24 Apr 2023
 | 24 Apr 2023

Past warm climate conditions show a shift in Northern Hemisphere winter variability towards a dominant North Pacific Oscillation

Arthur Merlijn Oldeman, Michiel L. J. Baatsen, Anna S. von der Heydt, Aarnout J. van Delden, and Henk A. Dijkstra

Abstract. 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.

Arthur Merlijn Oldeman et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-757', Anonymous Referee #1, 13 Jun 2023
    • AC1: 'Reply on RC1', Arthur Oldeman, 25 Jul 2023
  • RC2: 'Comment on egusphere-2023-757', Anonymous Referee #2, 21 Jun 2023
    • AC2: 'Reply on RC2', Arthur Oldeman, 25 Jul 2023
  • EC1: 'Comment on egusphere-2023-757', David Battisti, 16 Jul 2023
    • AC3: 'Reply on EC1', Arthur Oldeman, 25 Jul 2023

Arthur Merlijn Oldeman et al.

Arthur Merlijn Oldeman et al.


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Short summary
The mid-Pliocene, a geological period around 3 million years ago, is sometimes considered the best analog for near-future climate. It saw similar CO2 concentrations as the present-day, but also a slightly different geography. In this study, we use climate model simulations and find that the Northern Hemisphere winter responds very different to increased CO2 or to the mid-Pliocene geography. Our results weaken the potential of the mid-Pliocene as future analog.