Those artifacts look like the result of JPEG compression. Such compression artifacts are particularly likely to be visible in large flat areas with slowly changing intensity, like your blue sky.
If they are really sensor artifacts, then they would be in the raw files. Take a look. Make sure you are examining true raw files, not compressed raw files.
If this is due to compression, as it appears to be, compress less. At least don't compress until you make the final image, then chose the level of compression so that these artifacts are not visible. Any lossless compression is OK. Note that JPG has both lossless and lossy compression. The "quality" setting adjusts the tradeoff of how aggressive the lossy compression is versus the artifacts it causes. Only you can be the judge of what the right level is.
If you need really good quality, like to send out to have a large poster made, consider using only lossless compression. The forward differencing with LZW compression of TIFF files does a good job on flat areas like your sky, without introducing any artifacts at all.