Introduction
A persistent feature of the subpolar North Atlantic is
under-saturation of carbon dioxide (CO2) in surface waters throughout the
year, which gives rise to a perennial CO2 sink (Körtzinger et al.,
2008). This makes the north east Atlantic a region of great importance in the
global carbon cycle. There is evidence for inter-annual variation in the
CO2 sink (1–3 molm-2a-1) due to changes in wintertime mixing
and stratification (Schuster and Watson, 2007). Changes in the amount of
CO2 absorbed by the ocean may have implications for the global carbon
cycle now and for the role of the ocean as a carbon sink in the future.
Studies of the physical and biological processes regulating surface water
p(CO2) (partial pressure of CO2) are required to estimate future
trends in the ability of the ocean to act as a sink for increasing CO2 in
the atmosphere. Frequent observations from fixed positions are critical to
make these calculations (McGillicuddy et al., 1998).
Accurate, high-resolution, long-term data sets are offered by time series
studies such as the Porcupine Abyssal Plain sustained Observatory (PAP-SO) in
the northeast Atlantic at 49∘ N, 16.5∘ W (4850 m water
depth) where a fixed-point mooring has been in place since 2002 (Hartman et
al., 2012). The PAP-SO is in the North Atlantic Drift Region, a
biogeographical province defined by deep winter convective mixing (Longhurst,
2006; Monterey and Levitus, 1997). The surface mixed layer depth can change
from 25 m in the summer to over 400 m in winter. A 2-fold decrease in
winter nitrate concentration over a 3-year period from 2003 has been
attributed to a combination of shallower winter convective mixing and changes
in surface circulation (Hartman et al., 2010). The PAP-SO is in an area with
relatively high wind speeds, frequently greater than 10 ms-1.
High wind speeds have a significant effect on CO2 flux (Takahashi et
al., 2002). The CO2 flux at the PAP-SO was calculated from p(CO2),
between 2003 and 2005 as a net flux into the ocean of over
3 molm-2a-1 (Körtzinger et al., 2008). This is a
significant sink compared with subtropical time series sites such as ESTOC
(near the Canary Islands, 29.17∘ N, 15.5∘ W), which is an
overall annual CO2 source region (0.05 molm-2a-1,
González-Dávila et al., 2003).
Recently, the decline in North Atlantic CO2 uptake from 1994–1995 to
2002–2005 has been linked to a variation in the North Atlantic Oscillation
(Schuster and Watson, 2007; Padin et al., 2011). The decreased uptake may be
a consequence of declining rates of wintertime mixing and ventilation between
surface and subsurface waters due to increasing stratification. Enhanced
stratification forms a barrier to nutrient exchange, which may result in a
progressive decline in primary production (Field et al., 1998), as was seen
in the North Atlantic between 1999 and 2004 (Behrenfeld et al., 2006). The
observed decrease in nitrate concentration and productivity in this region
(Behrenfeld et al., 2006), may in turn affect the oceanic uptake of
p(CO2) .
In this paper, we present recent year round time series data of temperature,
salinity, nitrate concentration, chlorophyll a fluorescence and p(CO2)
collected at 30 m depth from May 2010 to June 2012. The data are compared
with an earlier published data set (from July 2003 to July 2005) and
additional p(CO2) measurements made from a ship of opportunity. The in
situ data set is considered in relation to convective mixing processes using
mixed layer depth (MLD) estimates calculated from profiling Argo floats. The
weekly air–sea CO2 flux at the PAP-SO site was calculated from in situ
p(CO2) measurements and ancillary satellite wind speed data sets. The
objective of this study is to examine the biogeochemical variations at the
PAP-SO in the northeast Atlantic over different periods from weekly, seasonal
to annual.
Materials and methods
Study site
The position of the PAP-SO at
49∘ N, 16.5∘ W is shown in Fig. 1. Lampitt et al. (2001) has
summarised the hydrography, meteorology and upper mixed layer dynamics in the
region.
