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2019
Sutton, AJ, Feely RA, Maenner-Jones S, Musielwicz S, Osborne J, Dietrich C, Monacci N, Cross J, Bott R, Kozyr A, Andersson AJ, Bates NR, Cai WJ, Cronin MF, DeCarlo EH, Hales B, Howden SD, Lee CM, Manzello DP, McPhaden MJ, Melendez M, Mickett JB, Newton JA, Noakes SE, Noh JH, Olafsdottir SR, Salisbury JE, Send U, Trull TW, Vandemark DC, Weller RA.  2019.  Autonomous seawater pCO(2) and pH time series from 40 surface buoys and the emergence of anthropogenic trends. Earth System Science Data. 11:421-439.   10.5194/essd-11-421-2019   AbstractWebsite

Ship-based time series, some now approaching over 3 decades long, are critical climate records that have dramatically improved our ability to characterize natural and anthropogenic drivers of ocean carbon dioxide (CO2) uptake and biogeochemical processes. Advancements in autonomous marine carbon sensors and technologies over the last 2 decades have led to the expansion of observations at fixed time series sites, thereby improving the capability of characterizing sub-seasonal variability in the ocean. Here, we present a data product of 40 individual autonomous moored surface ocean pCO(2) (partial pressure of CO2) time series established between 2004 and 2013, 17 also include autonomous pH measurements. These time series characterize a wide range of surface ocean carbonate conditions in different oceanic (17 sites), coastal (13 sites), and coral reef (10 sites) regimes. A time of trend emergence (ToE) methodology applied to the time series that exhibit well-constrained daily to interannual variability and an estimate of decadal variability indicates that the length of sustained observations necessary to detect statistically significant anthropogenic trends varies by marine environment. The ToE estimates for seawater pCO(2) and pH range from 8 to 15 years at the open ocean sites, 16 to 41 years at the coastal sites, and 9 to 22 years at the coral reef sites. Only two open ocean pCO(2) time series, Woods Hole Oceanographic Institution Hawaii Ocean Time-series Station (WHOTS) in the subtropical North Pacific and Stratus in the South Pacific gyre, have been deployed longer than the estimated trend detection time and, for these, deseasoned monthly means show estimated anthropogenic trends of 1.9 +/- 0.3 and 1.6 +/- 0.3 mu atm yr(-1), respectively. In the future, it is possible that updates to this product will allow for the estimation of anthropogenic trends at more sites; however, the product currently provides a valuable tool in an accessible format for evaluating climatology and natural variability of surface ocean carbonate chemistry in a variety of regions. Data are available at https.//doi. org/10.7289/V5DB8043 and https.//www.nodc.noaa.gov/ocads/oceans/Moorings/ndp097.html (Sutton et al., 2018).

2016
Sutton, AJ, Sabine CL, Feely RA, Cai WJ, Cronin MF, McPhaden MJ, Morell JM, Newton JA, Noh JH, Olafsdottir SR, Salisbury JE, Send U, Vandemark DC, Weller RA.  2016.  Using present-day observations to detect when anthropogenic change forces surface ocean carbonate chemistry outside preindustrial bounds. Biogeosciences. 13:5065-5083.   10.5194/bg-13-5065-2016   AbstractWebsite

