Drivers of neritic water intrusions at the subtropical front along a narrow shelf

Abstract

The near-ubiquitous presence of freshwater over the inner to mid-continental shelf off of the Otago Peninsula in southeast Aotearoa/New Zealand has been previously identified in long-term cross-shelf transects. Occasional influxes of this silicate-rich neritic water past the shelf break and beyond the Subtropical Front have been observed, potentially supporting elevated chlorophyll-a concentrations in SubAntarctic waters. Nearshore salinity variability has been connected to flow rates of the Clutha River, Aotearoa’s largest river by discharge, however, other physical mechanisms influencing the structure of freshwater along this coast have not been explicitly explored. A numerical model of Aotearoa’s southeast continental shelf forced with realistic discharge from the Clutha River is used to assess the drivers of river plume variability and identify conditions associated with the transport of neritic water beyond the shelf break. Event scale, lagged correlation, EOF, and composite analysis of a model passive dye tracer revealed three generalised plume structures that are dependent on river discharge, along-front wind stress, and shelf current velocities. Downfront wind stress suppresses bulge formation at the river mouth and constrains the river plume against the coast, whereas upfront wind stress causes the river plume to move offshore over the shelf and spread across the upper water column. Rarely, upfront wind stress generates a nearshore current reversal over the shelf, causing the plume to deflect straight off-shelf and across the shelf edge before meeting the frontal current and advecting to the northeast. Future observational work is required to validate the mechanisms described and address the biological implications of these findings.