The sensitivity of the Greenland Ice Sheet to glacial-interglacial oceanic forcing

Observations suggest that during the last decades the Greenland Ice Sheet (GrIS) has experienced a gradually accelerating mass loss, in part due to the observed speed-up of several of Greenland's marine-terminating glaciers. Recent studies directly attribute this to warming North Atlantic temperatures, which have triggered melting of the outlet glaciers of the GrIS, grounding-line retreat and enhanced ice discharge into the ocean, contributing to an acceleration of sea-level rise. Reconstructions suggest that the influence of the ocean has been of primary importance in the past as well. This was the case not only in interglacial periods, when warmer climates led to a rapid retreat of the GrIS to land above sea level, but also in glacial periods, when the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes. However, the GrIS response to palaeo-oceanic variations has yet to be investigated in detail from a mechanistic modelling perspective. In this work, the evolution of the GrIS over the past two glacial cycles is studied using a three-dimensional hybrid ice-sheet-shelf model. We assess the effect of the variation of oceanic temperatures on the GrIS evolution on glacial-interglacial timescales through changes in submarine melting. The results show a very high sensitivity of the GrIS to changing oceanic conditions. Oceanic forcing is found to be a primary driver of GrIS expansion in glacial times and of retreat in interglacial periods. If switched off, palaeo-atmospheric variations alone are not able to yield a reliable glacial configuration of the GrIS. This work therefore suggests that considering the ocean as an active forcing should become standard practice in palaeo-ice-sheet modelling

Impact of millennial-scale oceanic variability on the Greenland ice-sheet evolution throughout the last glacial period

Temperature reconstructions from Greenland icesheet (GrIS) ice cores indicate the occurrence of more than 20 abrupt warmings during the last glacial period (LGP) known as Dansgaard-Oeschger (D-O) events. Although their ultimate cause is still debated, evidence from both proxy data and modelling studies robustly links these to reorganisations of the Atlantic Meridional Overturning Circulation (AMOC). During the LGP, the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes than in the present. Therefore oceanic temperature fluctuations on millennial timescales could have had a non-negligible impact on the GrIS. Here we assess the effect of millennial-scale oceanic variability on the GrIS evolution from the last interglacial to the present day. To do so, we use a three-dimensional hybrid ice-sheet–shelf model forced by subsurface oceanic temperature fluctuations, assumed to increase during D-O stadials and decrease during D-O interstadials. Since in our model the atmospheric forcing follows orbital variations only, the increase in total melting at millennial timescales is a direct result of an increase in basal melting. We show that the GrIS evolution during the LGP could have been strongly influenced by oceanic changes on millennial timescales, leading to oceanically induced icevolume contributions above 1 m sea level equivalent (SLE). Also, our results suggest that the increased flux of GrIS icebergs as inferred from North Atlantic proxy records could have been triggered, or intensified, by peaks in melting at the base of the ice shelves resulting from increasing subsurface oceanic temperatures during D-O stadials. Several regions across the GrIS could thus have been responsible for ice mass discharge during D-O events, opening the possibility of a non-negligible role of the GrIS in oceanic reorganisations throughout the LGP

Submarine melt as a potential trigger of the North East Greenland Ice Stream margin retreat during Marine Isotope Stage 3

The Northeast Greenland Ice Stream (NEGIS) has been suffering a significant ice mass loss during the last decades. This is partly due to increasing oceanic temperatures in the subpolar North Atlantic, which enhance submarine basal melting and mass discharge. This demonstrates the high sensitivity of this region to oceanic changes. In addition, a recent study suggested that the NEGIS grounding line was 20–40 km behind its present-day location for 15 ka during Marine Isotope Stage (MIS) 3. This is in contrast with Greenland temperature records indicating cold atmospheric conditions at that time, expected to favour ice-sheet expansion. To explain this anomalous retreat a combination of atmospheric and external forcings has been invoked. Yet, as the ocean is found to be a primary driver of the ongoing retreat of the NEGIS glaciers, the effect of past oceanic changes in their paleo evolution cannot be ruled out and should be explored in detail. Here we investigate the sensitivity of the NEGIS to the oceanic forcing during the last glacial period using a three-dimensional hybrid ice-sheet–shelf model. We find that a sufficiently high oceanic forcing could account for a NEGIS ice-margin retreat of several tens of kilometres, potentially explaining the recently proposed NEGIS groundingline retreat during Marine Isotope Stage 3

Comparing different methods to retrieve cloud top height from Meteosat satellite data

