I worked on a home that had this happen much more severely. The home was built on fill dirt and over the years one corner of this Washington DC townhome built in the 1920's sunk 5". To remedy it took the same thing that most likely will take to fix your issue.
In your case, if it is from the freezing of the ground, the dirt is expanding under the foundation, in essence lifting it up, then thawing out, and coming back down. The movement may be minute, but masonry does not get along with movement at all. Without a larger view of the home and other cracks elsewhere, this does not look like what is happening. What is seen in the picture to me looks like the wall under the window is dropping, not just cracking and separating. Checking the level of the floor inside that part of the house will confirm this, AND this depends on the direction that the floor joists run matters too. If they run parallel with the wall the window is in, you may not detect any drop in the floor.
The reason the wall would sink under the window is the foundation may be failing. Whether the freezing ground is lifting, or the window dropping, underpinning is the remedy. It is costly, and a company that does this type of work needs to be brought in to do it.
If this has been going on for the life of the home and has only dropped 1" or less, I would not jump into this type of repair just yet. If it opens up another 1/8" in the next 10 years, then the settling is consistent for the time the home has been standing, more or less.