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Lagrangian particle tracking, based on currents derived from hydrodynamic models, is an important tool in quantifying bio-physical transports in the ocean. Particle tracking in the unstructured grids typically used in coastal regions is computationally slow, limiting the number of particles and ranges of behaviours that can be modeled. Techniques used in a new offline particle tracker “OceanTracker” are shown to be two orders of magnitude faster than those used in an existing ocean particle tracker for unstructured grids when run on a single computer core. More significantly, its computational speed can exceed that achieved when particle tracking on a regular grid. The techniques for unstructured grids make it possible to routinely calculate the trajectories of millions of particles. This large number of particles allows much better estimates of dispersion and transport statistics, particularly when the probability of connection is low but the consequences are significant, e.g. the spread of invasive species. It also enables wider exploration of parameter sensitivity and particles’ bio-physical behaviours to provide more robust results. The speed increases result largely from exploiting history and reuse within the spatial interpolation of the hydrodynamic model’s output. Using multiple computer cores further increased the speed to track a given number of particles.

Large turbine farms in strong tidal flows could contribute significantly to the global demand for renewable energy. Key to developing large scale power generation is determining how many turbines are required to deliver a given amount of power from proposed sites. Answering this question is computationally difficult, as large-scale power extraction changes the strength of the currents driving the turbines. As a consequence, the large hydrodynamic models used to assess the tidal current resource must be run many times to optimize power output for every potential site and farm size. This paper outlines an approach that can rapidly assess farm power output using an existing hydrodynamic model. This approach is aimed at rapidly determining the most promising farm sites, sizes and shapes within a region, enabling work with more detailed, realistic and slower models to focus on a smaller number of farms. The approach is used to assess how much of Cook Strait, New Zealand’s 15,000 MW potential could be realized with current generation turbines. A basic economic analysis suggests that a 90 MW farm with 95 20 m-diameter turbines might be viable in Cook Strait, if turbine manufacturing costs fall, or energy prices increase, by around 25%.

Bottom friction causes a vertical shear in the water column resulting in secondary circulation causing a helical flow pattern around curved open channels. Secondary circulation has cross-stream and...

A simple diagnostic narrow channel shallow water model for the cross-channel distribution of the depth average velocity is developed. It is diagnostic, because the slope of the stream-wise free sur...

Mussel aquaculture is heavily reliant on wild mussel populations that supply juveniles (spat) for seeding farms. However, little is often known about parent populations, representing a risk for the sustainability of the industry. We used hydrodynamic back-tracking models to identify potential parental areas that provision green-lipped mussel (Perna canaliculus) spat across a range of settlement sites in New Zealand's largest aquaculture area. Median parental area varied considerably between 19 km2 for sites located in enclosed bays and a maximum of >1150 km2 for sites located in open bays. Median distance to parent populations ranged between 1.8 and 21.4 km, with a maximum larval dispersal estimated to be ca. 100 km. Small seasonal variations in parental area and dispersal distance were detected in some regions, whereas inter-annual variability was relatively minor. Regional connectivity between settlement and parental regions ranged between a minimum of 45% of larvae originating in the same parental region, to maximum retention rates of 99.9% for sites in enclosed bays, implying a considerable regional variation in the potential for self-seeding and exporting mussel larvae other areas. Our results also delineate areas that support spatfall by identifying likely locations for wild or farmed parental populations, and by establishing the spatial extent where mussel reproduction and larval development through to settlement take place. These dispersal and connectivity patterns are crucial to support management decisions for the conservation and restoration of parental populations, and other environmental constraints, such as water quality, which are necessary to ensure the sustainability of spat catching operations that enable shellfish farming.

Vessel-mounted ADCP measurements were conducted to describe the transverse structure of flow between the two headland tips in Khuran Channel, south of Iran (26° 45′ N), where the highest tidal velocities in spring tides were ~ 1.8 m/s. Current profiles were obtained using a 614.4 kHz TRDI WorkHorse Broadband ADCP over nine repetitions of three cross-channel transects during one semidiurnal tidal cycle. The 2.2-km-long transects ran north/south across the channel. A least-square fit to semidiurnal, quarter-diurnal, and sixth diurnal harmonics was used to separate the tidal signals from the observed flow. Spatial gradients showed that the greatest lateral shears and convergences were found over the northern channel and near the northern headland tip due to very sharp bathymetric changes in this area. Contrary to the historical assumption, the across-channel momentum balance in the Khuran Channel was ageostrophic. The current study represents one of the few examples reported where the lateral friction influences the across-channel momentum balance.

