Australia: The Land Where Time Began

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Global Tidal Impacts Resulting from Large-Scale Ice Sheet Collapses

It has been shown by recent studies that ice loss from the glaciers that drain the West Antarctic and Greenland Ice Sheets has accelerated, which highlights the possibility of large-scale ice sheet retreat and rising sea level over the comring centuries and millennia. Wilmes et al. suggest these sea level changes would vary spatially and could alter global tides significantly as the tides are highly dependent on bathymetry, which is the thickness of the water column beneath the ice shelves, and the shape of basins. In this paper Wilmes et al. investigated how the semidiurnal (M₂) tidal amplitudes and dissipation of energy respond to the sea level changes that are not uniform that would be induced by complete ice sheet collapses. Gravitationally self-consistent sea level theory was used to calculate these changes in sea level, and an established tidal model was used to simulate the tides. The results of the simulations show global and spatially heterogeneous changes in tidal amplitudes. Also, pronounced changes in the dissipation of tidal energy occur in the open ocean and well as the shelf seas, and also altering the location of tidal mixing fronts. These changes could potentially impact ocean mixing, and therefore large-scale currents and climate patterns, and the contribution of the shelf-sea to the global carbon cycle. The importance of considering changes in tides when predicting future climate and reconstructions of past climate phases, such as the Last Interglacial, is highlighted by these new results.

Tides have an important role in the global Earth system. They provide energy for abyssal mixing through tidal conversion, which is important for the meridional overturning circulation (MOC; e.g. Ledwell et al., 2000; Munk & Wunsch, 1998). Vertical mixing balances the formation of deep water and influences the strength of the MOC, and therefore supports the key pathway for the redistribution of heat, momentum, and freshwater across the globe (e.g. Green et al., 2009; Wunsch & Ferrari, 2004). The high primary production rates in the temperate and polar shelf seas are sustained by tides by determining the location of tidal mixing fronts which separate waters that are stratified seasonally, depleted of nutrients, and fully mixed waters rich in nutrients (Pingree et al., 1978; Simpson & Pingree, 1978). The balance between solar heating and mixing by tidal currents, and wind, control the location of the tidal mixing fronts (Simpson & Hunter, 1974), which means that the location of the fronts can be modified by changes in the tides. Vertical nutrient flux at the shelf break and around sea mounts (Sharples et al., 2007), which controls primary production, which is sustained by tidal conversion, and tidal mixing fronts and conversion areas are therefore valuable fishing grounds because nutrients are resupplied there (Sharples et al., 2007). A dynamic component of the global carbon budget is represented by the continental shelf-seas. In areas of shelf-seas that are stratified seasonally, primary production and respiration are depth separated by the thermocline. The development of a continental shelf pump, which exports carbon from the atmosphere to the deep ocean by horizontal advection or vertical mixing, is facilitated by the separation of production and respiration. Areas that are stratified seasonally therefore act as sinks of CO₂ while areas that are fully mixed are supersaturated with respect to CO₂ and are weak sources of CO₂ outgassing. This process is what is known of as the continental shelf pump (Rippeth et al., 2008; Thomas et al., 2004; Tsunogai et al., 1999).

Tides are affected strongly by changes in the depth of the water (sea level) that can change the speed of propagation of the tidal wave and change the resonant properties of the basin, as tides propagate as shallow-water waves. Over the history of the Earth sea level and the aerial extent of shelf seas have changed greatly, with associated changes in the tides (see e.g. Green & Huber, 2013; for the Eocene, ⁓55 Ma, and Green et al., 2017 for tidal changes over the 250 My). Investigations of the impact of se level changes on tides have focused mostly on the Last Glacial Maximum (LGM), ⁓25,000 ago; (e.g. Egbert et al., 2004; Green, 2010; Griffiths & Peltier, 2009; Wilmes & Green, 2014) or on regional responses to rising sea level in the future (Carless et al., 2016; Clara et al., 2015; Pelling & Green, 2013; Pickering et al., 2012; Ward et al., 2012). Attempts have been made to model secular changes that were observed in the global tides during the 20^th century and the early 21^st century (Müller et al., 2011) Pickering et al., 2017) and to simulate responses of sea level increases that are expected to occur in the next centuries. In this paper Wilmes et al. examined the global impacts of sea level changes that are induced by large-scale collapses of ice sheets on the tides and the associated tidally driven processes.

