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-revision 1.4 by mlosch,
Thu Jan 10 15:47:32 2008 UTC
+revision 1.15 by mlosch,
Tue Feb 26 17:21:48 2008 UTC
 Line 1
+ % $Header$
+ % $Name$
  \documentclass[12pt]{article}
- \usepackage{graphicx,subfigure}
+ \usepackage[]{graphicx}
+ \usepackage{subfigure}
  \usepackage[round,comma]{natbib}
  \bibliographystyle{bib/agu04}
-Line 49
+Line 52
  \maketitle
  \begin{abstract}
-   Some blabla
+ As part of ongoing efforts to obtain a best possible synthesis of most
+ available, global-scale, ocean and sea ice data, a dynamic and thermodynamic
+ sea-ice model has been coupled to the Massachusetts Institute of Technology
+ general circulation model (MITgcm).  Ice mechanics follow a viscous plastic
+ rheology and the ice momentum equations are solved numerically using either
+ line successive relaxation (LSR) or elastic-viscous-plastic (EVP) dynamic
+ models.  Ice thermodynamics are represented using either a zero-heat-capacity
+ formulation or a two-layer formulation that conserves enthalpy.  The model
+ includes prognostic variables for snow and for sea-ice salinity.  The above
+ sea ice model components were borrowed from current-generation climate models
+ but they were reformulated on an Arakawa C-grid in order to match the MITgcm
+ oceanic grid and they were modified in many ways to permit efficient and
+ accurate automatic differentiation.  This paper describes the MITgcm sea ice
+ model; it presents example Arctic and Antarctic results from a realistic,
+ eddy-permitting, global ocean and sea-ice configuration; it compares B-grid
+ and C-grid dynamic solvers in a regional Arctic configuration; and it presents
+ example results from coupled ocean and sea-ice adjoint-model integrations.
  \end{abstract}
  \section{Introduction}
  \label{sec:intro}
- more blabla
+ The availability of an adjoint model as a powerful research
+ tool complementary to an ocean model was a major design
- \section{Model}
+ requirement early on in the development of the MIT general
- \label{sec:model}
+ circulation model (MITgcm) [Marshall et al. 1997a,
+ Marotzke et al. 1999, Adcroft et al. 2002]. It was recognized
+ that the adjoint permitted very efficient computation
+ of gradients of various scalar-valued model diagnostics,
+ norms or, generally, objective functions with respect
+ to external or independent parameters. Such gradients
+ arise in at least two major contexts. If the objective function
+ is the sum of squared model vs. obervation differences
+ weighted by e.g. the inverse error covariances, the gradient
+ of the objective function can be used to optimize this measure
+ of model vs. data misfit in a least-squares sense. One
+ is then solving a problem of statistical state estimation.
+ If the objective function is a key oceanographic quantity
+ such as meridional heat or volume transport, ocean heat
+ content or mean surface temperature index, the gradient
+ provides a complete set of sensitivities of this quantity
+ with respect to all independent variables simultaneously.
+ References to existing sea-ice adjoint models, explaining that they are either
+ for simplified configurations, for ice-only studies, or for short-duration
+ studies to motivate the present work.
  Traditionally, probably for historical reasons and the ease of
  treating the Coriolis term, most standard sea-ice models are
  discretized on Arakawa-B-grids \citep[e.g.,][]{hibler79, harder99,
-   kreyscher00, zhang98, hunke97}. From the perspective of coupling a
+ kreyscher00, zhang98, hunke97}. From the perspective of coupling a
  sea ice-model to a C-grid ocean model, the exchange of fluxes of heat
  and fresh-water pose no difficulty for a B-grid sea-ice model
  \citep[e.g.,][]{timmermann02a}. However, surface stress is defined at
-Line 71 
 velocities points and thus needs to be i
+Line 111 
 velocities points and thus needs to be i
  sea-ice model and a C-grid ocean model. While the smoothing implicitly
  associated with this interpolation may mask grid scale noise, it may
  in two-way coupling lead to a computational mode as will be shown. By
- choosing a C-grid for the sea-ice model, we circumvene this difficulty
+ choosing a C-grid for the sea-ice model, we circumvent this difficulty
  altogether and render the stress coupling as consistent as the
  buoyancy coupling.
-Line 79 
 A further advantage of the C-grid formul
+Line 119 
 A further advantage of the C-grid formul
  straits. In the limit of only one grid cell between coasts there is no
  flux allowed for a B-grid (with no-slip lateral boundary counditions),
  whereas the C-grid formulation allows a flux of sea-ice through this
- passage for all types of lateral boundary conditions. We (will)
+ passage for all types of lateral boundary conditions. We
  demonstrate this effect in the Candian archipelago.
+ Talk about problems that make the sea-ice-ocean code very sensitive and
+ changes in the code that reduce these sensitivities.
