Make sure you know the material specs for the tower and then consult the appropiate welder (as such) and make sure he / she knows the material compatibility with the tower and the filler metal and proper prep (not to insult anyone's intelligence, but make sure you find a welder in complete understanding of the applicable metallurgy). That and proper preparation is the success of a proper repair. That crack should be repairable from first glance. Don't just cover the crack with filler metal but prep the joint (should require minimal prep from looking at the pic), place a proper stress relief end point on each end of the crack (I'd drill a small hole on each end of the crack), then lay in the filler as needed. MIG, TIG, or GTAW.
I would even consider additional overlay beyond the crack, just for good measures.
Please do not take this as the proper procedure but use as a thought starter guideline. Your skilled welder should know exactly how to do this properly.