Homemade sauerkraut liquid level below cabbage. Should I add brine?
As you know, the salt in the brine keeps bad things from growing. The cabbage above the brine is at risk of mold or undesirable bacteria growing.

When your cabbage is actively fermenting it generates a good amount of CO2. Your lid is designed to bleed off the extra pressure without letting any O2 leak back in. Additionally, the CO2 is heavier than air and will displace residual oxygen in the jar.

You can open your lid and top off your brine. If it is fermenting as it is supposed to, the little oxygen in the jar will be consumed or displaced in short order and you will have your lovely anaerobic environment back. Just don't open it often.

In the future, I would recommend using a larger jar (or less cabbage) and give yourself a good inch or two of brine above the cabbage. Use a weight to keep the cabbage submerged. (I can't tell from your picture if your cabbage is weighted or not)