+ This paper describes the MITgcm sea ice
+ model; it presents example Arctic and Antarctic results from a realistic,
+ eddy-permitting, global ocean and sea-ice configuration; it compares B-grid
+ and C-grid dynamic solvers in a regional Arctic configuration; and it presents
+ example results from coupled ocean and sea-ice adjoint-model integrations.
+ \section{Model}
+ \label{sec:model}
  \subsection{Dynamics}
  \label{sec:dynamics}
- The momentum equations of the sea-ice model are standard with
+ The momentum equation of the sea-ice model is
  \begin{equation}
    \label{eq:momseaice}
    m \frac{D\vek{u}}{Dt} = -mf\vek{k}\times\vek{u} + \vtau_{air} +
    \vtau_{ocean} - m \nabla{\phi(0)} + \vek{F},
  \end{equation}
- where $\vek{u} = u\vek{i}+v\vek{j}$ is the ice velocity vectory, $m$
+ where $m=m_{i}+m_{s}$ is the ice and snow mass per unit area;
- the ice mass per unit area, $f$ the Coriolis parameter, $g$ is the
+ $\vek{u}=u\vek{i}+v\vek{j}$ is the ice velocity vector;
- gravity accelation, $\nabla\phi$ is the gradient (tilt) of the sea
+ $\vek{i}$, $\vek{j}$, and $\vek{k}$ are unit vectors in the $x$, $y$, and $z$
- surface height potential beneath the ice. $\phi$ is the sum of
+ directions, respectively;
- atmpheric pressure $p_{a}$ and loading due to ice and snow
+ $f$ is the Coriolis parameter;
- $(m_{i}+m_{s})g$. $\vtau_{air}$ and $\vtau_{ocean}$ are the wind and
+ $\vtau_{air}$ and $\vtau_{ocean}$ are the wind-ice and ocean-ice stresses,
- ice-ocean stresses, respectively.  $\vek{F}$ is the interaction force
+ respectively;
- and $\vek{i}$, $\vek{j}$, and $\vek{k}$ are the unit vectors in the
+ $g$ is the gravity accelation;
- $x$, $y$, and $z$ directions.  Advection of sea-ice momentum is
+ $\nabla\phi(0)$ is the gradient (or tilt) of the sea surface height;
- neglected. The wind and ice-ocean stress terms are given by
+ $\phi(0) = g\eta + p_{a}/\rho_{0}$ is the sea surface height potential
+ in response to ocean dynamics ($g\eta$) and to atmospheric pressure
+ loading ($p_{a}/\rho_{0}$, where $\rho_{0}$ is a reference density);
+ and $\vek{F}=\nabla\cdot\sigma$ is the divergence of the internal ice stress
+ tensor $\sigma_{ij}$.
+ When using the rescaled vertical coordinate system, z$^\ast$, of
+ \citet{cam08}, $\phi(0)$ also includes a term due to snow and ice
+ loading, $mg/\rho_{0}$.
+ Advection of sea-ice momentum is neglected. The wind and ice-ocean stress
+ terms are given by
  \begin{align*}
-   \vtau_{air} =& \rho_{air} |\vek{U}_{air}|R_{air}(\vek{U}_{air}) \\
+   \vtau_{air}   = & \rho_{air}  C_{air}   |\vek{U}_{air}  -\vek{u}|
-   \vtau_{ocean} =& \rho_{ocean} |\vek{U}_{ocean}-\vek{u}|
+                    R_{air}  (\vek{U}_{air}  -\vek{u}), \\
+   \vtau_{ocean} = & \rho_{ocean}C_{ocean} |\vek{U}_{ocean}-\vek{u}|
                     R_{ocean}(\vek{U}_{ocean}-\vek{u}), \\
  \end{align*}
  where $\vek{U}_{air/ocean}$ are the surface winds of the atmosphere
- and surface currents of the ocean, respectively. $C_{air/ocean}$ are
+ and surface currents of the ocean, respectively; $C_{air/ocean}$ are
- air and ocean drag coefficients, $\rho_{air/ocean}$ reference
+ air and ocean drag coefficients; $\rho_{air/ocean}$ are reference
- densities, and $R_{air/ocean}$ rotation matrices that act on the
+ densities; and $R_{air/ocean}$ are rotation matrices that act on the
- wind/current vectors. $\vek{F} = \nabla\cdot\sigma$ is the divergence
+ wind/current vectors.
- of the interal stress tensor $\sigma_{ij}$.
