Answer:
 ascend
Step-by-step explanation:
There are a couple of ways to figure this. One is to look at the partial derivative ...
 [tex]\dfrac{\partial z}{\partial y}=-0.02y[/tex]
The sign of it is negative, so when y decreases, z increases. Going south means decreasing the y-coordinate, so moving in that direction will cause you to ascend.
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Another way to figure this is to evaluate z for some small movement in the southerly direction. In the attached, we find that moving 1 m south to (120, 79, 1165.59) causes an elevation increase of about 1.6 m. Walking south causes you to ascend.
Note that this is consistent with our first result, as -0.02y = -0.02(80) = -1.6. So a change in y of -1 should cause a change in z of about (-1.6)(-1) = 1.6.
