Characterization of double diffusive convection steps and heat budget in the deep Arctic Ocean

摘要

In this paper, we explore the hydrographic structure and heat budget in the deep Canada Basin by using data measured with McLane-Moored-Profilers (MMP), bottom pressure recorders (BPR), and conductivity-temperature-depth (CTD) profilers. Upward from the bottom, a homogeneous bottom layer and its overlaying double diffusive convection (DDC) steps are well identified at Mooring A ( 75 degrees N,150 degrees W). We find that the deep water is in weak diapycnal mixing because the effective diffusivity of the bottom layer is approximate to 1.8x10-5m2s-1, while that of the other steps is approximate to 10-6m2s-1. The vertical heat flux through the DDC steps is evaluated by using different methods. We find that the heat flux ( 0.1-11mWm-2) is much smaller than geothermal heating ( approximate to 50mWm-2). This suggests that the stack of DDC steps acts as a thermal barrier in the deep basin. Moreover, the temporal distributions of temperature and salinity differences across the interface are exponential, whereas those of heat flux and effective diffusivity are found to be approximately lognormal. Both are the result of strong intermittency. Between 2003 and 2011, temperature fluctuations close to the sea floor were distributed asymmetrically and skewed toward positive values, which provide a direct observation that geothermal heating was transferred into the ocean. Both BPR and CTD data suggest that geothermal heating and not the warming of the upper ocean is the dominant mechanism responsible for the warming of deep water. As the DDC steps prevent vertical heat transfer, geothermal heating is unlikely to have a significant effect on the middle and upper Arctic Ocean.