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Johnson, KS, Plant JN, Dunne JP, Talley LD, Sarmiento JL.  2017.  Annual nitrate drawdown observed by SOCCOM profiling floats and the relationship to annual net community production. Journal of Geophysical Research-Oceans. 122:6668-6683.   10.1002/2017jc012839   AbstractWebsite

Annual nitrate cycles have been measured throughout the pelagic waters of the Southern Ocean, including regions with seasonal ice cover and southern hemisphere subtropical zones. Vertically resolved nitrate measurements were made using in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) optical nitrate sensors deployed on profiling floats. Thirty-one floats returned 40 complete annual cycles. The mean nitrate profile from the month with the highest winter nitrate minus the mean profile from the month with the lowest nitrate yields the annual nitrate drawdown. This quantity was integrated to 200 m depth and converted to carbon using the Redfield ratio to estimate annual net community production (ANCP) throughout the Southern Ocean south of 30 degrees S. A well-defined, zonal mean distribution is found with highest values (3-4 mol C m(-2) yr(-1)) from 40 to 50 degrees S. Lowest values are found in the subtropics and in the seasonal ice zone. The area weighted mean was 2.9 mol C m(-2) yr(-1) for all regions south of 40 degrees S. Cumulative ANCP south of 50 degrees S is 1.3 Pg C yr(-1). This represents about 13% of global ANCP in about 14% of the global ocean area. Plain Language Summary This manuscript reports on 40 annual cycles of nitrate observed by chemical sensors on SOCCOM profiling floats. The annual drawdown in nitrate concentration by phytoplankton is used to assess the spatial variability of annual net community production in the Southern Ocean. This ANCP is a key component of the global carbon cycle and it exerts an important control on atmospheric carbon dioxide. We show that the results are consistent with our prior understanding of Southern Ocean ANCP, which has required decades of observations to accumulate. The profiling floats now enable annual resolution of this key process. The results also highlight spatial variability in ANCP in the Southern Ocean.

Johnson, KS, Plant JN, Coletti LJ, Jannasch HW, Sakamoto CM, Riser SC, Swift DD, Williams NL, Boss E, Haentjens N, Talley LD, Sarmiento JL.  2017.  Biogeochemical sensor performance in the SOCCOM profiling float array. Journal of Geophysical Research-Oceans. 122:6416-6436.   10.1002/2017jc012838   AbstractWebsite

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program has begun deploying a large array of biogeochemical sensors on profiling floats in the Southern Ocean. As of February 2016, 86 floats have been deployed. Here the focus is on 56 floats with quality-controlled and adjusted data that have been in the water at least 6 months. The floats carry oxygen, nitrate, pH, chlorophyll fluorescence, and optical backscatter sensors. The raw data generated by these sensors can suffer from inaccurate initial calibrations and from sensor drift over time. Procedures to correct the data are defined. The initial accuracy of the adjusted concentrations is assessed by comparing the corrected data to laboratory measurements made on samples collected by a hydrographic cast with a rosette sampler at the float deployment station. The long-term accuracy of the corrected data is compared to the GLODAPv2 data set whenever a float made a profile within 20 km of a GLODAPv2 station. Based on these assessments, the fleet average oxygen data are accurate to 1 +/- 1%, nitrate to within 0.5 +/- 0.5 mu mol kg(-1), and pH to 0.005 +/- 0.007, where the error limit is 1 standard deviation of the fleet data. The bio-optical measurements of chlorophyll fluorescence and optical backscatter are used to estimate chlorophyll a and particulate organic carbon concentration. The particulate organic carbon concentrations inferred from optical backscatter appear accurate to with 35 mg C m(-3) or 20%, whichever is larger. Factors affecting the accuracy of the estimated chlorophyll a concentrations are evaluated.

