Variability of North Sea pH and CO
2
in response
to North Atlantic Oscillation forcing
Lesley A. Salt,
1
Helmuth Thomas,
2
A. E. Friederike Prowe,
3,4
Alberto V. Borges,
5
Yann Bozec,
6
and Hein J. W. de Baar
1
Received 5 February 2013; revised 20 October 2013; accepted 25 October 2013.
[1] High biological activity causes a distinct seasonality of surface water pH in the North
Sea, which is a strong sink for atmospheric CO
2
via an effective shelf pump. The intimate
connection between the North Sea and the North Atlantic Ocean suggests that the variability
of the CO
2
system of the North Atlantic Ocean may, in part, be responsible for the observed
variability of pH and CO
2
in the North Sea. In this work, we demonstrate the role of the
North Atlantic Oscillation (NAO), the dominant climate mode for the North Atlantic, in
governing this variability. Based on three extensive observational records covering the
relevant levels of the NAO index, we provide evidence that the North Sea pH and CO
2
system strongly responds to external and internal expressions of the NAO. Under positive
NAO, the higher rates of inflow of water from the North Atlantic Ocean and the Baltic
outflow lead to a strengthened north-south biogeochemical divide. The limited mixing
between the north and south leads to a steeper gradient in pH and partial pressure of CO
2
(pCO
2
) between the two regions in the productive period. This is exacerbated further when
coinciding with higher sea surface temperature, which concentrates the net community
production in the north through shallower stratification. These effects can be obscured by
changing properties of the constituent North Sea water masses, which are also influenced by
NAO. Our results highlight the importance of examining interannual trends in the North Sea
CO
2
system with consideration of the NAO state.
Citation: Salt, L. A., H. Thomas, A. E. F. Prowe, A. V. Borges, Y. Bozec, and H. J. W. de Baar (2013), Variability of North Sea
pH and CO
2
in response to North Atlantic Oscillation forcing, J. Geophys. Res. Biogeosci., 118, doi:10.1002/2013JG002306.
1. Introduction
[2] Coastal and marginal seas play an important role in the
atmosphere-ocean carbon exchange, responsible for a dispro-
portionately large amount of primary production relative to
their surface area [Gattuso et al., 1998], which is triggered
by large inputs of nutrients and organic carbon from the adja-
cent ocean, land, and atmosphere [Wollast, 1998; Thomas
et al., 2005a; Thomas et al., 2008b]. The export of this car-
bon into the adjacent open ocean, thus sequestering large
quantities of anthropogenic CO
2
, is known as the continental
shelf pump [Tsunogai et al., 1999; Thomas et al., 2004].
The effectiveness of this pump is related to the physical and
biological conditions governing the CO
2
disequilibrium
between the atmosphere and the sea surface, which in turn
is thermodynamically responsible for the CO
2
uptake and
the subsequent variation in pH and partial pressure of CO
2
(pCO
2
). The codependence of this variability on physical
and biological factors makes it difficult to discern the
increase in CO
2
solely attributable to atmospheric pCO
2
increases [Santana-Casiano et al., 2007].
[
3] The North Sea is a shelf sea on the northwest European
continent with links to the North Atlantic Ocean in the south
and the north. The majority of water exchange with the North
Atlantic occurs in the northern North Sea, where inflowing
waters enter through the Orkneys-Shetland shelf, Shetland
shelf, and the Norwegian channel, with the No rwegian
Trench providing the main exit pathway of circulated water
out of the North Sea [Otto et al., 1990; Winther and
Johannessen, 2006]. The total net carbon export to the
North Atlantic via the Norwegian Trench has been estimated
to be 6 ± 1 × 10
12
mol C yr
1
[Wakelin et al., 2012], which
includes more than 90% of the CO
2
drawn down from the
atmosphere in the North Sea [Thomas et al., 2005a]. The
effectiveness of the North Sea CO
2
pump is determined prin-
cipally by water mass exchange between the North Sea and
the North Atlantic Ocean in combination with the export of
carbon out of the surface layer, predominantly as sinking par-
ticulate organic matter [Thomas et al., 2004; Bozec et al.,
2005]. The latter applies most significantly to the deeper
1
Royal Netherlands Institute for Sea Research, Texel, Netherlands.
