The Southern Ocean plays a disproportionately large role in the global carbon cycle. Over the past few decades, the ocean has absorbed approximately 26 % of the carbon dioxide (CO2) emissions from fossil fuel burning and land use change (Le Quéré et al., 2016, 2018), and since the preindustrial era, the ocean has been the primary sink for anthropogenic emissions (McKinley et al., 2017; Ciais et al., 2013). The Southern Ocean (south of 30∘ S) accounts for almost half of the total oceanic sink of anthropogenic CO2 (Frölicher et al., 2015; Gruber et al., 2009; Takahashi et al., 2009). Though the importance of this region is widely understood, the relative scarcity of surface ocean carbon-related observations in the Southern Ocean hampers our ability to understand how this anthropogenic CO2 uptake occurs against the background of natural variability.

Observations and models suggest large variability in the strength of Southern Ocean CO2 uptake on decadal timescales. Several studies have reported a slow-down or reduction in the efficiency of Southern Ocean CO2 uptake from the 1980s to the early 2000s (Le Quéré et al., 2007; Lovenduski et al., 2008, 2015; Metzl 2009; Takahashi et al., 2012; Fay and McKinley, 2013; Landschützer et al., 2014a, 2015a), followed by a substantial strengthening of the Southern Ocean CO2 sink since 2002 (Fay and McKinley, 2013; Fay et al., 2014; Landschützer et al., 2015a; Munro et al., 2015a; Xue et al., 2015). Continued observational sampling efforts and coordination are required for quantifying and understanding decadal changes in this important CO2 sink region.

Initiated in 2002 and continuing to the present, the Drake Passage Time-series is unique among Southern Ocean research programs in both its spatial and temporal coverage. High-frequency underway observations of the surface ocean partial pressure of CO2 (pCO2) are collected on Antarctic Research and Supply Vessel Laurence M. Gould on up to 20 crossings per year from the southern tip of South America to the Antarctic Peninsula, spanning the Antarctic Circumpolar Current (ACC) and its associated Antarctic Polar Front (Munro et al., 2015a, b). The DPT is also notable for sampling surface ocean pCO2 during the austral winter in all years from 2002 to the present, providing valuable information about the full seasonal cycle of pCO2 in the poorly sampled Southern Ocean. Other ships have contributed observations in the Drake Passage region, including the Polarstern and the Nathaniel B. Palmer; however, none has the consistent temporal coverage as provided by the DPT.

The surface ocean pCO2 observations from the DPT have provided the foundation for larger datasets, which have been extensively used to examine variability and trends in CO2 uptake in the broader Southern Ocean (Fay and McKinley, 2013; Fay et al., 2014; Majkut et al., 2014; Landschützer et al., 2014b, 2015b; Rödenbeck et al., 2015, Gregor et al., 2018). In many of these studies, interpolated estimates of Southern Ocean pCO2 are used in conjunction with measurements of atmospheric pCO2 to estimate variability and trends in the air–sea pCO2 gradient and, when combined with wind speed, air–sea CO2 fluxes.

The physical oceanography of the Drake Passage region is unique in the Southern Ocean. Here, the strong flow of the zonally unbounded ACC is funneled through a narrow constriction (∼ 800 km), making it an ideal location for sampling across the entire ACC system over a relatively short distance (Sprintall et al., 2012). At the same time, the unique nature of this circulation could potentially reduce the degree to which the Drake Passage region is representative of the broader subpolar region. The DPT program takes advantage of frequent Gould crossings to conduct physical and biogeochemical sampling of the ACC system. Thus, before conclusions can be drawn about large-scale Southern Ocean carbon uptake and its variability using data from the DPT, it is important to document how pCO2 in this particular region compares with pCO2 measured elsewhere in the subpolar Southern Ocean. In this study, we utilize available ship-based surface ocean pCO2 observations collected in the subpolar Southern Ocean to evaluate how the seasonal cycle, interannual variability, and long-term trends of surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Further, we highlight the opportunity for post-deployment assessment of autonomous observational platforms passing through the Drake Passage utilizing the high-frequency, underway pCO2 measurements from the DPT.

This study uses several observational datasets and data products of surface ocean pCO2 in the Southern Ocean: measurements from the Surface Ocean CO2 Atlas (SOCAT), which includes underway measurements from the DPT, interpolated estimates of the SOCAT data using a self-organizing map feed-forward neural network (SOM-FFN) approach, and calculated pCO2 estimates from biogeochemical Argo floats. While the SOCAT database reports the fugacity of carbon dioxide (fCO2), for our analysis we consider datasets reporting pCO2 and fCO2 to be interchangeable. This is an acceptable assumption for surface ocean observations as CO2 behaves closely to an ideal gas. Globally, the difference between these parameters is less than 2 µatm, with fCO2 being smaller than pCO2 by no more than 2 µatm due to temperature dependence. This is roughly the reported uncertainty of shipboard observations of pCO2 and well within the uncertainty of the observation-based pCO2 estimates. Below, we describe each of these data sources in turn.

