Comment on egusphere-2023-757

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> In this study, we address the question whether the mid-Pliocene climate can act as an analog for a future warm climate with elevated CO₂ concentrations, specifically regarding Northern Hemisphere winter variability. We use a set of sensitivity experiments with the global coupled climate model CESM1.0.5, that is a part of PlioMIP2, to separate the response to a CO₂ doubling and to mid-Pliocene boundary conditions other than CO₂. In the CO₂ doubling experiment, the Aleutian low deepens, and the Pacific-North American pattern (PNA) strengthens. In response to the mid-Pliocene boundary conditions, sea-level pressure variance decreases over the North Pacific, the PNA becomes weaker, and the North Pacific Oscillation (NPO) becomes the dominant mode of variability. The mid-Pliocene simulation shows a weak North Pacific jet stream that is less variable in intensity, but has a high level of variation in jet latitude, consistent with a dominant NPO, and indicating that North Pacific atmospheric dynamics become more North Atlantic-like. We show that the weakening of the Aleutian low, and subsequent relative dominance of the NPO over the PNA, is related to the mean surface temperature field in the mid-Pliocene. Variability in the North Atlantic shows little variation between all simulations. The differences between the mid-Pliocene and pre-industrial surface temperature fields are likely caused by differences in orography, which includes the closure of Arctic gateways, rather than a reduced Greenland Ice Sheet. The opposite response in North Pacific winter variability to elevated CO₂ or mid-Pliocene boundary conditions demonstrate that the mid-Pliocene climate cannot serve as a future analog in this regard.