Effects of CO2 and Ocean Mixing on Miocene and Pliocene Temperature Gradients
Authors: Lohmann Gerrit , Knorr Gregor , Hossain Akil , Stepanek Christian 
Published: February 2023

During the Cenozoic Era hothouse climate transformed to a state that allowed establishment of extensive ice-sheets. The transformation towards an overall cooler climate encompassed periods of relatively steady change of global temperatures which were interrupted by short-term aberrations of relatively rapid cooling or warming. Various drivers have been found to contribute to this complex process of climate cooling - among these drawdown of carbon dioxide, reorganized ocean circulation related to ocean gateway evolution, varying amplitude and geographic location of deep water upwelling and formation processes, and internal feedbacks related to changes in environmental and land surface conditions in particular at high latitudes and on the continents.The fact that carbon dioxide as the most important current driver of climate change is not always proportionally linked to past changes in global temperatures underlines the importance of mechanisms beyond greenhouse gas drawdown that contributed to Cenozoic climate cooling. Several questions remain regarding mechanisms and drivers of climate evolution as reconstructed from Cenozoic proxy recorders: How can a low meridional temperature gradient be maintained at carbon dioxide concentrations that are in line with reconstructions and inference on relatively modest tropical tempatures? Which mechanisms contributed to extremely high deep sea temperatures ?Here we propose that during the Miocene and the Pliocene enhanced vertical mixing in the ocean may provide potential explanations to some of these enigmas. We employ the global general circulation model, which contributed to PlioMIP, MioMIP, and DeepMIP (e.g., St&#228;rz et al., 2017; Stepanek et al., 2020; Hossain et al., 2020), and study the impact of variations in vertical mixing in the ocean on large-scale climate patterns, meridional temperature gradient, and deep sea ocean temperatures. We find that both carbon dioxide and enhanced vertical mixing cause increased radiative feedback by reducing effective emissivity and surface albedo. For the Miocene, enhanced oceanic heat uptake due to invigorated vertical mixing causes intense warming of the deep ocean (5-10&#176;C) and of the Arctic (>12&#176;C). For the Pliocene we find that the impact of radiative forcing and enhanced vertical mixing is less relevant. This hints to a dependency of carbon dioxide and mixing sensitivity to background climate and ocean dynamics.While our work is focused on climate modelling, we highlight that consideration of enhanced vertical mixing leads in our Miocene and Pliocene climate simulations to large-scale climate patterns that are in better agreement with specific aspects of proxy-based inference on past warm climates. To further corroborate our results we must compare our simulations with reconstructions of thermocline depth and seasonality - lower seasonality in reconstructions would be in line with higher heat capacity as facilitated by enhanced vertical mixing. Our ad-hoc enhanced mixing formulation for the Pliocene and Miocene (Lohmann et al., 2022) can be motivated by recent simulations with a strongly eddying ocean and an altered heat transport (Nooteboom et al, 2022). In the future, we make use of the eddy resolving model to evaluate the polar amplification of the system with respect to model resolution, gateway configuration, and background CO2.