  We describe an atom interferometric gravitational wave detector design that
can operate in a resonant mode for increased sensitivity. By oscillating the
positions of the atomic wavepackets, this resonant detection mode allows for
coherently enhanced, narrow-band sensitivity at target frequencies. The
proposed detector is flexible and can be rapidly switched between broadband and
narrow-band detection modes. For instance, a binary discovered in broadband
mode can subsequently be studied further as the inspiral evolves by using a
tailored narrow-band detector response. In addition to functioning like a
lock-in amplifier for astrophysical events, the enhanced sensitivity of the
resonant approach also opens up the possibility of searching for important
cosmological signals, including the stochastic gravitational wave background
produced by inflation. We give an example of detector parameters which would
allow detection of inflationary gravitational waves down to $\Omega_\text{GW}
\sim 10^{-14}$ for a two satellite space-based detector.
