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\authorrunninghead{COULDREY ET AL.}

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\titlerunninghead{CARBON FLUX CONTROLS}

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\authoraddr{Corresponding author: M. P. Couldrey,
University of Southampton, Waterfront Campus, European Way, Southampton, Hampshire, SO143ZH (M.P.Couldrey@soton.ac.uk)}

\begin{document}

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\title{Supporting Information for ``On which timescales do gas transfer velocities control North Atlantic CO$_{2}$ flux variability?"}
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DOI: 10.1002/2015GB005267

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\authors{Matthew P. Couldrey\altaffilmark{1}, Kevin I. C. Oliver\altaffilmark{1}, Andrew Yool\altaffilmark{2}, Paul R. Halloran\altaffilmark{3} and Eric P. Achterberg\altaffilmark{1,4}}

\altaffiltext{1}{Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, UK}
\altaffiltext{2}{National Oceanography Centre, Southampton, UK}
\altaffiltext{3}{Geography, College of Life and Environmental Sciences, University of Exeter, UK}
\altaffiltext{4}{GEOMAR Helmholtz-Zentrum f{\"u}r Ozeanforschung, Kiel, Germany}

\begin{article}

\noindent\textbf{Contents of this file}
\begin{enumerate}
\item Text S1
\item Figure S1
\item Figure S2
\end{enumerate}

\noindent\textbf{Additional Supporting Information (Files uploaded separately)}
\begin{enumerate}
\item Software S1 to S13
\end{enumerate}

\noindent\textbf{Introduction}
This appendix contains some additional investigation into alternative gas transfer velocities and the model output used in this work (attached separately). We calculated critical timescales (Figure \ref{fig:s01}) using different gas transfer parameterizations (Figure \ref{fig:s02}) and offline model output, rather than running new simulations. Although this approach means that calculated critical timescales are not directly comparable with those in the main manuscript, we find that these results are qualitatively consistent with our main findings. The model output consists of 12 zipped netcdf archives (named 2015GB005267-sftsxx.nc.zip, where xx are the numbers 01 to 12 for the different files) and one .txt README file (2015GB005267-sfts13.txt) to describe the output.

\noindent\textbf{Text S1.}
To investigate the sensitivity of the critical timescale of $\Delta$\textit{p}CO$_{2}$ dominance on flux variability to the choice of gas transfer parameterization, we recalculated its value using alternative functions (Figure \ref{fig:s01}). To do this thoroughly, would be necessary to run additional simulations, each with a different \textit{k}. Since this is not possible within the scope of this work, we instead calculated \textit{k} using offline wind speeds. This will introduce some error, since the ocean's DIC field will not accurately respond to the differences in CO$_{2}$ flux imparted by the use of a different \textit{k} than that with which the model was run. As a result, this is not directly comparable with the analysis shown in the main manuscript, but it nevertheless indicates that the result is qualitatively similar. The other parameterizations chosen were selected on the basis that they were functionally distinct from the one used in our model (\citep{nightingale2000}) and commonly used: \citep{wanninkhof2014}, \citep{weiss2007}, \citep{sweeney2007} and \citep{mcgillis2001} (shown in Figure \ref{fig:s02}). Broadly, the spatial structures of all five are quite similar, and there is good agreement between timescales, no matter which parameterization is used (Figure \ref{fig:s01}). The main difference that the choice of parameterization makes is in the magnitude of the critical timescale when the timescale is large, making the highs higher, especially when the \citep{mcgillis2001} equation is used. 

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\begin{thebibliography}{5}
\providecommand{\natexlab}[1]{#1}
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\bibitem[{\textit{McGillis et~al.}(2001)\textit{McGillis, Edson, Hare, and
  Fairall}}]{mcgillis2001}
McGillis, W., J.~Edson, J.~Hare, and C.~Fairall (2001), Direct covariance
  air-sea {CO$_{2}$} fluxes, \textit{Journal of Geophysical Research: Oceans
  (1978--2012)}, \textit{106}(C8), 16,729--16,745.

\bibitem[{\textit{Nightingale et~al.}(2000)\textit{Nightingale, Malin, Law,
  Watson, Liss, Liddicoat, Boutin, and Upstill-Goddard}}]{nightingale2000}
Nightingale, P.~D., G.~Malin, C.~S. Law, A.~J. Watson, P.~S. Liss, M.~I.
  Liddicoat, J.~Boutin, and R.~C. Upstill-Goddard (2000), In situ evaluation of
  air-sea gas exchange parameterizations using novel conservative and volatile
  tracers, \textit{Global Biogeochemical Cycles}, \textit{14}(1), 373--387.

\bibitem[{\textit{Sweeney et~al.}(2007)\textit{Sweeney, Gloor, Jacobson, Key,
  McKinley, Sarmiento, and Wanninkhof}}]{sweeney2007}
Sweeney, C., E.~Gloor, A.~R. Jacobson, R.~M. Key, G.~McKinley, J.~L. Sarmiento,
  and R.~Wanninkhof (2007), Constraining global air-sea gas exchange for
  {CO$_{2}$} with recent bomb 14c measurements, \textit{Global Biogeochemical
  Cycles}, \textit{21}(2).

\bibitem[{\textit{Wanninkhof}(2014)}]{wanninkhof2014}
Wanninkhof, R. (2014), Relationship between wind speed and gas exchange over
  the ocean revisited, \textit{Limnology and Oceanography: Methods},
  \textit{12}(6), 351--362.

\bibitem[{\textit{Weiss et~al.}(2007)\textit{Weiss, Kuss, Peters, and
  Schneider}}]{weiss2007}
Weiss, A., J.~Kuss, G.~Peters, and B.~Schneider (2007), Evaluating transfer
  velocity-wind speed relationship using a long-term series of direct eddy
  correlation {CO$_{2}$} flux measurements, \textit{Journal of Marine Systems},
  \textit{66}(1), 130--139.

\end{thebibliography}



\end{article}

\begin{figure}
\setfigurenum{S1}
\centering
\noindent\includegraphics[width=30pc]{2015GB005267-FigS01}
\caption{Critical timescale of flux variability (purple colors) calculated using four alternative gas transfer velocities (\citep{wanninkhof2014}, \citep{weiss2007}, \citep{sweeney2007} and \citep{mcgillis2001}). Orange: areas where \textit{k} variability is important on all timescales. Compare with Figure 8a in the main manuscript}
\label{fig:s01}
\end{figure}

\begin{figure}
\setfigurenum{S2}
\centering
\noindent\includegraphics[width=30pc]{2015GB005267-FigS02}
\caption{Relationships between wind speed and gas transfer velocity (\textit{k}) for the parameterizations shown in Figure \ref{fig:s01} and the one used in the main simulations (\citep{nightingale2000})}
\label{fig:s02}
\end{figure}

\end{document}

