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This experimental study explores the physical mechanisms by which a transverse jet’s upstream shear layer can transition from being a convective instability to an absolute/global instability as the jet-to-cross-flow momentum flux ratio $$J$$ is reduced. As first proposed in computational studies by Iyer & Mahesh ( J. Fluid Mech. , vol. 790, 2016, pp. 275–307), the upstream shear layer just beyond the jet injection may be analogous to a local counter-current shear layer, which is known for a planar geometry to become absolutely unstable at a large enough counter-current shear layer velocity ratio, $$R_{1}$$ . The present study explores this analogy for a range of transverse jet momentum flux ratios and jet-to-cross-flow density ratios $$S$$ , for jets containing differing species concentrations (nitrogen, helium and acetone vapour) at several different jet Reynolds numbers. These studies make use of experimental data extracted from stereo particle image velocimetry as well as simultaneous stereo particle image velocimetry and acetone planar laser-induced fluorescence imaging. They provide experimental evidence for the relevance of the counter-current shear layer analogy to upstream shear layer instability transition in a nozzle-generated transverse jet.