A comparative study of large-scale atmospheric circulation in the context of a future scenario (RCP4.5) and past warmth (mid-Pliocene)

Abstract. The mid-Pliocene warm period (~ 3.3–3.0 Ma) is often considered as the last sustained warm period with close enough geographic configurations compared to the present one associated with atmospheric CO₂ concentration (405 ± 50 ppm) higher than the modern level. For this reason, this period is often considered as a potential analogue for the future climate warming, with the important advantage that for mid-Pliocene many marine and continental data are available. To investigate this issue, we selected the RCP4.5 scenario, one of the current available future projections, to compare the pattern of tropical atmospheric response with the past warm mid-Pliocene climate.

We use three Atmosphere-Ocean General Circulation Model (AOGCM) simulations (RCP4.5 scenario, mid-Pliocene and present-day simulation) carried out with the IPSL-CM5A model and investigate atmospheric tropical dynamics through Hadley and Walker cell responses to warmer conditions, considering that the analysis can provide some assessment of how these circulations will change in the future. Our results show that there is a damping of the Hadley cell intensity in the northern tropics and an increase in both subtropics. Moreover, northern and southern Hadley cells expand poleward. The response of the Hadley cells is stronger for the RCP4.5 scenario than for the mid-Pliocene, but in very good agreement with the fact that the atmospheric CO₂ concentration is higher in the future scenario than in the mid-Pliocene (543 versus 405 ppm). Concerning the response of the Walker cell, we show that despite very large similarities, there are also some differences. Common features to both scenarios are: weakening of the ascending branch, leading to a suppression of the precipitation over the western tropical Pacific. The response of the Walker cell is stronger in the RCP4.5 scenario than in the mid-Pliocene but also depicts some major differences, as an eastward shift of its rising branch in the future scenario compared to the mid-Pliocene.

In this paper, we explain the dynamics of the Hadley and Walker cells, and show that despite a minor discrepancy, the mid-Pliocene is certainly an interesting analogue for future climate changes in tropical areas.