Utilizing the Drake Passage Time-series to understand variability and change in subpolar Southern Ocean &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Amanda R Fay,Galen A Mckinley,Colm Sweeney,David R Munro,Nancy Williams,Nicole S Lovenduski,Britton B Stephens,Taro Takahashi,Peter Landschützer,Alison R Gray

doi:10.5194/bg-15-3841-2018

Abstract

Abstract. The Southern Ocean is highly under-sampled for the purpose of assessing total carbon uptake and its variability. Since this region dominates the mean global ocean sink for anthropogenic carbon, understanding temporal change is critical. Underway measurements of pCO2 collected as part of the Drake Passage Time-series (DPT) program that began in 2002 inform our understanding of seasonally changing air–sea gradients in pCO2, and by inference the carbon flux in this region. Here, we utilize available pCO2 observations to evaluate how the seasonal cycle, interannual variability, and long-term trends in surface ocean pCO2 in the Drake Passage region compare to that of the broader subpolar Southern Ocean. Our results indicate that the Drake Passage is representative of the broader region in both seasonality and long-term pCO2 trends, as evident through the agreement of timing and amplitude of seasonal cycles as well as trend magnitudes both seasonally and annually. The high temporal density of sampling by the DPT is critical to constraining estimates of the seasonal cycle of surface pCO2 in this region, as winter data remain sparse in areas outside of the Drake Passage. An increase in winter data would aid in reduction of uncertainty levels. On average over the period 2002–2016, data show that carbon uptake has strengthened with annual surface ocean pCO2 trends in the Drake Passage and the broader subpolar Southern Ocean less than the global atmospheric trend. Analysis of spatial correlation shows Drake Passage pCO2 to be representative of pCO2 and its variability up to several hundred kilometers away from the region. We also compare DPT data from 2016 and 2017 to contemporaneous pCO2 estimates from autonomous biogeochemical floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) so as to highlight the opportunity for evaluating data collected on autonomous observational platforms. Though SOCCOM floats sparsely sample the Drake Passage region for 2016–2017 compared to the Drake Passage Time-series, their pCO2 estimates fall within the range of underway observations given the uncertainty on the estimates. Going forward, continuation of the Drake Passage Time-series will reduce uncertainties in Southern Ocean carbon uptake seasonality, variability, and trends, and provide an invaluable independent dataset for post-deployment assessment of sensors on autonomous floats. Together, these datasets will vastly increase our ability to monitor change in the ocean carbon sink.

Highlights

The Southern Ocean plays a disproportionately large role in the global carbon cycle
SOCCOM float files were downloaded on 2 April 2018 and reported pressure of CO2 (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
Surface ocean pCO2 levels reach a maximum in austral winter (June to August), when deep mixing delivers carbon-rich water to the surface, and a minimum in austral summer (December to February), when biological production draws down the inorganic carbon from the surface (Takahashi et al, 2009)

Summary

Introduction

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. Continued observational sampling efforts and coordination are required for quantifying and understanding decadal changes in this important CO2 sink region

Methods

Results

Conclusion