Role of Vertical Mixing in the Upper Ocean in the Seasonal Variation of Arctic Amplification.

Reanalysis data and a numerical model are employed to explore how diffusion in the upper ocean evolves in different seasons and to understand its contribution to seasonal heat storage (SHS) and Arctic amplification. The numerical simulation results are closely consistent with observations. First, Arctic solar radiation absorption anomaly (maximum in June) due to ice‐loss is mainly stored as SHS (maximum in June and July) or warms the surface (minimum in July) in observations. Furthermore, the numerical simulation suggests that vertical diffusion dominates SHS formation/discharge. Second, surface ocean becomes warmer than surface air in cold season and releases SHS in Arctic. Sea‐ice loss allows more ocean be directly driven by wind; surface high pressure over Arctic may enhance the wind stress; increased water freeze leads to more salt rejection and additional dense water being produced and sinking downward; all of the occurrences strengthen vertical mixing and release more SHS to the atmosphere. As a result, Arctic surface warming reaches its maximum in cold season. In observations, much stronger outgoing longwave radiation due to the warmer surface in October causes Arctic surface warming to reach its maximum in November, although increased carbon dioxide forcing also contributes to maintain longwave radiation in November. Finally, Arctic surface (air) warming in cold season exhibits different spatial patterns from SHS discharge due to heat convergence/divergence induced by anomalous surface wind, which is determined by the variation in surface high pressure over the Arctic. Plain Language Summary: Due to little seasonal variation in global surface warming, seasonality of Arctic amplification (AA) is mainly determined by seasonal variation of Arctic surface warming. Seasonal heat storage (SHS) in the subsurface Arctic Ocean contributes to Arctic surface warming by holding anomalous heat in summer and releasing it in autumn and winter. As a result, there is significant seasonal variation in AA. In this study, we directly estimate the SHS with ocean data and focus on spatial feature discussions. (a) In summer, additional solar radiation is stored as SHS via vertical diffusion in the upper ocean, resulting in weak Arctic warming. (b) In cold season, more current is driven directly by wind due to sea‐ice loss; variation in surface high pressure over the Arctic enhances the wind stress; more dense water, which is produced due to additional water freezes, sink downward; all of which strengthen the vertical mixing and release more SHS, resulting in the Arctic warming maximum. (c) Although SHS discharge dominates Arctic warming in cold season, anomalous surface wind induces heat convergence/divergence and influences the spatial distribution of surface air temperature, which in turn impacts surface warming via air‐sea/air‐land interactions. Finally, the SHS anomaly and surface warming share different spatial patterns. Key Points: With (without) land surface warming occurring in June, the observational (numerical) Arctic warming/Arctic amplification reaches its minimum in July (June)Though seasonal heat storage (SHS) discharge dominates Arctic warming in cold season, anomalous wind makes their spatial patterns differentLess insulation by sea‐ice loss, wind acceleration or more dense water sinking increases vertical mixing and SHS discharge in cold season [ABSTRACT FROM AUTHOR]