Secondary circulation is a mechanism that increases vertical exchange between the horizontal layers in open channels. Vertical shear in curved flows induces secondary circulation in the pla...

Tidal channels with narrow constrictions are attractive sites for electricity generation due to energy dense, high velocity flow through the constricted zones. This work uses a 1D model to systematically examine the effect of constriction on channel potential (theoretical maximum power) and transport in channels connecting two large water bodies (Type I) and channels connecting a large water body to an embayment/lagoon (Type II). Type I channels showed a monotonic decrease in potential and transport with increasing constriction due to enhanced bottom drag from the resultant high velocity zone. The effect of constriction on potential and transport in Type II channels varies depending on the relative geometries of the channel and embayment. Type II channels may be geometrically modified to increase power and in some instances this would simultaneously produce a high velocity zone. The flow reduction required to achieve a channel’s potential was invariant to changes in constriction for Type I channels and also Type II channels if a “lagoon geometry factor” is less than about 4. A simple approximation for potential provided in the literature for unconstricted channels is extended for use with constricted channels and an approximation for the drag coefficient required to achieve potential is provided.

Large arrays of tidal turbines are critical to realise the potential of tidal current power. This study is a systematic exploration of large tidal array optimisation in channels with numerically modelled array layouts in 2-D. Crucially, flow along channels is driven by head loss leading to significantly more realistic results than previous models which assume constant velocity. The 2-D adaptive mesh approach bridges the gap between large- and small-scale array models. Hundreds of layouts and turbine tunings have been simulated using LES of turbulent flow in tidally reversing currents to explore channel-scale optimisation and tuning of large arrays. Simulations show that total power capture increases as rows are added to the array although there are diminishing returns on additional turbines. Each turbine in 1 (7), optimally blocked row in a small channel captures 2.5 × ( 0.5 × ) the power of an isolated turbine. There is an optimum blockage for maximum power per turbine which decreases linearly from 1.0 as the number of rows increases. As array size increases individual turbine wakes become less important than stepped head loss across each row. Free-stream velocity reduces linearly with total power capture, with the gradient increasing with channel size.

Much of the global tidal current energy resource lies in the accelerated flows along narrow channels. These channels have the potential to produce 10–1000 s of MW of electricity. However, realizing 100 MW of a channel׳s potential is much more complex than installing 100 1-MW turbines because large scale power extraction reduces tidal currents throughout the channel, changing the resource. This synthesis and review gives an overview of the issues and compromises in designing the layout of the large tidal turbine arrays required to realize this potential. The paper focuses on macro- and micro-design of arrays. Macro-design relates to the total number of turbines and their gross arrangement into rows, while micro-design adjusts the relative positions of the turbines within a grid and the spacing between rows. Interdependent macro-design compromises balance the total number of turbines, array power output, the power output of each turbine, the loads the turbines experience, turbine construction costs, maintaining navigability along the channel and any environmental impacts due to flow reduction. A strong emphasis is placed on providing physical insights about how “channel-scale dynamics” and the “duct-effect” impact on the compromises in array design. This work is relevant to the design of any “large” array which blocks more than 2–5% of a channel׳s cross-section, be it 2 turbines in a small channel or 100 turbines in a large channel.

High-resolution observations from the curved tidal channel of the Otago Harbour shows secondary flows up to 20% of the primary flow and vertical velocity inferred from secondary flow up to 1% of the primary flow. This vertical velocity is inferred on a much finer scale than previous works. The spatial pattern of this vertical velocity is upward on the inside and downward on the outside of the bend, consistent with previous laboratory flume measurements. Linear regression, rp = 0.95, shows the cross-channel distribution of the observed secondary flow can be resolved from the horizontal ADCP measurements well enough to be consistent with the cross-channel distribution of secondary flow derived from the observed primary flow using the model of Kalkwijk and Booij (1986). Linear regression, rp = 0.80, shows the vertical velocity inferred from observed secondary flow is consistent with vertical velocity derived from the observed primary flow using the model of Kalkwijk and Booij (1986). This also shows the cross-channel distribution of the observed secondary flow is resolved well enough from the horizontal ADCP measurements to be able to infer the vertical velocity from mass continuity. The required horizontal resolution is made possible by Radial Basis Function (RBF) smoothing and spatial interpolation that allows for continuity of the spatial derivatives. Modeling trajectories using channel dimensions and velocity field values equivalent to the observations show that 3-dimensional secondary circulation forms a loose helical flow pattern.

