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With climate heating, Aotearoa New Zealand is expected to experience more marine heatwaves (MHW) in the coming decades. These extreme events are already impacting the island nation's marine and coastal environments and marine industries at a variety of scales. There will potentially be substantial benefits in developing an early warning system–specifically ocean seasonal forecast tools. This near-term 2,030 horizon scan reviews studies supporting the development of this capability and notes work needed to enable stakeholders to benefit from this knowledge. Review findings congregate around six themes; (1) MHW impacts, (2) mechanistic understanding, (3) observational basis, (4) seasonal forecast tools, (5) supporting Te Tiriti (The Treaty of Waitangi) and Māori aspirations, and (6) end-user engagement. The primary recommendation is a cross-institutional, cross-sector MHW Taskforce that would address, in a coordinated and effective fashion, the real, multi-faceted challenges associated with the committed pathway of warming. A range of sub-recommendations follow that connect with the United Nations Ocean Decade initiative.

Observations collected from a fast-flowing buoyant river plume entering the head of Doubtful Sound, New Zealand, were analysed to examine the drivers of plume lateral spreading. The near-field plume is characterised by flow speeds of over 2 ms−1, and strong stratification (N2 > 0.1 s−2), resulting in enhanced shear which supports the elevated turbulence dissipation rates (ϵ > 10−3 W kg−1). Estimates of plume lateral spreading rates were derived from the trajectories of Lagrangian GPS surface drifters and from cross-plume hydrographic transects. Lateral spreading rates derived from the latter compared favourably with estimates derived from a control volume technique in a previous study. The lateral spreading of the plume was driven by a baroclinic pressure gradient toward the base of the plume. However, spreading rates were underestimated by the surface drifters. A convergence of near-surface flow from the barotropic pressure gradient concentrated the drifters within the plume core. The combination of enhanced internal turbulence stress and mixing at the base of the surface layer, and the presence of steep fjord sidewalls likely reduced the rate of lateral spreading relative to the theoretical spreading rate. The estimates of plume width from the observations provided evidence of scale-dependent dispersion which followed a 4/3 power law. Two theoretical models of dispersion, turbulence and shear flow dispersion, were examined to assess which was capable of representing the observed spreading. An analytical horizontal shear-flow dispersion model generated estimates of lateral dispersion that were consistent with the observed 4/3 law of dispersion. Therefore, horizontal shear dispersion appeared to be the dominant mechanism of dispersion, thus spreading, in the surface plume layer.

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 16–32 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.