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(Abridged) Many of the observed CO line profiles exhibit broad linewidthsthat greatly exceed the thermal broadening expected within molecular clouds.These suprathermal CO linewidths are assumed to be originated from the presenceof unresolved supersonic motions inside clouds. Typically overlooked in theliterature, in this paper we aim to quantify the impact of the opacitybroadening effects on the current interpretation of the CO suprathermal lineprofiles. Without any additional contributions to the gas velocity field, alarge fraction of the apparently supersonic (${\cal M}\sim$2-3) linewidthsmeasured in both $^{12}$CO and $^{13}$CO (J=1-0) lines can be explained by thesaturation of their corresponding sonic-like, optically-thin C$^{18}$Ocounterparts assuming standard isotopic fractionation. Combined with thepresence of multiple components detected in our C$^{18}$O spectra, theseopacity effects seem to be also responsible of the highly supersonic linewidths(${\cal M}>$8-10) detected in the broadest $^{12}$CO and $^{13}$CO spectra inTaurus. Our results demonstrate that most of the suprathermal $^{12}$CO and$^{13}$CO linewidths could be primarily created by a combination of opacitybroadening effects and multiple gas velocity components blended in thesesaturated emission lines. Once corrected by their corresponding optical depth,each of these gas components present transonic intrinsic linewidthsconsistently traced by the three CO isotopologues within a factor of 2. Highlycorrelated and velocity-coherent at large scales, the largest and highlysupersonic velocity differences inside clouds are generated by the relativemotions between individual gas components. This highly discretized structure ofthe molecular gas traced in CO suggest that the gas dynamics inside molecularclouds could be better described by the properties of a fully-resolvedmacroscopic turbulence.

Opacity broadening and interpretation of suprathermal CO linewidths: Macroscopic Turbulence and Tangled Molecular Clouds