2
Department of Oceanography, Dalhousie University, Halifax, Nova
Scotia, Canada.
3
Centre for Ocean LifeNational Institute of Aquatic Resources, Technical
University of Denmark, Charlottenlund, Denmark.
4
GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany.
5
Unité d’Océanographie Chimique, Institut et Physique B5, Université
de Liège, Liège, Belgium.
6
CNRS,UMR 7144, Equipe Chimie Marine, Station Biologique de
Roscoff, Roscoff, France.
Corresponding author: L. A. Salt, Royal Netherlands Institute for Sea
Research, Texel, NL-1790 AB, Netherlands. (lesley.salt@nioz.nl)
©2013. American Geophysical Union. All Rights Reserved.
2169-8953/13/10.1002/2013JG002306
1
JOURNAL OF GEOPHYSICAL RESEARCH: BIOGEOSCIENCES, VOL. 118, 1–9, doi:10.1002/2013JG002306, 2013
(>50 m), seasonally stratified northern part of the North Sea
(>56°N), where seasonality in pH is controlled by produc-
tion of organic matter and its export [Thomas et al., 2009].
The seasonality of pH and pCO
2
in the southern North Sea
is also closely coupled to primary production; however, the ex-
port of organic matter is reduced by the rapid remineralization,
which takes place in the shallow (<50 m) and well-mixed
water column [Borges and Frankignoulle, 1999, 2002;
Schiettecatte et al., 2006, 2007].
[
4] Over the North Atlantic Ocean, a number of atmo-
spheric teleconnection patterns influence climate variability
of which the North Atlantic Oscillation (NAO) is the most
prominent. The NAO Index (NAOI) is defined as the differ-
ence of atmospheric sea level pressure (SLP) between the
Icelandic low and the Azores high and accounts for the
greatest proportion (>30%) of the observed SLP variance
in the region from December to March [Hurrell, 2003]. The
effects of the NAO control a number of large-scale processes
at different timescales [e.g. Hurrell, 1995; Hurrell and van
Loon, 1997; Greatbatch, 2000]. Since the atmospheric pres-
sure anomalies are most pronounced during northern hemi-
sphere winter [Greatbatch, 2000] and the ratio of signal to
noise is the highest [Hurrell and van Loon, 1997], commonly
(but not exclusively) the NAOI recorded during December,
January, and February (DJF), has been referred to in the liter-
ature yielding the most accentuated NAOI variability. Thus,
although the NAOI is commonly established for DJF conse-
quences of the NAO have been identified at various time-
scales. While the atmospheric realm responds at immediate,
shorter timescales, for example via variability of trajectory,
direction and strength of winds, the oceanic system responds
at times scales from seasons to decades, for example via
altered circulation patterns at various spatial scales [e.g.,
Hurrell, 1995; Hurrell and van Loon, 1997; Greatbatch,
2000; Thomas et al., 2008a]. Modeling studies [Thomas
et al ., 2008b; Levine et al., 2011; McKinley et al., 2011]
and long-term observations [Santana-Casiano et al., 2007;
Pérez et al., 2010; Bates, 2001] suggest that NAO-driven
changes exert significant control over the interannual vari-
ability of hydrographic properties and in turn the uptake
of CO
2
.
