ALBERT

Description: 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.

Description: Double high or low tides are usually explained by adding a higher harmonic to the dominating tide. In its simplest form, the criterion for a double tide is that the amplitude ratio between the higher harmonic and the dominating constituent is larger than 1/n2 where n is the ratio of their periods. However, it is not always clear how the higher harmonic becomes large enough to generate the double tide. This is rectified here by identifying three possible ways to enhance the higher harmonic enough to produce a double tide. Using TPXO9, the latest version of the altimetry constrained global tide database, potential locations for all three classes are identified and the existence of double tides are then evaluated using historic long-term tide gauge data from nearby locations. Thirteen locations with double tides were identified this way across the classes, of which seven are discussed further and shown to fit the classification scheme. The search criterion for classes 1 and 2, based on the amplitudes of M2, S2, and M4, work well with TPXO9 and suggests over 400 locations with double tides. The main reason we cannot identify more double tide locations is a lack of TG data, especially in the polar areas. Class 3, which requires an embayment resonant for the higher harmonic initially provided over 8000 potential locations, but only a few of these were in embayments. This class thus requires more manual work to identify the locations. It is concluded that the mechanism behind double tides in most textbooks needs to be revised because they are far more frequent in both space and time than previously thought.

Description: Doodson proposed a minimum criterion to predict the occurrence of double high (or double low) waters when a higher-frequency tidal harmonic is added to the semi-diurnal tide. If the phasing of the harmonic is optimal, the condition for a double high water can be written bn2∕a 〉 1 where b is the amplitude of the higher harmonic, a is the amplitude of the semi-diurnal tide, and n is the ratio of their frequencies. Here we expand this criterion to allow for (i) a phase difference ϕ between the semi-diurnal tide and the harmonic and (ii) the fact that the double high water will disappear in the event that b∕a becomes large enough for the higher harmonic to be the dominant component of the tide. This can happen, for example, at places or times where the semi-diurnal tide is very small. The revised parameter is br2∕a, where r is a number generally less than n, although equal to n when ϕ = 0. The theory predicts that a double high tide will form when this parameter exceeds 1 and then disappear when it exceeds a value of order n2 and the higher harmonic becomes dominant. We test these predictions against observations at Port Ellen in the Inner Hebrides of Scotland. For most of the data set, the largest harmonic of the semi-diurnal tide is the sixth diurnal component, for which n = 3. The principal lunar and solar semi-diurnal tides are about equal at Port Ellen and so the semi-diurnal tide becomes very small twice a month at neap tides (here defined as the smallest fortnightly tidal range). A double high water forms when br2∕a first exceeds a minimum value of about 1.5 as neap tides are approached and then disappears as br2∕a then exceeds a second limiting value of about 10 at neap tides in agreement with the revised criterion.

Description: Doodson proposed a criterion to predict the occurrence of double high (or double low) waters when a higher frequency tidal harmonic is added to the semi-diurnal tide. If the phasing of the harmonic is optimal, the condition for a double high water can be written bn2/a 〉 1 where b is the amplitude of the higher harmonic, a is the amplitude of the semi-diurnal tide and n is the ratio of their frequencies. Here we expand this criterion to allow for (i) a phase difference φ between the semi-diurnal tide and the harmonic and (ii) the fact that the double high water will disappear in the event that b/a becomes large enough for the higher harmonic to be the dominant component of the tide. This can happen, for example, at places or times where the semi-diurnal tide is very small. The revised parameter is br2/a, where r is a number generally less than n, although equal to n when φ = 0. The theory predicts that a double high tide will form when this parameter exceeds 1 and then disappear when it exceeds a value of order n2 and the higher harmonic becomes dominant. We test these predictions against observations at Port Ellen in the Inner Hebrides of Scotland. For most of the data set, the largest harmonic of the semi-diurnal tide is the 6th diurnal component, for which n = 3.The principal lunar and solar semi-diurnal tides are about equal at Port Ellen and so the semi-diurnal tide becomes very small twice a month at neap tides. A double high water forms when br2/a first exceeds a minimum value of about 1.5 as neap tides are approached and then disappears as br2/a then exceeds a second limiting value of about 10 at neap tides in agreement with the revised criterion.