According to Wilmes et al. the West Antarctic Ice Sheet (WAIS) and parts of the Greenland Ice Sheet (GIS) may have collapsed during the Last Interglacial (LIG) ⁓125 ka (125,000 years ago) (e.g. Kopp et al., 2009; Raymo & Mitrovica, 2012) which led to a highstand of the sea level of 6.6-9.4 m relative to the sea levels of the present (Kopp et al., 2009).  It was highlighted recently that partial ice sheet collapses of the WAIS and GIS could occur in the next centuries (e.g. DeConto & Pollard, 2016), and if certain emissions thresholds are exceeded the collapses are likely to occur in the coming millennium (Clark et al., 2016, and references therein). Widespread retreat of grounding lines in glaciers that drain the WAIS have been documented (Joughin et al., 2014; Rignot et al., 2014) and ice discharge rates have increased over the last decade. The result is mass balance of the ice sheet that is strongly negative (Velicogna et al., 2014). The WAIS is inherently unstable because it is predominantly a marine based ice sheet situated on a reverse slope (Clark & Lingle, 1977; Gomez et al., 2010; Joughin et al., 2014), these trends possibly being linked to the early phases of a marine ice sheet instability, which could possibly lead to a marine sectors collapse of the WAIS in the coming centuries (Joughin et al., 2014; Mouginot et al., 2014). When added to contributions from East Antarctic Ice Sheet the result could be an increase in sea level of more than 10 m by the year 2300 (DeConto & Pollard, 2016). The Greenland Ice Sheet has, similarly, experienced increased rates of loss of ice mass over the past decades which are associated with flow speeds that have increased regionally (Velicogna et al., 2014). It has been suggested that a full melting of the ice sheet could occur if certain warming thresholds are crossed (Clark et al., 2016; Robinson et al., 2012). Global heterogeneous sea level changes could result from a full ice sheet collapse, due to the loss of gravitational attraction of the ice sheet, changes in loading of the surface of the Earth, and perturbations in the rotation of the Earth (Clark et al., 2016; Clark & Lingle, 1977; Gomez et al., 2010; Mitrovica et al., 2009; and figs 1 & 2 of this paper). Sea level changes, that are glacially mediated, in the future climate as well as the Last Interglacial have yet to be investigated, in spite of studies that show large sea level rates of change that are spatially nonuniform that can have the opposite sign in some regions to that of the global average (Clark & Lingle, 1977; Gomez et al., 2010; Mitrovica et al., 2009, 2011).

This study was aimed at demonstrating how collapses of the West Antarctic Ice Sheet and the Greenland Ice Sheet would impact the tides, and how these impacts could propagate through to key processes and pathways in the global climate system. It was suggested (Clark et al., 2016) that if the emissions of greenhouse gas continue rising at rates similar to those of the present, it will commit the Earth to loss of ice sheets from large parts of the West Antarctic Ice Sheet and the Greenland Ice Sheet during the coming millennia and a full or partial collapses for the Greenland Ice Sheet has been highlighted by Kopp et al. (2009). Ice Sheets were deliberately collapsed in this study to provide a response to the most extreme scenario so the possible tidal changes for the future and for the Last Interglacial can be explored. According to Wilmes et al. all intermediate cases of melt are likely to evoke tidal changes lying between the dynamics of the present and these extremes. The changes in sea level are calculated with a sea level theory that is self-consistent that takes into account the elastic deformation of the Earth, changes in the rotation of the Earth, and shorelines that are migrating (see Gomez et al., 2010 for details).

Wilmes et al. investigated the impact of large-scale collapses of ice sheets on the tides and processes that are tidally driven. They concluded that these changes are applicable for changes possibly occurring in a world that is warming, and also for the Last Interglacial which is often considered to be an analogy of our climate system in the next few millennia (e.g., IPCC, 2013).

Simulations forced with spatially varying sea level projections that had been computed with a sea level model that included gravitational, Earth deformational and Earth rotational effects on sea levels and shorelines that were migrating were compared by Wilmes et al. to simulations that had been forced with the global average SLR associated with the ice loss event, and they found that there we large differences in amplitudes and dissipation of tides between the 2.

It was highlighted by the results that the differences in tides are particularly large in the vicinity of regions where ice is being lost, i.e., local to Greenland and West Antarctica. The drawdown of the ocean surface and the and uplift of the solid Earth in response to the unloading of ice leads to a fall in sea level that differs significantly from the global average value of sea level rise average that is associated with the loss of ice. Large differences in the bathymetry between the 2 scenarios are, therefore, seen in these areas. Intermediate cases between that of the present and a full collapse of an ice sheet will most likely result in tidal responses that are somewhere between the CTRL simulation and one of the extreme ice sheet collapse cases. The response of the tides, however, may not scale linearly with respect to the global mean sea level rise since:

1)                The geometry of the loss of the ice and associated geometry of sea level change during the collapse of the ice sheet, and

2)                The tidal responses are not linear with respect to the sea level change applied.

These intermediate cases will be considered in future research.

