ALBERT

Beschreibung: Highlights: • Lagrangian ocean analysis is a powerful way to analyse the output of ocean circulation models • We present a review of the Kinematic framework, available tools, and applications of Lagrangian ocean analysis • While there are unresolved questions, the framework is robust enough to be used widely in ocean modelling Abstract: Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.

Beschreibung: Highlights: • Arctic sea ice extent and solid freshwater in 14 CORE-II models are inter-compared. • The models better represent the variability than the mean state. • The September ice extent trend is reasonably represented by the model ensemble mean. • The descending trend of ice thickness is underestimated compared to observations. • The models underestimate the reduction in solid freshwater content in recent years. Abstract: The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE II) is analyzed. The focus is on the Arctic sea ice extent, the solid freshwater (FW) sources and solid freshwater content (FWC). Available observations are used for model evaluation. The variability of sea ice extent and solid FW budget is more consistently reproduced than their mean state in the models. The descending trend of September sea ice extent is well simulated in terms of the model ensemble mean. Models overestimating sea ice thickness tend to underestimate the descending trend of September sea ice extent. The models underestimate the observed sea ice thinning trend by a factor of two. When averaged on decadal time scales, the variation of Arctic solid FWC is contributed by those of both sea ice production and sea ice transport, which are out of phase in time. The solid FWC decreased in the recent decades, caused mainly by the reduction in sea ice thickness. The models did not simulate the acceleration of sea ice thickness decline, leading to an underestimation of solid FWC trend after 2000. The common model behavior, including the tendency to underestimate the trend of sea ice thickness and March sea ice extent, remains to be improved.

Beschreibung: Highlights: • Arctic liquid freshwater budget simulated in 14 CORE-II models is studied. • The models better represent the temporal variability than the mean state. • Multi-model mean (MMM) FW fluxes compare well with observations. • MMM FWC shows an upward trend in the recent years, with an underestimated rate. • FW flux interannual variability is more consistent where volume flux determines it. Abstract: The Arctic Ocean simulated in 14 global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II) is analyzed in this study. The focus is on the Arctic liquid freshwater (FW) sources and freshwater content (FWC). The models agree on the interannual variability of liquid FW transport at the gateways where the ocean volume transport determines the FW transport variability. The variation of liquid FWC is induced by both the surface FW flux (associated with sea ice production) and lateral liquid FW transport, which are in phase when averaged on decadal time scales. The liquid FWC shows an increase starting from the mid-1990s, caused by the reduction of both sea ice formation and liquid FW export, with the former being more significant in most of the models. The mean state of the FW budget is less consistently simulated than the temporal variability. The model ensemble means of liquid FW transport through the Arctic gateways compare well with observations. On average, the models have too high mean FWC, weaker upward trends of FWC in the recent decade than the observation, and low consistency in the temporal variation of FWC spatial distribution, which needs to be further explored for the purpose of model development.