The SOCATv5 database from 2002 to 2016 is considered here to match the years of overlap with DPT observations, which began in 2002. The SOCAT dataset is then subsampled to include only observations with reported salinity values in the 33.5–34.5 range and a distance-to-land value greater than or equal to 50 km. This step restricts our analysis to open-ocean observations, since coastal observations report lower salinity values, which correspond to low pCO2 values due to the influence of freshwater and ice melt. SOCCOM float files were downloaded on 2 April 2018 and reported pCO2 values are an average of all data collected in the top 20 m of water, calculated using alkalinity derived from the LIAR algorithm (Carter et al., 2016), to remain consistent with previous SOCCOM float analysis.

The Southern Ocean region of interest is the Southern Ocean Subpolar Seasonally Stratified (SPSS) biome as defined in Fay and McKinley (2014) as the region of the Southern Hemisphere with climatological SST < 8 ∘C but excluding areas with a sea-ice fraction greater than or equal to 50 % (Fig. 1). While the SPSS biome encompasses the Drake Passage, we further define a Drake Passage region as the portion of the Southern Ocean SPSS biome bounded by 55 and 70∘ W lines of longitude (Fig. 1, black box). This is similar to the region analyzed in Munro et al. (2015a); however, it extends the region of interest to the northern and southern extents of the SPSS biome.

In order to compare the seasonal cycle and long-term trends in the Drake Passage with the broader SPSS biome, we analyze surface ocean pCO2 from three subsets of the SOCAT database: SOCAT-all, which includes all available SOCATv5 data from 2002 to 2016 in the SPSS biome, SOCAT-DP, which includes SOCATv5 data within the longitudinally defined Drake Passage region (Fig. 1, with 62 % of these data obtained by the LDEO/Univ. Colorado group), and SOCAT-noDP, which excludes any data within the longitudinally defined Drake Passage region of the SPSS biome. All datasets are first averaged to monthly, 1∘×1∘ resolution. Monthly means are then calculated for the SPSS biome by first removing the background mean annual climatological value of pCO2 at each 1∘×1∘ location (Landschützer et al., 2014a) to aid in accounting for the potential of spatial aliasing in the sparsely sampled Southern Ocean (Fay and McKinley, 2013).

Alternate definitions of the larger Southern Ocean region of interest were considered during our analysis, including a subdivision of the SPSS into a Northern SPSS and Southern SPSS, with the boundary defined by the location of the mean position in the Antarctic Polar Front (Freeman and Lovenduski, 2016; Freeman et al., 2016; Munro et al., 2015b). As discussed in Munro et al. (2015a), Drake Passage Time-series observations north of the front report higher pCO2 values than to the south, and they find a larger trend in pCO2 in the north for years 2002–2015. Additionally, the seasonal cycle amplitude north of the front is much larger and well defined than south of the front. We see these patterns in the SOCAT dataset as well; however, given the goal of this research, we choose to consider the entire north-to-south extent of the SPSS as a whole. Outside of the Drake Passage region, available data are limited such that analysis over northern and southern subregions would be impossible.

Additionally, we consider analysis over the Polar Antarctic Zone (PAZ), defined as the area between the Subantarctic Front and the sea-ice zone (Williams et al., 2017) (Supplement Fig. S1). While differences exist in trends and seasonality when using the PAZ definition (Supplement Figs. S2–3), the overall conclusions of the relationship between SOCAT-DP and SOCAT-all remain largely unchanged when using this alternate regional definition.

Biome-scale monthly means are compared and used to calculate seasonal cycles and trends. Seasonal cycles are calculated by first removing a 1.95 µatm yr-1 trend to account for increasing atmospheric CO2 during the 2002–2015 period (Dlugokencky et al., 2015). Seasonal uncertainties (Fig. 2) are estimated as 1 standard error from the mean of all available biome mean values for a given month. This is a conservative estimate of the uncertainty in any given month because of inconsistent annual coverage and spatial undersampling biases. Reported trends are calculated by fitting a single harmonic and linear trend to the biome-scale monthly means as done in Fay and McKinley (2013). Trends are not statistically different if the calculated mean seasonal cycle is removed instead of the choice to fit a harmonic to the data. Seasonal trends are calculated with a simple linear fit to the seasonal monthly means.

