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F. I. M. Thomas and M. J. Atkinson

Abstract

A new type of oxygen sensor for use on oceanic profiling CTDs has a response time that ranges from 0.4 to 0.7 s, depending on temperature. This oxygen (O2) sensor was calibrated in the field against O2 concentrations that were determined from Winkler titrations of Niskin-bottle water samples. Calibration coefficients were obtained from single casts and composite casts. When casts were calibrated separately, the mean error in predicting O2 concentrations was 1.22 ± 0.72 µM O2, n = 19 casts, and was not significantly different from the error between Niskin-bottle samples, which was 1.01 ± 2.33 µm O2, n = 33 triplicate or duplicate bottles. When casts wore combined into a composite cast for an entire cruise, however, the mean error in the calibrations increased to 5.03 ± 4.06 µm O2, n = 19 casts. These results also indicate that only three Niskin bottles on a cast are required to obtain an error of 1.0 µm in predicted oxygen. For routine use, we recommend calibrating each cast separately using either three or four measured O2 concentrations to obtain calibration coefficients.

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M. J. Atkinson, F. I. M. Thomas, and N. Larson

Abstract

To measure the effects of pressure on the output of a membrane oxygen sensor and a nonmembrane oxygen sensor, the authors pressure cycled a CTD sensor package in a laboratory pressure facility. The CTD sensor package was cycled from 30 to 6800 db over a range of temperatures from 2° to 38°C. Pressure decreased the output of the membrane sensor and increased the output of the microhole sensor. The pressure terms for both types of oxygen sensors were affected by temperature. The effect of pressure on both types of sensors can be quantified as exp (VP/RT), where V is a coefficient (cm3 mol−1), P is decibars, R′ is the gas constant (831.47 cm3 mol−1 db K−1), and T is kelvins. As water gets colder, V for both sensors increases. For temperatures less than 21°C, V for the membrane sensor is −33.7±0.54 cm3 mol−1, and V for the microbole sensor is 0.29±0.31 cm3 mol−1. The V's for calibrations of four oceanic casts had larger ranges than the laboratory experimental data: −27.6 to −34.9 cm3 mol−1 for the membrane sensor, and −0.4 to −2.9 cm3 mol−1 for the microhole sensor. At 10°C, increasing pressure to depths of 5000 m decreases current output of a membrane sensor approximately 50% and increases output of a microhole sensor about 0.6%. For field calibrations, the authors recommend using a constant V obtained by iterations of linear fits. The use of a pressure term with the form exp(VP/RT) appears to improve field calibrations of membrane oxygen sensors.

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F. I. M. Thomas, S. A. McCarthy, J. Bower, S. Krothapalli, M. J. Atkinson, and P. Flament

Abstract

Response characteristics of a microhole potentiostatic oxygen sensor and a Beckman membrane oxygen sensor were measured in a laboratory over temperatures ranging from 1° to 21°C. The response term τ of the microhole sensor changed 1.7-fold over this temperature range, and τ of the membrane sensor changed 1.6-fold. For the microhole sensor, the effect of temperature on τ can be modeled as lnτ+−6.5 + 1618T −1. For the membrane sensor the temperature effect on τ can be modeled as lnτ = −5.8 + 2116T −1, where T is temperature in kelvins.

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T. Kanzow, S. A. Cunningham, W. E. Johns, J. J-M. Hirschi, J. Marotzke, M. O. Baringer, C. S. Meinen, M. P. Chidichimo, C. Atkinson, L. M. Beal, H. L. Bryden, and J. Collins