+ For an isotropic system the stress tensor $\sigma_{ij}$ ($i,j=1,2$) can
- For an isotropic system this stress tensor can be related to the ice
+ be related to the ice strain rate and strength by a nonlinear
- strain rate and strength by a nonlinear viscous-plastic (VP)
+ viscous-plastic (VP) constitutive law \citep{hibler79, zhang98}:
- constitutive law \citep{hibler79, zhang98}:
  \begin{equation}
    \label{eq:vpequation}
    \sigma_{ij}=2\eta(\dot{\epsilon}_{ij},P)\dot{\epsilon}_{ij}
-Line 129 
 The ice strain rate is given by
+Line 190 
 The ice strain rate is given by
      \frac{\partial{u_{i}}}{\partial{x_{j}}} +
      \frac{\partial{u_{j}}}{\partial{x_{i}}}\right).
  \end{equation*}
- The pressure $P$, a measure of ice strength, depends on both thickness
+ The maximum ice pressure $P_{\max}$, a measure of ice strength, depends on
- $h$ and compactness (concentration) $c$:
+ both thickness $h$ and compactness (concentration) $c$:
  \begin{equation}
-   P = P^{*}c\,h\,e^{[C^{*}\cdot(1-c)]},
+   P_{\max} = P^{*}c\,h\,e^{[C^{*}\cdot(1-c)]},
- \label{icestrength}
+ \label{eq:icestrength}
  \end{equation}
  with the constants $P^{*}$ and $C^{*}$. The nonlinear bulk and shear
  viscosities $\eta$ and $\zeta$ are functions of ice strain rate
-Line 141 
 invariants and ice strength such that th
+Line 202 
 invariants and ice strength such that th
  stress lie on an elliptical yield curve with the ratio of major to
  minor axis $e$ equal to $2$; they are given by:
  \begin{align*}
-   \zeta =& \frac{P}{2\Delta} \\
+   \zeta =& \min\left(\frac{P_{\max}}{2\max(\Delta,\Delta_{\min})},
-   \eta =& \frac{P}{2\Delta{e}^2} \\
+    \zeta_{\max}\right) \\
+   \eta =& \frac{\zeta}{e^2} \\
    \intertext{with the abbreviation}
    \Delta = & \left[
      \left(\dot{\epsilon}_{11}^2+\dot{\epsilon}_{22}^2\right)
-Line 150 
 minor axis $e$ equal to $2$; they are gi
+Line 212 
 minor axis $e$ equal to $2$; they are gi
 \dot{\epsilon}_{11}\dot{\epsilon}_{22} (1-e^{-2})
    \right]^{-\frac{1}{2}}
  \end{align*}
+ The bulk viscosities are bounded above by imposing both a minimum
+ $\Delta_{\min}=10^{-11}\text{\,s}^{-1}$ (for numerical reasons) and a
+ maximum $\zeta_{\max} = P_{\max}/\Delta^*$, where
+ $\Delta^*=(5\times10^{12}/2\times10^4)\text{\,s}^{-1}$. For stress
+ tensor computation the replacement pressure $P = 2\,\Delta\zeta$
+ \citep{hibler95} is used so that the stress state always lies on the
+ elliptic yield curve by definition.
+ In the so-called truncated ellipse method the shear viscosity $\eta$
+ is capped to suppress any tensile stress \citep{hibler97, geiger98}:
+ \begin{equation}
+   \label{eq:etatem}
+   \eta = \min\left(\frac{\zeta}{e^2},
+   \frac{\frac{P}{2}-\zeta(\dot{\epsilon}_{11}+\dot{\epsilon}_{22})}
+   {\sqrt{(\dot{\epsilon}_{11}+\dot{\epsilon}_{22})^2
+       +4\dot{\epsilon}_{12}^2}}\right).
+ \end{equation}
  In the current implementation, the VP-model is integrated with the
  semi-implicit line successive over relaxation (LSOR)-solver of
  \citet{zhang98}, which allows for long time steps that, in our case,
- is limited by the explicit treatment of the Coriolis term. The
+ are limited by the explicit treatment of the Coriolis term. The
  explicit treatment of the Coriolis term does not represent a severe
  limitation because it restricts the time step to approximately the
  same length as in the ocean model where the Coriolis term is also
  treated explicitly.
  \citet{hunke97}'s introduced an elastic contribution to the strain
- rate elatic-viscous-plastic in order to regularize
+ rate in order to regularize Eq.\refeq{vpequation} in such a way that
- Eq.\refeq{vpequation} in such a way that the resulting
+ the resulting elastic-viscous-plastic (EVP) and VP models are
- elatic-viscous-plastic (EVP) and VP models are identical at steady
+ identical at steady state,
- state,
  \begin{equation}
    \label{eq:evpequation}
    \frac{1}{E}\frac{\partial\sigma_{ij}}{\partial{t}} +
-Line 188 
 $\sigma_{12}$. Introducing the divergenc
+Line 267 
 $\sigma_{12}$. Introducing the divergenc
  \dot{\epsilon}_{11}+\dot{\epsilon}_{22}$, and the horizontal tension
  and shearing strain rates, $D_T =
  \dot{\epsilon}_{11}-\dot{\epsilon}_{22}$ and $D_S =
-\dot{\epsilon}_{12}$, respectively and using the above abbreviations,
+\dot{\epsilon}_{12}$, respectively, and using the above abbreviations,
  the equations can be written as:
  \begin{align}
    \label{eq:evpstresstensor1}
-Line 210 
 $E_{0} = \frac{1}{3}$.