The Betz limit sets a theoretical upper limit for the power production by turbines expressed as a maximum power coefficient of 16/27. While power production by wind turbines falls short of the Betz limit, tidal turbines in a channel can theoretically have a power coefficient several times larger than 16/27. However, power extraction by turbines in large tidal farms also reduces the flow along the channel, limiting their maximum output. Despite this flow reduction, turbines in tidal farms can produce enough power to meet a stricter definition of what it means to exceed the Betz limit, one where the maximum power output of a turbine at the reduced flow exceeds the maximum output from a single Betz turbine operating in the unreduced flow. While having a power coefficient >16/27 is easily achieved by turbines in a channel, generating enough power to meet this stricter definition of exceedance is much more difficult. Whether turbines meet this stricter definition depends on their number, how they are arranged and tuned, and the dynamical balance of the channel. Arranging a tidal turbine farm so that the turbines within it exceed the stricter Betz limit would give tidal turbine farms an economic advantage over similarly sized wind farms. However, exceeding the stricter limit comes at a cost of both higher structural loads on the tidal turbines and the need to produce power from weaker flows. Farms in a channel loosely based on the Pentland Firth are used to discuss exceedance and structural loads.

The subtropical front zone (STFZ) is a fundamental feature of the southwest Pacific Ocean. It separates waters in the subtropical gyres from subantarctic waters. In this study we examine a section of the global STFZ that passes around southern New Zealand, using hydrographic sections and remotely sensed sea surface temperature (SST) data collected during the austral autumns of 2007, 2008 and 2011. The STFZ was composed of a single southern subtropical front (S-STF) that marked the poleward edge of the STFZ, while the region north of the S-STF was characterised by a series of shallow fronts (

Far infragravity waves (FIGWs) with periods of 2–64 min can reduce the underkeel clearance for vessels in transit to ports and cause resonance in and around harbors and bays that amplifies low-frequency current flows. Sea level records from six tide gauges along the east coast of New Zealand and the Chatham Islands are compared with offshore weather systems and spectral statistics from the gravity wave (GW) band (including both sea and swell waves) from wave buoys and hindcast sources. An empirical formula to predict coastal FIGWs is developed, based on the offshore GW significant wave height, associated peak period, and spectral width parameter. This formula is able to explain the occurrence of most FIGW events, confirming that most of the FIGW energy arriving at the coast is directly associated with GW energy. However, several discrete events were not predicted by the technique, and reverse ray tracing was applied to explore the origin of these events. In the light of these findings, previously ...

Tuning wind and tidal turbines is critical to maximizing their power output. Adopting a wind turbine tuning strategy of maximizing the output at any given time is shown to be an extremely poor strategy for large arrays of tidal turbines in channels. This ‘impatient-tuning strategy’ results in far lower power output, much higher structural loads and greater environmental impacts due to flow reduction than an existing ‘patient-tuning strategy’ which maximizes the power output averaged over the tidal cycle. This paper presents a ‘smart patient tuning strategy’, which can increase array output by up to 35% over the existing strategy. This smart strategy forgoes some power generation early in the half tidal cycle in order to allow stronger flows to develop later in the cycle. It extracts enough power from these stronger flows to produce more power from the cycle as a whole than the existing strategy. Surprisingly, the smart strategy can often extract more power without increasing maximum structural loads on the turbines, while also maintaining stronger flows along the channel. This paper also shows that, counterintuitively, for some tuning strategies imposing a cap on turbine power output to limit loads can increase a turbine’s average power output.

Sea ice growth rates near ice shelves are influenced by ocean-ice shelf interactions. Sea ice growth rates and ocean observations from McMurdo Sound, Antarctica in 1999 and 2000 are presented in this paper. Growth rate measurements were made for an individual platelet crystal through video camera observations. It was found that the crystal grew in discontinuous, episodic bursts at rates of the order of 10 − 6 m s − 1 . Sea water 0.15 m beneath the lower ice surface was measured to be supercooled by 0.01 K. Indications are that supercooling was continuous over the period of episodic platelet ice crystal growth and the growth bursts are attributed to the influence of variable currents. Growth rates for bulk sea ice (i.e., columnar and incorporated platelet ice) and heat fluxes were derived from ice temperature measurements. The growth rates for bulk sea ice were found to be of the order of 10 − 7 m s − 1 , an order of magnitude less than the rates for the individual platelet ice crystal. The residual of the energy balance suggested that a negative oceanic heat flux (i.e., heat transport down into the ocean) occurred, in addition to conduction of heat up into the atmosphere. Both salinity-based growth rate models and an oxygen isotope-based growth rate model (Eicken, 1998) were found to under-predict growth rates compared to those derived from ice temperature measurements. In addition, inverting the growth rates predicted by the models and integrating over the depth of the core failed to accurately predict the date of initial sea ice formation. Modifications are proposed to the models for sea ice formation occurring near ice shelves, where platelet ice formation is likely. Differences between bulk and individual platelet ice crystal growth rates are discussed with reference to heat fluxes, oceanic flows and the Eicken (1998) model.

