This looks rather serious - should we replace or brace it ASAP?
I myself would prefer to replace it, as the crack/check goes down beyond the knot breakout or simply for aesthetics (compared to the treatment below). If that crack/check goes to the bottom or within 2-feet of it may split entirely, so replacement would be the only option.
However, if the crack/check goes a foot or less below the picture frame, then the installation of 1/2" nuts, washers and through-bolts in pairs every foot from the where it starts and ends will keep everything together, aligned and from growing.
This post is for up (wind lift) and down loads, it would only knee or elbow out if struck laterally. The goal is to just re-establish the trunk, leg or arm to full load capacity and avoid any shear-off or stepping-out when under load.