In situ data
The instrumentation of the PAP-SO has been described in detail by
Hartman et al., 2012 (see Table 1 and Fig. 1 therein) and is briefly
summarised here. Since 2002 instruments on a mooring at the PAP-SO
(49∘ N, 16.5∘ W) have recorded a suite of parameters in the
mixed layer. Temperature and salinity measurements were made on a frame at a
nominal depth of 30 m, using Seabird SBE 37-IM MicroCAT recorders (Sea-Bird
Electronics Inc., Bellevue, Washington, USA). Measurements of nitrate
concentration, chlorophyll a fluorescence and p(CO2), were also made
using biogeochemical sensors on the frame, often within the deep chlorophyll
maximum. Between 2002 and 2007 the sensor frame depth varied from 20 to
225 m, deflecting in response to local currents. A surface buoy was added in
2007 so that measurements were consistently made at 30 m depth. In 2010,
collaboration with the UK Met Office led to a redesigned infrastructure,
providing simultaneous surface physical and biogeochemical measurements with
surface meteorological data.
p(CO2) data during the two periods of time examined here were collected
using different instrumentation. From 2003 to 2005 it was measured using a
SAMI (Sunburst Sensors LLC, USA) sensor, which is based on equilibration of a
pH indicator solution, contained in a gas-permeable membrane, with ambient
p(CO2) and subsequent spectrophotometric determination in the
equilibrated solution (DeGrandpre et al., 1995). Twice daily p(CO2)
measurements, from 2010 to 2012, were made using a membrane-based PRO-CO2
sensor (Pro-Oceanus, Canada), which uses an infrared detector and is
internally calibrated through an auto-zero calibration function (Jiang et
al., 2014). Note that a measurement error of an early version of the PRO-CO2
sensor during the deployment, induced by the fluctuation of detector cell
temperature, was identified and corrected (see Jiang et al., 2014 for further
details). A pump was used (Seabird Inc.) to improve water flow across the
sensor membrane to accelerate attaining equilibrium. The surface in situ
p(CO2) time series ceased between 2006 and 2009 due to funding issues.
Map of the inter-gyre region of the northeast Atlantic showing the
bathymetry around the PAP observatory (white diamond) and the ship of
opportunity (SOO) sampling positions (red circles) from 2010 to 2012.
Although measured by different instruments, the two p(CO2) data sets
were calibrated in a similar way to make them comparable: the sensor outputs
were calibrated against p(CO2) values calculated from dissolved
inorganic carbon (DIC) and total alkalinity (TA) from discrete samples taken
at the mooring site during deployment/recovery cruises; and plausibility
checks were made with underway p(CO2) measurements around the PAP site.
The 2003–2005 data were previously published (see Körtzinger et
al., 2008 for details) with a precision of 1 µatm and accuracy
estimated as 6–10 µatm. The 2010–2012 data have a similar
precision (1 µatm) and accuracy (6 µatm).
A Hobi Labs Inc., HS-2 fluorometer (Arizona, USA) was used on the PAP-SO
mooring to estimate chlorophyll a concentration until 2005 when an
alternative ECO FLNTU (WETlab, USA) fluorometer came into use. The quoted
precision for fluorescence measured by these fluorometers is 0.04 %,
however as described by Hartman et al. (2010), fluorescence output can only
provide an approximation of the chlorophyll a concentration. The
fluorescence/chlorophyll a concentration ratio changes throughout the year,
due to variations in the phytoplankton species composition. On the mooring,
chlorophyll a fluorescence measurements were taken every 2 h over
the 1 year deployments and biofouling was controlled using motorised copper
shutters on each of the fluorometers.
Nitrate concentration measurements were initially made using wet chemical NAS
Nitrate Analysers (EnviroTech LLC, USA), precision
0.2 µmolL-1, as described in Hydes et al. (2000), with twice
daily sampling frequency and internal calibration as described by Hartman et
al. (2010). From 2010 additional higher frequency inorganic nitrate
measurements were made using UV detection methods (ISUS, Satlantic), with a
precision of 1 µmolL-1.
For each instrument, the manufacturer's calibration was checked at the start
of each deployment and a correction for instrument drift was made using a
second calibration check on recovery of the instruments. Biogeochemical data
from the PAP-SO are available from EuroSITES (2014) and the British
Oceanographic Data Centre (BODC). Data presented here cover the period when
p(CO2) measurements are available, July 2003 to the end of June 2005
(with deployments in July 2003, November 2003, June 2004) and the period from
May 2010 to June 2012 (with sensor deployment in May 2010, September 2010,
July 2011, May 2012). All of the measurements are within the mixed layer
although the depth of measurements is closer to the 30 m nominal depth after
mooring redesign to incorporate a surface float in 2007.
