Double-diffusive Staircase Structure In Arctic Ocean And Special Physical Process In Staircesa-free Area

The CTD and LADCP data obtained in the third Chinese Arctic Expedition of 2008 are used to analyze the double-diffusive staircase structure in the Canadian Basin of the Arctic Ocean and the special physical processes at staircase-free area. The double-diffusive mixing events occur in most area of Canadian Basin, which generate the staircases of both temperature and salinity, especially in the central and northern Canadian Basin. Moreover, the distribution of double-diffusive staircases appears spatially inhomogeneous: the staircase structure could be observed between 100 - 5 00 m, especially at the depth of thermocline with the staircase thickness of 1 - 5 m, while there exist composite staircases in the deeper range.The double-diffusive staircases exist within the main halocline (also pycnocline), which is the interface between the upper Arctic water and the Arctic Intermediate water (AIW). The enhanced stratification and the barrier layer inhibit the vertical turbulent mixing, resulting in a quite weak vertical heat flux. The feeble heat loss through the interface maintains the higher temperature of the AIW during its long distance flow. A warming pulse entered into the Canadian Basin in 1999 and became a tracer to indicate the flow path of the AIW, which is also maintained by the low heat loss caused by the double-diffusive staircase.On the basis of double-diffusive flux law, estimated vertical heat fluxes through the staircases between 200 m and 300 m are in the range 0.05-0.22 W m^-2, only about one tenth of the estimated mean surface mixed layer heat flux to the sea ice. It is thus concluded that the vertical heat transport from the AIW is unlikely to have a significant impact to the upper ocean heat budget in the central and northern Canadian Basin. However, in the southern Canadian Basin and the basin boundaries where the well-formed staircase is absent, the heat from the AIW might be of importance to the upper ocean. The vertical heat flux between 200 m and 300 m at staircase-free area is about 0.64-0.89 W m-2, 4-10 times larger than the former. Turbulent mixing that disrupts the staircase drive greater flux from the AIW in the southern Canadian Basin and possibly dominate the regionally averaged heat flux. It is just the enhanced heat flux lowers the temperature of the AIW and reduces the warming pulse.The double-diffusive staircases appear in most area of Canadian Basin, which maintains the warming pulse of the AIW for long distance. However, double-diffusion does not exist in the slope area of Beaufort Sea south of Canadian Basin. Also, the warming signal there tends to disappear based on the field observation. It is speculated that there is a stronger vertical heat flux to release the heat inside the AIW. To study the physical process for enhanced heat loss, the "thermal dissipation zone" is demarcated as an area without double-diffusive staircase. Two parameters are calculated to estimate the heat loss. One is the vertical turbulent thermal dissipation rate from the AIW, below the pycnocline, estimated based on the parameterization of Pacanowsky and Philander, which is about 3.59x1010 J/s. The other is the heat loss rate of the AIW, about 6.70x1010 J/s, which is estimated by the difference of the heat contents entering and leaving the thermal dissipation zone. Both heat loss amounts are similar magnitude, indicating that the loss of the heat content of the AIW is mainly released by the vertical turbulent thermal dissipation.The persistence and coherence of the double-diffusive staircase point to double diffusion as the dominant mixing mechanism in the Canadian Basin, resulting in a quite weak vertical heat flux. However, at staircase-free area where the turbulent mixing disrupts the staircase, it is possible that the heat from the AIW escapes vertically. The enhanced heat loss indicates the stronger turbulence in the thermal dissipation zone with two possible reasons. The turbulence is enhanced by the stronger lateral friction close to the continental slope. The other reason is the upwelling appeared along the Beaufort slope, which is a result of compensation to the surface sea ice export. The upwelled pycnocline becomes more close to the surface, the turbulent mixing there should be stronger and the double-diffusive staircase stratification is absent or disrupted.