The components of the energy lost by currents flowing along tidal channels with large turbine arrays provide insights about developing farms to generate electricity from tidal streams. The performance and economics of a farm are profoundly affected by where a farm lies on the curve of the total power lost by the flow in relation to its peak at a total drag coefficient of 2. Farms in shallow channels lie well above the peak, where a falling ceiling in the total power lost leads to a diminishing return on optimally tuned turbines. Farms in large tidal straits lie well below the peak. They grow initially in the context of a rapidly rising ceiling in the total power lost and may benefit from an increasing return on turbines added to the cross-section, a power production per turbine well above that of the first turbine installed and have turbines which may exceed the Betz limit. Surprisingly farms in tidal straits have proportionately higher mixing losses behind the turbines, thus the benefits are not due to a higher turbine conversion efficiency as the blockage ratio increases. There is an optimal number of rows for a farm, however a harsh diminishing return on new rows near this optimum will result in farms being significantly smaller than the optimum size. Energy losses due to drag on turbine support structure can be significant in multi-row farms, altering their performance and limiting farm size.

Tidal turbines in strong flows have the potential to produce significant power. However, not all of this potential can be realized when gaps between turbines are required to allow navigation along a channel. A review of recent works is used to estimate the scale of farm required to realize a significant fraction of a channel's potential. These works provide the first physically coherent approach to estimating the maximum power output from a given number of turbines in a channel. The fraction of the potential realizable from a number of turbines, a farm's fluid dynamic efficiency, is constrained by how much of the channel's cross-section the turbines are permitted to occupy and an environmentally acceptable flow speed reduction. Farm efficiency increases as optimally tuned turbines are added to its cross-section, while output per turbine increases in tidal straits and decreases in shallow channels. Adding rows of optimally tuned turbines also increases farm efficiency, but with a diminishing return on additional rows. The diminishing return and flow reduction are strongly influenced by how much of the cross-section can be occupied and the dynamical balance of the undisturbed channel. Estimates for two example channels show that realizing much of the MW potential of shallow channels may well be possible with existing turbines. However unless high blockage ratios are possible, it will be more difficult to realize the proportionately larger potential of tidal straits until larger turbines with a lower optimum operating velocity are developed.

Applying a two-dimensional (2D) divergence-free (DF) interpolation to a one-person deployable unmanned underwater vehicle’s (UUV) noisy moving-vessel acoustic Doppler current profiler (MV-ADCP) measurements improves the results and increases the utility of the UUV in tidal environments. For a 3.5-h MV-ACDP simulation that spatially and temporally varies with the M2 tide, the 2D DF-estimated velocity magnitude and orientation improves by approximately 85%. Next the 2D DF method was applied to velocity data obtained from two UUVs that repeatedly performed seven 1-h survey tracks in Bear Cut Inlet, Miami, Florida. The DF method provides a more realistic and consistent representation of the ADCP measured flow field, improving magnitude and orientation estimates by approximately 25%. The improvement increases for lower flow velocities, when the ADCP measurements have low environmental signal-to-noise ratio. However, near slack tide when flow reversal occurs, the DF estimates are invalid because the flows are not steady state within the survey circuit.

