30 m) and wind speed. There is a persistent under saturation of carbon dioxide in surface waters throughout the year so this region is a year round sink for carbon dioxide. Comparison with an earlier dataset collected at the site (2003 to 2005) confirms seasonal and inter-annual changes in surface seawater chemistry. The observed seawater carbon dioxide increased from (339 ± 17) µatm in 2003-2005 to (353 ± 15) µatm in 2010-2012, which largely agrees with the increasing rate of surface seawater carbon dioxide observed in the North Atlantic basin. However the mean air-sea carbon dioxide flux did not show a significant change. It varied from (-5.7 ± 2.8) mmol m−2 d−1 in 2003-2005 to (-5.0 ± 2.2) mmol m−2 d−1 in 2010-2012.
The monthly calculations of carbon dioxide flux at the PAP-SO is shown in Figure 4.
One year of carbon dioxide flux calculations (calculated with in situ wind data) are shown in Figure 5.
Temperature, Salinity oxygen, nutrients and chlorophyll could all be used to assess the trends in the timing and quantity of open ocean productivity. There is a year-to-year variability in the timing of deep winter mixing at the PAP-SO (calculated from in situ temperature and salinity) and the intensity of the spring bloom (shown by chlorophyll measurements). Despite similar maximum winter mixed layer depths in 2003–2005 and 2010–2012, the timing of the spring bloom was different between years (as can be seen in Figure 6) as was the winter nitrate.
New production can be calculated from the average monthly drawdown of nitrate. This requires a calculation of the monthly nitrate concentration (from the in situ sensor measurements) and the mixed layer depth. Productivity was calculated up to the start of 2013 using the seasonal drawdown of nitrate to quantify the new production. Summing up the months with a net drawdown in nitrate (from February to August, figure 7) gives the mixed layer depth integrated seasonal new production as 0.37± 0.14 mol N m-2.