On the causes of the mid-Pliocene ITCZ shifts

Gabriel Pontes¹, Andréa Taschetto², Ilana Wainer¹

1. Oceanographic Institute, University of Sao Paulo, Sao Paulo, SP, Brazil
2. Climate Change Research Center, University of New South Wales, Sydney, NSW, Australia

Since the establishment of the modern geographic distribution of the continents at ~3.5 Ma BP, the Pliocene was the period with lowest global ice volume.  It was also a period with global temperatures higher than the present climate and CO2 concentration at similar levels. Therefore, the Pliocene allows us to evaluate the impact of reduced global ice volume on a warmer climate and provide insights for future climate studies. Here we investigate the climate of the Pliocene using model simulations from the Pliocene Model Intercomparison Project (PlioMIP), numerical experiments with the NCAR CESM (EXPAND) model, and sea surface temperature (SST) reconstructions. We look into the tropics with particular focus on the Atlantic Inter-Tropical Convergence Zone (ITCZ). We find that in 7, out of 8 models analyzed, the ITCZ migrates northwards by 1.5º (±0.5º) under the PlioMIP boundary conditions with respect to its mean piControl position. It illustrates an amplified warming in the Northern Hemisphere in comparison to the Southern Hemisphere warming and readjustment of the atmospheric circulation. The northern hemisphere Hadley cell weakens and migrates northwards while the southern cell strengthens and expands northwards accompanied by intensified southward heat transport at the equator. Here we show that the northward position of the ITCZ in the Pliocene is not related to changes in the Atlantic Meridional Overturning Circulation, and we explore instead a possible role of reduced sea-ice volume in changing the inter-hemispheric heat balance. To understand the driving factor of the changes in the ITCZ we perform sensitivity experiments with the NCAR-CESM model, where we can disentangle the differing role of reduced ice volume and SST distribution during the Pliocene in driving ITCZ shifts. The results of the numerical experiments will be discussed.