Additivity of handle number and Morse–Novikov number of a-small knots

A knot is an a-small knot if its exterior does not contain closed incompressible surfaces disjoint from some incompressible Seifert surface for the knot. Using circular thin position for knots we prove that the handle number is additive under the connected sum of two a-small knots. As a consequence the Morse–Novikov number turns out to be additive under the connected sum of two a-small knots.