Submesoscale Dynamics Revealed by Glider Observations in Central New Zealand

New Zealand hosts a large shallow shelf region in the middle of its North and South Islands, connecting the Tasman Sea to the Pacific Ocean through the 23 km wide Cook Strait. Cook Strait is not only a bottleneck for strongest wind speeds, but it is also an amphidrome. Greater Cook Strait (GCS), on the western side of Cook Strait, can be subject to enhanced primary production, which can be large enough to sustain the energy demands of blue whales. This relatively shallow continental shelf is regularly impacted by freshwater influx from large rivers. Combining glider-based observations with a numerical model, we show that submesoscale low salinity plumes, originating and separating from river runoffs, can propagate up to a distance of 100km offshore. Such Low Salinity Submesoscale Features (LSMF) generate and maintain strong salinity gradients of ΔS~0.45 that define density fronts and stratification in the upper ~30 m. We identified three different regimes of feature propagation that depend on the wind strengths and the intensity of the key ocean current in the region – the D’urville Current (dUC). 1) In the presence of intermediate conditions, with a strong dUC and moderate winds, LSMFs propagate offshore but get entrained as they interact with the dUC. 2) The presence of strong winds and a strong dUC create LSMF-unfavourable conditions which inhibit the propagation of LSMFs offshore in GCS. The water column remains unstable to double diffusive convection which enhances mixing. 3) The presence of moderate winds and a weak dUC are LSMF-favourable conditions which enable the propagation of LSMFs furthest offshore in GCS, where the water column becomes stably stratified, inhibiting mixing. A stably stratified regime has implications on the supply of nutrients and subsequent phytoplankton growth. Moreover, the features’ fronts are likely places of enhanced primary production in the region. We therefore provide the basis of investigation of how submesoscale variability in the region can at times lead to increased primary production through frontogenesis.