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This study analyses data assimilative numerical simulations in an eddy-dominated western boundary current: the East Auckland Current (EAuC). The goal is to assess the impact of assimilating surface and subsurface data into a model of the EAuC via running observing system experiments (OSEs). We used the Regional Ocean Modeling System (ROMS) in conjunction with the 4-dimensional variational (4D-Var) data assimilation scheme to incorporate sea surface height (SSH) and temperature (SST), as well as subsurface temperature, salinity and velocity from three moorings located at the upper, mid and lower continental slope using a 7 d assimilation window. Assimilation of surface fields (SSH and SST) reduced SSH root mean square deviation (RMSD) by 25 % in relation to the non-assimilative (NoDA) run. The inclusion of velocity subsurface data further reduced SSH RMSD up- and downstream the moorings by 18 %–25 %. By improving the representation of the mesoscale eddy field, data assimilation increased complex correlation between modelled and observed velocity in all experiments by at least three times. However, the inclusion of temperature and salinity slightly decreased the velocity complex correlation. The assimilative experiments reduced the SST RMSD by 36 % in comparison to the NoDA run. The lack of subsurface temperature for assimilation led to larger RMSD (>1 ∘C) around 100 m in relation to the NoDA run. Comparisons to independent Argo data also showed larger errors at 100 m in experiments that did not assimilate subsurface temperature data. Withholding subsurface temperature forces near-surface average negative temperature increments to the initial conditions that are corrected by increased net heat flux at the surface, but this had limited or no effect on water temperature at 100 m depth. Assimilation of mooring temperature generates mean positive increments to the initial conditions that reduces 100 m water temperature RMSD. In addition, negative heat flux and positive wind stress curl were generated near the moorings in experiments that assimilated subsurface temperature data. Positive wind stress curl generates convergence and downwelling that can correct interior temperature but might also be responsible for decreased velocity correlations. The few moored CTDs (eight) had little impact in correcting salinity in comparison to independent Argo data. However, using doubled decorrelation length scales of tracers and a 2 d assimilation window improved model salinity and temperature in comparison to Argo profiles throughout the domain. This assimilation configuration, however, led to large errors when subsurface temperature data were not assimilated due to incorrect increments to the subsurface. As all reanalyses show improved model-observation skill relative to HYCOM–NCODA (the model boundary conditions), these results highlight the benefit of numerical downscaling to a regional model of the EAuC.

Satellite-derived estimates of sea surface temperature (SST), chlorophyll (Chl), and fluorescence line height (FLH), from 22 sites ∼20 km offshore from the West Coast of the South Island, New Zeala...

An along-channel momentum budget is quantified in the near-field plume region of a controlled river flow entering Doubtful Sound, New Zealand. Observations include highly resolved density, velocity and turbulence, enabling a momentum budget to be constructed over a control volume. Estimates of internal stress (τ) were made from direct measurements of turbulence dissipation rates (ϵ) using vertical microstructure profiles. High flow speeds of the surface plume over 2 m s−1 and strong stratification (N2∼10-1 s−2) resulted in enhanced turbulence dissipation rates (ϵ>10-3 W kg−1) and internal stress (τ>10-2 m2 s−2) at the base of the surface layer. Internal waves were observed propagating along the base of the plume, potentially released subsequent to a hydraulic jump in the initial 1 km downstream of the plume discharge point. The momentum flux divergence of these internal waves suggests that almost 15 % of the total plume momentum can be transported out of the system by wave radiation, therefore playing a crucial role in the redistribution of momentum within the near-field plume. Observations illustrate that the evolution of the momentum budget components vary between the distinct surface plume layer and the turbulent, shear-stratified interfacial layer. Within the surface plume, a momentum balance was achieved. The dynamical balance demonstrates that the deceleration of the plume, driven by along-channel advection, is controlled by turbulence stress from the plume discharge point to as far as 3 km downstream. In the interfacial layer, however, the momentum equation was dominated by the turbulence stress term and the balance was not closed. The redistribution of momentum within the shear-stratified layer by internal wave radiation and other hydraulic features could account for the discrepancy in the budget.

Intra-annual variability in the East Auckland Current (EAuC) was studied using a year-long timeseries of in situ and remotely-sensed velocity, temperature and salinity observations. Satellite-derived velocities correlated well ($$\hbox {r} > 0.75$$ r > 0.75 ) with in situ observations and well-represent the long-term ($$> 30$$ > 30 days) variability of the upper ocean circulation. Four mesoscale eddies were observed during the year (for 260 days) which generated distinct flows between the continental slope and rise. The EAuC dominated the circulation in the continental shelf break, slope and rise for 110 days and generated the most energetic events associated with wind forcing. Current variability on the continental slope was coherent with along-slope wind stress (wind stress curl) at periods between 4 and 12 days (16 and 32 days). We suggest that along-slope winds generated offshore Ekman transport, uplift on the shelf-break, and a downwind geostrophic jet on the slope. In contrast, positive wind stress curl caused convergence of water, downwelling, and increased the current speed in the region. Bottom Ekman transport, generated by the EAuC, was suggested to have caused the largest temperature anomaly ($$-1.5 ^{\circ }\hbox {C}$$ - 1 . 5 ∘ C ) at the continental shelf-break.

The shelf seas surrounding New Zealand/Aotearoa, a region nominally extending out to the 250 m depth contour, are complex and varied as they sit above the submerged continent of Zealandia. ...