Other observational data sources
Temperature and salinity data were taken from Argo floats
(http://www.coriolis.eu.org), extracting (30±5) m depth data. To
obtain a continuous seasonal description, a large region around the PAP site
was selected (45–52∘ N and 26.08–8.92∘ W, excluding the
shelf area). The Argo data have a potentially lower accuracy
(0.005 ∘C for temperature and 0.1 for salinity) than the in situ
MicroCAT data (0.002 for salinity and 0.002 ∘C for temperature).
However the Argo data were chosen over the in situ data for all calculations
as they have a larger temporal coverage and are internally consistent.
The p(CO2) time series was compared with surface data from a ship of
opportunity (SOO) running from Portsmouth, UK, to the Caribbean (Schuster and
Watson, 2007). Onboard the SOO continuous p(CO2) measurements are made
using a calibrated system with a showerhead equilibrator (Schuster et
al., 2009). Data are available from the Surface Ocean CO2 Atlas (SOCAT;
http://www.socat.info/). Discrete nutrient samples were collected at
4 h intervals along the same route and were analysed ashore (Hartman et
al., 2008). This provides an approximately monthly nutrient sample and
p(CO2) data points close to the PAP-SO on the return route of the ship.
The nominal depth of these samples is 5 m, which is shallower than the 30 m
samples from the PAP-SO. We selected SOO data between 52 and 45∘ N
and 8.92 and 26.08∘ W, and then took the average p(CO2) values
that were within that area on the same day as the sample from the PAP-SO
site.
Through collaboration with the UK Met Office in situ wind speed data are
available since 2010. However for consistency in calculations of CO2 flux
between the two time periods (2003–2005 and 2010–2012) considered here we
took wind speed data from weekly satellite data: Fleet Numerical Meteorology
and Oceanography Center (FNMOC) 1 ∘ by 1 ∘. We calculated a
weekly mean from the 6 hourly, 10 m height data; available from
http://las.pfeg.noaa.gov/.
The air–sea CO2 flux (in mmolm-2d-1) was calculated from
the air–sea p(CO2) difference, temperature and salinity (30 m) and
wind speed at 10 m height, using the following equation:
F(CO2)=k⋅K0p(CO2,sea)-p(CO2,air),
where k is the transfer coefficient based on the wind speed-dependent
formulation of Nightingale et al. (2000), scaled to the temperature-dependent
Schmidt number according to Wanninkhof (1992), K0 is the CO2 solubility
at the in situ temperature and salinity after Weiss (1974). While p(CO2,
sea) and p(CO2, air) are the CO2 partial pressures of seawater and
average CO2 dry mole fraction measured in air, respectively. As
p(CO2) was reported throughout this manuscript, we used p(CO2) for
the air–sea flux calculation. Using fugacity for the calculation would
generate the same results of flux estimates. The atmospheric p(CO2) is
calculated from monthly averaged p(CO2) measured at Mace Head
(53.33∘ N, 9.90∘ W) assuming 100 % water vapour
saturation under 1 atm air pressure. Please note that 1 atm =1.01325 bar. This is an appropriate pressure to use at the PAP-SO as the
average (and standard deviation) of the air pressure, measured on the buoy at
the PAP-SO between September 2010 and July 2011, was (1.01354±13.14)
bar.
Total alkalinity (TA) was calculated from Argo temperature and salinity
(30 m), following the relationship for the North Atlantic developed by Lee
et al. (2006) with an uncertainty of ±6.4 µmolkg-1 (Lee
et al., 2006). The DIC concentration was then calculated from TA and
p(CO2) using the “seacarb” package (Lavigne and Gattuso, 2011), with
Argo temperature and salinity (30 m) and nutrient concentrations set to
zero. The chosen constants were Lueker et al. (2000) for K2 and K2,
Perez and Fraga (1987) for Kf and the Dickson (1990) constant for
Ks, as recommended by Dickson et al. (2007). We followed
Körtzinger et al. (2008)'s method to correct the DIC changes driven by
air–sea exchange:
ΔDICgas=F(CO2)/MLD.
