PNNL

Abstract

The degree of Arctic Amplification (AA) varies strongly with the analysis period, being ~1.9 over 1970-2005 but ~3.8 over 1985-2020. Zhou, Leung and Lu 2024 showed that this period-dependent AA reflects natural variability superimposed on a steady threefold forced AA. In particular, the interdecadal Pacific oscillation modulates global temperature, while an Arctic mode (AM) modulates Arctic temperature. Pithan and Hubner 2025 argued that AM identified in Zhou, Leung and Lu 2024 is an artefact arising from applying linear detrending to a period with little Arctic warming before 1985. Here, we rebut their argument and present additional evidence for the existence of the Arctic mode. We emphasize that the period before 1985 should be included because global temperature had already warmed notably and the muted Arctic warming indicates the effect of natural variability. We show that the Arctic mode is consistently identified from Empirical Orthogonal Function (EOF) analysis when the forced response is not assumed to be linear, when alternative longer and shorter periods are considered, and when analysis is restricted to the Arctic region with area weighting. The Arctic mode differs notably from the long-term warming trend in both spatial pattern and dynamic fingerprints. Moreover, it emerges in the PiControl simulations of climate models, strongly supporting its existence as an internally generated physical mode. We conclude that the Arctic mode is not an artefact and its physics warrant further investigation for a better understanding of Arctic variability.