Abstract

The Atlantic meridional overturning circulation (AMOC) makes the strongest oceanic contribution to the meridional redistribution of heat. Here, an observation-based, 48-month-long time series of the vertical structure and strength of the AMOC at 26.5°N is presented. From April 2004 to April 2008, the AMOC had a mean strength of 18.7 ± 2.1 Sv (1 Sv ≡ 106 m3 s−1) with fluctuations of 4.8 Sv rms. The best guess of the peak-to-peak amplitude of the AMOC seasonal cycle is 6.7 Sv, with a maximum strength in autumn and a minimum in spring. While seasonality in the AMOC was commonly thought to be dominated by the northward Ekman transport, this study reveals that fluctuations of the geostrophic midocean and Gulf Stream transports of 2.2 and 1.7 Sv rms, respectively, are substantially larger than those of the Ekman component (1.2 Sv rms). A simple model based on linear dynamics suggests that the seasonal cycle is dominated by wind stress curl forcing at the eastern boundary of the Atlantic. Seasonal geostrophic AMOC anomalies might represent an important and previously underestimated component of meridional transport and storage of heat in the subtropical North Atlantic. There is evidence that the seasonal cycle observed here is representative of much longer intervals. Previously, hydrographic snapshot estimates between 1957 and 2004 had suggested a long-term decline of the AMOC by 8 Sv. This study suggests that aliasing of seasonal AMOC anomalies might have accounted for a large part of the inferred slowdown.

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A. Duchez, J. J.-M. Hirschi, S. A. Cunningham, A. T. Blaker, H. L. Bryden, B. de Cuevas, C. P. Atkinson, G. D. McCarthy, E. Frajka-Williams, D. Rayner, D. Smeed, and M. S. Mizielinski

Abstract

The Atlantic meridional overturning circulation (AMOC) has received considerable attention, motivated by its major role in the global climate system. Observations of AMOC strength at 26°N made by the Rapid Climate Change (RAPID) array provide the best current estimate of the state of the AMOC. The period 2004–11 when RAPID AMOC is available is too short to assess decadal variability of the AMOC. This modeling study introduces a new AMOC index (called AMOCSV) at 26°N that combines the Florida Straits transport, the Ekman transport, and the southward geostrophic Sverdrup transport. The main hypothesis in this study is that the upper midocean geostrophic transport calculated using the RAPID array is also wind-driven and can be approximated by the geostrophic Sverdrup transport at interannual and longer time scales. This index is expected to reflect variations in the AMOC at interannual to decadal time scales. This estimate of the surface branch of the AMOC can be constructed as long as reliable measurements are available for the Gulf Stream and for wind stress. To test the reliability of the AMOCSV on interannual and longer time scales, two different numerical simulations are used: a forced and a coupled simulation. Using these simulations the AMOCSV captures a substantial fraction of the AMOC variability and is in good agreement with the AMOC transport at 26°N on both interannual and decadal time scales. These results indicate that it might be possible to extend the observation-based AMOC at 26°N back to the 1980s.

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J. D. Fuentes, M. Lerdau, R. Atkinson, D. Baldocchi, J. W. Bottenheim, P. Ciccioli, B. Lamb, C. Geron, L. Gu, A. Guenther, T. D. Sharkey, and W. Stockwell

Nonmethane hydrocarbons are ubiquitous trace atmospheric constituents yet they control the oxidation capacity of the atmosphere. Both anthropogenic and biogenic processes contribute to the release of hydrocarbons to the atmosphere. In this manuscript, the state of the science concerning biosynthesis, transport, and chemical transformation of hydrocarbons emitted by the terrestrial biosphere is reviewed. In particular, the focus is on isoprene, monoterpenes, and oxygenated hydrocarbons. The generated science during the last 10 years is reviewed to explain and quantify hydrocarbon emissions from vegetation and to discern impacts of biogenic hydrocarbons on local and regional atmospheric chemistry. Furthermore, the physiological and environmental processes controlling biosynthesis and production of hydrocarbon compounds are reported on. Many advances have been made on measurement and modeling approaches developed to quantify hydrocarbon emissions from leaves and forest ecosystems. A synthesis of the atmospheric chemistry of biogenic hydrocarbons and their role in the formation of oxidants and aerosols is presented. The integration of biogenic hydrocarbon kinetics and atmospheric physics into mathematical modeling systems is examined to assess the contribution of biogenic hydrocarbons to the formation of oxidants and aerosols, thereby allowing us to study their impacts on the earth's climate system and to develop strategies to reduce oxidant precursors in affected regions.

