J. A. Mattias Green

A Comparison of Tidal Conversion Parameterizations for Tidal Models

AbstractThe conversion of barotropic to baroclinic tidal energy in the global abyssal ocean is calculated using three different formulations. The calculations are done both “offline,” that is, using externally given tidal currents to estimate the energy conversion, and “online,” that is, by using the formulations to parameterize linear wave drag in a prognostic tidal model. All three schemes produce globally integrated offline dissipation rates beneath 500-m depth of ~0.6–0.8 TW for the M2 constituent, but the spatial structures vary significantly between the parameterizations. Detailed investigations of the energy transfer in local areas confirm the global results: there are large differences between the schemes, although the horizontally integrated conversion rates are similar. The online simulations are evaluated by comparing the sea surface elevation with data from the TOPEX/Poseidon database, and the error is then significantly lower when using the parameterization provided by Nycander than with the ...

Tides, sea-level rise and tidal power extraction on the European shelf

An established numerical tidal model has been used to investigate the impact of various sea-level rise (SLR) scenarios, as well as SLR in combination with large-scale tidal power plants on European shelf tidal dynamics. Even moderate and realistic levels of future SLR are shown to have significant impacts on the tidal dynamics of the area. These changes are further enhanced when SLR and tidal power plants are considered in combination, resulting in changes to tidal amplitudes, currents and associated tidal dissipation and bed shear stresses. Sea-level rise is the dominant influence on any far-field impacts, whereas tidal power plants are shown to have the prevailing influence over any changes close to the point of energy extraction. The spatial extent of the impacts of energy extraction is shown to be affected by the sea level when more than one tidal power plant in the Irish Sea was considered. Different ways to implement SLR in the model are also discussed and shown to be of great significance for the response of the tides.

Deep draft icebergs from the Barents Ice Sheet during MIS 6 are consistent with erosional evidence from the Lomonosov Ridge, central Arctic

[1] Iceberg scour marks on the Lomonosov Ridge in the central Arctic imply that bergs with drafts exceeding 850 m once existed in the Arctic, whereas similar erosion patterns observed on the Yermak Plateau suggest that some of these deep-keeled icebergs exited the Arctic through the Fram Strait into the Nordic Seas. An intermediate complexity climate model with a dynamic and thermodynamic iceberg model is used to simulate the collapse of the Barents Ice Sheet during MIS 6 with the purpose of investigating if deep-draft �mega�-icebergs can reach and ground on the Lomonosov Ridge. A number of simulations with different iceberg sizes show that deep-draft bergs seeded from near the Franz Victoria Trough could indeed have reached the Ridge, whereas other bergs crossed the Yermak Plateau and exited through the Fram Strait. The draft of the bergs mainly determines the trajectory, with only minor influence from the mass, and length and width of the bergs. This work is consistent with previous speculation that these icebergs originated from the disintegration of the Barents Ice Sheet during Marine Isotope Stage 6 (MIS 6; 140 ka BP), but suggests their origin was more likely to be from the western, rather than eastern, major ice stream of that ice sheet. The long-term impact on the overturning circulation from melting icebergs is more severe than that from a freshwater pulse of equivalent magnitude, and a correct representation of icebergs in paleoclimate models is therefore of importance.

Global Tidal Impacts of Large‐Scale Ice Sheet Collapses

Recent studies show that the glaciers draining both the West Antarctic and the Greenland ice sheets are experiencing an accelerated ice loss, highlighting the possibility of large-scale ice-sheet retreat and sea-level rise in the coming centuries and millennia. These sea-level changes would vary spatially, and could significantly alter global tides as the latter are highly dependent on bathymetry (or water column thickness under ice shelves) and basin shape. This paper investigates how the principal semi-diurnal (M2) tidal amplitudes and energy dissipation respond to the non-uniform sea-level changes induced by complete ice-sheet collapses. The sea-level changes are calculated using gravitationally self-consistent sea-level theory, and the tides are simulated using an established tidal model. Results from the simulations show global and spatially heterogeneous changes in tidal amplitudes. In addition, pronounced changes in tidal energy dissipation occur in both the open ocean and in shelf seas, also altering the location of tidal mixing fronts. These changes have the potential to impact ocean mixing, and hence large-scale currents and climate patterns, and the contribution of shelf-sea to the global carbon cycle. The new results highlight the importance of considering changes in the tides in predictions of future climate and reconstructions of past climate phases such as the Last Interglacial.

Baroclinic energy flux at the continental shelf edge modified by wind‐mixing

Temperature and current measurements from two moorings onshore of the Celtic Sea shelf break, a well-known hot spot for tidal energy conversion, show the impact of passing summer storms on the baroclinic wavefield. Wind-driven vertical mixing changed stratification to permit an increased on-shelf energy transport, and baroclinic energy in the semidiurnal band appeared at the moorings 1–4 days after the storm mixed the upper 50 m of the water column. The timing of the maximum in the baroclinic energy flux is consistent with the propagation of the semidiurnal internal tide from generation sites at the shelf break to the moorings 40 km away. Also, the ∼3 day duration of the peak in M2 baroclinic energy flux at the moorings corresponds to the restratification time scale following the first storm.

