antimonsoon

from The Century Dictionary.

• noun A current of air moving in a direction opposite to that of the monsoon: it may lie above the monsoon proper, but is generally strongest on the west side of it; the northerly wind which opposes the principal southwest monsoon of India, and is overcome by it.
• noun A northerly wind of the Gulf States opposed to the southeasterly winds that draw inward from the Gulf toward the dome of the western continent in the spring and summer seasons and constitute light monsoon winds.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Meteor.) The upper, contrary-moving current of the atmosphere over a monsoon.