I cannot recall the exact source of the information, but I remember that I was taught to put a figure caption at the bottom of a figure while put a table caption at the top of a table.
The reason for this arrangement in scientific/engineering article is :
1) a table is usually read from top to bottom, while
2) a figure is usually read from bottom to top.
Hence having a caption just before your eyes start scan a figure or a table would help reading them.