+Line 289 
 $E_{0} = \frac{1}{3}$.
  For details of the spatial discretization, the reader is referred to
  \citet{zhang98, zhang03}. Our discretization differs only (but
  importantly) in the underlying grid, namely the Arakawa C-grid, but is
- otherwise straightforward. The EVP model in particular is discretized
+ otherwise straightforward. The EVP model, in particular, is discretized
  naturally on the C-grid with $\sigma_{1}$ and $\sigma_{2}$ on the
  center points and $\sigma_{12}$ on the corner (or vorticity) points of
  the grid. With this choice all derivatives are discretized as central
-Line 218 
 differences and averaging is only involv
+Line 297 
 differences and averaging is only involv
  $P$ at vorticity points.
  For a general curvilinear grid, one needs in principle to take metric
- terms into account that arise in the transformation a curvilinear grid
+ terms into account that arise in the transformation of a curvilinear
- on the sphere. However, for now we can neglect these metric terms
+ grid on the sphere. For now, however, we can neglect these metric
- because they are very small on the cubed sphere grids used in this
+ terms because they are very small on the \ml{[modify following
- paper; in particular, only near the edges of the cubed sphere grid, we
+   section3:] cubed sphere grids used in this paper; in particular,
- expect them to be non-zero, but these edges are at approximately
+ only near the edges of the cubed sphere grid, we expect them to be
-\degS\ or 35\degN\ which are never covered by sea-ice in our
+ non-zero, but these edges are at approximately 35\degS\ or 35\degN\
- simulations.  Everywhere else the coordinate system is locally nearly
+ which are never covered by sea-ice in our simulations.  Everywhere
- cartesian.  However, for last-glacial-maximum or snowball-earth-like
+ else the coordinate system is locally nearly cartesian.}  However, for
- simulations the question of metric terms needs to be reconsidered.
+ last-glacial-maximum or snowball-earth-like simulations the question
- Either, one includes these terms as in \citet{zhang03}, or one finds a
+ of metric terms needs to be reconsidered.  Either, one includes these
- vector-invariant formulation fo the sea-ice internal stress term that
+ terms as in \citet{zhang03}, or one finds a vector-invariant
- does not require any metric terms, as it is done in the ocean dynamics
+ formulation for the sea-ice internal stress term that does not require
- of the MITgcm \citep{adcroft04:_cubed_sphere}.
+ any metric terms, as it is done in the ocean dynamics of the MITgcm
+ \citep{adcroft04:_cubed_sphere}.
+ Lateral boundary conditions are naturally ``no-slip'' for B-grids, as
+ the tangential velocities points lie on the boundary. For C-grids, the
+ lateral boundary condition for tangential velocities is realized via
+ ``ghost points'', allowing alternatively no-slip or free-slip
+ conditions. In ocean models free-slip boundary conditions in
+ conjunction with piecewise-constant (``castellated'') coastlines have
+ been shown to reduce in effect to no-slip boundary conditions
+ \citep{adcroft98:_slippery_coast}; for sea-ice models the effects of
+ lateral boundary conditions have not yet been studied.
  Moving sea ice exerts a stress on the ocean which is the opposite of
  the stress $\vtau_{ocean}$ in Eq.\refeq{momseaice}. This stess is
-Line 249 
 velocity and the ice velocity leading to
+Line 339 
 velocity and the ice velocity leading to
  temperature and salinity are different from the oceanic variables.
  Sea ice distributions are characterized by sharp gradients and edges.
- For this reason, we employ a positive 3rd-order advection scheme
+ For this reason, we employ positive, multidimensional 2nd and 3rd-order
- \citep{hundsdorfer94} for the thermodynamic variables discussed in the
+ advection scheme with flux limiter \citep{roe85, hundsdorfer94} for the
- next section.
+ thermodynamic variables discussed in the next section.
  \subparagraph{boundary conditions: no-slip, free-slip, half-slip}
-Line 288 
 content by means of enthalphy conservati
+Line 378 
 content by means of enthalphy conservati
  addition to ice-thickness and compactness (fractional area) additional
  state variables to be advected by ice velocities, namely enthalphy of
  the two ice layers and the thickness of the overlying snow layer.
