Vertical heat transfer based on direct microstructure measurements in the ice-free Pacific-side Arctic Ocean: the role and impact of the Pacific water intrusion

Abstract

This study quantifies diapycnal mixing and vertical heat transfer in the Pacific side of the Arctic Ocean, where sea-ice cover has disappeared between July and September in the last few decades. We conducted microstructure measurements in the open water region around the Canada Basin from late summer to fall in 2009 and 2010 using R/V Mirai. In the study domain, the dissipation rate of turbulent kinetic energy, ε, is typically as low level as O(10⁻¹⁰) W kg⁻¹, resulting in vertical heat diffusivity of O(10⁻⁷) m² s⁻¹, which is close to the molecular diffusivity of heat, suggesting comparatively little predominance of mechanical turbulent mixing. An exception is the case at the Barrow Canyon, where the strong baroclinic throughflow generates substantial vertical mixing, producing ε > O(10⁻⁷) W kg⁻¹, because of the shear flow instability. Meanwhile, in the confluence region, where the warm/salty Pacific water and the cold/fresh Arctic basin water encounter, the micro-temperature profiles revealed a localized enhancement in vertical diffusivity of heat, reaching O(10⁻⁵) m² s⁻¹ or greater. In this region, an intrusion of warm Pacific water creates a horizontally interleaved structure, where the double-diffusive mixing facilitates vertical heat transfer between the intruding Pacific water and the surrounding basin waters.