J. A. M. Green

Glacial ocean overturning intensified by tidal mixing in a global circulation model

Due to lower sea levels during the Last Glacial Maximum (LGM), tidal energy dissipation was shifted from the shallow margins into the deep ocean. Here using a high-resolution tide model, we estimate that global energy fluxes below 200 m depth were almost quadrupled during the LGM. Applying the energy fluxes to a consistent tidal mixing parameterization of a global climate model results in a large intensification of mixing. Global mean vertical diffusivity increases by more than a factor of 3, and consequently, the simulated meridional overturning circulation accelerates by ~21–46%. In the model, these effects are at least as important as those from changes in surface boundary conditions. Our findings contrast with the prevailing view that the abyssal LGM circulation was more sluggish. We conclude that changes in tidal mixing are an important mechanism that may have strongly increased the glacial deep ocean circulation and should no longer be neglected in paleoclimate simulations.

Boils and Turbulence in a Weakly Stratified Shallow Tidal Sea

Abstract Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually...

The evolution of tides and tidal dissipation over the past 21,000 years

The 120 m sea-level drop during the Last Glacial Maximum (LGM; 18�22 kyr BP) had a profound impact on the global tides and lead to an increased tidal dissipation rate, especially in the North Atlantic. Here, we present new simulations of the evolution of the global tides from the LGM to present for the dominating diurnal and semidiurnal constituents. The simulations are undertaken in time slices spanning 500�1000 years. Due to uncertainties in the location of the grounding line of the Antarctic ice sheets during the last glacial, simulations are carried out for two different grounding line scenarios. Our results replicate previously reported enhancements in dissipation and amplitudes of the semidiurnal tide during LGM and subsequent deglaciation, and they provide a detailed picture of the large global changes in M2 tidal dynamics occurring over the deglaciation period. We show that Antarctic ice dynamics and the associated grounding line location have a large influence on global semidiurnal tides, whereas the diurnal tides mainly experience regional changes and are not impacted by grounding line shifts in Antarctica.

Storms modify baroclinic energy fluxes in a seasonally stratified shelf sea: Inertial‐tidal interaction

Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind-generated near-inertial oscillations and the semidiurnal internal tide. Linear, baroclinic energy fluxes within the near-inertial (f) and semidiurnal (M2) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the 2 week deployment period, the semidiurnal tide drove 28–48 W m−1 of energy directly on-shelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25–43% increase in positive on-shelf energy flux. The phase relationship between f and M2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2 day counter-clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations.

Climatic Consequences of a Pine Island Glacier Collapse

AbstractAn intermediate-complexity climate model is used to simulate the impact of an accelerated Pine Island Glacier mass loss on the large-scale ocean circulation and climate. Simulations are performed for preindustrial conditions using hosing levels consistent with present-day observations of 3000 m3 s−1, at an accelerated rate of 6000 m3 s−1, and at a total collapse rate of 100 000 m3 s−1, and in all experiments the hosing lasted 100 years. It is shown that even a modest input of meltwater from the glacier can introduce an initial cooling over the upper part of the Southern Ocean due to increased stratification and ice cover, leading to a reduced upward heat flux from Circumpolar Deep Water. This causes global ocean heat content to increase and global surface air temperatures to decrease. The Atlantic meridional overturning circulation (AMOC) increases, presumably owing to changes in the density difference between Antarctic Intermediate Water and North Atlantic Deep Water. Simulations with a simultane...

Is There a Tectonically Driven Supertidal Cycle

15 Earth is 180 Myr into the current Supercontinent cycle and the next Supercontinent is pre16 dicted to form in 250 Myr. The continuous changes in continental configuration can move 17 the ocean between resonant states, and the semi-diurnal tides are currently large compared 18 to the past 252 Myr due to tidal resonance in the Atlantic. This leads to the hypothesis 19 that there is a “super-tidal” cycle linked to the Supercontinent cycle. Here, this is tested 20 using new tectonic predictions for the next 250 Myr as bathymetry in a numerical tidal 21 model. The simulations support the hypothesis: a new tidal resonance will appear 150 22 Myr from now, followed by a decreasing tide as the supercontinent forms 100 Myr later. 23 This affects the dissipation of tidal energy in the oceans, with consequences for the evo24 lution of the Earth-Moon system, ocean circulation and climate, and implications for the 25 ocean’s capacity of hosting and evolving life. 26