Observing the disintegration of the Thwaites B30 Iceberg with CryoSat-2 and Satellite Imagery

<p>Icebergs account for half of all ice loss from Antarctica and, once released, present a hazard to maritime operations. Their melting leads to a redistribution of cold fresh water around the Southern Ocean which, in turn, influences water circulation, promotes sea ice formation, and fosters primary production.</p><p>To quantify the total volume loss of icebergs both changes in area and in thickness have to be considered. In this study, we combine CryoSat-2 satellite altimetry with MODIS and Sentinel-1 satellite imagery to track changes in the area, freeboard, thickness, and volume of the B30 tabular iceberg between 2012 and 2018. Since it calved the iceberg’s area has decreased from 1500 +/- 60 to 426 +/- 27 km^2 , its mean freeboard has fallen from 49.0 +/- 4.6 to 38.8 +/- 2.2 m, and its mean thickness has reduced from 315 ± 36 to 198 ± 14 m. The combined loss amounts to an 80 +/- 16 % reduction in volume, two thirds (69 ± 14 %) of which is due to fragmentation and the remainder (31 ± 11 %) is due to basal melting.</p><p>The quantification of fresh water released from icebergs will help both the risk assessment of maritime operators and the improvement of ocean models by including a realistic – spatially and temporally variable - fresh water flux from iceberg melting in the Southern Ocean. Icebergs can also be used to study the reaction of glacial ice to warming environmental conditions, which they experience when they drift. These conditions might also become present at the ice shelf front in the future and therefore iceberg studies can inform the prediction of ice shelf response to warmer conditions.</p>

Systematic study of the impact of fresh water fluxes on the glacial carbon cycle

During glacial periods, atmospheric CO2 concentration increases and decreases by around 15 ppm. At the same time, the climate changes gradually in Antarctica. Such climate changes can be simulated in models when the AMOC (Atlantic Meridional Oceanic Circulation) is weakened by adding fresh water to the North Atlantic. The impact on the carbon cycle is less straightforward, and previous studies give opposite results. Because the models and the fresh water fluxes were different in these studies, it prevents any direct comparison and hinders finding whether the discrepancies arise from using different models or different fresh water fluxes. In this study we use the CLIMBER-2 coupled climate carbon model to explore the impact of different fresh water fluxes. In both preindustrial and glacial states, the addition of fresh water and the resulting slow-down of the AMOC lead to an uptake of carbon by the ocean and a release by the terrestrial biosphere. The duration, shape and amplitude of the fresh water flux all have an impact on the change of atmospheric CO2 because they modulate the change of the AMOC. The maximum CO2 change linearly depends on the time integral of the AMOC change. The different duration, amplitude, and shape of the fresh water flux cannot explain the opposite evolution of ocean and vegetation carbon inventory in different models. The different CO2 evolution thus depends on the AMOC response to the addition of fresh water and the resulting climatic change, which are both model dependent. In CLIMBER-2, the rise of CO2 recorded in ice cores during abrupt events can be simulated under glacial conditions, especially when the sinking of brines in the Southern Ocean is taken into account. The addition of fresh water in the Southern Hemisphere leads to a decline of CO2, contrary to the addition of fresh water in the Northern Hemisphere.

Systematic study of the fresh water fluxes impact on the carbon cycle

During glacial periods, atmospheric CO2 concentration rapidly increases and decreases by around 15 ppm at the same time as climate experiments an abrupt cooling in the North Hemisphere and warming in the South Hemisphere. Such a climate change can be triggered in models by adding fresh water fluxes (FWFs) in the North Atlantic. Yet the impact on the carbon cycle is less straightforward, and previous studies give opposite results. Because both models and added fresh water fluxes were different in these studies, it prevents any direct comparison and hinders finding an explanation for these discrepancies. In this study we use the CLIMBER-2 coupled climate carbon model to explore the impact of different additional fresh water fluxes in various conditions, including the experiments previously performed with other models. We show that the CO2 changes caused by the fresh water flux events should be interpreted as a combination of oceanic and terrestrial processes. The initial state of the Atlantic Meridional Overturning Circulation (AMOC) prior to the addition of fresh water fluxes appears to play a crucial role. The rapid increase of CO2 observed in ice core data can only be accounted for when the export of North Atlantic Deep Water (NADW) is relatively slow. Additionally, the terrestrial and oceanic carbon reservoirs responses are a consequence of the climate change and most importantly of the "seesaw" effect. As the latter is different in the various models it results in widely different evolution of the vegetation and oceanic carbon reservoirs. The discrepancies between the different studies can thus be explained by a combination of these factors: initial climatic and carbon cycle states, characteristics of the added fresh water flux, AMOC initial state and model "seesaw" pattern.

Symmetry breaking and overturning oscillations in thermohaline-driven flows

The bifurcation structure of thermohaline-driven flows is studied within one of the simplest zonally averaged models which captures thermohaline transport: a Boussinesq model of surface-forced thermohaline flow in a two-dimensional rectangular basin. Under mixed boundary conditions, i.e. prescribed surface temperature and fresh-water flux, it is shown that symmetry breaking originates from a codimension-two singularity which arises through the intersection of the paths of two symmetry-breaking pitchfork bifurcations. The physical mechanism of symmetry breaking of both the thermally and salinity dominated symmetric solution is described in detail from the perturbation structures near bifurcation. Limit cycles with an oscillation period in the order of the overturning time scale arise through Hopf bifurcations on the branches of asymmetric steady solutions. The physical mechanism of oscillation is described in terms of the most unstable mode just at the Hopf bifurcation. The occurrence of these oscillations is quite sensitive to the shape of the prescribed fresh-water flux. Symmetry breaking still occurs when, instead of a fixed temperature, a Newtonian cooling condition is prescribed at the surface. There is only quantitative sensitivity, i.e. the positions of the bifurcation points shift with the surface heat transfer coefficient. There are no qualitative changes in the bifurcation diagram except in the limit where both the surface heat flux and fresh-water flux are prescribed. The bifurcation structure at large aspect ratio is shown to converge to that obtained by asymptotic theory. The complete structure of symmetric and asymmetric multiple equilibria is shown to originate from a codimension-three bifurcation, which arises through the intersection of a cusp and the codimension-two singularity responsible for symmetry breaking.

A seasonal energy-balance climate model for coupling to ice-sheet models

An energy-balance climate model designed for coupling to ice-sheet models is presented. Its independent variables are longitude, latitude and time of the year. The model is based on the vertically integrated equations of conservation of energy and humidity. It can predict the vertically averaged temperature. Since it includes a hydrological cycle, it can also diagnose the net fresh-water flux and hence the annual snow budget at the atmosphere–ice-sheet interface. To this end, the model does not require observed precipitation rates. The computational cost is reduced by using an analytically computed Fourier–Legendre representation of daily insolation. For a highly idealized test-case configuration, two simple sensitivity experiments are carried out.

A seasonal energy-balance climate model for coupling to ice-sheet models

An energy-balance climate model designed for coupling to ice-sheet models is presented. Its independent variables are longitude, latitude and time of the year. The model is based on the vertically integrated equations of conservation of energy and humidity. It can predict the vertically averaged temperature. Since it includes a hydrological cycle, it can also diagnose the net fresh-water flux and hence the annual snow budget at the atmosphere–ice-sheet interface. To this end, the model does not require observed precipitation rates. The computational cost is reduced by using an analytically computed Fourier–Legendre representation of daily insolation. For a highly idealized test-case configuration, two simple sensitivity experiments are carried out.