Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth's climate. In the deep ocean, tides supply much of ...the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate-the relative importance of their local versus remote breaking into turbulence-remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.
Abstract
The 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 other two parameterizations examined.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Weak tides during Cryogenian glaciations Green, J A Mattias; Davies, Hannah S; Duarte, Joao C ...
Nature communications,
12/2020, Volume:
11, Issue:
1
Journal Article
Peer reviewed
Open access
The severe "Snowball Earth" glaciations proposed to have existed during the Cryogenian period (720 to 635 million years ago) coincided with the breakup of one supercontinent and assembly of another. ...Whereas the presence of extensive continental ice sheets predicts a tidally energetic Snowball ocean due to the reduced ocean depth, the supercontinent palaeogeography predicts weak tides because the surrounding ocean is too large to host tidal resonances. Here we show, using an established numerical global tidal model and paleogeographic reconstructions, that the Cryogenian ocean hosted diminished tidal amplitudes and associated energy dissipation rates, reaching 10-50% of today's rates, during the Snowball glaciations. We argue that the near-absence of Cryogenian tidal processes may have been one contributor to the prolonged glaciations if these were near-global. These results also constrain lunar distance and orbital evolution throughout the Cryogenian, and highlight that simulations of past oceans should include explicit tidally driven mixing processes.
Distributional shifts in species ranges provide critical evidence of ecological responses to climate change. Assessments of climate‐driven changes typically focus on broad‐scale range shifts (e.g. ...poleward or upward), with ecological consequences at regional and local scales commonly overlooked. While these changes are informative for species presenting continuous geographic ranges, many species have discontinuous distributions—both natural (e.g. mountain or coastal species) or human‐induced (e.g. species inhabiting fragmented landscapes)—where within‐range changes can be significant. Here, we use an ecosystem engineer species (Sabellaria alveolata) with a naturally fragmented distribution as a case study to assess climate‐driven changes in within‐range occupancy across its entire global distribution. To this end, we applied landscape ecology metrics to outputs from species distribution modelling (SDM) in a novel unified framework. SDM predicted a 27.5% overall increase in the area of potentially suitable habitat under RCP 4.5 by 2050, which taken in isolation would have led to the classification of the species as a climate change winner. SDM further revealed that the latitudinal range is predicted to shrink because of decreased habitat suitability in the equatorward part of the range, not compensated by a poleward expansion. The use of landscape ecology metrics provided additional insights by identifying regions that are predicted to become increasingly fragmented in the future, potentially increasing extirpation risk by jeopardising metapopulation dynamics. This increased range fragmentation could have dramatic consequences for ecosystem structure and functioning. Importantly, the proposed framework—which brings together SDM and landscape metrics—can be widely used to study currently overlooked climate‐driven changes in species internal range structure, without requiring detailed empirical knowledge of the modelled species. This approach represents an important advancement beyond predictive envelope approaches and could reveal itself as paramount for managers whose spatial scale of action usually ranges from local to regional.
Many species have discontinuous ranges, both natural and human‐induced. Projected responses to climate‐driven range changes have so far overlooked changes in species internal range structure, providing only partial information on potential climate change effects. To overcome this limitation, we propose to combine landscape ecology metrics–traditionally used to describe landscape configuration–with predicted spatial distribution model outputs (i.e. presence–absence). We show that this novel unified framework provides additional quantitative information on species ranges that can be used for answering multiple scale‐dependent ecological questions.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
In this study, we employed a 3-D and two-way nested Regional Ocean Modeling System (ROMS) to address several important outstanding issues regarding tidal energy development in the Gulf of Maine. We ...investigated the impact of projected sea-level rise (SLR) on the energy resources of the region, and examined how tidal dynamics will be influenced by energy extraction and/or SLR. Further, we assessed whether the effect of SLR on the generation of tides in the ocean (hence at the boundary of the region) is significant in these assessments. We find that the impact of SLR exceeds the impact due to energy extraction in the Gulf of Maine - even when considering very large energy extraction, of order 3.0 GW, in the Minas Passage. Although results showed that energy extraction does not significantly increase the amplitude of the tides in the far-field, a drastic change in the Bay of Fundy (e.g. full blockage) can lead to considerably higher amplitudes of tides (around 35 cm, or 12%) in the western Gulf of Maine. As a result of 1 m SLR, the theoretical tidal energy resources in some areas, including the Bay of Fundy can increase noticeably while any significant change in extracted energy highly depends on the turbine technology.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Abstract
