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Centrifugal Instability in the Orkney Passage

Periodic Reporting for period 1 - CIOP (Centrifugal Instability in the Orkney Passage)

Reporting period: 2018-12-03 to 2020-12-02

Centrifugal Instability in the Orkney Passage (CIOP) is a collaborative effort to better understand turbulent processes within a deep-sea canyon near Antarctica. The main problem addressed is whether one of these turbulent processes--centrifugal instability by flow past topography—has an impact on turbulence and mixing in a deep-sea. The reason this is climatically important is that dense waters that are formed in the Weddell Sea (referred to as Weddell Sea Deep Water, WSDW) make their way through this canyon prior to joining the Antarctic Bottom Water (AABW). Since AABW helps drive the global overturning circulation, understanding turbulent processes in this Passage are critical for climate prediction in future years.

The main objective of the project is an improved understanding of physics associated with centrifugal instability in the Orkney Passage. However, a secondary objective of CIOP is the development of the Fellow's modelling skills. A significant component of the Marie-Skłodowska Curie Actions (MSCA) Individual Fellowship (IF) is synergy between the applicant and the host institution. CIOP achieves this objective through close synergy between observational and modelling expertise, using a branch of the Regional Ocean Modelling System (ROMS) as our tool. The third and final objective of CIOP is increased international collaborations across multiple institutions. These include French, UK, and US institutions. The result of these objectives is a suite of realistic, high-resolution numerical simulations that can be used for future analysis, development, and evaluation of parameterizations for coarser-resolution models.
We have created a set of high-resolution simulations using the Coastal and Regional Ocean COmmunity (CROCO) Model, a recent branch of ROMS. This work has begun with initial and boundary conditions kindly provided by M. Mazloff (Scripps) of the Southern Ocean State Estimate (http://ecco.ucsd.edu/). These data served as initial and boundary conditions for our 4-km regional model covering the Drake Passage and Weddell Sea region (Figure1). The simulation was run for four years, with one of these years serving to spin up the regional model. A 1-km resolution grid has then been nested within this solution and run for several years. Finally, a 300-m resolution simulation was run for brief periods within this domain. Approximately 50TB of data were created and presently reside at the regional supercomputing facility (Datarmor) in Brittany, France. Much of the Fellow's time has been spent properly forming these datasets. Future work will involve analysing these data at resolutions less than 100 m. (As an example, Figure2 illustrates kinetic energy in 4-km and 1-km simulations near the bottom boundary.)

The observational analysis has made use of in situ measurements in the Orkney Passage. The objective of this segment of the project focused on the analysis of moored observations provided by the British Antarctic Survey (BAS). These data were calibrated together with Dr. Povl Abrahamsen (BAS) and compared against short-term in situ measurements collected as part of a large-scale field campaign. These data were then used to publish as a group paper. While moored measurements unfortunately did not enter this publication, the results were nonetheless used during peer-review. Additional to this paper, the observations proved pivotal in two additional ways: they served as benchmarks for model realisations described below and provided constraints on a mixing budget paper.

The COVID-19 epidemic reduced the efficiency of the project. Nevertheless, we managed to analytically investigate centrifugal instability, an important step in the project since centrifugal effects are typically neglected in present-day analysis. This has led to two manuscripts, with a third in preparation. An unexpected finding brought to light by the Fellow and Host during the study is a possible explanation for the predominance of anticyclones at small horizontal scales in the ocean. Essentially, the curvature of fronts cause the wind shear to be perturbed in a way that is different for cyclonic and anticyclonic curved fronts. At low Richardson numbers, anticyclones become weakly stable while cyclones become strongly unstable. This is something that is counter to intuition and is a major finding of this MSCA-IF project.

All of these finding have been published in open access papers. Moreover, this work has been presented at a major international conference (held virtually) in December 2020. Finally, it is likely that the Host and colleague will apply these results to model output within the Atlantic. This could help parameterise, for example, the impact of coherent vortices on transport of tracers across major ocean basins.
Modelling centrifugal instability presently relies upon idealised model configurations. For cases in which realistic simulations exist, models are often coarse resolution. CIOP moves beyond this by examining output in a realistic configuration for a long duration (2 years) at high spatial resolution (<100 m after the project). Furthermore, the resulting dataset facilitates testing of future parameterizations.

These results above move beyond the state of the art in another way: by addressing a fundamental question not previously answered. Despite numerous studies that have attempted to explain this question, we have demonstrated that, at low Richardson numbers, anticyclonic and cyclonic curved fronts are more and less stable, respectively. Perhaps more importantly, they undergo centrifugal/symmetric instabilities in a manner different than previously understood. The wider societal implications are two-fold. While this has certainly helped better understand vortex formation in the Orkney Passage (explained below), it may also have an impact for ocean-atmosphere exchange. For example, vertical motion will be enhanced in curved cyclonic meanders due to centrifugal/symmetric instabilities, while vertical motion will be suppressed within curved anticyclonic fronts. Because meandering fronts are ubiquitous in the world ocean, it may have implications for tracer exchange between the deep ocean and atmosphere (e.g. biology). The applicant has been in communication with fellow scientists about this topic, encouraging use of these ideas in their research.

Abyssal flows will preferentially generate anticyclonic vorticity owing to the manner in which the mean flow moves past topography: the flow exits canyons with steep topography on the left, regardless of hemisphere. This generates anticyclonic vortices. If such anticyclonic (stable) vortices trap fluid, this will act as a bolus transport of dense waters. In the Orkney Passage, this water is climatically important: it is Weddell Sea Deep Water (WSDW), some of which originates from shelf waters around Antarctica. However, such dense waters reside in other ocean basins with important implications for transport of water masses. Thus, the finding that intense anticyclones are weakly stable is a significant result of this MSCA-IF.

In summary, the wider societal implications of these results are that it will help to improve understanding and parameterizations of centrifugal instability within global climate models. This includes tracer exchange in the upper ocean, as well as bolus transport of watermasses.
Schematic of vortex generation and evolution
Example model output
Proposed modelling strategy