Most studies that looked at impacts on tides of future sea level changes, especially those focused regionally, assume the global sea level increase to be uniform and the open ocean tides that interact with the tidal dynamics of shelf-seas show no or little change with respect to the present. The results of Wilmes et al., as well as those of previous studies (see Arbic et al., 2009; Arbic & Garrett, 2010), indicate, however, that open ocean tidal changes can impact shelf sea tides and vice versa. Tidal changes in shelf-seas can still be influenced by far field tidal changes due to deviations from the global mean sea level change, even if the sea level forcing on the shelf corresponds to the increase in global mean sea level. Wilmes et al. suggest, therefore, that regional studies should apply adequate boundary forcing reflecting potential far-field changes in tidal dynamics.

Along the coastlines the largest amplitude changes, which have a heterogeneous nature, occur. In particular the margins of the Pacific, eastern and western, experiences large increases in the amplitudes of the tides while along the coastline of the Atlantic the changes in amplitude of the tides tend to be smaller or even decrease. The coastal morphology and intertidal ecosystems, such as salt marshes or mangrove swamps, are among the most diverse ecosystems at present, and are important zones of carbon sequestration (e.g. Saintilan et al., 2013). It is implied by the present results that there were considerable tidal amplitude changes in the past which could cause problems for the reconstruction of LIG as sea level index points often rely on the tidal amplitudes not varying in magnitude in the past (e.g. Scourse, 2013).

The structure of the local water column will be affected profoundly by the regional changes in the tidal energy dissipation level in the shelf seas. It is predicted that the extent of water that is seasonally stratified will increase in the Yellow Sea and East China Sea and the Patagonian Shelf (with the exception of No WAIS), though the area of the Barents Sea, which is where there is a fishery that is globally important, the areal extent is predicted to shrink. The location of the mixing fronts will experience large shifts for all scenarios. It is suggested by the results of this study that tidally driven changes in the oceanography of shelf seas could be large enough to impact significantly ecosystems and the carbon and nutrient cycling via the shelf-pump in these waters.

The dissipation rates in deep water seen for the central and northern Pacific may affect the dynamics of the ocean, climate patterns, and consequently biochemical cycles. The diapycnal mixing could be intensified by the enhancements in the Pacific and Southern Ocean which would therefore influence the overturing circulation in the Pacific Ocean and the Antarctic Circumpolar Current (Egbert & Ray, 2001; Munk & Wunsch, 1998). The transfer of heat and momentum across the globe would be affected by this, as has been hypothesised to have occurred during the LGM (Green et al., 2009; Schmittner et al., 2015) and Eocene (Green & Huber, 2013). There are predicted to be major dissipation changes for the Indonesian Seas which could potentially affect the Indonesian Throughflow current, which is a major transporter of heat and freshwater to the Indian Ocean (Sprintall et al., 2009), which has implications for ENSO and the Indian Ocean Dipole, and therefore variability of regional climate (e.g. Zhou et al., 2015).

Wilmes et al. assumed that the stratification of the global ocean remains unaffected by the addition of meltwater from the ice sheets in spite for the potential of it changing the rate of tidal conversion rates. It is suggested by a sensitivity simulation, which is not shown, that changing γ in equation (4, Wilmes et al., 2017) has effects that are relatively small on the response, and for simplicity the conversion coefficient between simulations is not changed.

It was suggested by Kopp et al (2009) that global mean sea level during the Last Interglacial, the Eemian, was about 8 m higher than at present with contributions from the West Antarctic Ice Sheet and the Green land Ice Sheet to the rise in sea level. The simulations by Wilmes et al. suggest that tides and tidal processes are very sensitive to losses of ice sheets from both the West Antarctic Ice Sheet and the Greenland Ice Sheet and future work should examine the tidal dynamics under realistic conditions of the Last Interglacial ice sheet extent and the land-ocean configuration, e.g., the work of Hay et al. (2014) should be taken into account, as they have shown the time period over which loss from the ice sheet occurred during the Last Interglacial affects the fingerprint of the ice loss.

It was concluded by Wilmes et al. that sea level changes in the past and future have the potential to alter sea level variability (via the tides) as well as lead to important feedbacks in the climate system which could be superimposed on the variations that that were discussed previously (Clark et al., 2016). It is therefore suggested by Wilmes et al. that parameterisations of tidal effects in climate models need to include, and represent accurately, the impacts of the changes of sea level on the tides. It is also emphasised by the results obtained by Wilmes et al. that there is a need for high-resolution regional tide studies which address local impacts of changes in sea level on tides, better descriptions of the mechanisms that are behind these changes, as well as feedbacks with different components of the climate system. Also, Wilmes et al. suggest such simulations should use global simulations as boundary forcing, as the arguments that were used previously of limited (?feed)back effects in the deep ocean may not hold.

Sources & Further reading

1. Wilmes, S.-B., et al. (2017). "Global Tidal Impacts of Large-Scale Ice Sheet Collapses." Journal of Geophysical Research: Oceans 122(11): 8354-8370.