By comparison with both experimental and numerical data, Dysthe's (1979) O(ε4)
modified nonlinear Schrödinger; equation has been shown to model the evolution
of a slowly varying wavetrain well (here ε is the wave steepness). In this work, we
extend the equation to include a prescribed, large-scale, O(ε2) surface current which
varies about a mean value. As an introduction, a heuristic derivation of the O(ε3)
current-modified equation, used by Bakhanov et al. (1996), is given, before a more
formal approach is used to derive the O(ε4) equation. Numerical solutions of the new
equations are compared in one horizontal dimension with those from a fully nonlinear
solver for velocity potential in the specific case of a sinusoidal surface current, such
as may be due to an underlying internal wave. The comparisons are encouraging,
especially for the O(ε4) equation.