Characteristics and Drivers of Deep Water Properties in a Temperate Fjord in Aotearoa/New Zealand

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Abstract
Fiordland, Aotearoa/New Zealand (AoNZ) is host to 14 named temperate fjords, however the magnitude and timescales of oceanographic variability in the deep basins remain poorly understood due to limited oceanographic sampling in these basins over the past five decades. Fjords have been identified as globally significant carbon burial sites and oceanographic processes and properties are important components of the sequestration process. Deep‐water renewal (DWR) and vertical mixing are key processes in governing deep‐water conditions. In this study, we present a near‐monthly, 2 yr, in situ timeseries of deep‐water properties in Patea/Doubtful Sound, Fiordland, AoNZ, and determine the roles of DWR and vertical mixing in controlling deep‐water properties and timescales of their variation. In conjunction with pre‐ and post‐winter CTD casts collected in four other AoNZ fjords, these data provide the first direct observations of DWR in the southwest fjords of AoNZ. Using two years of near‐monthly repeat CTD survey data (2023–2025), we estimate vertical diffusivities at two sites using a budget method and use these to hindcast properties in the fjord between January 2012 and April 2025 using a 1‐D model. Hindcast output suggests renewal occurs every winter in both entrance and fjord‐head basins. Comparisons between hindcast output and coastal water properties reveal that annual DWR in AoNZ results from deepening and cooling of the winter mixed layer, contrasting with other fjord systems where DWR is often caused by upwelling, providing a novel renewal preconditioning mechanism.
Funding Information
  • Ministry of Business, Innovation and Employment (UOOX2101)
  • University of Otago

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