The Eocene–Oligocene transition (EOT) is marked by a sudden δ18O excursion occurring in two distinct phases approximately 500 kyr apart. These phases signal a shift from the warm middle to late Eocene greenhouse climate to cooler conditions, with global surface air temperatures decreasing by 3–5 °C and the emergence of the first continent-wide Antarctic ice sheet (AIS). While ice sheet modelling suggests that ice sheet growth can be triggered by declining pCO2, it remains unclear how this transition was initiated, particularly the first growth phase that appears to be related to oceanic and atmospheric cooling rather than ice sheet growth. Recent climate model simulations of the late Eocene show improved accuracy but depict climatic conditions that are not conducive to the survival of incipient ice sheets throughout the summer season. This study therefore examines whether it is plausible to develop ice sheets of sufficient scale to trigger the feedback mechanisms required to disrupt the atmospheric regime above the Antarctic continent during warm and moist late Eocene summers and establish more favourable conditions for ice expansion. We aim to assess the sustainability of an incipient AIS under varying radiative, orbital and cryospheric forcing. To do so, we evaluate Community Earth System Model 1.0.5 simulations, using a 38 Ma geographical and topographical reconstruction, considering different radiative and orbital forcings. The climatic conditions prevailing during (and leading up to) the EOT can be characterised as extremely seasonal and monsoon-like, featuring a short yet intense summer period and contrasting cold winters. A narrow convergence zone with moist convection around the region with high sub-cloud equivalent potential temperature exhibits a ring-like structure, advecting moist surface air from the Southern Ocean in both summer and winter. This advection leads to high values of moist static energy and subsequent precipitation in coastal regions. Paradoxically, this atmospheric regime – particularly its coastal precipitation in winter – appears to be necessary for the sustenance of the moderately sized regional ice sheets we imposed on the continent, contrary to our assumption that these ice sheets would disrupt the atmospheric regime. This underscores a hysteresis effect for regional ice sheets on the Antarctic continent, suggesting the potential for a significant volume of ice on the continent without imminent full glaciation prior to the EOT. ... Read more

The Atlantic Meridional Overturning Circulation (AMOC) is vitally important for regulating global climate through the redistribution of heat, salt, carbon and other tracers across latitudes, yet the precise role of its governing physical processes in the subpolar North Atlantic (SPNA) remains poorly understood. This knowledge gap is significant to address, given the AMOC’s sensitivity to anthropogenic climate change and its potential for dramatic weakening or collapse, with profound global implications. Here, we adopt a three-dimensional dynamical perspective, focusing on lateral exchange between the boundary current (BC) and basin interiors, ridge exchange over sills bordering the north of the SPNA, and shallow convection within the BC itself. We do so by analysing water mass transformation in density space, using segmented volume transport budgets in the eddy-resolving global ocean reanalysis GLORYS12 (1/12°). Our findings reveal that: (1) during the BC’s circumvention around the subpolar gyre alongstream intensification and densification takes place, and overflow waters from the Iceland–Scotland Ridge and Denmark Strait are added to the system; (2) vertical recirculation cells due to lateral exchange are not immediately evident; (3) residual water mass transformation can be partly explained by shallow convection driven by surface buoyancy fluxes; and (4) substantial spatial variability in local overturning contributions exists. These insights highlight the importance of further quantification of the relative contribution of each governing process to water mass transformation and subsequent overturning in the SPNA. ... Read more

The Eocene-Oligocene Transition is marked by a sudden δ18O excursion occurring in two distinct phases: a precursor event at 34.15±0.04 Ma and the Earliest Oligocene oxygen Isotope Step at 33.65±0.04 Ma. These events signal a shift from the warm Late-Eocene greenhouse climate to cooler conditions, with temperature decreases of 3-5 °C, and the emergence of the first continent-wide Antarctic Ice Sheet (AIS). Despite clear evidence from proxy data, general circulation models (GCMs) struggle to replicate this Antarctic transition accurately, failing to capture the shift from warm, ice-free to cold, glaciated conditions. Even with unrealistically low pCO2 levels, Late-Eocene Antarctic summers in GCMs remain too warm and moist for snow or ice to survive. This study evaluates CESM1.0.5 simulations conducted by Baatsen et al. (2020), using a 38 Ma geo- and topographical reconstruction, considering different radiative (4 pre-industrial carbon levels (PIC) and 2 PIC) and orbital (present-day insolation and low Antarctic summer insolation) forcings. The climate is found to be highly seasonal, characterised by hot and wet summers and cold and dry winters. While reduced radiative and summer insolation forcing weaken this seasonality, the persistent atmospheric circulation still impedes ice sheet growth by limiting summer snow survival. For that reason, a new simulation is conducted with regional, moderately-sized ice sheets imposed on the continent, in order to investigate their stability and their influence on the atmospheric circulation. These ice sheets demonstrate self-sustaining and even expansion potential under 2 PIC and low summer insolation conditions. However, correlating resulting temperature and precipitation patterns with proxy data proves challenging, given the absence of terrestrial proxies. Extended simulations with coupled GCM-ISM models are therefore recommended, allowing for more dynamic atmosphere-ice-ocean-vegetation feedback mechanisms and dynamic radiative and orbital forcing. ... Read more

The Atlantic Meridional Overturning Circulation (AMOC) transports vast amounts of heat to high latitudes, and is largely responsible for Western Europe’s relatively mild climate. Climate models project the AMOC will weaken substantially over the 21st century, which impacts weather, climate, sea level and the oceanic carbon cycle. In many studies, the AMOC state is described in a condensed two-dimensional view or even by means of a single metric, which leaves many aspects of its complex 3D-structure underexposed. By revealing the sharp contrast in overturning strength between the western and eastern subpolar gyre (SPG), the recent OSNAP observations emphasized the importance of considering the AMOC in 3D.