A 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 M
2
barotropic ...tidal current amplitude is of similar size to the phase speed of the M
2
baroclinic tide, spectral and harmonic analysis techniques lead to erroneous estimates of the amplitude, phase, and energy in the M
2
internal tide. In general, harmonic fits and bandpass or low-pass filters that attempt to isolate the lowest M
2
harmonic significantly underestimate the strength of M
2
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.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Measurements of the intra-tidal and spring-neap variation in the vertical flux of nitrate into the base of the sub-surface chlorophyll maximum (SCM) were made at the shelf edge of the Celtic Sea, a ...region with strong internal mixing driven by an internal tide. The neap tide daily mean nitrate flux was 1.3 (0.9-1.8, 95% confidence interval) $mmol m^{-2} d^{-1}$. The spring tide flux was initially estimated as 3.5 (2.3-5.2, 95% confidence interval) $mmol m^{-2} d^{-1}$. The higher spring tide nitrate flux was the result of turbulent dissipation occurring within the base of the SCM as compared to deeper dissipation during neap tides and was dominated by short events associated with the passage of internal solitons. Taking into account the likely under-sampling of these short mixing events raised the spring tide nitrate flux estimate to about $9 mmol m^{-2} d^{-1}$. The neap tide nitrate flux was sufficient to support substantial new production and a considerable fraction of the observed rates of carbon fixation. Spring tide fluxes were potentially in excess of the capacity of the phytoplankton community to uptake nitrate. This potential excess nitrate flux during spring tides may be utilized to support new production during the lower mixing associated with the transition toward neap tide. The shelf edge is shown to be a region with a significantly different phytoplankton community as compared to the adjacent Celtic Sea and northeast Atlantic Ocean, highlighting the role of gradients in physical processes leading to gradients in ecosystem structure.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NMLJ, NUK, OILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
During past geological times, the Earth experienced several intervals of
global warmth, but their driving factors remain equivocal. A careful
appraisal of the main processes controlling past warm ...events is essential to
inform future climates and ultimately provide decision makers with a clear
understanding of the processes at play in a warmer world. In this context,
intervals of greenhouse climates, such as the thermal maximum of the
Cenomanian–Turonian (∼94 Ma) during the Cretaceous Period,
are of particular interest. Here we use the IPSL-CM5A2 (IPSL: Institut Pierre et Simon Laplace) Earth system model to
unravel the forcing parameters of the Cenomanian–Turonian greenhouse
climate. We perform six simulations with an incremental change in five major
boundary conditions in order to isolate their respective role on climate
change between the Cenomanian–Turonian and the preindustrial. Starting with
a preindustrial simulation, we implement the following changes in boundary
conditions: (1) the absence of polar ice sheets, (2) the increase in
atmospheric pCO2 to 1120 ppm, (3) the change in vegetation and soil parameters, (4) the 1 % decrease in the Cenomanian–Turonian value of the
solar constant and (5) the Cenomanian–Turonian palaeogeography. Between the
preindustrial simulation and the Cretaceous simulation, the model simulates
a global warming of more than 11 ∘C. Most of this warming is
driven by the increase in atmospheric pCO2 to 1120 ppm.
Palaeogeographic changes represent the second major contributor to global
warming, whereas the reduction in the solar constant counteracts most of
geographically driven warming. We further demonstrate that the
implementation of Cenomanian–Turonian boundary conditions flattens
meridional temperature gradients compared to the preindustrial simulation.
Interestingly, we show that palaeogeography is the major driver of the
flattening in the low latitudes to midlatitudes, whereas pCO2 rise and polar ice sheet retreat dominate the high-latitude response.
The tidal response of the European Shelf to moderate (<1m) levels of sea level rise is investigated using a high resolution, well established tidal model. The model is validated for present day ...conditions and the tidal response to sea level rise by comparing the modelled response to long term tide gauge data. The effects of coastal defence schemes are tested, with three levels of present day coastal defences simulated. Full walls are added at the present day coastline, no coast defence schemes are used and a set of present day coastal defence schemes is simulated. The simulations show that there is a significant tidal response to moderate levels of SLR and that the response is strongly dependant on level of coastal defence simulated. The simulation using coastal defence data resulted in the strongest response as the tide was able to build up behind the coastal defence walls and create a patchwork of sea and land at the coastline. This had a strong impact on the spatial tidal energy dissipation field and in turn this has large effects on the tidal regime throughout the domain.
•The tides of the European Shelf are sensitive to moderate (<1m) levels of sea level rise.•The model is able to capture the observed tidal response to sea level rise.•Simulations with flood defences show the strongest tidal response.•We suggest that the tidal response to sea level change is controlled by the spatial distribution of tidal energy dissipation.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
PDF
