River-induced submesoscale processes in an Aotearoa New Zealand shelf sea

Abstract

This study determines how rivers connect terrestrial and oceanic environments beyond the immediate river mouth area through small-scale processes. Submesoscale processes, the role of freshwater inputs, river-influenced coastal systems, coastal upwelling and the importance of interdisciplinary studies are investigated. Key questions on the spatio-temporal structure of river-induced submesoscale features, their interactions with a coastal current, their seasonal and intra-seasonal variability and accompanying biological signals are addressed. The physical and biological characteristics of these submesoscale features are also compared to an upwelling system. Submesoscale features, characterised by a low salinity layer originating from river discharges, connect terrestrial and oceanic environments. Using a combination of data from multiple ocean glider surveys and regional modelling, this study showed that low salinity submesoscale features (LSMFs) can cause increased stratification on the order of 10−4 s−2 in a New Zealand shelf sea. Modelled oceanographic conditions compared well to observations, especially in austral spring. Stably strati_ed LSMFs can replace the previously well mixed layer in the water column up to a distance of 100 km o_shore before getting entrained by the regional barotropic current in Greater Cook Strait. LSMFs generated strong vertical and horizontal salinity differences of ΔS∼0.45 psu. These salinity differences defined density fronts and stratification in the upper ∼30 m. Temperature differences of up to ΔT∼1.4◦C associated with LSMFs were not large enough to entirely cancel the density effect of salinity. The offshore reach of LSMFs was partly constrained by the variability of the coastal barotropic d'Urville Current. Its presence and strong winds inhibited the advection of LSMFs offshore in Greater Cook Strait, enhancing mixing and deepening the mixed layer depth. In contrast, moderate winds and weak current enabled the advection of LSMFs furthest offshore in Greater Cook Strait, where the water column became stably stratified. A combination of glider and satellite observations revealed that seasonal and intra-seasonal variability in temperature, chlorophyll and stratification existed in LSMFs, even though they were persistent throughout all seasons with a characteristic salinity of ≤34.75 psu. Stronger stratification was observed at the end of spring, in summer and at the beginning of autumn both in ambient shelf conditions and within LSMFs. Subsurface chlorophyll maxima were found to be mostly characteristic of summer and spring LSMFs, when temperature had stronger contributions towards stratification. A crucial initiation threshold of 600 m3 s−1 in river discharge was identified as a requirement for LSMFs to be advected out of the coastal bays. When river flow was lower than this threshold, other drivers such as wind or currents were required to advect LSMFs offshore. Cyclones and increased rainfall led to more frequent and fresher LSMFs during seasons when they would theoretically be less frequent. The presence of extreme weather events increased river discharge and small mountainous rivers were more responsive to these extreme events. A two-gliders austral spring bloom, process-focused experiment was conducted to sample LSMFs and upwelling concurrently. LSMFs had strong salinity signals while upwelling had strong temperature signals. While upwelling was associated with persistent temperature of ≤14◦C, LSMFs showed varying temperature due to solar irradiance and seasonal fluctuations. Chlorophyll florescence was identified to be threefold higher in river-induced LSMFs than in the upwelling plume, shifting the paradigm of upwelling-associated primary production in shelf seas. LSMFs significantly altered the stratification which potentially caused rapid biological responses. This impact of LSMFs on stratification was sustained for days after their occurrence, even during their decay, because of the absence of external forces such as strong winds and currents. This may have implications for growth and sinking of phytoplankton in the stratified water column. This research provided critical information for understanding how the health of Aotearoa New Zealand's land and rivers affects the ocean. Understanding of the physical oceanography of a continental shelf sea that encompasses oil and gas interests, aquaculture, various wild fisheries, big mammals, transport (ferry) operations and energy infrastructure has been greatly improved. Chlorophyll-a observations both in the surface and subsurface reported in this study are relevant for biogeochemical models. The findings of this study are relevant for ongoing and future coastal observation and modelling projects. Moreover, multiple other shelf seas are regulated by interactions between barotropic and baroclinic processes globally, similar to Greater Cook Strait.