Sources and Variability of Weddell Sea Deep Water
Final Report Abstract
It is often suggested that the WSDW is formed in two regions of the WS, the FRIS region in the southern part of the basin and the in the eastern Antarctic Peninsula area near LIS. In situ measurements confirm that FRIS is a source of dense waters that can originate WSDW. The hypothesis that LIS area is a dense water source builds on observations made downstream, on the slope below the shelf break in front of LIS or further to the north. Results of a numerical experiment performed with FESOM with tracers for GMW indicate that only FRIS is source of the dense WSDW and the main contributor to the bottom waters of the BS, especially in the Eastern basin. The waters from the LIS region, i.e. with LIS GMW, are not dense enough to sink to the WS abyss but follow the continental shelf into BS where it plays a minor role in the ventilation of BS Central basin. The ventilation scheme proposed for the BS deep regions suggests that measurements from the area could be used as proxy for changes in the WS, this needs to be confirmed by in situ measurements. The combination of observation made directly in the WS and the findings obtained by this potential proxy could increase the time span that we can investigate changes in the WS, the main source of AABW. Past studies show that the boundary current system in the WS contributes to both the export of deep and bottom waters toward lower latitudes, and the flow of WDW toward the FRIS cavities. Although the seasonality along the continental slope of the WS has been investigated before at specific locations, here we present a coherent understanding of the entire boundary current system. This study combines multiplatform historical data at different locations, providing an interannual-mean view of the boundary current system and explores its adjustment on seasonal time scales. We confirm that the geostrophic flow reverses its interannual-mean vertical shear between KN and AP, which is characteristic for an overturning circulation forced by the dynamics on the continental shelf. The along-slope decrease in surface stress might also contribute to the shift in flow regime. A coherent seasonal acceleration of the barotropic flow is observed at all sites with a maximum speed in austral autumn, associated with a coastal rise of SLA. The comparison between the SLA and the surface stress field indicates that the barotropic flow adjusts to a momentum input on the eastern/northeastern side of the gyre. Such teleconnection implies that changes in the surface stress field trigger a fast large-scale response in the ocean. At the tip of the Antarctic Peninsula, the barotropic flow significantly contributes to the seasonal velocity in the dense plume. Thus, by controlling the seasonal acceleration of the barotropic flow, the surface stress might remotely modulate the export of dense water from the Weddell Sea on monthly time scales. Several methods of calculation of surface stress are compared to estimate its uncertainty, either considering the wind velocity only, accounting for the sea ice modulation of the stress, or using sea ice ocean model output. We find large differences in local-averages at the coast. However, the relationship between barotropic flow strength and surface stress variability on the eastern gyre is valid no matter the method. This shows that the main driver of barotropic seasonality is the wind even though sea ice significantly modulates the surface stress in marginal ice zones. Further research integrating the contribution of the seasonal variations of buoyancy forcing at the continental shelf is needed to understand the seasonal baroclinic fluctuations.
Publications
- On the ventilation of Bransfield Strait deep basins, Deep Sea Research Part II: Topical Studies in Oceanography, Volume 149, 2018, Pages 25-30
van Caspel, Mathias; Hartmut H. Hellmer; Mauricio M. Mata
(See online at https://doi.org/10.1016/j.dsr2.2017.09.006) - (2020). Coherent seasonal acceleration of the Weddell Sea boundary current system driven by upstream winds. Journal of Geophysical Research: Oceans, 125, e2020JC016316
Le Paih, N., Hattermann, T., Boebel, O., Kanzow, T., Lüpkes, C., Rohardt, G., et al.
(See online at https://doi.org/10.1029/2020JC016316)