+ \ml{[Jean-Michel, your turf: ]Care must be taken in advecting these
+   quantities in order to ensure conservation of enthalphy. Currently
+   this can only be accomplished with a 2nd-order advection scheme with
+   flux limiter \citep{roe85}.}
- \section{Funnel Experiments}
- \label{sec:funnel}
- For a first/detailed comparison between the different variants of the
- MIT sea ice model an idealized geometry of a periodic channel,
-\,km long and 500\,m wide on a non-rotating plane, with converging
- walls forming a symmetric funnel and a narrow strait of 40\,km width
- is used. The horizontal resolution is 5\,km throughout the domain
- making the narrow strait 8 grid points wide. The ice model is
- initialized with a complete ice cover of 50\,cm uniform thickness. The
- ice model is driven by a constant along channel eastward ocean current
- of 25\,cm/s that does not see the walls in the domain. All other
- ice-ocean-atmosphere interactions are turned off, in particular there
- is no feedback of ice dynamics on the ocean current. All thermodynamic
- processes are turned off so that ice thickness variations are only
- caused by convergent or divergent ice flow. Ice volume (effective
- thickness) and concentration are advected with a third-order scheme
- with a flux limiter \citep{hundsdorfer94} to avoid undershoots. This
- scheme is unconditionally stable and does not require additional
- diffusion. The time step used here is 1\,h.
- \reffig{funnelf0} compares the dynamic fields ice concentration $c$,
- effective thickness $h_{eff} = h\cdot{c}$, and velocities $(u,v)$ for
- five different cases at steady state (after 10\,years of integration):
- \begin{description}
- \item[B-LSRns:] LSR solver with no-slip boundary conditions on a B-grid,
- \item[C-LSRns:] LSR solver with no-slip boundary conditions on a C-grid,
- \item[C-LSRfs:] LSR solver with free-slip boundary conditions on a C-grid,
- \item[C-EVPns:] EVP solver with no-slip boundary conditions on a C-grid,
- \item[C-EVPfs:] EVP solver with free-slip boundary conditions on a C-grid,
- \end{description}
- \ml{[We have not implemented the EVP solver on a B-grid.]}
- \begin{figure*}[htbp]
-   \includegraphics[width=\widefigwidth]{\fpath/all_086280}
-   \caption{Ice concentration, effective thickness [m], and ice
-     velocities [m/s]
-     for 5 different numerical solutions.}
-   \label{fig:funnelf0}
- \end{figure*}
- At a first glance, the solutions look similar. This is encouraging as
- the details of discretization and numerics should not affect the
- solutions to first order. In all cases the ice-ocean stress pushes the
- ice cover eastwards, where it converges in the funnel. In the narrow
- channel the ice moves quickly (nearly free drift) and leaves the
- channel as narrow band.
- A close look reveals interesting differences between the B- and C-grid
- results. The zonal velocity in the narrow channel is nearly the free
- drift velocity ( = ocean velocity) of 25\,cm/s for the C-grid
- solutions, regardless of the boundary conditions, while it is just
- above 20\,cm/s for the B-grid solution. The ice accelerates to
-\,cm/s after it exits the channel. Concentrating on the solutions
- B-LSRns and C-LSRns, the ice volume (effective thickness) along the
- boundaries in the narrow channel is larger in the B-grid case although
- the ice concentration is reduces in the C-grid case. The combined
- effect leads to a larger actual ice thickness at smaller
- concentrations in the C-grid case. However, since the effective
- thickness determines the ice strength $P$ in Eq\refeq{icestrength},
- the ice strength and thus the bulk and shear viscosities are larger in
- the B-grid case leading to more horizontal friction. This circumstance
- might explain why the no-slip boundary conditions in the B-grid case
- appear to be more effective in reducing the flow within the narrow
- channel, than in the C-grid case. Further, the viscosities are also
- sensitive to details of the velocity gradients. Via $\Delta$, these
- gradients enter the viscosities in the denominator so that large
- gradients tend to reduce the viscosities. This again favors more flow
- along the boundaries in the C-grid case: larger velocities
- (\reffig{funnelf0}) on grid points that are closer to the boundary by
- a factor $\frac{1}{2}$ than in the B-grid case because of the stagger
- nature of the C-grid lead numerically to larger tangential gradients
- across the boundary; these in turn make the viscosities smaller for
- less tangential friction and allow more tangential flow along the
- boundaries.
- The above argument can also be invoked to explain the small
- differences between the free-slip and no-slip solutions on the C-grid.
- Because of the non-linearities in the ice viscosities, in particular
- along the boundaries, the no-slip boundary conditions has only a small
- impact on the solution.
- The difference between LSR and EVP solutions is largest in the
- effective thickness and meridional velocity fields. The velocity field
- appears to be a little noisy. This noise has been address by
- \citet{hunke01}. It can be dealt with by reducing EVP's internal time
- step (increasing the number of iterations) or by regularizing the bulk
- and shear viscosities. We revisit the latter option by reproducing the
- results of \citet{hunke01} for the C-grid no-slip cases.