Cloud parameters such as the Cloud Top Height (CTH), Cloud Top Temperature (CTT), emissivity, particle size and optical depth have always been matter of interest for the atmospheric community. Particularly the CTH provides information leading to better understand the cloud radiative effects. Although there are many meteorological satellites providing the CTH, there are other sensors, not devoted to this purpose, that give some information from which this crucial parameter can be estimated. In this contribution we will describe three different methodologies to retrieve the CTH. The first technique is based on stereo-vision algorithms and requires two different views of the same scene and does not need of extra atmospheric information. In the second one, brightness temperatures in two IR spectral bands are converted to real cloud temperature by means of the proposed algorithms. From the CTT, the CTH is estimated using temperature vertical profiles (measured or modeled). The third technique retrieves the CTH from the output parameters of post event simulations performed by a Numerical Weather Prediction (NWP) model that in this work will be the mesoscale model WRF (Weather Research Forecast). This article presents a preliminary work, in which the heights retrieved by the three methodologies applied to the geostationary satellite Meteosat 10 are compared with the heights given by MODIS sensor installed on the polar satellite AQUA. This promising results show that valuable information about CTH can be retrieved from Meteosat which provide high frequency and large scale data useful for weather and climate research

Methods to Retrieve the Cloud-Top Height in the Frame of the JEM-EUSO Mission

The Japanese Experiment Module-Extreme Universe Space Observatory (JEM-EUSO) telescope will measure ultrahigh-energy cosmic ray properties by detecting the UV fluorescence light generated in the interaction between cosmic rays and the atmosphere. Therefore, information on the state of clouds in the atmosphere is crucial for a proper interpretation of the data. For a real-time observation of the clouds in the telescope field of view, the JEM-EUSO will use an atmospheric monitoring system composed of a light detection and ranging and an infrared (IR) camera. In this paper, the focus is on the IR camera data. To retrieve the cloud-top height (CTH) from IR images, three different methods are considered here. The first one is based on bispectral stereo vision algorithms and requires two different views of the same scene in different spectral bands. For the second one, brightness temperatures provided by the IR camera are converted to effective cloud-top temperatures, from which the CTH is estimated using the vertical temperature profiles. A third method that uses the primary numerical weather prediction model output parameters, such as the cloud fraction, has also been considered to retrieve the CTH. This paper presents a first analysis, in which the heights retrieved by these three methodologies are compared with the heights given by the Moderate Resolution Imaging Spectroradiometer sensor installed on the polar satellite Terra. Since all these methods are suitable for the JEM-EUSO mission, they could be used in the future in a complementary way to improve the accuracy of the CTH retrieval

El papel de la interacción hielo-océano en la evolución pasada del manto de hielo de Groenlandia

Tesis llevada a cabo para conseguir el grado de Doctor por la Universidad de Complutense de Madrid.--2019-01-10.--Sobresaliente[EN] Assessing the response of the Greenland Ice Sheet (GrIS) to climate variations is crucial to understand its past and constrain its future evolution under a changing climate. On one hand, the evidence of atmospheric warming observed since the last century has strongly demanded a major understanding of the sensitivity of the GrIS to atmospheric temperature variations, leading to extensive investigation of this topic in the past. On the other hand, only recent evidence connecting GrIS mass loss to warming North Atlantic waters has driven the attention to the potential role of a changing ocean on the GrIS evolution, an aspect that has been overlooked for a long time. Starting from this evidence, several studies have been dedicated to understanding the interactions between the current GrIS evolution and the surrounding ocean. However, although we are now aware that the ocean plays a non-negligible role in shaping the configuration of the present-day GrIS, many uncertainties remain about its role in the past...[ES] Comprender la respuesta del manto de hielo de Groenlandia (del ingles, GreenlandIce Sheet, GrIS) a las variaciones del clima es crucial para entender la evolucion pasada de dicho manto así como para constreñir su evolucion futura en un clima cambiante. Por un lado, la evidencia del calentamiento global observado desde el siglo pasado ha exigido una mejor comprension de la sensibilidad del GrIS frente a las variaciones de la temperatura del aire, lo que ha llevado a investigar ampliamente este asunto en el pasado. Por otra parte, recientemente las observaciones han permitido relacionar la perdida masiva de hielo del GrIS con el calentamiento del Atlantico Norte. Ello ha puesto el foco en el posible papel del oceano en la evolucion del GrIS, aspecto que se ha pasado por alto durante mucho tiempo. Desde entonces se ha hecho un esfuerzo importante por tratar de comprender mejor las interacciones entre el GrIS y el oceano en la actualidad. Sin embargo, aunque ahora somos conscientes de que el oceano desempeña un papel importante en la configuracion actual del GrIS, subsisten todavía incertidumbres importantes acerca de su papel en el pasado...This thesis was funded by the Spanish National Programme for the Promotion of Talent and Its Employability (grant no. BES-2015- 074097) of the Spanish Ministry of Science and Innovation under the project MOCCA (Modelling Abrupt Climate Change, grant no. CGL2014-59384-R). All model simulations analysed in this thesis were performed in EOLO, the High-Performance computing of Climate Change of the International Campus of Excellence of Moncloa, funded by MECD (Ministerio de Eucaci´on, Cultura y Deporte) and MICINN (Ministerio de Ciencias, Innovaci´on y Universidades). Special thanks go to Catherine Ritz for providing the original ice-sheet-shelf model GRISLI (GRenoble Ice Shelf and Land Ice model), subsequently extended by our research group (PalMA) and named as GRISLI-UCM model to be used in this thesis.Peer reviewe