Williams, NL, Juranek LW, Feely RA, Johnson KS, Sarmiento JL, Talley LD, Dickson AG, Gray AR, Wanninkhof R, Russell JL, Riser SC, Takeshita Y.  2017.  Calculating surface ocean pCO(2) from biogeochemical Argo floats equipped with pH: An uncertainty analysis. Global Biogeochemical Cycles. 31:591-604.   10.1002/2016gb005541   AbstractWebsite

More than 74 biogeochemical profiling floats that measure water column pH, oxygen, nitrate, fluorescence, and backscattering at 10 day intervals have been deployed throughout the Southern Ocean. Calculating the surface ocean partial pressure of carbon dioxide (pCO(2sw)) from float pH has uncertainty contributions from the pH sensor, the alkalinity estimate, and carbonate system equilibrium constants, resulting in a relative standard uncertainty in pCO(2sw) of 2.7% (or 11 mu atm at pCO(2sw) of 400 mu atm). The calculated pCO(2sw) from several floats spanning a range of oceanographic regimes are compared to existing climatologies. In some locations, such as the subantarctic zone, the float data closely match the climatologies, but in the polar Antarctic zone significantly higher pCO(2sw) are calculated in the wintertime implying a greater air-sea CO2 efflux estimate. Our results based on four representative floats suggest that despite their uncertainty relative to direct measurements, the float data can be used to improve estimates for air-sea carbon flux, as well as to increase knowledge of spatial, seasonal, and interannual variability in this flux. Plain Language Summary The Southern Ocean is a key player in the global flow of carbon, yet it is hard to reach, and there are relatively few measurements there, especially in winter. Measuring the amount of carbon dioxide gas in seawater is key to advancing our understanding of the Southern Ocean. More than 74 robotic floats that use sensors to measure seawater properties have been deployed throughout the Southern Ocean, and each has a lifetime of around 5 years. It is currently not possible to directly measure carbon dioxide gas from these floats; however, it is possible to estimate carbon dioxide from things that the float can measure, like pH, a measure of ocean acidity. Here surface ocean carbon dioxide is estimated from several floats and compared to two ship-based estimates. In some locations, the floats closely match the existing estimates, but in other locations the floats see significantly higher surface ocean carbon dioxide in the wintertime, reinforcing the idea that the Southern Ocean's role in the global carbon cycle needs a closer look. Our results show that despite not measuring carbon dioxide directly, these floats will help scientists learn a lot about the Southern Ocean's part in the global flow of carbon.

Carter, BR, Feely RA, Mecking S, Cross JN, Macdonald AM, Siedlecki SA, Talley LD, Sabine CL, Millero FJ, Swift JH, Dickson AG, Rodgers KB.  2017.  Two decades of Pacific anthropogenic carbon storage and ocean acidification along Global Ocean Ship-lebased Hydrographic Investigations Program sections P16 and P02. Global Biogeochemical Cycles. 31:306-327.   10.1002/2016gb005485   AbstractWebsite

A modified version of the extended multiple linear regression (eMLR) method is used to estimate anthropogenic carbon concentration (C-anth) changes along the Pacific P02 and P16 hydrographic sections over the past two decades. P02 is a zonal section crossing the North Pacific at 30 degrees N, and P16 is a meridional section crossing the North and South Pacific at similar to 150 degrees W. The eMLR modifications allow the uncertainties associated with choices of regression parameters to be both resolved and reduced. Canth is found to have increased throughout the water column from the surface to similar to 1000 m depth along both lines in both decades. Mean column Canth inventory increased consistently during the earlier (1990s-2000s) and recent (2000s-2010s) decades along P02, at rates of 0.53 +/- 0.11 and 0.46 +/- 0.11 mol Cm-2 a(-1), respectively. By contrast, Canth storage accelerated from 0.29 +/- 0.10 to 0.45 +/- 0.11 mol Cm-2 a(-1) along P16. Shifts in water mass distributions are ruled out as a potential cause of this increase, which is instead attributed to recent increases in the ventilation of the South Pacific Subtropical Cell. Decadal changes along P16 are extrapolated across the gyre to estimate a Pacific Basin average storage between 60 degrees S and 60 degrees N of 6.1 +/- 1.5 PgC decade(-1) in the earlier decade and 8.8 +/- 2.2 PgC decade(-1) in the recent decade. This storage estimate is large despite the shallow Pacific Canth penetration due to the large volume of the Pacific Ocean. By 2014, Canth storage had changed Pacific surface seawater pH by -0.08 to -0.14 and aragonite saturation state by -0.57 to -0.82.

