At larger tube lengths, because of shape of the curve of absorption vs. wavelength, the transmission decreases rather slowly with the amount of CO2. (realclimate.org)
The exact shape of this curve varies from place to place and season to season, obviously. (vox.com)
The point I was making was that this version of ENSO index has what I think is an astonishing correlation with Roy Spencer's AMSR - E GLOBAL SST's, as compared here: I find it hard to wrap my head around a global lag of only 3 months, let alone the matching shape of the curves! (wattsupwiththat.com)