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J. C. Dietrich, S. Bunya, J. J. Westerink, B. A. Ebersole, J. M. Smith, J. H. Atkinson, R. Jensen, D. T. Resio, R. A. Luettich, C. Dawson, V. J. Cardone, A. T. Cox, M. D. Powell, H. J. Westerink, and H. J. Roberts

Abstract

Hurricanes Katrina and Rita were powerful storms that impacted southern Louisiana and Mississippi during the 2005 hurricane season. In , the authors describe and validate a high-resolution coupled riverine flow, tide, wind, wave, and storm surge model for this region. Herein, the model is used to examine the evolution of these hurricanes in more detail. Synoptic histories show how storm tracks, winds, and waves interacted with the topography, the protruding Mississippi River delta, east–west shorelines, manmade structures, and low-lying marshes to develop and propagate storm surge. Perturbations of the model, in which the waves are not included, show the proportional importance of the wave radiation stress gradient induced setup.

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S. Bunya, J. C. Dietrich, J. J. Westerink, B. A. Ebersole, J. M. Smith, J. H. Atkinson, R. Jensen, D. T. Resio, R. A. Luettich, C. Dawson, V. J. Cardone, A. T. Cox, M. D. Powell, H. J. Westerink, and H. J. Roberts

Abstract

A coupled system of wind, wind wave, and coastal circulation models has been implemented for southern Louisiana and Mississippi to simulate riverine flows, tides, wind waves, and hurricane storm surge in the region. The system combines the NOAA Hurricane Research Division Wind Analysis System (H*WIND) and the Interactive Objective Kinematic Analysis (IOKA) kinematic wind analyses, the Wave Model (WAM) offshore and Steady-State Irregular Wave (STWAVE) nearshore wind wave models, and the Advanced Circulation (ADCIRC) basin to channel-scale unstructured grid circulation model. The system emphasizes a high-resolution (down to 50 m) representation of the geometry, bathymetry, and topography; nonlinear coupling of all processes including wind wave radiation stress-induced set up; and objective specification of frictional parameters based on land-cover databases and commonly used parameters. Riverine flows and tides are validated for no storm conditions, while winds, wind waves, hydrographs, and high water marks are validated for Hurricanes Katrina and Rita.

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Elizabeth C. Kent, John J. Kennedy, Thomas M. Smith, Shoji Hirahara, Boyin Huang, Alexey Kaplan, David E. Parker, Christopher P. Atkinson, David I. Berry, Giulia Carella, Yoshikazu Fukuda, Masayoshi Ishii, Philip D. Jones, Finn Lindgren, Christopher J. Merchant, Simone Morak-Bozzo, Nick A. Rayner, Victor Venema, Souichiro Yasui, and Huai-Min Zhang

Abstract

Global surface temperature changes are a fundamental expression of climate change. Recent, much-debated variations in the observed rate of surface temperature change have highlighted the importance of uncertainty in adjustments applied to sea surface temperature (SST) measurements. These adjustments are applied to compensate for systematic biases and changes in observing protocol. Better quantification of the adjustments and their uncertainties would increase confidence in estimated surface temperature change and provide higher-quality gridded SST fields for use in many applications.

Bias adjustments have been based on either physical models of the observing processes or the assumption of an unchanging relationship between SST and a reference dataset, such as night marine air temperature. These approaches produce similar estimates of SST bias on the largest space and time scales, but regional differences can exceed the estimated uncertainty. We describe challenges to improving our understanding of SST biases. Overcoming these will require clarification of past observational methods, improved modeling of biases associated with each observing method, and the development of statistical bias estimates that are less sensitive to the absence of metadata regarding the observing method.

New approaches are required that embed bias models, specific to each type of observation, within a robust statistical framework. Mobile platforms and rapid changes in observation type require biases to be assessed for individual historic and present-day platforms (i.e., ships or buoys) or groups of platforms. Lack of observational metadata and high-quality observations for validation and bias model development are likely to remain major challenges.

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