Starting in late 2014, autonomous biogeochemical profiling floats have been deployed as part of the SOCCOM project, and as of December 2017, 10 floats had traveled through or were approaching the Drake Passage region (Fig. 7a). These floats offer a new opportunity to complement our oceanographic understanding that has been developed primarily with traditional shipboard observations. Results above show that a lack of observations outside of the Drake Passage region may contribute to the large uncertainties in both seasonality and trends, which limits the conclusions we are able to make with currently available shipboard data. As floats provide autonomous, near-real-time observations covering existing spatial and temporal gaps throughout the Southern Ocean and ship-based systems provide high density observations at higher accuracy (±2.7 % or 11 µatm at a pCO2 of 400 µatm for floats compared to ±2 µatm for ships), there is great potential for these two platforms to work in concert to provide a whole Southern Ocean carbon observing system. However, there are limitations of float observations, notably the indirect estimate of pCO2 from pH and the requirement to adjust the sensor calibrations post-deployment by reference to deep (near 1500 m) pH values estimated from multiple linear regression equations fitted to high-quality, spectrophotometric pH observations made on repeat hydrography cruises (Williams et al., 2016, 2017; Johnson et al., 2016, 2017). Further comparisons between float-estimated pCO2 and shipboard observations are clearly warranted, and the complementary strengths of the Drake Passage Time-series make it an ideal dataset to help address these issues.

Here we utilize the underway Drake Passage Time-series pCO2 data to conduct comparisons to nearby SOCCOM floats, considering both seasonality as well as fine-scale crossovers. Note that in this section of the analysis we utilize data only from the Drake Passage Time-series (Takahashi et al., 2017, available at https://www.nodc.noaa.gov/ocads/data/0160492.xml) instead of the SOCATv5 dataset because SOCAT data are not available after 2016 at the time of writing.

A strong benefit of autonomous observation systems is their ability to sample regions and times that are not often surveyed by ships. SOCCOM floats collect data throughout the year, and especially important are the additional observations in austral winter, a time when there are limited opportunities for ship-based measurements. While currently only 2 full years of data are available from floats within the Drake Passage (2016–2017), they span the full width of the region (Fig. 7a) and are able to observe during each month of the year. A seasonal comparison of monthly mean pCO2 values for the DPT data and float pCO2 estimates within the defined Drake Passage region show that both platforms capture the expected seasonal cycle for the subpolar Southern Ocean with a wintertime peak and summertime low (Fig. 7b). All datasets shown have been adjusted to 2017 using the mean atmospheric trend (1.95 µatm yr-1) (Dlugokencky et al., 2015), and thus mean values are higher than shown in the seasonal curves of Fig. 2. Standard error shading on the seasonal cycles (Fig. 7b) includes considerations of measurement accuracy as this differs substantially between these two platforms. Shading represents 1 standard error accounting for the spatial and temporal heterogeneity of the sample and measurement error (2.7 % or ±11 µatm at a pCO2 of 400 µatm for floats; ±2 µatm for DPT data), combined using the square root of the sum of squares.

The seasonal cycle derived from float-estimated pCO2 has a larger seasonal amplitude compared to the DPT data from 2002 to 2017, due to an earlier and much lower observed summertime minimum. The difference in summertime minima is smaller however when DPT data from only 2016 and 2017 are considered (Fig. 7b). The remaining difference between the floats and the DPT 2016–2017 data might be an artifact of the specific locations sampled, as floats and ships are not exactly synchronous, as well as the conditions specific to 2016 and 2017. In summer 2016 for example, the floats appear to have captured a strong phytoplankton bloom to the north and upstream of the Drake Passage, not captured by the DPT, that resulted in strong inorganic carbon uptake and low pCO2; the floats did not sample in the southern region where pCO2 is substantially higher in the early spring (Fig. 8). However, there is no indication from the 2002–2017 seasonal cycle that this low excursion of the pCO2 persists when looking at the entire Drake Passage region (Fig. 7b). Since phytoplankton blooms typically progress southward during spring (Carranza and Gille, 2015) this difference in phasing likely results from the floats sampling preferentially the earlier northern uptake.

Underway Drake Passage Time-series pCO2 data in the SPSS biome have a large range, often spanning over 100 µatm each month as shown in the time series in Fig. 8, largely related to the 10 ∘C temperature gradient and associated physical and biological dynamics that are captured over the region. pCO2 from all floats east of 90 and west of 55∘ W is also plotted on the time series (Fig. 8, diamonds), with the reported pCO2 value being an average for all depths shallower than 20 m. Float-based pCO2 surface estimates largely fall within the range of the direct underway pCO2 observations; however, notable differences do exist when spatial and temporal differences are taken into consideration. Float estimates from the central Drake Passage in winter (JJA) 2017 (Fig. 8b) are higher than nearby DPT observations, though cruise data do not precisely overlap in time. Overall, the range of the DPT observations is far larger than the range of estimated pCO2 from floats inside the Drake Passage region because they regularly span across the full width of the Drake Passage where meridional decorrelation length scales are relatively short (Eveleth et al., 2017). Conversely, floats tend to sample along the path of the ACC.