Using TA and p(CO2) to calculate DIC, and taking the various
uncertainties in the calculation into account, introduces an error in the
order of 7.0 µmolkg-1.
The MLD was calculated from density profiles using global gridded fields of
temperature and salinity collected by Argo floats, XBTs, CTDs and moorings.
These data are collected and made freely available by the Coriolis project
and programmes that contribute to it (http://www.coriolis.eu.org). We
used the near real-time mode data as these data sets have been quality control
checked. Before deciding on a MLD definition an inter-comparison of many
definitions commonly used in the literature was done such as density
differences, temperature differences and density gradients (Kara et
al., 2000; Thomson and Fine, 2003; Montegut et al., 2004). A subset of the
global density profiles calculated from the gridded temperature and salinity
fields was used to compare the different methods. The depth of the mixed
layer was estimated through visual inspection of over 3000 profiles,
following a similar approach used by Fiedler (2010). The Holte and Talley
(2009) density difference algorithm gave the closest match with the visually
estimated MLD (RMSD 29.38 m). The depth of the mixed layer was defined by a
density difference of 0.03 kgm-3 from the density at a reference
depth (in this case 10 m to avoid diurnal changes in temperature and
salinity at the surface). This Holte and Talley (2009) density difference
algorithm incorporates linear interpolation to estimate the depth at which
the density difference is crossed.
The North Atlantic Oscillation (NAO) index (after Hurrell, 1995) was
obtained from the University of East Anglia web site
http://www.cru.uea.ac.uk/cru/data/nao/.
Results
Figure 2a–c show the in situ observations from the PAP-SO at 30 m depth,
including p(CO2), chlorophyll a fluorescence and nitrate
concentration. Figure 2a shows the range of p(CO2) from 2003 to 2005,
which was also shown in Körtzinger et al. (2008). The range was
74 µatm (300–374 µatm) and the mean was
339 µatm. In comparison, p(CO2) between 2010 and 2012 had a
57 µatm range (327 to 384 µatm) with a higher mean of
353 µatm. The p(CO2) data for the 2010–2012 period are
confirmed by SOO data from the Portsmouth to Caribbean route in Fig. 2a (see
Fig. 1 for positions of the SOO samples). Körtzinger et al. (2008) also
reported a good comparison with a SOO route from Kiel, to the north of the
Portsmouth to Caribbean route, for the 2003–2005 data. The SOO data fill in
the gap in the time series when PAP-SO p(CO2) data were not available
due to failure of the instrument logger. The higher p(CO2) values in the
2010–2012 period are confirmed by the SOO data.
In situ chlorophyll data in Fig. 2b shows the characteristic chlorophyll
a fluorescence increase for this area during the spring bloom. There is
large inter-annual variability in both the timing and magnitude of the spring
bloom for the two time periods shown. For example the spring bloom in 2004
started in late May compared with an earlier bloom in 2011 (that started in
April). The increase in chlorophyll a fluorescence during the 2011 spring
bloom was also larger compared with the other years shown.
Nitrate concentration data in Fig. 2c shows the characteristic seasonality,
with increased winter nitrate concentrations and depletion following the
spring bloom (seen in Fig. 2b). The seasonality in the nitrate concentration
is similar for the two periods shown (2003–2005 and 2010–2012). SOO nitrate
concentration data show a good agreement with the PAP-SO data throughout
2010–2012 and fill in the gaps in early 2011 when nutrient measurements at
the PAP-SO are not available. Overall, the in situ data show a characteristic
increase in inorganic nitrate concentrations, and p(CO2), through the
winter as fluorescence decreases. However, winter nitrate concentrations are
significantly lower in the 2004/05 winter compared with other years as has
been discussed in Hartman et al. (2010).
In situ 30 m PAP-SO data (blue circles) from 2003 to 2005 and 2010
to 2012 and 5 m SOO data (red squares) with vertical lines to represent the
start of each year showing: (a) p(CO2);
(b) chlorophyll a concentration; (c) weekly-averaged
nitrate concentration.
