Step-by-step guide
You need to use the mask() macro function. Let us suppose that the field is stored in fieldset g. Then the macro code goes like this:
#Define a mask for the area (Southern Hemisphere in this case). It contains #1s for the points within the area and 0s for points outside the area. mask_sh=mask(g,[0,-180,-90,180]) #We take the "inverse" of this mask and multiply the original field with it. #Now, r contains the original values from g outside the mask area, and #0s inside the mask area. r=g*(1-mask_sh)