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)