Spectra obtained over the region of the four boxes defined in Fig. 6, for the following lines: ¹²CO (in black), ¹³CO (in blue), and C¹⁸O (in red). First row: (1–0) transition, second row: (2–1) transition, and third row: (3–2) transition for: A, the cloudlet; B, the ring-likestructure; C, the shocked clump; and D, the shocked knot. Spectral cubes were resampled to allow a direct comparison between the different spectra. Spatial resolutions of all transitions were modified to the nominal resolution of C¹⁸O (2–1), θ = 30.2′′. Spectral resolution were set to 0.5 km s⁻¹ for the IRAM 30 m data, and 0.25 km s⁻¹ for APEX data. On both panels, the v_LSR of IC443 is indicated with a vertical dashed line (at − 4.5 km s⁻¹). The black data points with error bars represent the measurements of the optical depth τ corresponding to each channel of these spectra resampled with a spectral resolution of 2 km s⁻¹, using the [¹²CO]/[¹³CO] ratio as a tracer, and the red data points represent the measurements of the optical depth obtained using the [¹²CO]/[C¹⁸O] ratio. The adopted values for the isotopic ratios are based on the results of Sect. 4.2.1 and indicated in Table 5.