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

Abstract

[1] 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 N; 150 W). We find that the deep water is in weak diapycnal mixing because the effective diffusivity of the bottom layer is 1:8 3 10 25 m 2 s 21 , while that of the other steps is 10 26 m 2 s 21 . The vertical heat flux through the DDC steps is evaluated by using different methods. We find that the heat flux (0:1211 mWm 22 ) is much smaller than geothermal heating (50 mWm 22 ). 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.

The instability of diffusive convection and its implication for the thermohaline staircases in the deep Arctic Ocean

“…Note that these data measured by conductivity-temperature-depth (CTD), not by MMP, are used here because data of CTD covers a larger depth range. As reported in Zhou and Lu (2013), both CTD and MMP detected exactly the same staircase structures at the same depth range. The potential temperature, θ, profile, is shown in Fig.…”

“…4) indicate that the Ra TI of each interface is distributed approximately lognormal, which suggests that Ra TI is strongly intermittent. Similar distributions have been found in vertical heat flux, eddy diffusivity and other properties (Zhou and Lu, 2013). These results imply that the deep Arctic Ocean exhibits certain turbulent behaviors (Frisch, 1995).…”

“…As h and θ vary with time (Zhou and Lu, 2013), the deduced Ra TI is a function of time too. Its temporal distributions (shown in Fig.…”

“…3b, due to the instrument resolution. As analyzed in our previous work (Zhou and Lu, 2013), the interface properties could be determined with an averaging technique. In Fig.…”

“…According to the definitions of Ra T and Ra S , R ρ can be rewritten as R ρ = Ra S /Ra T . For detailed analysis of the determination of the diffusive interface properties, readers are referred to Zhou and Lu (2013). The mean properties of the four interfaces are listed in Table 1.…”