Hernandez-Guerra, A, Talley LD.  2016.  Meridional overturning transports at 30 degrees S in the Indian and Pacific Oceans in 2002-2003 and 2009. Progress in Oceanography. 146:89-120.   10.1016/j.pocean.2016.06.005   AbstractWebsite

The meridional circulation and transports at 30 degrees S in the Pacific and Indian Oceans for the years 20022003 and 2009 are compared, using GO-SHIP hydrographic section data with an inverse box model and several choices of constraints. Southward heat transport across the combined Indian-Pacific sections, reflecting net heating north of these sections, doubled from -0.7 +/- 0.2 PW in 2002-2003 to -1.4 +/- 0.1 PW in 2009 (negative sign is southward), with the increase concentrated in the Indian Ocean (-0.6 PW compared with similar to 0.2 PW in the Pacific), and was insensitive to model choices for the Indonesian Throughflow. Diagnosed net evaporation also more than doubled in the Indian Ocean, from 0.21-0.27 Sv in 2002-2003 to 0.51-0.58 in 2009, with a smaller but significant increase in net evaporation in the Pacific, from 0.06-0.08 Sv to 0.16-0.32 Sv. These increased heat and freshwater exports coincided with Indian Ocean warming, a shift in the Indian's shallow gyre overturning transport to lower densities, and an increase in southward Agulhas Current transport from 75 Sv in 2002 to 92 Sv in 2009. The Indian's deep overturn weakened from about 11 Sv in 2002 to 7 Sv in 2009. In contrast, the Pacific Ocean overturning circulation was, nearly unchanged from 2003 to 2009, independent of model within the uncertainties. The East Australian Current transport decreased only slightly, from 52 Sv to 46 Sv. The southward Pacific Deep Water transport was at a higher density than the southward Indian Deep Water transport in both years and all models, similar to prior results. Estimated diapycnal diffusivity and velocity are strongly enhanced near the ocean bottom and are higher farther up in the water column in the Indian than in the Pacific, likely extending the reach of Indian Ocean overturning up to shallower depths than in the Pacific. The horizontal distribution of transports in the Pacific at all depths changed notably from 2003 to 2009, despite the stability of its meridional overturning structure. The 2009 horizontal structure resembles a "bowed gyre"; the hydrographic section data show that this disturbance extends to the abyss and disrupts the Deep Western Boundary Current structure in the Southwest Pacific Basin. Satellite altimetry suggests association with slow westward Rossby wave propagation generated in the eastern Pacific, with no apparent effect on the net overturning circulation. The Indian Ocean's upper ocean horizontal structure was stable between the two years even though its shallow gyre overturning transports changed significantly. On the other hand, northward abyssal transports concentrated' in the central Indian Ocean (Crozet Basin) in 2002 shifted westward to the Mozambique and Madagascar Basins in 2009, although the Crozet Basin's Deep Western Boundary Current existed in both years. (C) 2016 Elsevier Ltd. All rights reserved.

Whalen, CB, MacKinnon JA, Talley LD, Waterhouse AF.  2015.  Estimating the mean diapycnal mixing using a finescale strain parameterization. Journal of Physical Oceanography. 45:1174-1188.   10.1175/jpo-d-14-0167.1   AbstractWebsite

Finescale methods are currently being applied to estimate the mean turbulent dissipation rate and diffusivity on regional and global scales. This study evaluates finescale estimates derived from isopycnal strain by comparing them with average microstructure profiles from six diverse environments including the equator, above ridges, near seamounts, and in strong currents. The finescale strain estimates are derived from at least 10 nearby Argo profiles (generally <60 km distant) with no temporal restrictions, including measurements separated by seasons or decades. The absence of temporal limits is reasonable in these cases, since the authors find the dissipation rate is steady over seasonal time scales at the latitudes being considered (0 degrees-30 degrees and 40 degrees-50 degrees). In contrast, a seasonal cycle of a factor of 2-5 in the upper 1000m is found under storm tracks (30 degrees-40 degrees) in both hemispheres. Agreement between the mean dissipation rate calculated using Argo profiles and mean from microstructure profiles is within a factor of 2-3 for 96% of the comparisons. This is both congruous with the physical scaling underlying the finescale parameterization and indicates that the method is effective for estimating the regional mean dissipation rates in the open ocean.

