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

Abstract

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.

Similar North Pacific variability despite suppressed El Niño variability in the warm mid-Pliocene climate

“…CCSM4-Utr, on the other hand, performs well in terms of ENSO variability but substantially overestimates AL variability, compared to NOAA observations. However, as shown in Oldeman et al (2023), the patterns of North Pacific atmospheric variability (i.e. the spatial patterns, amplitudes, and variance fractions of the PNA and NPO) are very well reproduced, even though the total amplitude is overestimated.…”

“…The PDO is studied through the dominant principal component (PC) of SST anomalies in the North Pacific (20 • N -70 • N, Newman et al (2003)). We study precipitation variability in the West Equatorial Pacific (WEP), which is the region that has the strongest ENSO-related precipitation anomalies (Deser and Wallace, 1990;Williams et al, 2024), here defined as 6 Oldeman et al (2023). In all cases we take area averages using grid weights based on the cosine of the latitude.…”

“…However, there is a considerable model spread regarding both ENSO and PDO change in the PlioMIP2. In one PlioMIP2 model, it is the mid-Pliocene boundary conditions that cause an ENSO suppression and substantial suppression of the AL variability (specifically the PNA), while elevated CO 2 causes increased AL variability without any change of ENSO (Oldeman et al, 2023). It could indicate a tug-ofwar in ENSO teleconnection responses to different conditions of the mid-Pliocene.…”

“…It could indicate a tug-ofwar in ENSO teleconnection responses to different conditions of the mid-Pliocene. However, the model used in Oldeman et al (2023) showed the largest ENSO reduction of the PlioMIP2, so might not be representative for the rest of the ensemble.…”