[
5] Recently, observations from the North Sea [Thomas
et al., 2007] and the North Atlantic Ocean [Watson et al.,
2009] indicate that the surface water pCO
2
has risen faster
than the atmospheric pCO
2
, which has been linked to the
effects of the NAO, with varying time scales of effect across
the region [Santana-Casiano et al., 2007; Thomas et al.,
2008a]. In-depth discussions of the role of the NAO in regu-
lating the climate and weather go beyond the scope of the
present paper, and can be found elsewhere, for example in
Hurrell [1995], or Greatbatch [2000]. However, within the
North Sea, many processes have demonstrated significant
correlations with the wintertime (DJF) NAOI, which, as we
later show, impact the carbonate system. The strength of
the water mass exchange between the North Atlantic and
the North Sea is regulated by the NAO, which in turn affects
physical and chemical characteristics of the North Sea water
column for the annual cycle. As a consequence of enhanced
water mass exchange between North Atlantic and North
Sea during years of positive NAOI (NAO+)[Winther and
Johannessen, 2006; Kühn et al., 2010], the corresponding
increase of the North Sea’s nutrient inventory leads to
higher productivity throughout the productive season from
spring until the end of summer [Pätsch and Kühn, 2008].
Characteristic in the North Sea’s response to NAO forcing
can also be a hysteresis between cause and effect: In the
North Sea, NAO+ has further been associated with higher
precipitation across Scandinavia with drier conditions over
central Europe [Ionita et al., 2011]. Changes in precipitation
patterns over the drainage area of the Baltic Sea during winter
will affect the runoff from the Baltic Sea into the North Sea
over the relevant (runoff) seasons. Additionally, stronger
westerly winds during winter, correlated with a positive win-
tertime NAOI, push North Sea water into the Baltic Sea, a
process that in turn leads to an enhanced outflow from the
Baltic Sea into the North Sea during the subsequent spring
and summer [Hordoir and Meier, 2010]. These patterns are
generally reversed during a NAO negative (NAO) state.
From these few examples, it is evident that despite the fact
that the NAOI is commonly established for winter (DJF),
the consequences for the North Sea are complex, not re-
stricted to the winter season, and can be (partly) masked
or even overridden by local or regional weather. One of the
primary aims of this paper is to unravel this complex situation
and to understand the variability of the North Sea CO
2
system
in front of this background.
[
6] The intra-annual variability of pCO
2
and dissolved
inorganic carbon (DIC) has been well documented in the
North Sea [Frankignoulle and Borges, 2001; Thomas
et al., 2005b; Prowe et al., 2009; Bozec et al., 2006; Omar
et al., 2010; Artioli et al., 2012]; however, on long time
scales little work has been done [Thomas et al., 2005a;
Thomas et al., 2007; Borges and Gypens, 2010] and despite
the proximity of the North Atlantic, the drivers for
interannual variability of the CO
2
system in relation to the
North Atlantic variability have not yet been investigated
from field observations. Further, the full scope of variations
in pH and pCO
2
is stil l difficult to constrain when
attempting to reproduce them in models [Prowe et al.,
2009; Gypens et al., 2011; Lorkowski et al., 2012; Artioli
et al., 2012]. Relying on a unique data set covering three
basin-wide occupations of the North Sea during all relevant
NAO phases, we are now able to examine the influences of
NAO forcing on the North Sea carbonate system.
2. Materials and Methods
[7] The North Sea was sampled during August/September
2001, 2005, and 2008 using a station grid of approximately
90 identical stations each time [Bozec et al., 2005, 2006].
These years experienced NAO (DJF) indices of 1.9, 0.12,
and 2.1, respectively (http://www.cgd.ucar.edu/cas/jhurrell/
indices.html, 2012). As we examine the influence of the
NAO in the North Sea on different time scales, we assume
that the wintertime NAO forcing will be responsible for
producing the most prominent signal in the data on a basin-
wide scale.
[
8] Due to greater spatial coverage of DIC and pCO
2
data,
in all three years, compared to that of total alkalinity (A
T
) and
pH, the latter two were calculated from the former two, as
previously done in intercomparison studies [Thomas et al.,
2009]. Internal consistency studies from cruises with full
carbonate parameter coverage in late summer indicate that
A
T
and pH can be predicted, using DIC and pCO
2
,withan
SALT ET AL.: VARIABILITY OF NORTH SEA PH AND CO
2
2
accuracy of ± 9 μmol kg
1
and ± 0.008, respectively
(L.A. Salt, manuscript in preparation, 2013). Only stations
with valid values for DIC and pCO
2
for all of the three
years were used, resulting in a total of 85 stations worth
of data for comparison.
