Alfred Wegener Institute for Polar and Marine Research
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Evaluating the Mechanism of Tropical Expansion Using Idealized Numerical Experiments
Get PDFA wide range of evidence reveals that the tropical belt is expanding. Several mechanisms have been proposed to contribute to this expansion, some of which even contradict each other. The study of Yang etal. suggests that the poleward advancing mid-latitude meridional temperature gradient (MTG), originating from enhanced subtropical ocean warming, plays a leading role in driving tropical expansion. However, the abrupt4xCO2 experiment indicates that tropical expansion occurs at a faster rate than is indicated by changes related to ocean temperature rise. The idealized amip4K experiment illustrates that without introducing any ocean warming pattern, uniform ocean surface warming also drives tropical expansion. The results based on these idealized experiments seem to contradict the hypothesis proposed by Yang et al. In this study, we revisit these 2 experiments and show that both experiments actually support the hypothesis that MTG migration is driving tropical expansion. More specifically, in the abrupt4xCO2 experiment, although the rate of ocean warming is relatively slow, the poleward shift of the MTG is as rapid as tropical expansion. In the amip4K experiment, although ocean surface warming is uniform, the heating effect of the ocean on the atmosphere is nonuniform because of the nonlinear relationship between temperature, evaporation, and thermal radiation. The nonuniform oceanic heating to the atmosphere introduces a poleward shift of the MTG within the upper troposphere and drives a shift in the jet streams. By conducting an additional idealized experiment in which tropical expansion occurs under both a migrating MTG and a cooling climate, we argue that the migration of the MTG, rather than global warming, is the key mechanism in driving tropical expansion
No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years
Get PDFThe Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid-to-late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment
Southern ocean carbon and heat impact on climate
Get PDFThe Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea-ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'
Editorial: Impact of climate change on coastal environmental variability and aquatic physiology
Get PDFA Southern Ocean supergyre as a unifying dynamical framework identified by physics-informed machine learning
Get PDFThe Southern Ocean closes the global overturning circulation and is key to the regulation of carbon, heat, biological production, and sea level. However, the dynamics of the general circulation and upwelling pathways remain poorly understood. Here, a physics-informed unsupervised machine learning framework using principled constraints is used. A unifying framework is proposed invoking a semi-circumpolar supergyre south of the Antarctic circumpolar current: a massive series of leaking sub-gyres spanning the Weddell and Ross seas that are connected and maintained via rough topography that acts as scaffolding. The supergyre framework challenges the conventional view of having separate circulation structures in the Weddell and Ross seas and suggests that idealized models and zonally-averaged frameworks may be of limited utility for climate applications. Machine learning was used to reveal areas of coherent driving forces within a vorticity-based analysis. Predictions from the supergyre framework are supported by available observations and could aid observational and modelling efforts to study this climatologically key region undergoing rapid change
Were climatic forcings the main driver for mid-holocene changes in settlement dynamics on the Varamin Plain (Central Iranian Plateau)?
