  We study the spectral energy transfer due to wave-triad interactions in the
Garrett-Munk spectrum of internal gravity waves (IGWs) based on a numerical
evaluation of the collision integral in the wave kinetic equation. Our
numerical evaluation builds on the reduction of the collision integral on the
resonant manifold for a horizontally isotropic spectrum. We directly evaluate
the downscale energy flux available for ocean mixing, whose value is in close
agreement with the empirical finescale parameterization. We further decompose
the energy transfer into contributions from different mechanisms, including
local interactions and three types of nonlocal interactions, namely parametric
subharmonic instability (PSI), elastic scattering (ES) and induced diffusion
(ID). Through analysis on the role of each type of interaction, we resolve two
long-standing paradoxes regarding the mechanism for forward cascade in
frequency and zero ID flux for GM76 spectrum. In addition, our analysis
estimates the contribution of each mechanism to the energy transfer in each
spectral direction, and reveals new understanding of the importance of local
interactions and ES in the energy transfer.
