Decoding the dynamic of poleward shifting climate zones using aqua-planet model simulation

Growing evidence implies that the atmospheric and oceanic circulation experiences a systematic poleward shift in a warming climate. However, the complexity of climate system, including the coupling between the ocean and the atmosphere, natural climate variability and land-sea distribution, tends to obfuscate the causal mechanism underlying the circulation shift. Here, using an idealized coupled aqua-planet model, we explore the mechanism of the shifting circulation, by isolating the contributing factors from the direct CO2 forcing, the indirect ocean surface warming, and the wind-stress feedback from the ocean dynamics. We find that, in contrast to direct CO2 forcing, an enhanced subtropical ocean warming plays a leading role in driving the circulation shift. This enhanced subtropical ocean warming emerges from the background Ekman convergence of surface anomalous heat in the absence of the ocean dynamical change. It expands the tropical warm water zone, causes a poleward shift of the meridional temperature gradients, hence forces a corresponding shift in the atmospheric circulation. The shift in the atmospheric circulation in turn drives a shift in the ocean circulation. Our simulations, despite being idealized, capture the main features of observed climate changes, for example, the enhanced subtropical ocean warming, poleward shift of the patterns of near-surface wind, sea level pressure, storm tracks, precipitation and large-scale ocean circulation, implying that increase in greenhouse gas concentrations not only raises the temperature, but can also systematically shift the climate zones poleward.