2.1. DIC and pCO
2
[9] Samples for the carbonate parameters were obtained
following the operating procedures outlined in DOE [2007].
Carbonate system parameters, DIC, A
T
, and pH (in 2005)
were determined at 8–15 depths per station, yielding approx-
imately 700 samples per cruise. All samples were analyzed
within 12 h of sampling, and were verified for quality control
using certified reference material (CRM) supplied by Prof.
Andrew Dickson (Scripps Institute of Oceanography, USA).
A single sample was obtained for both DIC and A
T
and
these were determined by coulometric and potentiometric ti-
trations, respectively. For further details, please see Thomas
et al. [2007].
[
10] Surface water pCO
2
was measured every minute using
a flow-through system with continuous equilibration and
infrared detection [Körtzinger et al., 1996], yielding approx-
imately 20,000 measurements per cruise with an accuracy of
±1 μatm. A temperature normalization was applied to the
pCO
2
data to obtain pCO
2
@16°C, which is independent
of temperature differences between the years [Takahashi
et al., 1993].
2.2. Calculations
2.2.1. Water Mass Analysis
[
11] The North Sea surface waters (5 m depth) were sepa-
rated into three simpler constituents using a mixing analysis
of the dominant water masses in the North Sea [Kempe
and Pegler, 1991; Shadwick et al., 2011], with North
Atlantic water, Baltic Sea water, and German Bight water
as end-members. The latter two consist of a fraction of
North Atlantic water; however, the large freshwater contribu-
tion makes both very distinct from North Atlantic water and
allows us to track them into the central North Sea. In order
to differentiate water mass fractions from n number of con-
tributors, n 1 end-member variables are required. As this
decomposition was done for the surface waters, tempera-
ture cannot be used, because due to seasonal heating and
cooling it is not a conservative tracer as used in traditional
multiparametric optimizations for analysis of deep waters.
Salinity,DIC,andA
T
all offer suf ficiently distinct end-
member concentrations to be used; however, the codepen-
dence of A
T
on salinity makes DIC favorable. The DIC
is nonconservative due to biological uptake/release of CO
2
;
however, the end-members used (Table 1) are sufficiently
distinct that changes in DIC concentration affected by mixing
are much greater than the potential interference of primary
production/respiration. This was confirmed by similar results
being obtained using salinity and A
T
, which is more conserva-
tive with respect to primary production.
[
12] The end-members for the North Atlantic and German
Bight were determined individually for each year, with only
the Baltic end-member remaining constant. As we lacked
observations in the Baltic Proper, DIC and A
T
values were
taken from literature for a representative salinity of 8 [Thomas
and Schneider, 1999; Hjalmarsson et al., 2008] (Table 1).
The North Atlantic end-member was determined by the max-
imum surface salinity found in the northwest North Sea
(Latitudes > 58°N and Longitudes < 0°E) and its corre-
sponding DIC value. The German Bight end-member was
determined by the salinity and DIC value found at the station
closest to the Elbe river mouth (54.75°N, 8.25°E). By using
annually determined end-members for the three years, we
can rule out any change in water mass fractions occurring
due to changes in the chemical signal of end-members, which
we later show does occur in the North Sea. Statistics describing
the calculated water mass fractions are shown in Table 2.