The sensitivity of the Greenland Ice Sheet to glacial–interglacial oceanic forcing

Observations suggest that during the last decades the Greenland Ice Sheet (GrIS) has experienced a gradually accelerating mass loss, in part due to the observed speed-up of several of Greenland's marine-terminating glaciers. Recent studies directly attribute this to warming North Atlantic temperatures, which have triggered melting of the outlet glaciers of the GrIS, grounding-line retreat and enhanced ice discharge into the ocean, contributing to an acceleration of sea-level rise. Reconstructions suggest that the influence of the ocean has been of primary importance in the past as well. This was the case not only in interglacial periods, when warmer climates led to a rapid retreat of the GrIS to land above sea level, but also in glacial periods, when the GrIS expanded as far as the continental shelf break and was thus more directly exposed to oceanic changes. However, the GrIS response to palaeo-oceanic variations has yet to be investigated in detail from a mechanistic modelling perspective. In this work, the evolution of the GrIS over the past two glacial cycles is studied using a three-dimensional hybrid ice-sheet–shelf model. We assess the effect of the variation of oceanic temperatures on the GrIS evolution on glacial–interglacial timescales through changes in submarine melting. The results show a very high sensitivity of the GrIS to changing oceanic conditions. Oceanic forcing is found to be a primary driver of GrIS expansion in glacial times and of retreat in interglacial periods. If switched off, palaeo-atmospheric variations alone are not able to yield a reliable glacial configuration of the GrIS. This work therefore suggests that considering the ocean as an active forcing should become standard practice in palaeo-ice-sheet modelling.This work was funded by the Spanish Ministry of Science and Innovation under the project MOCCA (Modelling Abrupt Climate Change, grant no. CGL2014-59384-R). Ilaria Tabone is funded by the Spanish National Programme for the Promotion of Talent and Its Employability (grant no. BES-2015-074097). Alexander Robinson is funded by the Marie Curie Horizon2020 project CONCLIMA (grant no. 703251). All of these simulations were performed in EOLO, the HPC of Climate Change of the International Campus of Excellence of Moncloa, funded by MECD and MICINN.Peer reviewe

The Antarctic Ice Sheet response to glacial millennial-scale variability

The Antarctic Ice Sheet (AIS) is the largest ice sheet on Earth and hence a major potential contributor to future global sea-level rise. A wealth of studies suggest that increasing oceanic temperatures could cause a collapse of its marine-based western sector, the West Antarctic Ice Sheet, through the mechanism of marine ice-sheet instability, leading to a sea-level increase of 3–5 m. Thus, it is crucial to constrain the sensitivity of the AIS to rapid climate changes. The last glacial period is an ideal benchmark period for this purpose as it was punctuated by abrupt Dansgaard–Oeschger events at millennial timescales. Because their center of action was in the North Atlantic, where their climate impacts were largest, modeling studies have mainly focused on the millennial-scale evolution of Northern Hemisphere (NH) paleo ice sheets. Sea-level reconstructions attribute the origin of millennial-scale sea-level variations mainly to NH paleo ice sheets, with a minor but not negligible role of the AIS. Here we investigate the AIS response to millennial-scale climate variability for the first time. To this end we use a three-dimensional, thermomechanical hybrid, ice sheet–shelf model. Different oceanic sensitivities are tested and the sea-level equivalent (SLE) contributions computed. We find that whereas atmospheric variability has no appreciable effect on the AIS, changes in submarine melting rates can have a strong impact on it. We show that in contrast to the widespread assumption that the AIS is a slow reactive and static ice sheet that responds at orbital timescales only, it can lead to ice discharges of around 6 m SLE, involving substantial grounding line migrations at millennial timescales.This work was funded by the Spanish Ministry of Science and Innovation under the project MOCCA (Modelling Abrupt Climate Change, grant no. CGL2014-59384-R). Ilaria Tabone is funded by the Spanish National Programme for the Promotion of Talent and its Employability (grant no. BES-2015- 074097). Alexander Robinson is funded by the Ramón y Cajal Programme of the Spanish Ministry for Science, Innovation and Universities. All of these simulations were performed in EOLO, the HPC of Climate Change of the International Campus of Excellence of Moncloa, funded by MECD and MICINN.Peer reviewe

Criosfera: el hielo polar y su papel en el clima terrestre

21 pages, 8 figuresPeer reviewe