East Auckland Current (EAuC) variability was studied using a year-long timeseries of in situ and remotely-sensed velocity, temperature and salinity observations. Satellite-derived velocities correlated well (r > 0.75) with in situ observations and well-represent the long-term (> 30 day) variability of the upper ocean circulation. Four mesoscale eddies were observed during the year (for 260 days) which generated distinct flows between the continental slope and rise. The EAuC dominated the circulation in the continental shelf break, slope and rise for 110 days and generated the most energetic events associated with wind forcing. Current variability on the continental slope was coherent with along-slope wind stress (wind stress curl) at periods between 4 and 12 days (16 and 32 days). We suggest that along-slope winds generated offshore Ekman transport, uplift on the shelf-break, and a downwind geostrophic jet on the slope. In contrast, positive wind stress curl caused convergence of water, downwelling, and increased the current speed in the region. Bottom Ekman transport, generated by the EAuC, was suggested to have caused the largest temperature anomaly (-1.5°C) at the continental shelf-break.

Submesoscale features, characterised by a low salinity layer originating from river discharges, enhance water column stability in a New Zealand shelf sea - the Greater Cook Strait. Using a combination of data from multiple ocean glider surveys and regional modelling, we show that low salinity submesoscale features (LSMFs) can cause increased stratification on the order of 10^-4. Modelled oceanographic conditions compared well to observations, especially in austral spring. Stably stratified LSMFs can replace the previously well mixed layer in the water column up to a distance of 100 km offshore before getting entrained by the regional barotropic current in Greater Cook Strait. LSMFs observed from glider surveys and reproduced from modelled results generate strong vertical and horizontal salinity differences of ~0.45 psu. These salinity differences define density fronts and stratification in the upper ~30 m. Temperature differences of up to ~1.4°C associated with LSMFs were not large enough to entirely cancel the density effect of salinity. The offshore advection reach of LSMFs is partly constrained by the variability of the barotropic d'Urville Current. Its presence and strong winds inhibit the propagation of LSMFs offshore in Greater Cook Strait, leaving the water column unstable while enhancing mixing and deepening the mixed layer depth. In contrast, moderate winds and weak current enable the propagation of LSMFs furthest offshore in Greater Cook Strait, where the water column becomes stably stratified. A stably stratified regime inhibits vertical mixing of nutrients and phytoplankton, which may lead to enhanced primary production in the region.

Using moorings with a surface expression, responses of buoyant river outflows were examined in the Firth-Hauraki Gulf system, New Zealand. The Firth region of freshwater influence (ROFI) behaves as a gulf-type ROFI where the balance of key processes lies between rotation and advection. The latter process is largely regulated by the frequency and amplitude of freshwater discharges. Using 12-months of observations from two inner Firth sites, the system was found to be salinity stratified for up to 4 months of a year. During the largest event (274 m3s-1, mean daily maximum) in 2012 near-oceanic surface salinity (34 psu) freshened substantially down to 28 psu over the upper 3 m. Time lags between freshwater inflows and ROFI response were 4 and 7 days at the Waiheke and WilsonB locations, respectively. Faster seaward advection of surface layers occurred for a minimum of 3 days and up to a week after peak discharges. High frequency winds (~ 3-hourly) were persistent in regulating surface flows over the annual cycle. Surface salinity had peaks in energy that were coherent with surface flows at periods of 16 to 32 days over the Austral winter. The duration of discharge events was O(days), yet responses by buoyancy and advection were evident for weeks to a month in the Firth ROFI.

International audience; The influence of sea swell (SS) waves, infragravity (IG) waves, and wave setup on maximum runup (Rmax) is investigated across different tidal stages on Fatato Island, Funafuti Atoll, Tuvalu. Field results illustrate that SS waves are tidally modulated at the shoreline, with comparatively greater wave attenuation and setup occurring at low tide versus high tide. A shoreward increase in IG wave height is observed across the 100 m wide reef flat at all tidal elevations, with no tidal modulation of IG wave height at the reef flat or island shoreline. A 1-D shock-capturing Green-Naghdi solver is used to replicate the field deployment and analyze Rmax. Model outputs for SS wave height, IG wave height and setup at the shoreline match field results with model skill >0.96. Model outputs for Rmax are used to identify the temporal window when geomorphic activity can occur on the beach face. During periods of moderate swell energy, waves can impact the beach face at spring low tide, due to a combination of wave setup and strong IG wave activity. Under mean wave conditions, the combined influence of setup, IG waves and SS waves results in interaction with island sediment at midtide. At high tide, SS and IG waves directly impact the beach face. Overall, wave activity is present on the beach face for 71% of the study period, a significantly longer duration than is calculated using mean water level and topographic data.

International audience; Two-dimensional, non-linear, Boussinesq, non-hydrostatic simulations of internalsolitary waves breaking and running up uniform slopes have been performed usingan adaptive, finite volume fluid code “Gerris”. It is demonstrated that the Gerrisdynamical core performs well in this specific but important geophysical context.The “semi-structured” nature of Gerris is exploited to enhance model resolutionalong the slope where wave breaking and run-up occur. Comparison with labora-tory experiments reveals that the generation of single and multiple turbulent surges(“boluses”) as a function of slope angle is consistently reproduced by the model,comparable with observations and previous numerical simulations, suggesting as-pects of the dynamical energy transfers are being represented by the model in twodimensions. Adaptivity is used to explore model convergence of the wave breakingdynamics, and it is shown that significant cpu memory and time savings are possiblewith adaptivity.