Why did Luffy ring the ox bell?

Luffy rang the golden bell to let Montblanc Cricket and the monkeys below hear that the City of Gold really existed in the sky. Luffy had just defeated Enel and stood before the massive ruins while reflecting on the man whose life had been ruined by the search for the city. He shouted that the city being up in the sky was not a lie and that they had to tell the people below. ch. 294, p. 16 Otherwise Cricket would keep diving until he wrecked his body and died. ch. 294, p. 17 Luffy decided the only way to reach them was to ring the giant bell whose sound could travel really far. ch. 294, p. 17 He declared he would stay up there and ring the golden bell no matter what Enel tried to do. ch. 294, p. 17 "IF I CAN RING THE BELL, THEY SHOULD BE ABLE TO HEAR ME!" ch. 294, p. 17 "A GIANT BELL'S RING SHOULD BE ABLE TO TRAVEL REALLY FAR!" ch. 294, p. 17 "SO I'M GONNA STAY UP HERE! AND RING THE GOLDEN BELL!" ch. 294, p. 17 No other sources address the bell or its purpose.