The overarching goal of our research is to understand the significance of the polar ocean in the Prydz Bay region to the global climate system through estimating the formation and export of dense shelf water in the region and illustrating the dynamic processes involved. Specific scientific questions that we investigated include: (1) What are mechanisms that control circumpolar deep water (CDW) intrusions and its synoptic, seasonal and interannual variations? (2) What are the spatial/temporal variations in distributions of dense shelf water formed in the Prydz Bay shelf region? (3) What are the dynamic processes controlling the export and fate of the dense shelf water formed in the Prydz Bay shelf region?

To address these questions, a high-resolution regional ocean-sea ice coupled model was developed to provide time and space continuous three-dimensional ocean state estimation. Both in-situ and remote sensing observations and modeling simulation results were used to investigate (i) the local atmosphere-ocean-sea ice interaction and shelf processes that produce dense shelf water and (ii) the dynamic processes that control the shelf water export.

Intellectual Merit : The Antarctic Bottom Water (AABW) formation is a key element in global thermohaline circulation. Recent studies infer a reduction of AABW production from reduced volume of AABW in global deep basins during recent decades. It is important to obtain better estimates of AABW production rate at its source areas to investigate whether the overturning circulation is slowing-down at the bottom branch. The Prydz Bay region has been suggested as a key AABW production site. However great uncertainties remain as to the volume of dense shelf water and how they are exported. Our model development and analyses allow us to systematically examine the seasonal and interannual variability of water masses on the shelf , isolate contributions from sea ice formation and CDW intrusion that affect the properties of shelf water, and produce a better assessment of AABW formation in the Prydz Bay region

Broader Impacts : Because AABW is an important component in global climate system, findings from this project benefit the larger climate research community. Observations collected and model developed in this project set a foundation for follow-up biogeochemistry studies.  The project has supported a postdoctoral researcher and research associate at NCSU. They were trained in polar physical oceanography and coupled ocean modeling and data analyses. PI. R. He had participated in various public outreach activities and intergrated research findings in the marine science courses he has been teaching at NC State.

Our study and analyses resulted in several singificant findings:

1) We used a high-resolution coupled ocean-sea ice-ice shelf model to hindcast regional ocean conditions in 2010-2015 to determine the spatiotemporal variability of water type transformations and the mixing dynamics that lead to dense shelf water formation. Comparisons between model simulations and in situ observations show that the model successfully reproduces the bulk characteristics of the period and previously described water type interactions in Prydz Bay. We further diagnosed individual mixing terms from the K-profile parameterization vertical mixing scheme to quantify the seasonal contributions to mixing from double diffusion, internal waves, and convective diffusivity due to static instability. The mixing analyses suggest a novel pathway for dense shelf water preconditioning and export independent of the Amery Ice Shelf. This pathway depends less on polynya activity and more on the mixing of modified Circumpolar Deep Water with overlying winter water through double diffusive convection

 2) One of the defining features of the Southern Ocean is the stark potential temperature-salinity contrast of its cold, fresh surface layer with the warm, salty water masses which lie beneath. These water masses are often studied using the water mass transformation framework, in which surface fluxes of freshwater and heat are used to compute the rate that one water mass is transformed into another. This transformation rate, as it is known, is nearly always computed as a function of density. However, density alone cannot be used to distinguish water masses because waters of different compositions can have the same density. We reviewed prior studies and demonstrated how they conflate surface waters with subsurface water masses. To reconcile this problem, we used the Southern Ocean model reanalysis to illustrate a pair of complimentary techniques for understanding water mass transformation: computing transformations as a function of potential temperature and salinity, rather than density, and comparing these transformations to a census of the Southern Ocean water masses. We found that Circumpolar Deep Water is usually insulated from the sea surface and is thus not typically transformed by surface fluxes into upper and lower branches, as is commonly believed. We also demonstrated that Subantarctic Mode Water and Antarctic Intermediate Water are formed by density gain in the vicinity of the Subantarctic Front.

Last Modified: 11/13/2021
Modified by: Ruoying He