- \begin{figure*}[htbp]
-   \includegraphics[width=\widefigwidth]{\fpath/hun12days}
-   \caption{Hunke's test case.}
-   \label{fig:hunke01}
- \end{figure*}
- \begin{itemize}
- \item B-grid LSR no-slip
- \item C-grid LSR no-slip
- \item C-grid LSR slip
- \item C-grid EVP no-slip
- \item C-grid EVP slip
- \end{itemize}
- \subsection{B-grid vs.\ C-grid}
- Comparison between:
- \begin{itemize}
- \item B-grid, lsr, no-slip
- \item C-grid, lsr, no-slip
- \item C-grid, evp, no-slip
- \end{itemize}
- all without ice-ocean stress, because ice-ocean stress does not work
- for B-grid.
  \subsection{C-grid}
  \begin{itemize}
-Line 445 
 differences between the two main options
+Line 429 
 differences between the two main options
  \subsection{Arctic Domain with Open Boundaries}
  \label{sec:arctic}
- The Arctic domain of integration is illustrated in Fig.~\ref{???}.  It is
+ The Arctic domain of integration is illustrated in Fig.~\ref{fig:arctic1}.  It
- carved out from, and obtains open boundary conditions from, the global
+ is carved out from, and obtains open boundary conditions from, the
- cubed-sphere configuration of the Estimating the Circulation and Climate of
+ global cubed-sphere configuration of the Estimating the Circulation
- the Ocean, Phase II (ECCO2) project \cite{men05a}.  The domain size is 420 by
+ and Climate of the Ocean, Phase II (ECCO2) project
-grid boxes horizontally with mean horizontal grid spacing of 18 km.
+ \citet{menemenlis05}.  The domain size is 420 by 384 grid boxes
+ horizontally with mean horizontal grid spacing of 18 km.
+ \begin{figure}
+ %\centerline{{\includegraphics*[width=0.44\linewidth]{\fpath/arctic1.eps}}}
+ \caption{Bathymetry of Arctic Domain.\label{fig:arctic1}}
+ \end{figure}
  There are 50 vertical levels ranging in thickness from 10 m near the surface
  to approximately 450 m at a maximum model depth of 6150 m. Bathymetry is from
  the National Geophysical Data Center (NGDC) 2-minute gridded global relief
  data (ETOPO2) and the model employs the partial-cell formulation of
- \cite{adc97}, which permits accurate representation of the bathymetry. The
+ \citet{adcroft97:_shaved_cells}, which permits accurate representation of the bathymetry. The
  model is integrated in a volume-conserving configuration using a finite volume
  discretization with C-grid staggering of the prognostic variables. In the
- ocean, the non-linear equation of state of \cite{jac95}.  The ocean model is
+ ocean, the non-linear equation of state of \citet{jackett95}.  The ocean model is
  coupled to a sea-ice model described hereinabove.
- This particular ECCO2 simulation is initialized from rest using the January
+ This particular ECCO2 simulation is initialized from rest using the
- temperature and salinity distribution from the World Ocean Atlas 2001 (WOA01)
+ January temperature and salinity distribution from the World Ocean
- [Conkright et al., 2002] and it is integrated for 32 years prior to the
+ Atlas 2001 (WOA01) [Conkright et al., 2002] and it is integrated for
--2001 period discussed in the study. Surface boundary conditions are from
+years prior to the 1996--2001 period discussed in the study. Surface
- the National Centers for Environmental Prediction and the National Center for
+ boundary conditions are from the National Centers for Environmental
- Atmospheric Research (NCEP/NCAR) atmospheric reanalysis [Kistler et al.,
+ Prediction and the National Center for Atmospheric Research
-]. Six-hourly surface winds, temperature, humidity, downward short- and
+ (NCEP/NCAR) atmospheric reanalysis [Kistler et al., 2001]. Six-hourly
- long-wave radiations, and precipitation are converted to heat, freshwater, and
+ surface winds, temperature, humidity, downward short- and long-wave
- wind stress fluxes using the Large and Pond [1981, 1982] bulk
+ radiations, and precipitation are converted to heat, freshwater, and
- formulae. Shortwave radiation decays exponentially as per Paulson and Simpson
+ wind stress fluxes using the \citet{large81, large82} bulk formulae.
