The position of the southern boundary of the Pacific warm pool is shown to have been stable since the early Pleistocene, based upon a planktic foraminiferal Mg/Ca-derived reconstruction of subtropical sea surface temperature in the Coral Sea. This contrasts with previous reconstructions showing warm pool contraction from the north and east and means that the early Pleistocene warm pool was more hemispherically asymmetric than its present configuration. The latter was not established until similar to 1Ma, supporting a strengthening of the northern Hadley Cell, which was not replicated in its southern counterpart, prior to the Mid-Pleistocene Transition.