As floats offer autonomous, frequent observations and ships offer data of the highest quality, it is ideal for these two platforms to work in partnership. Analysis of direct comparisons between DPT data and SOCCOM floats at crossover points indicates more precisely this potential (Fig. 9). As of December 2017, there have been six occurrences of floats surfacing near DPT observations within a window of 75 km, 3 days, and have a reported SST within 0.3 ∘C of each other (Fig. 9a). This window is consistent with the crossover criteria used by the SOCAT community to quality control shipboard data (Pfeil et al., 2013; Olsen et al., 2013). Figure 9a shows locations of the floats and the nearby DPT observations that fit this crossover window. As DPT offers high-frequency observations, all available measurements over the 3-day window are shown (Fig. 9b). Also indicated are DPT observations that cross over within a 50 km and 2-day window and 25 km and 1-day window (Fig. 9b, black “x” and squares, respectively), both also with the 0.3 ∘C SST criteria.

This comparison of the calculated pCO2 from the floats and observed DPT pCO2 reveals a broad correspondence (passing through the 1:1 line) in all six crossover instances within the ±2.7 % relative standard uncertainty of the SOCCOM float measurements and ±2 µatm DPT uncertainty (Fig. 9b shading). While all float crossovers do intersect the 1:1 line given their stated uncertainties, these comparisons reveal the large range of pCO2 captured by high-frequency shipboard measurements in a relatively small region and illustrate that this range cannot be fully captured by floats surfacing only once every 10 days. Further investigation of crossovers in the entire Southern Ocean region is needed; the DPT provides the most likely occurrence for this, although other regions with frequent ship traffic and autonomous platforms with biogeochemical capabilities should also be utilized when feasible. Additional post-deployment data quality checks using the underway surface pCO2 data from DPT and other ship-based programs should be conducted, and more thorough assessments could be achieved if hydrocast observations were planned to occur in the vicinity of a passing biogeochemical float. Such coordinated efforts would significantly advance monitoring of the carbon cycle in the Southern Ocean.

The Drake Passage Time-series illustrates the large variability of surface ocean pCO2 and exemplifies the value of sustained observations for understanding changing ocean carbon uptake in the Southern Ocean. This is the only location where carbon measurements throughout the entire annual cycle in the subpolar Southern Ocean have been made regularly over the past 2 decades. The available observations to date indicate that the Drake Passage seasonal cycle is representative of the seasonality observed for the entire SPSS biome, but increased observations outside of the Drake Passage, specifically during austral winter, are needed to provide a more robust comparison. Uncertainties in the seasonality for all datasets studied remain considerable given the dynamic nature of this region and the short time series considered. Specifically, a lack of winter data in all years limits the direct conclusions for differences between the Drake Passage and the larger SPSS biome where we see a discrepancy in the timing of the winter maxima. These findings can direct specific goals for focus regions of future observations. Specifically, insufficient wintertime data in regions outside of the Drake Passage limit our assessment of how representative Drake Passage data are of the larger subpolar region.

The magnitude of interannual variability is comparable for SOCAT pCO2 data within and outside of the Drake Passage region of the SPSS biome, a finding that conflicts with results from previous modeling and analysis of the SOM-FFN product. A clear idea of whether the Drake Passage is more or less variable in pCO2 will require increased data, particularly during the austral winter, outside of the Drake Passage. Given these data restrictions, the representativeness of the larger SPSS biome is also investigated using the SOM-FFN product. Within this gap-filled data product, monthly anomalies in the Drake Passage region are representative of broad swaths of the Southern Ocean, specifically regions upstream of the Drake Passage, but strong relationships are also evident in regions in the Indian Ocean sector of the Southern Ocean. Consistent with this finding, estimates of long-term trends do not change substantially if observations in the Drake Passage are removed from the SOM-FFN analysis. Across approaches to data analysis, trends in annual oceanic pCO2 trends for 2002–2016 are less than the atmospheric pCO2 trend, confirming previous findings that the Southern Ocean has, on average, been a growing sink for atmospheric carbon over this period.

Comparisons between underway DPT measurements and SOCCOM float estimates taken within the Drake Passage show broad agreement, while a fine-scale crossover investigation demonstrates their direct correspondence given uncertainty ranges for SOCCOM float pCO2 estimates. Continuation of high-temporal measurements of the DPT, in addition to expanded programs to target floats with both underway observations and frequent hydrocasts serving as independent datasets for post-deployment, will provide high-value comparisons, improving community confidence in float-based pCO2 estimates. Coordinated monitoring efforts that combine a well-calibrated array of autonomous biogeochemical floats with a robust ship-based observational network will improve and expand monitoring of the carbon cycle in the Southern Ocean in the future.