Initial transformation and export of dense shelf water from the Prydz Bay and Cape Darnley regions: A Lagrangian perspective using clustered drifters

Antarctic Bottom Water (AABW) forms the deepest limb of the meridional overturning circulation (MOC) and is a key control in global heat, freshwater, and carbon exchanges. AABW originates in part from Dense Shelf Water (DSW) produced in Antarctic coastal polynyas. Prydz Bay and Cape Darnley host several productive polynyas that supply AABW to the Atlantic and Indian Ocean sectors. However, the pathways by which DSW travels from the shelf to the abyss, and the transformations it undergoes over its first year, remain poorly understood. We use a 10 km circum-Antarctic ocean-sea ice model (the Whole Antarctic Ocean Model) and Lagrangian particle tracking with a novel clustering analysis to examine how DSW from Prydz Bay region is exported and modified before contributing to AABW. Our results reveal that shelf mixing influences DSW’s fate. When DSW flows beneath the Amery Ice Shelf, it mixes with fresher Ice Shelf Water (ISW), becoming less dense and reducing its potential to sink. Along the shelf break, intrusions of Circumpolar Deep Water (CDW) mix with DSW to form modified CDW (mCDW), further altering its properties. We find that the DSW-to-AABW pathway is sensitive to these mixing processes, which can either facilitate or preclude descent. Cluster analysis of Lagrangian trajectories reveals distinct groups of particle pathways and transformations, highlighting the diverse outcomes of DSW export. This Lagrangian approach demonstrates a powerful tool for diagnosing shelf-to-deep water mass transformation, and provides insights into the processes regulating AABW formation in the Prydz Bay region.