2.2.2. DIC Inventory Calculation of the North Sea
[
13] To quantify the changes in DIC between years, differ-
ences in biological activity and remineralization must be
accounted for. Here we use the apparent oxygen utilization
(AOU = [O
2
]
sat
[O
2
]
obs
) to account for production and
remineralization, applying the Redfield ratio [Anderson and
Sarmiento, 1994] in the following equation:
DIC* ¼ DIC AOU*0:7ðÞ:
[14] These values were integrated throughout the water
column to a common maximum depth per station, and
then station totals were extrapolated over the entire North
Sea basin. To help us better understand the changes, the
International Council for the Exploration of the Sea (ICES)
defined boxes [ICES, 1983] were used to examine the
regional changes in DIC and salinity. Unless specified other-
wise, boxes 1–5 refer to the entire water column, not just the
upper 30 m, whereas boxes 11–15 refer to the deep (>30 m)
portion of these boxes, respectively.
Table 1. Dissolved Inorganic Carbon and Salinity End-Members
a
DIC ( μmol kg
1
) and
Salinity End- Members 2001 2005 2008
North Atlantic 2047 (35.06) 2078 (35.30) 2065 (35.13)
Baltic 1530 (8) 1530 (8) 1530 (8)
German Bight 2090 (30.36) 2012 (30.77) 2149 (32.28)
a
The DIC end-members used for the water mass fraction calculation for
the three years, with the corresponding salinity values in parentheses.
Table 2. Statistics of the Different Water Mass Fractions Present in the North Sea in 2001, 2005, and 2008
a
2001 2005 2008
Water Mass Range Average Median Range Average Median Range Average Median
North Atlantic 0–100 83 91 0–100 81 87 0–100 84 91
Baltic 0–16 2 0 0–15 3 1 0-20 3 7
German Bight 0-100 15 4 0–100 16 10 0–100 13 1
a
The basin-wide range of values, average, and median are given with the units representing the % of the total water present in the North Sea.
SALT ET AL.: VARIABILITY OF NORTH SEA PH AND CO
2
3
2.2.3. Calculation of Brunt-Väisälä Frequency Squared
[
15] To assess the vertical stability of the water column in the
northern North Sea (>56°N), we utilized the Brunt-Väisälä
frequency squared:
N
2
¼g=ρðÞ: ∂ρ=∂zðÞ
where z is the depth (m), ρ is density (kg m
3
) computed fol-
lowing the density equation of Fofonoff and Millard [1983],
and g is the gravitational acceleration (9.807 m s
2
). The tem-
perature and salinity data from the conductivity-temperature-
depth (CTD) casts with a 1 m resolution were smoothed using
a cubic spline. The N
2
was calculated from the smoothed
dataset and the depth at which the maximum N
2
occurs we
defined as the mixed-layer depth.
3. Results
[16] The main distribution pattern of the carbonate param-
eters is relatively constant between years (Figure 1). The
brackish Baltic outflow around the Norwegian headland has
low A
T
and DIC signals. The German Bight, in the south-
west, is distinguished by its high A
T
and high DIC content.
The Shetland shelf represents the main North Atlantic inflow
Figure 1. Surface layer distribu tion of carbonate parameters with 50 and 100 m depth contours.
(a–c) Total alkalinity (μmol kg
1
) for the years 2001, 2005, and 2008, (d–f) dissolved inorganic carbon
(μmol kg
1
) for the years 2001, 2005, and 2008, (g– i) pH for the years 2001, 2005, and 2008, and (j– l) par-
tial pressure of CO
2
(pCO
2
; μatm) for the years 2001, 2005, and 2008. The DIC and pCO
2
are observations,
A
T
and pH are calculated parameters from DIC and pCO
2
, using carbonic acid dissociation constants of
Mehrbach et al., 1973, refitbyDickson and Millero [1987], and pH is given on the Total scale. Average
values for 2001, 2005, and 2008 are 2299, 2298, and 2291 μmol kg
1
for A
T
, respectively, 2034, 2052,
and 2055 μmol kg
1
for DIC, 8.129, 8.105, and 8.079 for pH, and 323, 344, and 369 μatm for pCO
2
.
