Trends and Variability in Depth and Spiciness of Subsurface Isopycnals on the Vancouver Island Continental Shelf and Slope

Abstract

The waters over the southern Vancouver Island shelf and slope are productive, economically important regions. The circulation is highly dynamic through variable mixing of both younger subarctic and older southern water masses, and significant local upwelling and downwelling regimes. We take advantage of three time series that cover more than three decades: the La-Perouse, Line-P, and CalCOFI time series, each covering at least 37 years (1984–2021). We investigate subsurface salinity, temperature, and depth observations to study seasonal and interannual variability in isopycnal heave and spiciness, and evaluate possible trends. The 26.6 kg m⁻³ isopycnal is the deepest isopycnal that is regularly upwelled onto the Vancouver Island shelf (above 200 m), shoaling over 100 m seasonally. Spiciness on the 26.6 isopycnal is highest in winter, but seasonal variability is much smaller than the interannual variability, which itself is two to three times smaller than the variability in adjacent subarctic waters and in the California Undercurrent further south, prior to their arrival in our study region. Spiciness in subsurface shelf waters appears to have been increasing over the past 37 years, by about half the magnitude of the increase in spiciness observed in the California Undercurrent water. In contrast, the subsurface slope shows no trend in spiciness but appears to be experiencing physical convergence, specifically an increase in the volume of water with a density of around 26.6 kg m⁻³, which has also been observed in the subarctic northeast Pacific, at Ocean Station Papa.