Beschreibung: As the planet cooled from peak warmth in the early Cenozoic, extensive Northern Hemisphere ice sheets developed by 2.6 Ma ago, leading to changes in the circulation of both the atmosphere and oceans. From not, vert, similar2.6 to not, vert, similar1.0 Ma ago, ice sheets came and went about every 41 ka, in pace with cycles in the tilt of Earth’s axis, but for the past 700 ka, glacial cycles have been longer, lasting not, vert, similar100 ka, separated by brief, warm interglaciations, when sea level and ice volumes were close to present. The cause of the shift from 41 ka to 100 ka glacial cycles is still debated. During the penultimate interglaciation, not, vert, similar130 to not, vert, similar120 ka ago, solar energy in summer in the Arctic was greater than at any time subsequently. As a consequence, Arctic summers were not, vert, similar5 °C warmer than at present, and almost all glaciers melted completely except for the Greenland Ice Sheet, and even it was reduced in size substantially from its present extent. With the loss of land ice, sea level was about 5 m higher than present, with the extra melt coming from both Greenland and Antarctica as well as small glaciers. The Last Glacial Maximum (LGM) peaked not, vert, similar21 ka ago, when mean annual temperatures over parts of the Arctic were as much as 20 °C lower than at present. Ice recession was well underway 16 ka ago, and most of the Northern Hemisphere ice sheets had melted by 6 ka ago. Solar energy reached a summer maximum (9% higher than at present) not, vert, similar11 ka ago and has been decreasing since then, primarily in response to the precession of the equinoxes. The extra energy elevated early Holocene summer temperatures throughout the Arctic 1–3 °C above 20th century averages, enough to completely melt many small glaciers throughout the Arctic, although the Greenland Ice Sheet was only slightly smaller than at present. Early Holocene summer sea ice limits were substantially smaller than their 20th century average, and the flow of Atlantic water into the Arctic Ocean was substantially greater. As summer solar energy decreased in the second half of the Holocene, glaciers re-established or advanced, sea ice expanded, and the flow of warm Atlantic water into the Arctic Ocean diminished. Late Holocene cooling reached its nadir during the Little Ice Age (about 1250–1850 AD), when sun-blocking volcanic eruptions and perhaps other causes added to the orbital cooling, allowing most Arctic glaciers to reach their maximum Holocene extent. During the warming of the past century, glaciers have receded throughout the Arctic, terrestrial ecosystems have advanced northward, and perennial Arctic Ocean sea ice has diminished. Here we review the proxies that allow reconstruction of Quaternary climates and the feedbacks that amplify climate change across the Arctic. We provide an overview of the evolution of climate from the hot-house of the early Cenozoic through its transition to the ice-house of the Quaternary, with special emphasis on the anomalous warmth of the middle Pliocene, early Quaternary warm times, the Mid Pleistocene transition, warm interglaciations of marine isotope stages 11, 5e, and 1, the stage 3 interstadial, and the peak cold of the last glacial maximum.

Beschreibung: Highlights: ► We imaged a 3 × 5-km giant fluid seep structure, the Giant Gjallar Vent, off mid-Norway. ► We combined neural network analysis and sandbox modeling. ► We define the internal geometries of the underlying pipe. ► The Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development. An exploration 3D seismic data set from the Gjallar Ridge off mid-Norway images a giant fluid seep structure, 3 × 5 km wide, which connects to late Palaeocene magmatic sills at depth. Two of the pipes that have developed as hydrothermal vents reach all the way to the modern seafloor implying that they either were active much longer than the original hydrothermal activity or have been reactivated. We combine detailed seismic analysis of the northern pipe and sandbox modeling to constrain pipe initiation and propagation. Although both the seismic data and the sandbox models suggest that fluids at depth are focused through a vertical conduit, sandbox models show that fluids ascend and reach a critical depth migration where focused migration abruptly transforms into distributed fluid flow through unconsolidated sediments. This indicates that at this level the sediments are intensely deformed during pipe propagation, creating a V-shaped structure, i.e. an inverted cone at depth and a positive relief anomaly, 5 to 10 m high, at the seafloor, which is clearly identified on 3D seismic data. Comparison of the geometries observed in sandbox modeling with the seismically observed geometries of the Giant Gjallar Vent suggests that the Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development, preceding the future collapse of the structure forming a seafloor depression. Our results imply that the Gjallar Giant Vent can be used as a window into the geological processes active in the deep parts of the Vøring Basin.

Beschreibung: We present a magnesium (Mg) and strontium (Sr) record from an aragonitic speleothem (Grotte de Piste, Morocco, 34‬°N; 04°W) providing a reconstruction of effective rainfall from 619 to 1962 AD. The corresponding drip site was monitored over 2 yr for drip water Mg/Ca and Sr/Ca ratios. Results show evidence for prior aragonite precipitation, which can explain negative correlations between speleothem Mg and Sr concentrations. The data shown here have important climate implications concerning the evolution of the North Atlantic Oscillation (NAO). A comparison of the stalagmite data from Grotte de Piste with an updated tree ring based drought reconstruction from Morocco and other NAO related proxy records confirms that the Medieval Warm Period (MWP) was dominated by NAO+ conditions. The stalagmite record and multiple proxy records from the Iberian Peninsula, however, suggest that considerable rainfall variability occurred during the MWP. This implies that the NAO has been more variable during the MWP than formerly suggested.

