  Using recordings of swell from pitch-and-roll buoys, we have reproduced the
classic observations of long-range surface wave propagation originally made by
Munk et al. (1963) using a triangular array of bottom pressure measurements. In
the modern data, the direction of the incoming swell fluctuates by about $\pm
10^\circ$ on a time scale of one hour. But if the incoming direction is
averaged over the duration of an event then, in contrast with the observations
by Munk et al. (1963), the sources inferred by great-circle backtracking are
most often in good agreement with the location of large storms on weather maps
of the Southern Ocean. However there are a few puzzling failures of
great-circle backtracking e.g., in one case, the direct great-circle route is
blocked by the Tuamoto Islands and the inferred source falls on New Zealand.
Mirages like this occur more frequently in the bottom-pressure observations of
Munk et al. (1963), where several inferred sources fell on the Antarctic
continent.
  Using spherical ray tracing we investigate the hypothesis that the refraction
of waves by surface currents produces the mirages. With reconstructions of
surface currents inferred from satellite altimetry, we show that mesoscale
vorticity significantly deflects swell away from great-circle propagation so
that the source and receiver are connected by a bundle of many rays, none of
which precisely follow a great circle. The $\pm 10^\circ$ directional
fluctuations at the receiver result from the arrival of wave packets that have
travelled along the different rays within this multipath. The occasional
failure of great-circle backtracking, and the associated mirages, probably
results from partial topographic obstruction of the multipath, which biases the
directional average at the receiver.
