An Assessment of Density-Based Finescale Methods for Estimating Diapycnal Diffusivity in the Southern Ocean

Abstract

Finescale estimates of diapycnal diffusivity k are computed from CTD and expendable CTD (XCTD) data sampled in Drake Passage and in the eastern Pacific sector of the Southern Ocean and are compared against microstructure measurements from the same times and locations. The microstructure data show vertical diffusivities that are one-third to one-fifth as large over the smooth abyssal plain in the southeastern Pacific as they are in Drake Passage, where diffusivities are thought to be enhanced by the flow of the Antarctic Circumpolar Current over rough topography. Finescale methods based on vertical strain estimates are successful at capturing the spatial variability between the low-mixing regime in the southeastern Pacific and the high-mixing regime of Drake Passage. Thorpe-scale estimates for the same dataset fail to capture the differences between Drake Passage and eastern Pacific estimates. XCTD profiles have lower vertical resolution and higher noise levels after filtering than CTD profiles, resulting in XCTD k estimates that are, on average, an order of magnitude higher than CTD estimates. Overall, microstructure diffusivity estimates are better matched by strain-based estimates than by estimates based on Thorpe scales, and CTD data appear to perform better than XCTD data. However, even the CTD-based strain diffusivity estimates can differ from microstructure diffusivities by nearly an order of magnitude, suggesting that density-based fine-structure methods of estimating mixing from CTD or XCTD data have real limitations in low-stratification regimes such as the Southern Ocean.

Mechanisms for enhanced turbulence in the Drake Passage region of the Southern Ocean

“…2-2). These discrepancies are consistent with the results of Frants et al (2013). Finescale strain, defined as the vertical derivative of isopycnal displacements, may also be used to estimate dissipation from hydrographic data.…”

“…This is a significant issue in the Thompson et al (2007) study, as they used expendable CTD instrumentation which have notoriously poor salinity resolution. As a result, their dissipation and diffusivity estimates based on Thorpe scales were biased high by up to two orders of magnitude (Frants et al (2013)). …”

“…Turbulent quantities are important to the energy balance of the global ocean and are prevalently parameterized in largescale circulation models. Results from previous work using finestructure methods to interpret mixing rates have recently been shown to be biased high (Frants et al (2013), Mater et al (2015), Waterman et al (2014)), often by up to two orders of magnitude. This work has revisted the values and patterns of mid-depth mixing in this region to inform modeling efforts and large-scale estimates of vertical diffusivity.…”

“…Mater et al (2015) found Thorpe scale methods overestimate mixing rates when compared to direct measurements especially in the presence of large overturns. In the Southern Ocean, mixing rates inferred from finescale parameterizations are shown to overestimate those from microstructure measurements by as much as 1-2 orders of magnitude (Frants et al, 2013).…”

“…2-2). Frants et al (2013) found that the Thompson et al (2007) diffusivities were biased high by 1-2 orders of magnitude for reasons related to the detection of overturns with poor signal to noise resolution. Mixing rates derived from the DIMES US2 data (described in Section 1.5.1) show discrepancies with Thompson et al (2007)'s Thorpe scale estimates ( Fig.…”