When grid coordinates are used in the dimensions expressions, then they are converted to world coordinates for the boundary to be determined. This conversion is done using the scaling of the default file. Note that the conversion is done using the outside grid boundary, rather than the grid center. Here the boundary would be determined by using the grid values 0.5, 72.5, 0.5, and 46.5 which would be converted to world coordinates. In the Y dimension, when the boundary is beyond the pole, the aave function recognizes this and weights appropriately.
Examples See the tloop function for an example of creating a time series of area averages.
In this case, it is assumed the mask grid has negative values at ocean points.