A simple method for estimating the potential of currents in tidal channels to produce power is presented. The method only requires measurement of the peak tidal volume transport through the channel without turbines, along with a bottom drag coefficient and the channel’s dimensions. A recent existing method for estimating potential requires measurements of the undisturbed transport as well as water levels at both ends of the channel to give the head loss. The adaptation of the existing method presented here exploits analytic solutions for the transport and optimal farm drag coefficient and does not require measurement of the head loss. The equations presented allow both the channel’s potential and the flow reduction due to power extraction to be estimated using a calculator. Thus the presented method has much of the ease of use of the older KE flux method, but is more reliable as it includes retardation of the flow by the turbines. The presented method can be used for the initial assessment of channels to determine if the additional measurements required to use the existing method are warranted. It can also be used where the headloss in the channel is too small to measure reliably. The presented equations enable the maximum power available to be simply traded off against environmentally acceptable flow speed reduction. The presented method is applied to two example channels. Cook Strait NZ has an estimated potential of 15 GW, while the entrance channel to Kaipara Harbour has a potential between 110 MW and 240 MW.

The oceanic forced wave beneath a moving atmospheric disturbance is amplified by Proudman resonance. When modified by the Earth's rotation this classical resonance only occurs if the disturbance time scale is smaller than the inertial period. With or without Coriolis effects, free transients generated by storm forced waves obliquely crossing step changes in water depth at particular angles are shown to resonate by exciting a range of long barotropic free waves. Rotationally influenced slow atmospherically forced waves crossing a vertical coast at a critical angle lead to a form of subcritical resonance, which occurs only when the component of the disturbances' phase velocities along the coast matches that of a free Kelvin wave (KW). In a rotating ocean, transients generated by disturbances crossing a step at a particular angle are shown to excite a free double Kelvin wave (DKW). This new type of resonance only occurs for sufficiently large steps and disturbances with time scale greater than the inertial period. A storm crossing a step shelf can result in the excitation of an infinite set of edge waves, a single KW, a unique DKW and a first-mode continental shelf wave, depending on the topography and the disturbance time scale, translation speed and incident angle. The study of resonances and wave mode excitations generated by storms crossing a coast or a continental shelf may contribute to understanding how a particular combination of the storm characteristics can result in destructive coastal events with time scales encompassing the typical meteotsunami period band (tens of minutes) and storm surges with periods of several hours or days.

Tuning is essential to maximise the output of turbines extracting power from tidal currents. To realise a large fraction of a narrow channel's potential, rows of turbines not only have to be tuned for a particular tidal channel, they must also be tuned in the presence of all the other rows, i.e. ‘tuned in-concert’. The necessity for in-concert tuning to maximise farm efficiency occurs because the tuning of any one row affects a channel's total drag coefficient and hence the flow through all other rows. Surprisingly, in several circumstances the optimal in-concert tunings are the same or almost the same for all rows. Firstly, in both constricted and unconstricted channels, rows with the same turbine density have the same optimal tuning. Secondly, turbine rows in channels with a quasi-steady dynamical balance typically have almost the same optimal in-concert tunings, irrespective of their turbine density or any channel constrictions. Channel constrictions, occupying a large fraction of the cross-section or adding more rows of turbines, also make optimal tunings more uniform between rows. Adding turbines to a cross-section increases a farm's efficiency. However, in a law of diminishing returns for quasi-steady channels, turbine efficiency (the output per turbine) decreases as turbines are added to a cross-section. In contrast, for inertial channels with only moderate constrictions, turbine efficiency increases as turbines are added to a cross-section.

As tidal turbine farms grow they interact with the larger scale flow along a channel by increasing the channel's drag coefficient. This interaction limits a channel's potential to produce power. A 1D model for a tidal channel is combined with a theory for turbines in a channel to show that the tuning of the flow through the turbines and the density of turbines in a channel's cross-section also interact with the larger scale flow, via the drag coefficient, to determine the power available for production. To maximise turbine efficiency, i.e. the power available per turbine, farms must occupy the largest fraction of a channel's cross-section permitted by navigational and environmental constraints. Maximising of power available with these necessarily densely packed farms requires turbines to be tuned for a particular channel and turbine density. The optimal through-flow tuning fraction varies from near 1/3 for small farms occupying a small fraction of the cross-section, to near 1 for large farms occupying most of the cross-section. Consequently, tunings are higher than the optimal through-flow tuning of 1/3 for an isolated turbine from the classic turbine theory. Large optimally tuned farms can realise most of a channel's potential. Optimal tunings are dependent on the number of turbines per row, the number of rows, as well as the channel geometry, the background bottom friction coefficient and the tidal forcing.