SALT ET AL.: VARIABILITY OF NORTH SEA PH AND CO
2
4
site, and is clearly identified by notably higher A
T
values than
the basin-wide average (Figure 1b). The anticorrelated pH
and pCO
2
both show a strong gradient at the 50 m depth con-
tour, representing the boundary at which the dominant con-
trol on pCO
2
changes from temperature, in the well-mixed
south, to biology, in the stratified north [Prowe et al., 2009;
Thomas et al., 2005b]. The gradient between the north and
south is steepest in 2008, closely follo wed by 2001, and
weakest in 2005. This pattern reflects the measured surface
water temperatures in the North Sea, 2001 and 2008 being
the warmest (mean surface temperatures of 16.2°C (±1.3)
and 16.1°C (±1.1), respectively), and 2005 the coldest
(15.4°C (±1.0)). The highest northern pH values and lowest
pCO
2
values are observed in the northern North Sea in
2001, with progressively decreasing mean pH and increasing
mean pCO
2
trends over time. The average surface DIC in-
creased by 21 μmol kg
1
, although not uniformly, and the av-
erage surface A
T
remained relatively constant, ± 4 μmol kg
1
,
throughout all three years.
[
17] The North Sea CO
2
system is largely governed by
the relative contribution of different water masses composing
the North Sea water as well as the rate at which these are
circulated within the North Sea shelf. The mixing analysis
(Figure 2a) clearly identifies the North Atlantic Ocean wa ter
as the dominant water mass, constituting an average 83%
fraction throughout all three years (Table 2). The average,
basin-wide fractio n of Baltic water increases from 2% in
2001 to 3% in 2005 and 2008 (Table 2). The dominant
North Atlantic inflow follows an anticlockwise circulation
from the north, mixing in the south with German Bight
water. In the northeastern areas, the Baltic Sea outflow plays
an additional role where it is introduced to the circulation on
its way out of the North Sea. This leads to the presence of
two main mixing regimes, one consisting of North Atlantic
and German Bight water, and the other of North Atlantic
and Baltic water (Figure 2a). It can be seen that in 2008 these
two regimes are most distinguishable, which corresponds to
the year in which the maximum Baltic fraction was recorded
(20%; Table 2). The formation of the two mixing environ-
ments is reflected in the formation of a dichotomy in pH
measurements in 2008, which is absent in 2001 and 2005
(Figures 2b–2d). The dichotomy divides the North Sea, with
more acidic waters in the south and higher pH values in
the north. The divide is formed at, approximately, the 50 m
depth contour accounting for the strong gradient visible in
Figures 1i and 1l.
Figure 2. Water type contributions to the North Sea.(a) The fractions of North Atlantic water, Baltic
water, and German Bight water in the North Sea in 2001 (blue), 2005 (green), and 2008 (red). (b–d)
Histograms of the frequency of calculated pH values binned in 0.01 pH unit intervals with the mean value
per year marked by the red line.
SALT ET AL.: VARIABILITY OF NORTH SEA PH AND CO
2
5
![Figure 1. Surface layer distribution of carbonate parameters with 50 and 100 m depth contours. (a–c) Total alkalinity (μmol kg 1) for the years 2001, 2005, and 2008, (d–f) dissolved inorganic carbon (μmol kg 1) for the years 2001, 2005, and 2008, (g– i) pH for the years 2001, 2005, and 2008, and (j– l) partial pressure of CO2 (pCO2; μatm) for the years 2001, 2005, and 2008. The DIC and pCO2 are observations, AT and pH are calculated parameters from DIC and pCO2, using carbonic acid dissociation constants of Mehrbach et al., 1973, refit by Dickson and Millero [1987], and pH is given on the Total scale. Average values for 2001, 2005, and 2008 are 2299, 2298, and 2291 μmol kg 1 for AT, respectively, 2034, 2052, and 2055 μmol kg 1 for DIC, 8.129, 8.105, and 8.079 for pH, and 323, 344, and 369 μatm for pCO2.](/figures/figure-1-surface-layer-distribution-of-carbonate-parameters-3uo507iy.webp)