Generally, ocean waves are thought to act as a drag on the surface
wind so that momentum is transferred downwards, from the atmosphere
into the waves. Recent observations have suggested that when long
wavelength waves, characteristic of remotely generated swell,
propagate faster than the surface wind momentum can also be
transferred upwards. This upward momentum transfer acts to accelerate
the near-surface wind, resulting in a low-level wave-driven wind
jet. Previous studies have suggested that the sign reversal of the
momentum flux is well predicted by the inverse wave age, the ratio of
the surface wind speed to the speed of the waves at the peak of the
spectrum. ECMWF ERA-40 data has been used here to calculate the global
distribution of the inverse wave age to determine whether there are
regions of the ocean that are usually in the wind-driven wave regime
and others that are generally in the wave-driven wind regime. The
wind-driven wave regime is found to occur most often in the
mid-latitude storm tracks where wind speeds are generally high. The
wave-driven wind regime is found to be prevalent in the tropics where
wind speeds are generally light and swell can propagate from storms at
higher latitudes. The inverse wave age is also a useful indicator of
the degree of coupling between the local wind and wave fields. The
climatologies presented emphasise the non-equilibrium that exists
between the local wind and wave fields and highlight the importance of
swell in the global oceans
