ALBERT

Description: Air-sea interaction in late boreal winter is studied over the extratropical North Atlantic (NA) during 1960–2020 by examining the relationship between sea-surface temperature (SST) and total turbulent heat flux (THF). The two quantities are positively correlated on interannual timescales over the central-midlatitude and subpolar NA, suggesting the atmosphere on average drives SST and THF variability is independent of SST. On decadal timescales and over the central-midlatitude NA the correlation is negative, suggesting ocean processes on average drive SST and THF variability is sensitive to SST. The correlation is positive over the subpolar NA. There, interannual and decadal THF variability is governed by the North Atlantic Oscillation (NAO). During the major late 20th and early 21st century SST increase in the subpolar NA diminishing oceanic heat loss associated with a weakening NAO was observed. This study suggests that the atmosphere is more sensitive to SST over the central-midlatitude than subpolar NA. Key Points: - Regional variation in the nature of air-sea interaction over the extratropical North Atlantic (NA) north of 35°N - Timescale dependence in relationship between sea-surface temperature (SST) and turbulent heat flux over the central-midlatitude NA - The atmosphere is more sensitive to SST variability over the central-midlatitude than subpolar NA

Description: Key Points: - We reconstruct the temporal evolution of seawater isotope ratios of boron, strontium, lithium, and osmium over the last 65 million years - The evolution of seawater boron isotope ratio shows similarity to the evolution of strontium, lithium and osmium isotope ratios - Randomly drawn, smooth time series are provided for use in uncertainty propagation in calculation of palaeo pH The boron isotope ratio of seawater (δ11Bsw) is a parameter which must be known to reconstruct palaeo pH and CO2 from boron isotope measurements of marine carbonates. Beyond a few million years ago, δ11Bsw is likely to have been different to modern. Palaeo δ11Bsw can be estimated by simultaneously constraining the vertical gradients in foraminiferal δ11B (Δδ11B) and pH (ΔpH). A number of subtly different techniques have been used to estimate ΔpH in the past, all broadly based on assumptions about vertical gradients in oxygen, and/or carbon, or other carbonate system constraints. In this work we pull together existing data from previous studies, alongside a constraint on the rate of change of δ11Bsw from modeling. We combine this information in an overarching statistical framework called a Gaussian Process. The Gaussian Process technique allows us to bring together data and constraints on the rate of change in δ11Bsw to generate random plausible evolutions of δ11Bsw. We reconstruct δ11Bsw, and by extension palaeo pH, across the last 65Myr using this novel methodology. Reconstructed δ11Bsw is compared to other seawater isotope ratios, namely ,87/86 Sr, 187/188 Os , and δ7Li, which we also reconstruct with Gaussian Processes. Our method provides a template for incorporation of future δ11Bsw constraints, and a mechanism for propagation of uncertainty in δ11Bsw into future studies.

Description: Scientists are using a variety of geochemical, geophysical, and numerical methods to study offshore freshened groundwater and better understand its role in the global water cycle.

Description: Low-level jets (LLJs), vertical profiles with a wind speed maxima in the lowest hundred meters of the troposphere, have multiple impacts in the Earth system, but a global present-day climatology based on contemporary data does not exist. We use the spatially and temporally complete data set from ERA5 reanalysis to compile a global climatology of LLJs for studying the formation mechanisms, characteristics, and trends during the period of 1992–2021. In the global mean, LLJs are detected 21% of the time with more cases over land (32%) than over the ocean (15%). We classified the LLJs into three categories: non-polar land (LLLJ), polar land (PLLJ), and coastal (CLLJ) LLJs. For LLLJ, the averaged frequency of occurrence is 20% and 75% of them are associated with a near-surface temperature inversion as a prerequisite for an inertial oscillation. PLLJs are also associated with a temperature inversion and occur even more frequently with 59% of the time. These are also the lowest and the strongest LLJs among the three categories. CLLJs are particularly frequent in some marine hotspots, situated along the west coast of continents, with neutral to unstable stratification close to the surfaces and a stably stratified layer aloft. We found distinct regional trends in both the frequency and intensity of LLJs over the past decades, which can have implications for the emission and transport of aerosols, and the transport of atmospheric moisture. Future studies could address changes in LLJs and the associated implications in more detail, based on the here released ERA5-based LLJ data. Key Points: - First global comprehensive low-level jet (LLJ) climatology using ERA5 - Polar LLJs are the strongest and most frequent among the detected types - Distinct past trends in regional LLJ frequency and intensity

Description: The Banda Sea is of crucial importance for the circulation of the world's oceans, as it is part of the connection between the Pacific to the Indian Ocean. One peculiarity of the upper ocean hydrography in the Banda Sea is the occurrence of barrier layers. The regionality and temporal variability of barrier layer thickness (BLT) in the Banda Sea are examined in this study utilizing in-situ observations and ocean reanalysis output. It is found that a barrier layer occurs in over 90 % of the observational data profiles, and in over 72 % of those profiles, the BLT is shallower than 10 m. Furthermore, we find a seasonal cycle in BLT with a maximum thickness of about 60 m occurring during austral autumn and winter and coinciding with the presence of low saline waters fed by the regional river discharge and rainfall from the Java Sea and Makassar Strait. In addition, we identify the existence of a quasi-permanent anticyclonic circulation cell in the Banda Sea that may support the trapping of surface freshwater by retention. The anticyclonic circulation is most likely wind-driven because it coincides with the regional Ekman pumping pattern. Modulation of the anticyclone is via seasonal variability in the wind stress curl which in turn may explain the efficiency of freshwater retention and thus the BLT. The annual mean BLT distribution in the Banda Sea shows a preferential region of thickened barrier layers around 6o-8oS and 124o-126oE and resampling the pattern of the monthly mean climatology. Key Points: - First study estimating barrier layer thickness (BLT) in the Banda Sea using comprehensive observations - A quasi-permanent barrier layer exists in the Banda Sea with seasonal variation in occurrence and thickness - The intrusion of low saline waters and anticyclonic circulation are identified as the main mechanisms for creating and modulating the local BLT

