  It is speculated that the high star-formation efficiency observed in
spiral-arm molecular clouds is linked to the prevalence of compressive
(curl-free) turbulent modes, while the shear-driven solenoidal
(divergence-free) modes appear to be the main cause of the low star-formation
efficiency that characterises clouds in the Central Molecular Zone. Similarly,
analysis of the Orion B molecular cloud has confirmed that, although turbulent
modes vary locally and at different scales within the cloud, the dominant
solenoidal turbulence is compatible with its low star formation rate. This
evidence points to inter-and intra-cloud fluctuations of the solenoidal modes
being an agent for the variability of star formation efficiency. We present a
quantitative estimation of the relative fractions of momentum density in the
solenoidal modes of turbulence in a large sample of plane molecular clouds in
the \ce{^{13}CO}/\ce{C^{18}O} ($J=3\rightarrow 2$) Heterodyne Inner Milky Way
Plane Survey (CHIMPS). We find a negative correlation between the solenoidal
fraction and star-formation efficiency. This feature is consistent with the
hypothesis that solenoidal modes prevent or slow down the collapse of dense
cores. In addition, the relative power in the solenoidal modes of turbulence
(solenoidal fraction) appears to be higher in the Inner Galaxy declining with a
shallow gradient with increasing Galactocentric distance. Outside the Inner
Galaxy, the slowly, monotonically declining values suggest that the solenoidal
fraction is unaffected by the spiral arms.