Figure 3a shows the Argo temperature data extracted at 30 m depth and the in
situ MicroCAT temperature data at the PAP-SO. Temperature shows opposite
seasonal variations to the p(CO2) and nitrate concentration from in situ
data. A comparison of Argo temperature with in situ 30 m MicroCAT data (n=112, comparison not shown) suggests errors of up to 1 % for temperature
in the Argo data compared with the in situ data (when available). Both
data sets show that the temperature variations in these years are very
similar, showing a summer–winter difference of 6 ∘C (Fig. 3a).
The seasonality of the in situ data can be put in context when looking at the
MLD in Fig. 3b. The increase in p(CO2) and nitrate concentration
corresponds to deeper convective mixing in winter. The MLD range varies
little over the winters considered here (Fig. 3b) and the maximum MLD does
not exceed 260 m. However the timing of the maximum winter mixed layer depth
at PAP-SO varies from year to year. For example the maximum MLD (Fig. 3b) for
the 2010/11 winter reached 215 m in February 2011 compared with earlier and
deeper mixing (to 257 m) in the following 2011/12 winter (December 2011).
The calculated DIC concentrations (Fig. 3c) show a closer relationship to the
MLD seasonality than nitrate concentration data. Seasonal variation in the
concentration of both DIC and nitrate is similar apart from the 2004/05
winter; when low DIC concentrations were not seen at the same time as the low
nitrate concentrations (Fig. 3c).
The interrelation between DIC and nitrate concentrations can be considered by
comparing the C : N ratios to the Redfield ratio (Redfield, 1958). The
2003–2005 time period has already been considered in Körtzinger et
al. (2008) so is not reproduced here. Following Körtzinger et al. (2008)
we calculated DIC, corrected for gas exchange. DIC concentrations were
plotted against the in situ nitrate concentrations in different seasons for
2010–2012 (Fig. 4). The C : N ratio differed from the Redfield ratio of
6.6 with especially high values in spring (14.3).
Data from 2003 to 2005 and 2010 to 2012 (blue circles) with vertical
lines to represent the start of each year showing: (a) Argo
temperature data from 30 m depth around the PAP-SO and in situ MicroCAT
temperature data at 30 m (red circles); (b) calculated mixed layer
depth (MLD) data; (c) calculations of weekly dissolved inorganic
carbon (DIC) concentrations based on in situ PAP-SO p(CO2) and
salinity-based TA parameterisations (see text for details) with additional
DIC calculations based on SOO data (red circles).
The relationship between concentrations of gas exchange-corrected
DIC and nitrate (2010–2012) at the PAP-SO showing 4 different seasons:
winter (January–March, red squares); spring (April–June, green triangles);
summer (July–September, blue diamonds); autumn (October–December, dark blue
crosses). The green line shows the ratio in spring (14.3) and the blue line
is the ratio in autumn (6.4), with the Redfield ratio of 6.6 shown for
reference as a dashed line.
Data from 2003 to 2005 and 2010 to 2012 (blue circles) for
(a) weekly satellite wind data in the region of the PAP-SO;
(b) calculations of weekly sea-to-air CO2 flux (negative: into
the ocean) from in situ PAP-SO p(CO2) data and satellite wind (see text
for details) with additional flux calculations from SOO data (red circles);
(c) the monthly NAO index.
Figure 5a shows weekly satellite wind speed data used to calculate the CO2
flux. The wind speeds were similar in the two periods. There is an earlier
period of days with high wind speeds towards the end of 2011 that can be
compared with the CO2 data presented. The annual average wind speed was
8.2 ms-1 for both time periods. The maximum was
14 ms-1, although in situ winds of up to 20 ms-1
were seen from the Met Office data (eurosites.info/pap), this is not
seen in the weekly averaged satellite wind speed data presented.
Figure 5b shows the sea-to-air CO2 flux (where a positive flux is defined
as from sea to the atmosphere). This was calculated from in situ p(CO2)
data and satellite wind speed data (Fig. 5b). The week by week variation in
CO2 flux is shown and an overall average for the two periods of time has
been calculated as (-5.7±2.8) mmolm-2d-1 for the
2003–2005 period and (-5.0±2.2) mmolm-2d-1 for the
2010–2012 period. SOO data have been used at the start of 2011 when in situ
p(CO2) data were unavailable. The start and end months of the two
periods of time differs, which will contribute to the errors in the flux
measurements. However the errors are comparable for the two periods of time
considered and overall the average for the two time periods is similar.
