function [AONB]= aonb(S, T, P, keyword)

% SW_AONB    Calculate alpha/beta (a on b)
%================================================================
% SW_AONB   $Revision$   $Date$
%           Copyright (C) CSIRO, Nathan Bindoff 1993
%
% USAGE: [AONB] = aonb(S, T, P, {keyword} )
%
%        [AONB] = aonb(S, T,    P, 'temp' )      %default
%        [AONB] = aonb(S, PTMP, P, 'ptmp' )
%
% DESCRIPTION
%    Calculate alpha/beta.  See sw_alpha.m and sw_beta.m
%
% INPUT:  (all must have same dimensions)
%   S       = salinity              [psu      (PSS-78) ]
% * PTMP    = potential temperature [degree C (ITS-90)]
% * T       = temperature           [degree C (ITS-90)]
%   P       = pressure              [db]
%             (P may have dims 1x1, mx1, 1xn or mxn for S(mxn) )
%
%   keyword = optional string to identify if temp or ptmp passed.
%           = No argument defaults to 'temp'
%           = 'temp' assumes (S,T,P) passed.    Will execute slower
%                    as ptmp will be calculated internally.
%           = 'ptmp' assumes (S,PTMP,P) passed. Will execute faster.
%
% OUTPUT
%   AONB  = alpha/beta [psu/degree_C]
%
% AUTHOR:   N.L. Bindoff  1993, Lindsay Pender (Lindsay.Pender@csiro.au)
%
% DISCLAIMER:
%   This software is provided "as is" without warranty of any kind.
%   See the file sw_copy.m for conditions of use and licence.
%
% REFERENCE:
%    McDougall, T.J. 1987. "Neutral Surfaces"
%    Journal of Physical Oceanography vol 17 pages 1950-1964,
%
% CHECK VALUE:
%    aonb=0.34763 psu C^-1 at S=40.0 psu, ptmp=10.0 C, p=4000 db
%

% Modifications
% 93-04-22. Phil Morgan,  Help display modified to suit library
% 93-04-23. Phil Morgan,  Input argument checking
% 94-10-15. Phil Morgan,  Pass S,T,P and keyword for 'ptmp'
% 99-06-25. Lindsay Pender, Fixed transpose of row vectors.
% 03-12-12. Lindsay Pender, Converted to ITS-90.

% CHECK INPUT ARGUMENTS
if ~(nargin==3 | nargin==4)
  error('sw_aonb.m: requires 3 input arguments')
end %if
if nargin == 3
  keyword = 'temp';
end %if

% CHECK S,T,P dimensions and verify consistent
[ms,ns] = size(S);
[mt,nt] = size(T);
[mp,np] = size(P);


% CHECK THAT S & T HAVE SAME SHAPE
if (ms~=mt) | (ns~=nt)
   error('check_stp: S & T must have same dimensions')
end %if

% CHECK OPTIONAL SHAPES FOR P
if     mp==1  & np==1      % P is a scalar.  Fill to size of S
   P = P(1)*ones(ms,ns);
elseif np==ns & mp==1      % P is row vector with same cols as S
   P = P( ones(1,ms), : ); %   Copy down each column.
elseif mp==ms & np==1      % P is column vector
   P = P( :, ones(1,ns) ); %   Copy across each row
elseif mp==ms & np==ns     % PR is a matrix size(S)
   % shape ok
else
   error('check_stp: P has wrong dimensions')
end %if

%***check_stp

% ENSURE WE USE PTMP IN CALCULATIONS
if ~strcmp(lower(keyword),'ptmp')
  T = sw_ptmp(S,T,P,0); % now have ptmp
end %if

T = T * 1.00024;

% BEGIN
     c1=fliplr([ 0.665157e-1, 0.170907e-1, ...
        -0.203814e-3, 0.298357e-5, ...
            -0.255019e-7]);
         c2=fliplr([ 0.378110e-2, ...
            -0.846960e-4]);
         c2a=fliplr([0.0 -0.164759e-6, ...
            -0.251520e-11]);
         c3=[-0.678662e-5];
         c4=fliplr([+0.380374e-4, -0.933746e-6, ...
            +0.791325e-8]);
         c5=[0.512857e-12];
         c6=[-0.302285e-13];
%
% Now calaculate the thermal expansion saline contraction ratio adb
%
        [m,n] = size(S);
        sm35  = S-35.0*ones(m,n);
        AONB  = polyval(c1,T) + sm35.*(polyval(c2,T)...
               + polyval(c2a,P)) ...
               + sm35.^2*c3 + P.*polyval(c4,T) ...
               + c5*(P.^2).*(T.^2) + c6*P.^3;

return
