We present estimates of gravity wave momentum fluxes calculated from
Project Loon superpressure balloon data collected between 2013 and 2021.
In total, we analyzed more than 5000 days of data from balloon flights
in the lower stratosphere, flights often over regions or during times of
the year without any previous in-situ observations of gravity waves.
Maps of mean momentum fluxes show significant regional variability; we
analyze that variability using the statistics of the momentum flux
probability distributions for six regions: the Southern Ocean, the
Indian Ocean, and the tropical and extratropical Pacific and Atlantic
Oceans. The probability distributions are all approximately log-normal,
and using only their geometric means and geometric standard deviations
we explain the sign and magnitude of regional mean and 99th percentile
zonal momentum fluxes, and regional momentum flux intermittencies. We
study the dependence of the zonal momentum flux on the background zonal
wind and argue that the increase of the momentum flux with the wind
speed over the Southern Ocean is likely due to a varying combination of
both wave sources and filtering. Finally, we show that as the magnitude
of the momentum flux increases, the fractional contributions by
high-frequency waves increases, waves which need to be parameterized in
large-scale models of the atmosphere. In particular, the
near-universality of the log-normal momentum flux probability
distribution, and the relation of its statistical moments to the mean
momentum flux and intermittency, offer useful checks when evaluating
parameterized or resolved gravity waves in models.