Carter, BR, Talley LD, Dickson AG.  2014.  Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific. Journal of Geophysical Research-Oceans. 119:4001-4028.   10.1002/2013jc009355   AbstractWebsite

A hydrographic data set collected in the region and season of Subantarctic Mode Water and Antarctic Intermediate Water (SAMW and AAIW) formation in the southeastern Pacific allows us to estimate the preformed properties of surface water detrained into these water masses from deep mixed layers north of the Subantarctic Front and Antarctic Surface Water south of the front. Using 10 measured seawater properties, we estimate: the fractions of SAMW/AAIW that originate as surface source waters, as well as fractions that mix into these water masses from subtropical thermocline water above and Upper Circumpolar Deep Water below the subducted SAMW/AAIW; ages associated with the detrained surface water; and remineralization and dissolution rates and ratios. The mixing patterns imply that cabbeling can account for similar to 0.005-0.03 kg m(-3) of additional density in AAIW, and similar to 0-0.02 kg m(-3) in SAMW. We estimate a shallow depth (similar to 300-700 m, above the aragonite saturation horizon) calcium carbonate dissolution rate of 0.4 +/- 0.2 mmol CaCO3 kg(-1) yr(-1), a phosphate remineralization rate of 0.031 +/- 0.009 mu mol P kg(-1) yr(-1), and remineralization ratios of P:N:-O-2:C-org of 1:(15.5 +/- 0.6):(143 +/- 10):(104 +/- 22) for SAMW/AAIW. Our shallow depth calcium carbonate dissolution rate is comparable to previous estimates for our region. Our -O-2:P ratio is smaller than many global averages. Our model suggests neglecting diapycnal mixing of preformed phosphate has likely biased previous estimates of -O-2:P and C-org:P high, but that the C-org:P ratio bias may have been counteracted by a second bias in previous studies from neglecting anthropogenic carbon gradients.

Bourassa, MA, Gille ST, Bitz C, Carlson D, Cerovecki I, Clayson CA, Cronin MF, Drennan WM, Fairall CW, Hoffman RN, Magnusdottir G, Pinker RT, Renfrew IA, Serreze M, Speer K, Talley LD, Wick GA.  2013.  High-latitude ocean and sea ice surface fluxes: Challenges for climate research. Bulletin of the American Meteorological Society. 94:403-423.   10.1175/bams-d-11-00244.1   AbstractWebsite

Polar regions have great sensitivity to climate forcing; however, understanding of the physical processes coupling the atmosphere and ocean in these regions is relatively poor. Improving our knowledge of high-latitute surface fluxes will require close collaboration among meteorologists, oceanographers, ice physicists, and climatologists, and between observationalists and modelers, as well as new combinations of in situ measurements and satellite remote sensing. This article describes the deficiencies in our current state of knowledge about air-sea surface fluxes in high latitutes, the sensitivity of various high-latitude processes to changes in surface fluxes, and the scientific requirements for surface fluxes at high latitutdes. We inventory the reasons, both logistical and physical, why existing flux products do not meet these requirements. Capturing an annual cycle in fluxes requires that instruments function through long periods of cold polar darkness, often far from support services, in situations subject to icing and extreme wave conditions. Furthermore, frequent cloud cover at high latitudes restricts the avilability of surface and atmospheric data from visible and infrared (IR) wavelength satellite sensors. Recommendations are made for improving high-latitude fluxes, including 1) acquiring more in situ observations, 2) developing improved satellite-flux-observing capabilities, 3) making observations and flux products more accessible, and 4) encouraging flux intercomparisons.

Billheimer, S, Talley LD.  2013.  Near cessation of Eighteen Degree Water renewal in the western North Atlantic in the warm winter of 2011-2012. Journal of Geophysical Research-Oceans. 118:6838-6853.   10.1002/2013jc009024   AbstractWebsite