Beschreibung: To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

Beschreibung: Dissolved free amino acids (DFAA) in seawater are a form of nitrogen (N) available for marine microbes. In oligotrophic environments where N-containing nutrients are the limiting factor for microbial growth, N nutrition from DFAA could be crucial, but as yet it is poorly resolved. Measurements of individual DFAA are challenging as concentrations are typically in the low nmol L− 1 range. Here we report modifications to methodology using o-phthaldialdehyde (OPA) derivatization and reversed phase high performance liquid chromatography (HPLC) that provide a 30-fold improvement in sensitivity enabling the detection of 15 amino acids in seawater with a limit of detection as low as 10 pmol L− 1 with accuracy and precision of better than 10%. This analytical methodology is now suitable for the challenging quantitation of DFAA in oligotrophic seawaters. The method was successfully applied to a suite of seawater samples collected on a cruise crossing the South Atlantic Ocean, where concentrations of DFAAs were generally low (sub nmol L− 1), revealing basin-scale features in the oceanographic distributions of DFAA. This unique dataset implies that DFAAs are an important component of the N cycle in both near-coastal and open oceans. Further calculations suggest that the proportions of organic N originating from DFAA sources were significant, contributing between 0.2 and 200% that of NH4 + and up to 77% that of total inorganic nitrogen in the upper 400 m in some regions of the transect.

Beschreibung: Highlights • The Southern Hemisphere mid-latitudes are characterised by obliquity and semi-precession cycles during the Mi-1 deglaciation • The obliquity variability is attributed to polar influence and the semi-precessional variability to tropical influence • Semi-precession cycles do not appear until 23.01 Ma, corresponding to the onset of Antarctic deglaciation • The interaction between polar and tropical influence is related to the position and strength of the westerly wind belt Abstract It is well-known from geologic archives that Pleistocene and Holocene climate is characterised by cyclical variation on a wide range of timescales, and that these cycles of variation interact in complex ways. However, it is rarely possible to reconstruct sub-precessional (〈 20 kyr) climate variations for periods predating the oldest ice-core records (c. 800 ka). Here we present an investigation of orbital to potentially sub-precessional cyclicity from an annually resolved lake sediment core dated to a 100-kyr period in the earliest Miocene (23.03–22.93 Ma) and spanning a period of major Antarctic deglaciation associated with the second half of the Mi-1 event. Principal component analysis (PCA) of sediment bulk density, magnetic susceptibility (MS), and CIELAB L* and b* with a resolution of ~10 years indicates two major environmental processes governing the physical properties records, which we interpret as changes in wind strength and changes in precipitation. Spectral analysis of the principal components indicates that both processes are strongly influenced by obliquity (41 kyr). We interpret this 41-kyr cycle in wind strength and precipitation as related to the changing position and strength of the Southern Hemisphere westerly winds. Precipitation is also influenced by an 11-kyr cycle. The 11-kyr periodicity is potentially related to orbital cyclicity, representing the equatorial semi-precessional maximum insolation cycle. This semi-precession cycle has been identified in a number of records from the Pleistocene and Holocene and has recently been suggested to indicate that insolation in low-latitude regions may be an important driver of millennial-scale climate response to orbital forcing (Feretti et al., 2015). This is the first time this cycle has been identified in a mid-latitude Southern Hemisphere climate archive, as well as the first identification in pre-Pleistocene records. The 11-kyr cycle appears at around 23.01 Ma, which coincides with the initiation of a major phase of Antarctic deglaciation, and strengthens during the subsequent period of rapid ice decay. This pattern suggests that the westerly winds may have expanded north of 50°S at the height of Mi-1, excluding tropical influence from the Foulden Maar site, and subsequently contracted polewards in tandem with warming deep-sea temperatures and Antarctic deglaciation, allowing the advection of tropical waters further south.