One of the major challenges to assessing the impact of ocean acidification on marine life is detecting and interpreting long-term change in the context of natural variability. This study addresses this need through a global synthesis of monthly pH and aragonite saturation state (Omega(arag)) climatologies for 12 open ocean, coastal, and coral reef locations using 3-hourly moored observations of surface seawater partial pressure of CO2 and pH collected together since as early as 2010. Mooring observations suggest open ocean subtropical and subarctic sites experience present-day surface pH and Omega(arag) conditions outside the bounds of preindustrial variability throughout most, if not all, of the year. In general, coastal mooring sites experience more natural variability and thus, more overlap with preindustrial conditions; however, present-day Omega(arag) conditions surpass biologically relevant thresholds associated with ocean acidification impacts on Mytilus californianus (Omega(arag) < 1.8) and Crassostrea gigas (Omega(arag) < 2.0) larvae in the California Current Ecosystem (CCE) and Mya arenaria larvae in the Gulf of Maine (Omega(arag) < 1.6). At the most variable mooring locations in coastal systems of the CCE, subseasonal conditions approached Omega(arag) = 1. Global and regional models and data syntheses of ship-based observations tended to underestimate seasonal variability compared to mooring observations. Efforts such as this to characterize all patterns of pH and Omega(arag) variability and change at key locations are fundamental to assessing present-day biological impacts of ocean acidification, further improving experimental design to interrogate organism response under real-world conditions, and improving predictive models and vulnerability assessments seeking to quantify the broader impacts of ocean acidification.

2002
Send, U, Eden C, Schott F.  2002.  Atlantic equatorial deep jets: Space-time structure and cross-equatorial fluxes. Journal of Physical Oceanography. 32:891-902.   10.1175/1520-0485(2002)032<0891:aedjst>2.0.co;2   AbstractWebsite

The so-called equatorial stacked jets are analyzed with ship-board observations and moored time series from the Atlantic Ocean. The features are identified and isolated by comparing vertical wavenumber spectra at the equator with those a few degrees from the equator. Mode-filtering gives clear views of the jets in meridional sections, the typical extent being +/-1degrees in latitude. The vertical structure can be well described (explaining 82% of the variance) by N-1-stretched cosines, with a Gaussian amplitude tapering in the vertical. The stretched wavelengths are somewhat variable. Fitting jets of a fixed (stretched) wavelength to four moored sensors in the depth range 1300-1900 m, allows one to track the vertical phase of the jets with an rms error of 30degrees-45degrees. The resulting fit from a 20-month moored time series shows long periods of unchanging jet conditions and intermittent times of high variability. There is no significant vertical propagation on these timescales nor a seasonal reversal. Using a composite from many different experiments, interannual variability is visible, however. A possible mechanism for the stacked jets is inertial instability, resulting from background meridional shears at the equator. A condition is that the Ertel potential vorticity becomes zero somewhere, due to meridional asymmetries in the zonal flows. The ship-board observations show that this may be approximately fulfilled by the instantaneous zonal low-mode flows at various depths, resulting from an excess of zonal momentum south of the equator most of the time. Inertial instability should act to redistribute this zonal momentum, and our mooring data show indeed persistent northward momentum flux, but not at the depth levels expected. The momentum transport might suggest that the jets can also flux or mix other properties across the equator.

2000
Skarsoulis, EK, Send U.  2000.  One-step analysis of nonlinear traveltime data in ocean acoustic tomography. Journal of Atmospheric and Oceanic Technology. 17:240-254.   10.1175/1520-0426(2000)017<0240:osaont>2.0.co;2   AbstractWebsite

A new approach based on statistical estimation is proposed for the analysis of tomographic traveltime data in cases of significant nonlinear dependence of the traveltimes on the sound-speed variations. Traditional tomography schemes based on linear perturbative inversions about a single, a priori fixed background state cannot properly handle such cases since the linearized model relations will lend to considerable inversion errors, depending on the extent of nonlinearity. In contrast, the background state is considered here as a variable unknown quantity to be estimated from the traveltime data, simultaneously with the peak identification function and the sound-speed perturbation. Using the maximum likelihood approach and the Gaussian assumption, the statistical estimation problem reduces to a weighted least squares problem to be solved simultaneously for the three unknown quantities. A posteriori inversion-error estimates are derived accounting also for uncertainties in the background selection and the peak identification. The proposed method is applied to nine-month-long traveltime data from the Thetis-2 experiment, conducted from January to October 1994 in the Western Mediterranean Sea, where the variability of the ocean environment gives rise to significant nonlinear dependencies between sound-speed and traveltime variations. The recovered temporal variability and stratification compare well with independent XBT observations.