Proudman resonance amplifies the oceanic forced wave beneath moving atmospheric pressure disturbances. The amplification varies with water depth; consequently, the forced wave beneath a disturbance crossing topography radiates transient free waves. Transients are shown to magnify the effects of Proudman resonance for disturbances crossing the coast or shelf at particular angles. A Snell like reflection law gives rise to a type of resonance for relatively slow moving disturbances crossing a coast in an otherwise flat-bottomed ocean. This occurs for translation speeds less than the shallow water wave speed for disturbances approaching the coast at a critical angle given by the inverse sine of the Froude number of the disturbance. A disturbance crossing the shelf at particular angles can also excite seiche modes of the shelf via generation of a transient at the continental slope. Beyond a typically small angle of incidence, transients generated by a disturbance crossing the continental slope and coast will be trapped on the shelf by internal reflection. The refraction law for a fast-moving forced wave crossing an ocean ridge at greater than a small angle of incidence also results in trapped free-wave transients with tsunami-like periods propagating along the ridge. The subcritical resonance, excitation of shelf modes and trapping of the transients may have implications for storm surges and the generation of destructive meteotsunami.

We explored the relationship between mixing dynamics in the surface mixed layer and the photoprotective response of phytoplankton. The distribution of photoprotective pigments was determined at 15 stations in subantarctic waters southeast of New Zealand during austral autumn. The first-order kinetics of the phytoplankton photoprotective response to both light and dark was determined using in situ simulated (deck) incubations. The vertical mixing regime was deduced from two physical parameterizations based on CTD and ADCP profiles. A photophysiological index based on the vertical profile of de-epoxidation state (DES) of the diadinoxanthin-cycle to changes in light intensity was compared with the two physical estimates of vertical eddy diffusivity. For short time scales (,12 h) and within a similar water mass, the DES index provides within one order of magnitude an estimate of average vertical velocity representative of the bulk vertical eddy diffusivity. From the results, a conceptual model is presented of the quantitative relationship between vertical mixing and phytoplankton photoprotection in the water column. This relationship can provide additional insights into the effects of changes in vertical eddy diffusivity in the surface mixed layer on the photoprotective response of phytoplankton.

Wavelet and cross-wavelet power spectra of sea level records from tide gauges along the Atlantic coast of Canada showed a low-frequency barotropic response after Hurricane Florence crossed the Newfoundland shelf in September 2006. In comparison with two other storms, the results showed that Florence was the only one that excited a propagating sea level disturbance with a period range similar to the passage time of the storm over the shelf (26–30 h) and phase shifts consistent with a barotropic continental shelf wave (CSW). The high amplitude of the oscillations generated by Florence along the shore diminished from approximately 45 to 12 cm as the CSW propagated from the south coast of Newfoundland to the southern Nova Scotia seaboard. This paper presents the first direct measurement of a remarkably high alongshore group speed (11.4 ± 5.9 m s−1), in the manner of free-barotropic CSW, by examination of sea level wavelet power spectra at different locations. Furthermore, using cross-wavelet analysis of pairs of stations, an exceptional phase speed of 16.0 ± 5.1 m s−1 has been found, greater than had been previously observed for a free CSW. The results were consistent with dispersion curves for the first-mode barotropic CSW.

Using the ice algal community growing at the bottom of the annual sea ice in McMurdo Sound Antarctica, the response of the photoprotective diadinoxanthin (DD)-cycle to exposure to light was investigated. Changes in pigment concentration were detected using high-performance liquid chromatography. A light mixing simulator (LMS) was developed and used to simulate the pigment response to mixing in the upper water column. No DD-cycle was detected under the sea ice under natural light conditions. The DD-cycle was activated after exposure to surface natural light conditions and artificial light conditions. The first-order kinetic rates of the DD-cycle under constant artificial irradiance, natural irradiance and simulations with the LMS were found to be similar to other studies suggesting that ice algae do not vary the rate of deepoxidation depending on light history. Simulations under natural light using the LMS demonstrated that the response of the DD-cycle to static incubations and when subject to vertical mixing was not similar, and that static incubations overestimate DD-cycle activity over the long term. Algae in a simulated vertically mixed environment were able to increase the pool of xanthophyll pigments compared to static conditions where the pool remained the same or decreased. The recovery of DD in the dark or under low light was found to be significantly faster than in temperate algal communities. These results suggest that ice algae at the sea ice bottom can activate the photoprotective DD-cycle to regulate excess thermal energy. Unlike temperate species of diatoms, ice algae can rapidly reconstruct the pigment pool under low light or in the dark and is likely a particular adaptation to the unique light environment in Antarctica.