Get PDFSettlement crises in ancient cultures of Western Asia are commonly thought to be caused by climatic events such as severe droughts. However, the insufficient climate proxy situation in this region challenges the inference of clear relationships between climate and settlement dynamics. We investigate the Holocene climatic changes on the Varamin Plain in the context of the climatic history of Western Central Asia by using a transient comprehensive Earth System Model simulation (8 ka BP to pre-industrial), a high-resolution regional snapshot simulation and a synthesis of pollen-based climate reconstructions. In line with the reconstructions, the models reveal only slightly varying mean climatic conditions on the Varamin Plain but indicate substantial changes in seasonality during the Holocene. Increased precipitation during spring, combined with lower temperature and potentially stronger snow accumulation on the upstream Alborz mountains may have led to an increased water supply on the alluvial fan during the vegetation period and thus to more favourable conditions for agricultural production during the Mid-Holocene compared to modern times. According to the model, dry periods on the Central Iranian Plateau are related to particularly weak Westerly winds, fostering the subsidence in the mid-troposphere and hampering precipitation over the region. The model reveals that dry periods have spatially heterogenous manifestations, thus explaining why they do not appear in all proxy records in the wider study region. In fact, the climatic signal may depend on local environmental conditions. The interaction of the topography with the atmospheric circulation leads to additional spatial heterogeneity. Although our results provide several indications for a connection between climate and settlement dynamics, the small overall changes in moisture call into question whether climate is the main driver for settlement discontinuities on the Central Iranian Plateau. To shed further light on this issue, more high-resolution long-term proxy records are needed.</jats:p
Modeling Biogenic Aerosol Precursors in the Arctic Ocean: Occurrence patterns and long-term trends
Get PDFGlobal oceanic diazotroph database version 2 and elevated estimate of global oceanic N2 fixation
Get PDFAbstract. Marine diazotrophs convert dinitrogen (N2) gas into
bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the
first version of the global oceanic diazotroph database (version 1) was
published. Here, we present an updated version of the database (version 2),
significantly increasing the number of in situ diazotrophic measurements from
13 565 to 55 286. Data points for N2 fixation rates, diazotrophic cell
abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and
809 %, respectively. Version 2 includes two new data sheets for the nifH gene
copy abundance of non-cyanobacterial diazotrophs and cell-specific N2
fixation rates. The measurements of N2 fixation rates approximately
follow a log-normal distribution in both version 1 and version 2. However,
version 2 considerably extends both the left and right tails of the
distribution. Consequently, when estimating global oceanic N2 fixation
rates using the geometric means of different ocean basins, version 1 and
version 2 yield similar rates (43–57 versus 45–63 Tg N yr−1; ranges
based on one geometric standard error). In contrast, when using arithmetic
means, version 2 suggests a significantly higher rate of 223±30 Tg N yr−1 (mean ± standard error; same hereafter) compared to version 1
(74±7 Tg N yr−1). Specifically, substantial rate increases are
estimated for the South Pacific Ocean (88±23 versus 20±2 Tg N yr−1), primarily driven by measurements in the southwestern subtropics,
and for the North Atlantic Ocean (40±9 versus 10±2 Tg N yr−1). Moreover, version 2 estimates the N2 fixation rate in the
Indian Ocean to be 35±14 Tg N yr−1, which could not be estimated
using version 1 due to limited data availability. Furthermore, a comparison
of N2 fixation rates obtained through different measurement methods at
the same months, locations, and depths reveals that the conventional
15N2 bubble method yields lower rates in 69 % cases compared to
the new 15N2 dissolution method. This updated version of the
database can facilitate future studies in marine ecology and
biogeochemistry. The database is stored at the Figshare repository
(https://doi.org/10.6084/m9.figshare.21677687; Shao et
al., 2022).
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The iron “redox battery” in sandy sediments: Its impact on organic matter remineralization and phosphorus cycling
Get PDFPermeable sandy sediments cover 50-60% of the global continental shelf and are important bioreactors that regulate organic matter (OM) turnover and nutrient cycling in the coastal ocean. In sands, the dynamic porewater advection can cause rapid mass transfer and variable redox conditions, thus affecting OM remineralization pathways as well as the recycling of iron and phosphorus. In this study, North Sea sands were incubated in flow-through reactors (FTRs) to investigate biogeochemical processes under porewater advection and changing redox conditions. We found that the average rate of anaerobic OM remineralization was 12 times lower than the aerobic pathway, and Fe(III) oxyhydroxides were found as the major electron acceptors during 34 days of anoxic incubation. Abundant reduced Fe in the solid phase (expressed as Fe(II)) was measured before extensive Fe2+ release into porewater, and most of the reduced Fe (~96%) remained in the solid phase throughout the anoxic incubation. Fe(II) retained in the solid phase, either through the formation of authigenic Fe(II)-bearing minerals or adsorption, was easily re-oxidized upon exposure to O2 . Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe2+ with a constant P/Fe2+ ratio of 0.26. After 34 days of anoxic incubation, porewater was re-oxygenated and >99% of released P was coprecipitated through Fe2+ oxidation (so-called “Fe2+ curtain”). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as relatively immobile and rechargeable “redox battery” under dynamic porewater advection. Due to frequent oscillation of redox conditions, the Fe “redox battery” is characteristic for permeable sediments and plays an important role in coastal OM turnover. We also suggest that P liberated before Fe2+ release can escape the “Fe2+ curtain” in porewater advection, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions