Enhanced ocean mixing improves proxy-model agreement for an Early Pliocene seasonally ice-free Arctic Ocean

Abstract. The Arctic Ocean rapidly transitions towards ice-free summers, a "blue" (i.e., summer sea ice-free) state driven by Arctic amplification of abrupt global warming. To improve climate projections and better understand the consequences of reduced Arctic sea ice in a warming climate, it is essential to examine past warm intervals such as the Early Pliocene (~5–4 Ma), when diminished Arctic sea ice coincided with higher global temperatures. Proxy records from the Arctic-Atlantic Gateway (AAG) (~70–80° N), combined with climate model simulations, indicate periodically ice-free summers during this time. Yet, model-data comparisons for the central Arctic Ocean (>80° N) reveal winter sea ice as a persistent feature. Here, we show that proxy evidence for a "blue" Arctic is best reproduced when enhanced vertical ocean mixing is integrated into climate models. Under enhanced mixing, Arctic warming occurs through different mechanisms depending on the model geography, whereas simulations with standard mixing show substantially weaker agreement with proxy evidence. We do not interpret enhanced mixing as the unique solution but rather as a physically motivated sensitivity test that represents unresolved ocean processes. Our results demonstrate that modifying a single ocean parameterization within physically plausible boundaries substantially improves proxy-model agreement for a warmer than modern Early Pliocene, both at the AAG and for the wider Arctic. Combined with published proxy evidence for a warmer Arctic Ocean interior, our findings underscore the need for improved representation of ocean dynamics in paleoclimate simulations. Next-generation climate models should evaluate the sensitivity of simulated "blue" Arctic climates to ocean mixing alongside other key processes, including cloud microphysics and radiative feedbacks, to increase confidence in projections of seasonally ice-free Arctic conditions