Storms crossing topography are shown to radiate long surface gravity waves. The waves are transients generated by changes in the depth-dependent amplitude of the atmospherically forced pressure wave beneath a storm. This generation mechanism for long waves, known as “meteorological tsunamis” or rissaga, does not appear to have been previously discussed. The transients have periods equal to the passage time of the storm, of order 30 min for small fast-moving storms. A 1D model is used to give the amplitudes of the transient waves generated by a small fast-moving storm crossing a topographic step on to a continental shelf and across a ridge. Large transients are generated by storms whose translation speed is subcritical in deep water and supercritical in shallow water, that is, faster than the shallow-water wave speed. Surprisingly, when the depth difference between the deep water and the continental shelf is large, a gentle transition from deep to shallow water over 10 storm widths only slightly reduces the amplitudes of the transients. The influence of a finite-width shelf on the enhancement of coastal storm surge is also discussed. A 2D numerical model illustrates the topographic transients generated by sub- and supercritical storms moving across a ridge. Topographic transients are suggested as a source of energy for seiches on shelves and within embayments. The energy may come from a storm crossing the adjacent continental slope and possibly from distant open-ocean storms crossing multiple ridges and seamounts.

A broadband acoustic Doppler current profiler was used to measure the velocity field of the ebb tidal jet formed at the entrance to the Otago Harbour, South Island, New Zealand. Four separate surveys were conducted where various sets of transects were run continuously over a 13 hour period, producing time series of velocity profiles with 1 hour sampling over a grid with an across-jet resolution of 20 m and an along-jet resolution of 150 m. The velocity field of the ebb tidal jet is phase locked to the tidal response of the harbor, making it possible to combine the data from separate tidal cycle surveys to produce synoptic maps of the depth-averaged velocity field. The spatial structure of the ebb tidal jet at the time of peak flow exhibits many features of a classical plane jet. There is a strong asymmetry between the ebb and flood flow structures; the ebb flow extends beyond 2 km from the entrance, while the flood flow is limited to within 600 m of the coast. The flow along the jet axis becomes pulse-like in time with distance from the entrance. The pulse that develops propagates way from the entrance at a speed slower than the advective speed of the ebb jet. The formation of a propagating eddy observed along the eastern side of the jet suggests that this momentum pulse may be associated with the pair of separation eddies generated by the accelerating ebb tidal flow.

Time series measurements of velocity and salinity have been used to examine the salt flux components in the tidally-energetic regime of the Conwy Estuary in North Wales. The instantaneous volume transport, up to 400 m3 s−1, was deduced from a current meter located in mid-channel using a frictional model with a correction for the observed mean velocity shear. Transport estimates obtained in this way have been compared with independent measurements of integrated cross-section transport using a boat-mounted ADCP. The two determinations of the flow are found to be in satisfactory agreement confirming the validity of the frictional model. Estimates of the covariance of the tidally varying transport Qtand salinity Stindicate that a consistently upstream transport of salt occurred over the 9 day period of the observations. This upstream tidal pumping flux (≈60 kg s−1) was greater than the downstream transport (≈27 kg s−1) estimated from the freshwater river discharge Qf(≈3 m3 s−1). The difference in the salt fluxes implies a net upstream movement of salt and was consistent in sign with the observed general increase in mean salinity which occurred in the upper estuary prior to a rapid salinity decrease due to a large increase in freshwater discharge at the end of the measurement period. The observations have been interpreted to illustrate the problems involved in trying to determine the net flux of a dissolved constituent from such as estuary. Using estimates of the net inflow from river gauging to determine the volume dischargeQ0 , estimates of the net flux are realistic but, in the absence of river gauging, large errors are likely to arise from the uncertainties in Q0, which is the small difference between large flood and ebb transports.

The Subtropical Front (STF) is a global front which extends around the Southern Ocean and is the boundary where the Subantarctic Surface Water mass (SAW) converges with the Subtropical Surface Water mass (STW). The Southland Front (SF) is part of the STF, which lies off the east coast of the South Island, New Zealand. The SF is narrow, approximately 8 km, with a temperature difference of approximately 1.8°C which can be detected using remote sensing Advanced Very High Resolution Radiometer (AVHRR) sea surface temperature (SST) data. The work presented here is an application of remote sensing for the first detailed study of the surface spatial and temporal variability of the SF. The variability of the SF was quantified using an algorithm developed to follow the Front using AVHRR SST imagery. The algorithm used a new approach to determine the position, mean SST, SST difference, width, and gradient across the Front. Three time scales of variability were examined: long-term (3 years), annual, and seasonal. The algorithm efficiently followed the SF and consistently showed the 3-year mean position was stable and constrained by the bathymetry of the continental slope. Seasonally, the front moved inshore during summer. The temperature gradient across the front was strongest and the front narrowest in winter. The decrease in SST gradient during the 3-year data set coincided with the decrease in Southern Oscillation Index (SOI).