Systematic Bias in Baroclinic Energy Estimates in Shelf Seas

AbstractA simple model of an internal wave advected by an oscillating barotropic flow suggests flaws in standard approaches to estimating properties of the internal tide. When the M2 barotropic tidal current amplitude is of similar size to the phase speed of the M2 baroclinic tide, spectral and harmonic analysis techniques lead to erroneous estimates of the amplitude, phase, and energy in the M2 internal tide. In general, harmonic fits and bandpass or low-pass filters that attempt to isolate the lowest M2 harmonic significantly underestimate the strength of M2 baroclinic energy fluxes in shelf seas. Baroclinic energy flux estimates may show artificial spatial variability, giving the illusion of sources and sinks of energy where none are actually present. Analysis of previously published estimates of baroclinic energy fluxes in the Celtic Sea suggests this mechanism may lead to values being 25%–60% too low.

Tidal Conversion and Mixing Poleward of the Critical Latitude (an Arctic Case Study)

The tides are a major source of the kinetic energy supporting turbulent mixing in the global oceans. The prime mechanism for the transfer of tidal energy to turbulent mixing results from the interaction between topography and stratified tidal flow, leading to the generation of freely propagating internal waves at the period of the forcing tide. However, poleward of the critical latitude (where the period of the principal tidal constituent exceeds the local inertial period), the action of the Coriolis force precludes the development of freely propagating linear internal tides. Here we focus on a region of sloping topography, poleward of the critical latitude, where there is significant conversion of tidal energy and the flow is supercritical (Froude number, Fr > 1). A high-resolution nonlinear modeling study demonstrates the key role of tidally generated lee waves and supercritical flow in the transfer of energy from the barotropic tide to internal waves in these high-latitude regions. Time series of flow and water column structure from the region of interest show internal waves with characteristics consistent with those predicted by the model, and concurrent microstructure dissipation measurements show significant levels of mixing associated with these internal waves. The results suggest that tidally generated lee waves are a key mechanism for the transfer of energy from the tide to turbulence poleward of the critical latitude. Plain Language Summary The decline in aerial extent of sea ice covering the Arctic Ocean in the recent years is perhaps one of the leading indications of climate change. Warm water enters the Arctic Ocean at depths of 100–200 m; however, it is isolated from melting the ice by the lack of mixing in the Arctic Ocean. This lack of mixing has been attributed to the ocean being isolated from the wind by ice, and the fact that much of the Arctic Ocean is north of the critical latitude, beyond which the type of internal tide that is believed to drive mixing across other major oceans on the planet cannot occur. However, new evidence has been found that suggests that the tide might be important in driving mixing in certain areas of the Arctic Ocean. Here we combine state-of-the-art numerical modeling with new turbulence measurements to identify the mechanism by which the tide can drive mixing at these high latitudes.

The Lifecycle of Semidiurnal Internal Tides over the Northern Mid-Atlantic Ridge

AbstractThe life cycle of semidiurnal internal tides over the Mid-Atlantic Ridge (MAR) sector south of the Azores is investigated using in situ, a high-resolution mooring and microstructure profiler, and satellite data, in combination with a theoretical model of barotropic-to-baroclinic tidal energy conversion. The mooring analysis reveals that the internal tide horizontal energy flux is dominated by mode 1 and that energy density is more distributed among modes 1–10. Most modes are compatible with an interpretation in terms of standing internal tides, suggesting that they result from interactions between waves generated over the MAR. Internal tide energy is thus concentrated above the ridge and is eventually available for local diapycnal mixing, as endorsed by the elevated rates of turbulent energy dissipation e estimated from microstructure measurements. A spring–neap modulation of energy density on the MAR is found to originate from the remote generation and radiation of strong mode-1 internal tides fr...

The impact of sea-level rise on tidal characteristics aroundAustralasia

An established tidal model, validated for presentday conditions, is used to investigate the effect of large levels of sea-level rise (SLR) on tidal characteristics around Australasia. SLR is implemented through a uniform depth increase across the model domain, with a comparison between the implementation of coastal defences or allowing low-lying land to flood. The complex spatial response of the semi-diurnal M2 constituent does not appear to be linear with the imposed SLR. The most predominant features of this response are the generation of new amphidromic systems within the Gulf of Carpentaria and large-amplitude changes in the Arafura Sea, to the north of Australia, and within embayments along Australia’s north-west coast. Dissipation from M2 notably decreases along north-west Australia but is enhanced around New Zealand and the island chains to the north. The diurnal constituent, K1, is found to decrease in amplitude in the Gulf of Carpentaria when flooding is allowed. Coastal flooding has a profound impact on the response of tidal amplitudes to SLR by creating local regions of increased tidal dissipation and altering the coastal topography. Our results also highlight the necessity for regional models to use correct open boundary conditions reflecting the global tidal changes in response to SLR.

Scale-dependent spatial patterns in benthic communities around a tropical island seascape

Understanding and predicting patterns of spatial organization across ecological communities is central to the field of landscape ecology, and a similar line of inquiry has begun to evolve sub-tidal ...