- [1977]. Additionally the time-mean river run-off from Large and Nurser [2001]
+ Shortwave radiation decays exponentially as per Paulson and Simpson
- is applied and there is a relaxation to the monthly-mean climatological sea
+ [1977]. Additionally the time-mean river run-off from Large and Nurser
- surface salinity values from WOA01 with a relaxation time scale of 3
+ [2001] is applied and there is a relaxation to the monthly-mean
- months. Vertical mixing follows Large et al. [1994] with background vertical
+ climatological sea surface salinity values from WOA01 with a
- diffusivity of 1.5 � 10-5 m2 s-1 and viscosity of 10-3 m2 s-1. A third order,
+ relaxation time scale of 3 months. Vertical mixing follows
- direct-space-time advection scheme with flux limiter is employed and there is
+ \citet{large94} with background vertical diffusivity of
- no explicit horizontal diffusivity. Horizontal viscosity follows Leith [1996]
+ $1.5\times10^{-5}\text{\,m$^{2}$\,s$^{-1}$}$ and viscosity of
- but modified to sense the divergent flow as per Fox-Kemper and Menemenlis [in
+ $10^{-3}\text{\,m$^{2}$\,s$^{-1}$}$. A third order, direct-space-time
- press].  Shortwave radiation decays exponentially as per Paulson and Simpson
+ advection scheme with flux limiter is employed \citep{hundsdorfer94}
- [1977].  Additionally, the time-mean runoff of Large and Nurser [2001] is
+ and there is no explicit horizontal diffusivity. Horizontal viscosity
- applied near the coastline and, where there is open water, there is a
+ follows \citet{lei96} but
- relaxation to monthly-mean WOA01 sea surface salinity with a time constant of
+ modified to sense the divergent flow as per Fox-Kemper and Menemenlis
-days.
+ [in press].  Shortwave radiation decays exponentially as per Paulson
+ and Simpson [1977].  Additionally, the time-mean runoff of Large and
+ Nurser [2001] is applied near the coastline and, where there is open
+ water, there is a relaxation to monthly-mean WOA01 sea surface
+ salinity with a time constant of 45 days.
  Open water, dry
  ice, wet ice, dry snow, and wet snow albedo are, respectively, 0.15, 0.85,
-Line 512 
 ice, wet ice, dry snow, and wet snow alb
+Line 506 
 ice, wet ice, dry snow, and wet snow alb
  \item C-grid LSR slip
  \item C-grid EVP no-slip
  \item C-grid EVP slip
+ \item C-grid LSR + TEM (truncated ellipse method, no tensile stress, new flag)
  \item C-grid LSR no-slip + Winton
  \item  speed-performance-accuracy (small)
    ice transport through Canadian Archipelago differences
-Line 523 
 We anticipate small differences between
+Line 518 
 We anticipate small differences between
  \begin{itemize}
  \item advection schemes: along the ice-edge and regions with large
    gradients
- \item C-grid: more transport through narrow straits for no slip
+ \item C-grid: less transport through narrow straits for no slip
-   conditons, less for free slip
+   conditons, more for free slip
  \item VP vs.\ EVP: speed performance, accuracy?
  \item ocean stress: different water mass properties beneath the ice
  \end{itemize}
-Line 605 
 storing vs. recomputation of the model s
+Line 600 
 storing vs. recomputation of the model s
  checkpointing loop.
  Again, an initial code adjustment is required to support TAFs
  checkpointing capability.
- The code adjustments are sufficiently simply so as not to cause
+ The code adjustments are sufficiently simple so as not to cause
  major limitations to the full nonlinear parent model.
  Once in place, an adjoint model of a new model configuration
  may be derived in about 10 minutes.
-Line 628 
 may be derived in about 10 minutes.
+Line 623 
 may be derived in about 10 minutes.
  We demonstrate the power of the adjoint method
  in the context of investigating sea-ice export sensitivities through Fram Strait
  (for details of this study see Heimbach et al., 2007).
+ %\citep[for details of this study see][]{heimbach07}. %Heimbach et al., 2007).
  The domain chosen is a coarsened version of the Arctic face of the
  high-resolution cubed-sphere configuration of the ECCO2 project
- (see Menemenlis et al. 2005). It covers the entire Arctic,
+ \citep[see][]{menemenlis05}. It covers the entire Arctic,
  extends into the North Pacific such as to cover the entire
  ice-covered regions, and comprises parts of the North Atlantic
  down to XXN to enable analysis of remote influences of the
-Line 641 
 The adjoint models run efficiently on 80
+Line 637 
 The adjoint models run efficiently on 80
  (benchmarks have been performed both on an SGI Altix as well as an
  IBM SP5 at NASA/ARC).
- Following a 1-year spinup, the model has been integrated for four years
+ Following a 1-year spinup, the model has been integrated for four
- between 1992 and 1995.
+ years between 1992 and 1995. It is forced using realistic 6-hourly
- It is forced using realistic 6-hourly NCEP/NCAR atmospheric state variables.