The winter of 2011-2012 was a particularly weak season for the renewal of "Eighteen Degree Water" (EDW), the Subtropical Mode Water of the western North Atlantic, as demonstrated by Argo and repeat hydrography. Weak, late winter buoyancy forcing produced shallower than usual winter mixed layers throughout the subtropical gyre, failing to thoroughly ventilate the underlying mode water, and can likely be attributed to the coinciding high, positive phase of the North Atlantic Oscillation (NAO). The only region where EDW was renewed was in the far northeastern Sargasso Sea where it is understood that the Gulf Stream plays a central role in formation; no EDW formed over the large regions of the gyre where deep winter mixed layers driven by surface buoyancy loss normally create EDW. The present investigation evaluates 2011-2012 winter buoyancy content anomalies, surface buoyancy fluxes, and advection of buoyancy via the Gulf Stream and compares them with the previous seven winters that exhibited more vigorous EDW formation. The weak 2011-2012 formation did not result from increased Gulf Stream heat advection, and was also not driven by preconditioning as the buoyancy content of the region prior to the onset of winter forcing was not unusually high. Rather, the weak formation resulted from climatologically weak surface cooling late in winter. The winter of 2007-2008 also experienced particularly weak EDW formation under similar conditions, including a high NAO and weak late winter surface cooling.

Holte, JW, Talley LD, Chereskin TK, Sloyan BM.  2012.  The role of air-sea fluxes in Subantarctic Mode Water formation. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007798   AbstractWebsite

Two hydrographic surveys and a one-dimensional mixed layer model are used to assess the role of air-sea fluxes in forming deep Subantarctic Mode Water (SAMW) mixed layers in the southeast Pacific Ocean. Forty-two SAMW mixed layers deeper than 400 m were observed north of the Subantarctic Front during the 2005 winter cruise, with the deepest mixed layers reaching 550 m. The densest, coldest, and freshest mixed layers were found in the cruise's eastern sections near 77 degrees W. The deep. SAMW mixed layers were observed concurrently with surface ocean heat loss of approximately -200 W m(-2). The heat, momentum, and precipitation flux fields of five flux products are used to force a one-dimensional KPP mixed layer model initialized with profiles from the 2006 summer cruise. The simulated winter mixed layers generated by all of the forcing products resemble Argo observations of SAMW; this agreement also validates the flux products. Mixing driven by buoyancy loss and wind forcing is strong enough to deepen the SAMW layers. Wind-driven mixing is central to SAMW formation, as model runs forced with buoyancy forcing alone produce shallow mixed layers. Air-sea fluxes indirectly influence winter SAMW properties by controlling how deeply the profiles mix. The stratification and heat content of the initial profiles determine the properties of the SAMW and the likelihood of deep mixing. Summer profiles from just upstream of Drake Passage have less heat stored between 100 and 600 m than upstream profiles, and so, with sufficiently strong winter forcing, form a cold, dense variety of SAMW.

Brambilla, E, Talley LD.  2008.  Subpolar Mode Water in the northeastern Atlantic: 1. Averaged properties and mean circulation. Journal of Geophysical Research-Oceans. 113   10.1029/2006jc004062   AbstractWebsite

Subpolar Mode Waters (SPMW) in the eastern North Atlantic subpolar gyre are investigated with hydrographic and Lagrangian data (surface drifters and isopycnal floats). Historical hydrographic data show that SPMWs are surface water masses with nearly uniform properties, confined between the ocean surface and the permanent pycnocline. SPMWs represented by densities 27.3(sigma theta), 27.4(sigma theta), and 27.5(sigma theta) are present in the eastern subpolar gyre and are influenced by the topography and the regional circulation. Construction of an absolute surface stream function from surface drifters shows that SPMWs are found along the mean path of each of the several branches of the North Atlantic Current (NAC) and their density increases gradually downstream. The Rockall Trough branch of the NAC carries 27.3(sigma theta), 27.4(sigma theta), and 27.5(sigma theta) SPMW toward the Iceland-Faroe Front. In the Iceland Basin, the Subarctic Front along the western flank of the Rockall Plateau carries a similar sequence of SPMW. The western side of the Central Iceland Basin branch of the NAC, on the other hand, veers westward and joins the East Reykjanes Ridge Current, feeding the 27.5(sigma theta) SPMW on the Reykjanes Ridge. The separation among the various NAC branches most likely explains the different properties that characterize the 27.5(sigma theta) SPMW found on the Reykjanes Ridge and on the Iceland-Faroe Ridge. Since the branches of the NAC have a dominant northeastward direction, the newly observed distribution of SPMW combined with the new stream function calculation modify the original hypothesis of McCartney and Talley (1982) of a smooth cyclonic pathway for SPMW advection and density increase around the subpolar gyre.