This function takes a sum over an X-Y region. The syntax is:
asum(expr, xdim1, xdim2, ydim1, ydim2)
where:
expr    - any valid GrADS expression xdim1   - starting X dimension expression xdim2   - ending X dimension expression ydim1   - starting Y dimension expression ydim2   - ending Y dimension expression For global summing, a shorthand may be used:
asum(expr, global) or asum(expr, g)
asum(expr, lon=0, lon=360, lat=-90, lat=90)
asum function always does its average to the
exact boundaries specified, in world coordinates. If the boundaries
specified via the dimension expressions do not fall on grid box
boundaries, then the  values in the boundary grid boxes are weighted in the sum according to the length of the partial grid box in world coordinates. asum(expr,x=1,x=72,y=1,y=46)
  Here the boundary would be determined by using the X grid values ranging from 0.5 to 
  72.5 and Y grid values ranging from 0.5 to 46.5. These four grid boundary values would be converted to world coordinates using the scaling information from the default file. If
  we assume that x=1 is 0 degrees longitude and
  x=72 is 355 degrees longitude, then the averaging
  boundary would be -2.5 to 357.5 degrees, which would cover the
  earth. In the Y dimension, when the boundary is beyond the pole, the
  asum function recognizes this and weights
  appropriately. To calculate a sum without any weighting of partial grid boxes at the domain boundaries, use the
  asumg function.
  
  
asum. The atot function (in GrADS version 2.0.2+) will calculate a sum over an area and weight the grid box values according to the difference in the sine of the latitude at the northern and southern boundaries of each grid box.