  We address the formation of giant clumps in violently unstable gas-rich disc
galaxies at cosmic noon. While these are commonly thought to originate from
gravitational Toomre instability, cosmological simulations have indicated that
clumps form even in regions where the Toomre $Q$ parameter is well above unity,
which should be stable according to linear Toomre theory (Inoue et al., 2016).
Examining one of these cosmological simulations, we find that it exhibits an
excess in compressive modes of turbulence with converging motions. The energy
in converging motions within proto-clump regions is $\sim 70\%$ of the total
turbulent energy, compared to $\sim 17\%$ expected in equipartition. When
averaged over the whole disc, $\sim 32\%$ of the turbulent energy is in
converging motions, with a further $\sim 8\%$ in diverging motions. Thus, a
total of $\sim 40\%$ of the turbulent energy is in compressive modes, with the
rest in solenoidal modes, compared to the $(1/3)-(2/3)$ division expected in
equipartition. By contrast, we find that in an isolated-disc simulation with
similar properties, resembling high-$z$ star-forming galaxies, the energy in
the different turbulence modes are in equipartition, both in proto-clump
regions and over the whole disc. We conclude that the origin of the excessive
converging motions in proto-clump regions is external to the disc, and propose
several mechanisms that can induce them. This is an additional mechanism for
clump formation, complementary to and possibly preceding gravitational
instability.