There is little variation in CO2 flux and MLD between the years shown but
for completeness the NAO index is shown in Fig. 5c. The 2003/04 winter NAO
was near zero and the 2004/05 winter NAO was also low, between -2 to +1.
In contrast there is a large range in the winter NAO in the 2010/11 winter
when the NAO changed from -4 to +3. Overall the range in the NAO values
was larger for the 2010–2012 time period shown.
Discussion
PAP-SO seasonal variation
The 2003–2005 and 2010–2013 data sets show very similar seasonal patterns
between the years. Concentrations of nitrate and DIC exhibit seasonal
variations opposite to temperature. The seasonal variation in nitrate and
DIC concentrations is controlled by convective mixing (resulting in the
winter maximum) and biological uptake during the spring bloom period
(resulting in the summer minimum), which is similar to elsewhere in the
North Atlantic (Jiang et al., 2013).
The p(CO2) distribution pattern at the PAP-SO site is characterised by a
single annual peak (high in winter and low in summer), which is similar to
that of nutrient and DIC concentrations, but in antiphase to the temperature
signal. Jiang et al. (2013) compared seasonal carbon variability between
different sites in the North Atlantic and suggested a latitudinal change in
p(CO2) seasonality from the temperature-dominated oligotrophic
subtropical gyre to the subpolar region where p(CO2) is dominated by
changing concentrations of DIC. Our p(CO2) observations at the PAP-SO
site show the subpolar-like seasonal pattern, which is similar to that of the
ocean weather station M (Skjelvan et al., 2008). The surface p(CO2) is
mainly governed by the varying DIC concentration while the seasonal cooling
and warming have a contrasting effect.
The time integrated uptake of DIC and nitrate during the spring bloom is
reflected by the slope of the linear regression between them (Fig. 4). The
ratio of DIC and nitrate concentrations from 2010 to 2012 shows higher values
than the Redfield C : N ratio of 6.6. For example the spring-time ratio of
14.3 (±5) was considerably higher than the Redfield ratio, in agreement
with similar “carbon overconsumption” ratios seen for the North Atlantic
(e.g. 14.2, Sambrotto et al., 1993). This value is in agreement with the
single C : N ratio reported previously at the PAP-SO of 11.0
(Körtzinger et al., 2008). In both cases the DIC concentrations were
calculated and therefore associated with errors in the order of
7.0 µmolkg-1. We have demonstrated seasonal variation in the
C : N ratio at the PAP-SO, with an autumn C : N value that is closer to
the Redfield ratio and large deviations from the Redfield ratio in winter.
Air–sea CO2 flux
Wind speeds have an indirect impact on the biogeochemistry, in particular
p(CO2). In the North Atlantic the strength and frequency of wintertime
storms is significantly increasing (Donat et al., 2011). Wind speeds are
similar for the two time periods considered here. However there is some
suggestion of an earlier increase in winds at the start of the 2011/12 winter
(Fig. 5a) coinciding with an earlier increase in mixing (Fig. 3b). Although
the CO2 flux is not linked linearly to the wind speed there is a
corresponding decrease in CO2 flux into the ocean at this time.
It is well known that the northeast Atlantic is a strong CO2 sink with
large variability. The observations at the PAP-SO provide high frequency data
to follow the variability in CO2 exchange. The largest CO2 flux shown
here was in September 2004, as a combined result of low seawater p(CO2)
(Fig. 2a) and high wind speed (Fig. 5b). Larger CO2 flux into the ocean
may have occurred in 2011 considering the large, early spring bloom seen in
that year but we do not have in situ PAP-SO p(CO2) data to calculate the
flux at that time. However flux calculations from SOO data in early 2011 do
not suggest an increase in CO2 flux. Increases in productivity do not
necessarily result in enhanced oceanic CO2 uptake as the gas exchange is
also affected by other factors such as temperature and wind speed (Jiang et
al., 2013). The average is similar for the years presented with values of
-5.7 mmolm-2d-1 in 2003–2005 and
-5.0 mmolm-2d-1 from 2010 to 2012.
