Atlantic meridional overturning circulation

Résumé

The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean, although the extent of overturning in the Labrador Sea is disputed. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers. Climate change has the potential to weaken the AMOC through increases in ocean heat content and elevated freshwater flows from the melting ice sheets. Oceanographic reconstructions generally suggest that the AMOC is already weaker than it was before the Industrial Revolution, although there is a robust debate over the role of climate change ve

Source officielle

https://en.wikipedia.org/wiki/Atlantic_meridional_overturning_circulation

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Overview of the MOSAiC expedition—Atmosphere

Hélène Paule Angot, Ivo Fabio Beck, Thomas Krumpen, Julia Schmale

With the Arctic rapidly changing, the needs to observe, understand, and model the changes are essential. To support these needs, an annual cycle of observations of atmospheric properties, processes, and interactions were made while drifting with the sea ice across the central Arctic during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition from October 2019 to September 2020. An international team designed and implemented the comprehensive program to document and characterize all aspects of the Arctic atmospheric system in unprecedented detail, using a variety of approaches, and across multiple scales. These measurements were coordinated with other observational teams to explore cross-cutting and coupled interactions with the Arctic Ocean, sea ice, and ecosystem through a variety of physical and biogeochemical processes. This overview outlines the breadth and complexity of the atmospheric research program, which was organized into 4 subgroups: atmospheric state, clouds and precipitation, gases and aerosols, and energy budgets. Atmospheric variability over the annual cycle revealed important influences from a persistent large-scale winter circulation pattern, leading to some storms with pressure and winds that were outside the interquartile range of past conditions suggested by long-term reanalysis. Similarly, the MOSAiC location was warmer and wetter in summer than the reanalysis climatology, in part due to its close proximity to the sea ice edge. The comprehensiveness of the observational program for characterizing and analyzing atmospheric phenomena is demonstrated via a winter case study examining air mass transitions and a summer case study examining vertical atmospheric evolution. Overall, the MOSAiC atmospheric program successfully met its objectives and was the most comprehensive atmospheric measurement program to date conducted over the Arctic sea ice. The obtained data will support a broad range of coupled-system scientific research and provide an important foundation for advancing multiscale modeling capabilities in the Arctic.

2022

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This paper summarizes the findings of a long term experimental program corroborated with detailed finite element simulations that investigated the hysteretic behavior of wide-flange columns in steel moment-resisting frames (MRFs) designed in highly seismic regions. Several aspects of the steel column behavior are thoroughly investigated. It is shown that steel column axial shortening is a failure mode that strongly influences the steel column stability under earthquake-induced loading. The amount of axial shortening can be considerably different in interior columns compared to end (exterior) columns that experience transient axial load demands due to dynamic overturning effects. Axial shortening is typically followed by column out-of-plane deformations that become maximum near the dissipative plastic hinge zone and migrate near the column top end. This failure mode is strongly influenced by the considered column end boundary conditions. Routinely used symmetric loading histories provide insufficient information for modeling the cyclic deterioration in flexural strength and stiffness of steel columns near collapse. Modeling recommendations for updated backbone parameters for nonlinear modeling of steel columns are proposed inline with the current ASCE 41 nonlinear modeling recommendations for performance-based seismic assessment of steel moment frames.

2017

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The influence of atmospheric circulation on the mid-Holocene climate of Europe: a data-model comparison

Pamela Collins, Basil Andrew Stansfield Davis, Jed Oliver Kaplan

The atmospheric circulation is a key area of uncertainty in climate model simulations of future climate change, especially in mid-latitude regions such as Europe where atmospheric dynamics have a significant role in climate variability. It has been proposed that the mid-Holocene was characterized in Europe by a stronger westerly circulation in winter comparable with a more positive AO/NAO, and a weaker westerly circulation in summer caused by anticyclonic blocking near Scandinavia. Model simulations indicate at best only a weakly positive AO/NAO, whilst changes in summer atmospheric circulation have not been widely investigated. Here we use a new pollen-based reconstruction of European mid-Holocene climate to investigate the role of atmospheric circulation in explaining the spatial pattern of seasonal temperature and precipitation anomalies. We find that the footprint of the anomalies is entirely consistent with those from modern analogue atmospheric circulation patterns associated with a strong westerly circulation in winter (positive AO/NAO) and a weak westerly circulation in summer associated with anti-cyclonic blocking (positive SCAND). We find little agreement between the reconstructed anomalies and those from 14 GCMs that performed mid-Holocene experiments as part of the PMIP3/CMIP5 project, which show a much greater sensitivity to top-of-the-atmosphere changes in solar insolation. Our findings are consistent with data-model comparisons on contemporary timescales that indicate that models underestimate the role of atmospheric circulation in recent climate change, whilst also highlighting the importance of atmospheric dynamics in explaining interglacial warming.

Copernicus Gesellschaft Mbh2014

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Overview of the MOSAiC expedition—Atmosphere

Hélène Paule Angot, Ivo Fabio Beck, Thomas Krumpen, Julia Schmale

2022