In this study, we explore this further by analyzing the characteristics of the overturning in density space in the North Atlantic SPG on a regional scale, and over time periods ranging from seasons to decades. For this, we use model data from the high-resolution GLORYS12 reanalysis, spanning the period 1993-2020. Following the approach applied in OSNAP, the overturning is assessed from alongstream changes in boundary current transport in specific density classes. This analysis is performed for the entire SPG, for its major basins (Iceland Basin, Irminger Sea, and Labrador Sea) and for smaller segments along the boundary currents, thus providing detailed insights in variations of the overturning varies along the entire SPG boundary.

The mean overturning from GLORYS12 for 1993-2020 is 23.8 Sv, distributed as 41%, 29%, and 30% for the Iceland Basin, Irminger Sea, and Labrador Sea respectively, and peaking at increasingly higher densities in alongstream direction. Within each basin, a pronounced seasonal cycle can be identified, with the maximum overturning occurring in March and the minimum in September. Over the entire reanalysis period, the overturning strength in both the Iceland Basin and Irminger Sea exhibits a weak decreasing trend, whereas the Labrador Sea displays a weak increasing trend.

The subdivision in shorter segments reveals large spatial differences in overturning, both with regard to its overall strength and its distribution over density classes. However, these outcomes are less robust than the analyses on the scale of the major basins, as the flow is highly variable and numerical uncertainties associated with offline overturning calculations become more prominent.

Further research is needed to properly interpret these regional variations, and thereby improve our understanding of the AMOC dynamics and its sensitivity to changing oceanic and atmospheric forcing conditions. Linking them to local processes known to govern the overturning (i.e., formation of dense waters in the interior of marginal seas and their export, formation of dense waters within the boundary current system itself and the exchange of waters via overflows) seems a viable route. ... Read more

This study explores the relation between writing quality and contextualization and elaboration moves, two kinds of textual expansion devices crucial for building common ground between writers and readers. We ask whether elaboration and contextualization features differentially predict writing quality and whether their quality contributions differ between genres. We also ask to what extent elaboration and contextualization are tied to individual writers, and can be explained by writer characteristics. To examine these issues, we annotated descriptive and argumentative texts of Dutch adolescents. Text quality was rated holistically, using benchmark scales.

As regards elaboration, depth affects quality more than breadth does. It also contributes across genres, whereas breadth only contributes in argumentations. Depth shows a large individual consistency across tasks, which is substantially related to students’ school type, grade and gender. Breadth shows weaker links to individual writers and their characteristics.

With regard to contextualization, opening and closing moves play a modest role in text quality. Initial support moves contribute to quality across tasks; concluding moves contribute more in argumentations. Concluding moves are most consistent within writers; however, for all contextualization moves, the writer variance is substantially explained by writer characteristics. This study opens up new avenues for explicating writing quality and writing skill. ... Read more

Manually annotated corpora of writing products may greatly contribute to writing research: they offer detailed insights in the quality of these texts, in the text features actually attended to by human text raters, in possibilities and difficulties for the use of automatic writing analytics and writing tools, and in the relations between different text quality dimensions. This paper presents the Utrecht System for Annotation of Learner text (USALT), that covers both general features (orthography, punctuation, wording, coherence) and genre-specific elements (such as openings, endings, structuring devices and politeness). The annotations contain up to three items (annotation unit; problem type; part-of-speech tag). USALT reflects various text quality dimensions, notably correctness, comprehensibility and appropriateness (both stylistically and in terms of genre conventions).

We present an USALT analysis of 371 texts produced by Dutch students from grades 7-9 (aged 12-15 years), taken from the so-called Schrijfmeters-corpus. The assignment concerned a letter about ‘typically Dutch things’ to a Swedish girl about to emigrate to The Netherlands. USALT reliabilities were adequate. In terms of problem frequency, we were struck by the pervasiveness of punctuation problems. Furthermore, the orthography and punctuation problems together present considerable difficulties for automatic analysis of original learner texts at this level. A remarkable result regarding relations between various text quality dimensions is that the frequency of orthography problems correlates higher with genre convention problems than with lexico-grammatical problems. We also used the annotations as predictors of the holistic scores assigned to the texts by human raters. Standardized annotation frequencies by themselves may account for 45% of the score variance, with a prominent role for annotations regarding genre elements; text length by itself explains 52%. The best model includes both text length and annotations (65% explained variance). In ongoing work, USALT is being extended to handle argumentative writing assignments. ... Read more