Dendritic crystals of platelet ice appear beneath the columnar land-fast sea ice of McMurdo Sound, Antarctica. These leaf-like crystals are frozen into place by the advancing columnar growth. The platelets most probably begin to appear during July although in some parts of the Sound they may not appear at all. In addition, the amount and extent of platelet ice within the Sound varies from year to year. Previous authors have suggested that the formation of platelet ice is linked to the presence of the nearby ice shelf. It is a matter of debate whether these platelets form at depth and then float upwards or whether they grow in slightly supercooled water at the ice/water interface. The phenomenon is similar to that observed in the Weddell Sea region, but previous authors have suggested the two regions may experience different processes. This paper presents the results of field-work conducted in McMurdo Sound in 1999. Ice-structure analysis, isotopic analysis and salinity and temperature measurements near the ice/water interface are presented. Freezing points are calculated, and the possible existence of supercooling is discussed in relation to existing conjectures about the origin of platelets.

The Mernoo Saddle is situated c. 100 km east of Banks Peninsula, Canterbury, New Zealand at 44°S 174°E. The Saddle separates the South Island of New Zealand from an underwater ridge known as the Chatham Rise. The Rise acts as a partial barrier to the flow of the subantarctic surface water mass (SAW) and the subtropical surface water mass (STW), which are part of the global Subtropical Front extending around the Southern Ocean. This study examined the variability of the water masses flowing through the Mernoo Saddle using a 3‐year data set of Advanced Very High Resolution Radiometer (AVHRR) Sea Surface Temperature (SST) imagery. This investigation revealed that SAW extended north through the western edge of the Mernoo Saddle for most of the year, however in winter and early spring a southward extension of STW was observed.

This study examined the temporal variability of sea surface temperature (SST) of three water masses east of the South Island, New Zealand, where two of these water masses are part of the global Subtropical Front (STF). The STF is well defined and can be detected easily using remote sensing Advanced Very High Resolution Radiometer (AVHRR) SST data. Ten sites were used from a ten year AVHRR SST data set and they were compared with a manually sampled 45 year SST data set from Portobello Marine Laboratory, Dunedin. This investigation decomposed each SST series into a periodic seasonal component and an interannual trend. The latter component was further decomposed into low and mid-frequency bands using smoothing techniques. The results showed that the periodic component of offshore waters were in phase and reached their maximum temperature 10 days after the inshore neritic waters. The amplitudes of seasonal variations of Subantarctic and Subtropical water masses were similar while the neritic waters showed considerably higher seasonal variation. El Nino has a clear effect and lowers the temperatures of all water masses by 1.4oC at Portobello and 2.2-3.4oC for all offshore waters.

Intuitively, the transport of an oscillating flow in a short channel should be constrained to be approximately the same at all cross sections; that is, the transport is nondivergent. This intuitive constraint is quantified by developing a generalized definition of “shortness” given by the smallness of a nondimensional parameter e. This parameter assesses the shortness of channels of variable cross-sectional area, with linear or nonlinear dynamical balances and, thus, is applicable to a wide range of channels, from tidal flow through the entrance to an estuary to subinertial flow through a strait. This analysis leads to a general result for a barotropic oscillating flow, which states that the variation in the amplitude of the transport along the channel is of order 2e of the mean transport and the variation in the transport’s phase is of order 2 tan−1e. A diagnostic model for tidal flow within a “short” narrow channel of variable depth is developed. The model shows that the phase of the cross-sect...

The phase of the M2 surface elevation tide changes by 100° along a 40-km length of Cook Strait, New Zealand. Acoustic Doppler Current Profiler measurements are presented that show a change of only 12° in the phase of depth-averaged semidiurnal tidal currents along this same length. This phase change supports the general result developed by Vennell, which states that the phase of the cross-sectional average velocity is approximately constant along a “short” strait of variable cross-sectional area. Vennell also developed a relationship between the constant phase of tidal currents along the strait and the amplitude and phase of the surface elevation tide at the ends of a short strait. The velocity phase given by this relationship is shown to agree well with the observed tidal velocity phase in Cook Strait and one other strait.