+ NCEP/NCAR atmospheric state variables. Over the open ocean these are
- Over the open ocean these are converted into
+ converted into air-sea fluxes via the bulk formulae of
- air-sea fluxes via the bulk formulae of Large and Yeager (2004).
+ \citet{large04}.  Derivation of air-sea fluxes in the presence of
- Derivation of air-sea fluxes in the presence of sea-ice is handled
+ sea-ice is handled by the ice model as described in \refsec{model}.
- by the ice model as described in Section XXX.
  The objective function chosen is sea-ice export through Fram Strait
- computed for December 1995
+ computed for December 1995.  The adjoint model computes sensitivities
- The adjoint model computes sensitivities to sea-ice export back in time
+ to sea-ice export back in time from 1995 to 1992 along this
- from 1995 to 1992 along this trajectory.
+ trajectory.  In principle all adjoint model variable (i.e., Lagrange
- In principle all adjoint model variable (i.e. Lagrange multipliers)
+ multipliers) of the coupled ocean/sea-ice model are available to
- of the coupled ocean/sea-ice model
+ analyze the transient sensitivity behaviour of the ocean and sea-ice
- are available to analyze the transient sensitivity behaviour
+ state.  Over the open ocean, the adjoint of the bulk formula scheme
- of the ocean and sea-ice state.
+ computes sensitivities to the time-varying atmospheric state.  Over
- Over the open ocean, the adjoint of the bulk formula scheme
+ ice-covered parts, the sea-ice adjoint converts surface ocean
- computes sensitivities to the time-varying atmospheric state.
+ sensitivities to atmospheric sensitivities.
- Over ice-covered parts, the sea-ice adjoint converts
- surface ocean sensitivities to atmospheric sensitivities.
+ \reffig{4yradjheff}(a--d) depict sensitivities of sea-ice export
+ through Fram Strait in December 1995 to changes in sea-ice thickness
- Fig. XXX(a--d) depict sensitivities of sea-ice export through Fram Strait
+, 24, 36, 48 months back in time. Corresponding sensitivities to
- in December 1995 to changes in sea-ice thickness
+ ocean surface temperature are depicted in
-, 24, 36, 48 months back in time.
+ \reffig{4yradjthetalev1}(a--d).  The main characteristics is
- Corresponding sensitivities to ocean surface temperature are
+ consistency with expected advection of sea-ice over the relevant time
- depicted in Fig. XXX(a--d).
+ scales considered.  The general positive pattern means that an
- The main characteristics is consistency with expected advection
+ increase in sea-ice thickness at location $(x,y)$ and time $t$ will
- of sea-ice over the relevant time scales considered.
+ increase sea-ice export through Fram Strait at time $T_e$.  Largest
- The general positive pattern means that an increase in
+ distances from Fram Strait indicate fastest sea-ice advection over the
- sea-ice thickness at location $(x,y)$ and time $t$ will increase
+ time span considered.  The ice thickness sensitivities are in close
- sea-ice export through Fram Strait at time $T_e$.
+ correspondence to ocean surface sentivitites, but of opposite sign.
- Largest distances from Fram Strait indicate fastest sea-ice advection
+ An increase in temperature will incur ice melting, decrease in ice
- over the time span considered.
+ thickness, and therefore decrease in sea-ice export at time $T_e$.
- The ice thickness sensitivities are in close correspondence to
- ocean surface sentivitites, but of opposite sign.
- An increase in temperature will incur ice melting, decrease in ice thickness,
- and therefore decrease in sea-ice export at time $T_e$.
  The picture is fundamentally different and much more complex
  for sensitivities to ocean temperatures away from the surface.
- Fig. XXX (a--d) depicts ice export sensitivities to
+ \reffig{4yradjthetalev10??}(a--d) depicts ice export sensitivities to
  temperatures at roughly 400 m depth.
  Primary features are the effect of the heat transport of the North
  Atlantic current which feeds into the West Spitsbergen current,
-Line 730 
 sea-ice thickness at various prior times
+Line 721 
 sea-ice thickness at various prior times
  -48 months}]
  {\includegraphics*[width=0.44\linewidth]{\fpath/run_4yr_ADJtheta_arc_lev1_tim292_cmax5.0E+01.eps}}
  }
- \caption{Same as Fig. XXX but for sea surface temperature
+ \caption{Same as \reffig{4yradjheff} but for sea surface temperature
  \label{fig:4yradjthetalev1}}
  \end{figure}
-Line 755 
 parameters that we use here. What about
+Line 746 
 parameters that we use here. What about
  \paragraph{Acknowledgements}
  We thank Jinlun Zhang for providing the original B-grid code and many
- helpful discussions.
+ helpful discussions. ML thanks Elizabeth Hunke for multiple explanations.
  \bibliography{bib/journal_abrvs,bib/seaice,bib/genocean,bib/maths,bib/mitgcmuv,bib/fram}

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