Transverse asymmetry in the temperature profile of the regenerator in a Stirling-type pulse tube cooler as observed in experiments was analysed in a numerical study. The asymmetry was reproduced using a one-dimensional model of the cooler where the regenerator was modelled using two identical parallel regenerator channels. The asymmetry was caused by a circulating flow that was superimposed on the oscillating flow. The primary mechanism driving the circulating flow was due to the wave form of the pressure difference between the ends of the regenerator and the dependence of the instantaneous mass flow rate on the pressure difference and temperature.