Description: An established tidal model, validated for present-day 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 coastal boundary being treated as impenetrable or allowing low-lying land to flood. The complex spatial response of the semi-diurnal constituents, M2 and S2, is broadly similar, with the magnitude of M2's response being greater. The most predominant features of this response are large amplitude changes in the Arafura Sea and within embayments along Australia's north-west coast, and the generation of new amphidromic systems within the Gulf of Carpentaria and south of Papua, once water depth across the domain is increased by 3 and 7m respectively. Dissipation from M2 increases around the islands in the north of the Sahul shelf region and around coastal features along north Australia, leading to a notable drop in dissipation along Eighty Mile Beach. The diurnal constituent, K1, is found to be amplified within the Gulf of Carpentaria, indicating a possible change of resonance properties of the gulf. Coastal flooding has a profound impact on the response of tidal amplitudes to SLR, particularly K1, by creating local regions of increased tidal dissipation and altering the shape of coastlines.

Description: The tidal forcing of ice streams at their ocean boundary can serve as a natural experiment to gain an insight into their dynamics and constrain the basal sliding law. A 3-D visco-elastic full Stokes model of coupled ice-stream ice-shelf flow is used to investigate the response of ice streams to ocean tides. In agreement with previous results based on flow-line modeling and with a fixed grounding line position, we find that a non-linear basal sliding law can reproduce long period modulation of tidal forcing found in field observations, and the inclusions of lateral effects and grounding line migration do not alter this result. Further analysis of modeled ice stream flow shows a varying stress-coupling length scale of boundary effects upstream of the grounding line. We derive a visco-elastic stress coupling length scale from ice stream equations that depends on the forcing period and closely agrees with model output.

Description: The tidal forcing of ice streams at their ocean boundary can serve as a natural experiment to gain an insight into their dynamics and constrain the basal sliding law. A nonlinear 3-D viscoelastic full Stokes model of coupled ice stream ice shelf flow is used to investigate the response of ice streams to ocean tides. In agreement with previous results based on flow-line modelling and with a fixed grounding line position, we find that a nonlinear basal sliding law can qualitatively reproduce long-period modulation of tidal forcing found in field observations. In addition, we show that the inclusion of lateral drag, or allowing the grounding line to migrate over the tidal cycle, does not affect these conclusions. Further analysis of modelled ice stream flow shows a varying stress-coupling length scale of boundary effects upstream of the grounding line. We derive a viscoelastic stress-coupling length scale from ice stream equations that depends on the forcing period and closely agrees with model output.

Description: Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77 day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modeling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidal subglacial water pressure variations. Furthermore, the subglacial water system must be highly conductive and at low effective pressure, and the relationship between sliding velocity and effective pressure highly nonlinear in order for the model results to match GPS measurements. Hydrological and basal sliding model parameters that produced a best fit to observations were a mean effective pressure N of 105 kPa, subglacial drainage system conductivity K of 7 × 109 m2d-1, with sliding law exponents m = 3 and q =10. Coupled model results show the presence of tides result in a ~ 12% increase in mean surface velocity. Observations of tidally-induced variations in flow of ice-streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

Description: Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77-day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modelling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼12 % increase in mean horizontal velocity of the adjoining ice stream. Observations of tidally induced variations in flow of ice streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

Description: The Earth is currently 180 Myr into a supercontinent cycle that began with the break-up of Pangaea and which will end around 200–250 Myr (million years) in the future, as the next supercontinent forms. As the continents move around the planet they change the geometry of ocean basins, and thereby modify their resonant properties. In doing so, oceans move through tidal resonance, causing the global tides to be profoundly affected. Here, we use a dedicated and established global tidal model to simulate the evolution of tides during four future supercontinent scenarios. We show that the number of tidal resonances on Earth varies between one and five in a supercontinent cycle and that they last for no longer than 20 Myr. They occur in opening basins after about 140–180 Myr, an age equivalent to the present-day Atlantic Ocean, which is near resonance for the dominating semi-diurnal tide. They also occur when an ocean basin is closing, highlighting that within its lifetime, a large ocean basin – its history described by the Wilson cycle – may go through two resonances: one when opening and one when closing. The results further support the existence of a super-tidal cycle associated with the supercontinent cycle and gives a deep-time proxy for global tidal energetics.