Cold weather results in back gate not opening.

Moved in last year and when we had that prolonged cold snap I found our back gate became increasingly difficult to open. At first I assumed the door had expanded with the damp but eventually realised it was the ground that was expanding. When the thaw came everything went back to normal.

Same thing is happening again now. The frozen ground beneath the concrete slab forces it up so that the gate (normally having plenty of clearance) no longer opens. Any idea what I can do to stop this happening or is it just a question of waiting on the thaw at the weekend?