Observed bottom warming in the East Siberian Sea driven by the intensified vertical mixing

Abstract. The East Siberian Sea (ESS) features the broadest continental shelf on Earth and contains nearly 80 % of the world's subsea permafrost. A persistently cold bottom layer, with temperatures at freezing point, inhibits the downward transport of heat, thus preventing the thawing of permafrost and subsequent methane release from sediments. However, in early September 2016, we observed an unprecedented warming of over 3 °C at the bottom of the water column, approximately 46 m deep in the ESS, following a relatively moderate Arctic cyclone. We attribute this notable bottom warming to enhanced wave-induced vertical mixing, which facilitates the well-mixed Arctic marginal seas and allows surface heat to reach the bottom layer. As sea ice continues to retreat on the Arctic continental shelf, wind-driven waves have longer fetch to grow. Consequently, even moderate cyclones can trigger substantial vertical mixing, a phenomenon not previously documented. Given the accelerated warming of the Arctic and the rapid decline of sea ice, we anticipate that more open water will foster the growth of larger wind-driven waves and intensified vertical mixing, leading to greater heat influx to the bottom layers of Arctic shelves in the future.