PAP-SO inter-annual variations
It is suggested that NAO plays an important role in modulating the
inter-annual variability in the northeast Atlantic region by affecting the
intensity of winter convection (Bennington et al., 2009; Jiang et al., 2013).
The Gibraltar minus Iceland version of the NAO index is really most
applicable to the winter half of the year. During positive NAO periods, the
PAP-SO region experiences subpolar-like conditions, with strong wind stress
and deep mixed layers (Henson et al., 2012). However the MLD did not vary
significantly at the PAP-SO between the 2003–2005 and 2010–2012 time
periods shown here (with a range of only 215–257 m for deepest winter MLD
between the years. In previous years such as 2009/10 deep winter mixing of
390 m has been seen with an NAO reaching -3, (not shown). NAO is unlikely
to have a large role at the PAP-SO as winter sea surface temperature and MLD
were similar in the time periods 2003–2005 and 2010–2012. Data from a
winter with deeper mixing would need to be put into the comparison to resolve
this.
There was a 2-fold decrease in nitrate concentrations in the 2004–2005 winter
despite sea surface temperature and MLD values being close to other years.
The low values were confirmed by SOO data, also shown in Hartman et
al. (2010). As discussed in Hartman et al. (2010) the lower winter nitrate
concentration seen in 2004/05 did not correlate with a decrease in the MLD
and this showed the influence of horizontal mixing at the PAP-SO. It was
suggested that lateral advection to the site at that time introduced a
subtropical water mass with a lower nitrate concentration. Earlier
time series studies largely ignored circulation at the PAP-SO site, assuming
convective mixing is a dominant process influencing mixed layer temperature
and nitrate concentrations in the region (Williams et al., 2000;
Körtzinger et al., 2008). However, fixed-point time series observations
are influenced by spatial variability passing the point of observation
(McGillicuddy et al., 1998; Painter et al., 2010). It is clear from Hartman
et al. (2010) that lateral advection may significantly influence the surface
temperature and nitrate concentrations in the region of the PAP-SO site.
The observed seawater p(CO2) 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
p(CO2) observed in the North Atlantic basin of (1.84±0.4) µatma-1 (Takahashi et al., 2009). Despite similar
maximum winter MLD in 2003–2005 and 2010–2012, the timing and intensity of
the spring bloom is quite different and the cause of this requires further
investigation.
Conclusions and further work
We have presented recent year-round surface time series biogeochemical data at the PAP-SO and compared it
with previous observations. The surface p(CO2), and concentrations of
DIC and nitrate, at the PAP-SO all show a clear seasonal cycle, which is
mainly controlled by winter convective mixing and biological activity in the
spring bloom. However the suggestion that inter-annual variability is
dominated by convection (Bennington et al., 2009) is not clear as the MLD did
not vary significantly between the winter periods shown. An especially low
winter nitrate concentration in 2005 was observed, thought to be due to
surface advection and this highlights the need to consider advection when
dealing with time series data in the future. Despite the similar winter
physical conditions (temperature and MLD), there is a year to year difference
in the timing and intensity of the spring blooms, which requires further
investigation. At PAP-SO, increasing mean seawater p(CO2) from (339±17) µatm in 2003 to (353±15) µatm in 2011 was
observed. However the mean air–sea CO2 flux did not show a significant
change. It varied from (-5.7±2.8) mmolm-2d-1 in
2003–2005 to (-5.0±2.2) mmolm-2d-1 in 2010–2012.
In 2010, collaboration between the UK's Natural Environment Research Council
(NERC) and Meteorological Office led to the first simultaneous monitoring of
in situ meteorological and ocean variables at the PAP-SO (Hartman et
al., 2012). From 2013 additional measurements of p(CO2) will be made at
the site, at the shallower depth of 1 m, and should further improve the SOO
comparison. The site could be used to investigate the effect of different
parameterisations (Prytherch et al., 2010) and wind products on calculations
of CO2 flux, in particular during the high wind conditions seen. Using the
contemporaneous atmospheric and ocean data sets we will be able to investigate
the effect of storms on CO2 flux and resolve daily variability.