how to calculate the zonal mean, while the coordinates of the data are two dimension

Hi all,
I have one question on how to calculate the zonal mean of data, of which
the coordinates are all two dimension.
The data I used are Sea Ice Concentrations (SIC) form NASA Goddard and
NSIDC based on NASA Team algorithm, which are provided in the polar
stereographic projection at a grid cell size of 25 x 25 km.
Description of SIC data (part of coordinates ) , as follows:
.............
      double latitude ( ygrid, xgrid )
         standard_name : latitude
         long_name : latitude
         units : degrees_north
         valid_range : ( 0, 90 )
         _FillValue : -999

      double longitude ( ygrid, xgrid )
         standard_name : longitude
         long_name : longitude
         units : degrees_east
         valid_range : ( -180, 180 )
         _FillValue : -999

      character projection ( ncl_scalar )
         grid_boundary_top_projected_y : 5850000
         grid_boundary_bottom_projected_y : -5350000
         grid_boundary_right_projected_x : 3750000
         grid_boundary_left_projected_x : -3850000
         parent_grid_cell_row_subset_start : 0
         parent_grid_cell_row_subset_end : 448
         parent_grid_cell_column_subset_start : 0
         parent_grid_cell_column_subset_end : 304
.........
Thanks for the help in advance.
Tingting

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Received on Wed Dec 5 11:44:49 2012