Description: The recent severe European summer heat waves of 2015 and 2018 co-occurred with cold subpolar North Atlantic (NA) sea surface temperatures (SSTs). However, a significant connection between this oceanic state and European heat waves was not yet established. We performed two AMIP-like model experiments: (a) employing daily 2018 SSTs as observed and (b) applying a novel approach to remove the negative NA SST anomaly, while keeping SST daily and small-scale variability. Comparing these experiments, we find that cold subpolar NA SSTs significantly increase heat wave duration and magnitude downstream over the European continent. Surface temperature and circulation anomalies are connected by the upper-tropospheric summer wave pattern of meridional winds over the North Atlantic European sector, which is enhanced with cold NA SSTs. Our results highlight the relevance of the subpolar NA region for European summer conditions, a region that is marked by large biases in current coupled climate model simulations. Key Points: - Model study designed to investigate the ocean impact on European heat waves by prescribing observed and realistic ocean surface conditions - Cold subpolar North Atlantic sea surface temperatures significantly enhance heat wave intensity and duration over the European continent - North Atlantic ocean and European surface temperature and circulation anomalies are bridged by the upper-tropospheric summer mean wave

Description: In geoscience and other fields, researchers use models as a simplified representation of reality. The models include processes that often rely on uncertain parameters that reduce model performance in reflecting real-world processes. The problem is commonly addressed by adapting parameter values to reach a good match between model simulations and corresponding observations. Different optimization tools have been successfully applied to address this task of model calibration. However, seeking one best value for every single model parameter might not always be optimal. For example, if model equations integrate over multiple real-world processes which cannot be fully resolved, it might be preferable to consider associated model parameters as random parameters. In this paper, a random parameter is drawn from a wide probability distribution for every singe model simulation. We developed an optimization approach that allows us to declare certain parameters random while optimizing those that are assumed to take fixed values. We designed a corresponding variant of the well known Covariance Matrix Adaption Evolution Strategy (CMA-ES). The new algorithm was applied to a global biogeochemical circulation model to quantify the impact of zooplankton mortality on the underlying biogeochemistry. Compared to the deterministic CMA-ES, our new method converges to a solution that better suits the credible range of the corresponding random parameter with less computational effort.

Description: Flow of dense shelf water provide an efficient mechanism for pumping CO 2 to the deep ocean along the continental shelf slope, particularly around the Antarctic bottom water (AABW) formation areas where much of the global bottom water is formed. However, the contribution of the formation of AABW to sequestering anthropogenic carbon ( C ant ) and its consequences remain unclear. Here, we show prominent transport of C ant (25.0 ± 4.7 Tg C yr −1 ) into the deep ocean (〉2,000 m) in four AABW formation regions around Antarctica based on an integrated observational data set (1974–2018). This maintains a lower C ant in the upper waters than that of other open oceans to sustain a stronger CO 2 uptake capacity (16.9 ± 3.8 Tg C yr −1 ). Nevertheless, the accumulation of C ant can further trigger acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr −1 . Our findings elucidate the prominent role of AABW in controlling the Southern Ocean carbon uptake and storage to mitigate climate change, whereas its side effects (e.g., acidification) could also spread to other ocean basins via the global ocean conveyor belt. Plain Language Summary The Southern Ocean is thought to uptake and store a large amount of anthropogenic CO 2 ( C ant ), but little attention has been paid to the Antarctic coastal regions in the south of 60°S, mainly due to the lack of observations. Based on an integrated data set, we discovered the deep penetration of C ant and a visible pattern of relatively high concentration of C ant along the AABW formation pathway, and the concentration of C ant along the shelf‐slope is higher than that of other marginal seas at low‐mid latitudes, implying a highly effective C ant transport in AABW formation areas. We also found strong upper‐layer CO 2 uptake and a significant acidification rate in the deep waters of the Southern Ocean due to the AABW‐driven CO 2 transport, which is 3 times faster than those in other deep oceans. It is therefore crucial to understand how the Antarctic shelf regions affect the global carbon cycle through the uptake and transport of anthropogenic CO 2 , which also drives acidification in the other ocean basins. Key Points We show evidence for the accumulation of C ant along the Antarctic shelf‐slope into the deep ocean The process of AABW formation drives C ant downward transport at 25.0 ± 4.7 Tg C yr −1 , sustaining the CO 2 uptake in the surface ocean This further triggers acidification of AABW at a rate of −0.0006 ± 0.0001 pH unit yr −1 , which is faster than in other deep oceans

Description: To reach their net-zero targets, countries will have to compensate hard-to-abate CO2 emissions through carbon dioxide removal (CDR). Yet, current assessments rarely include socio-cultural or institutional aspects or fail to contextualize CDR options for implementation. Here we present a context-specific feasibility assessment of CDR options for the example of Germany. We assess 14 CDR options, including three chemical carbon capture options, six options for bioenergy combined with carbon capture and storage (BECCS), and five options that aim to increase ecosystem carbon uptake. The assessment addresses technological, economic, environmental, institutional, social-cultural and systemic considerations using a traffic-light system to evaluate implementation opportunities and hurdles. We find that in Germany CDR options like cover crops or seagrass restoration currently face comparably low implementation hurdles in terms of technological, economic, or environmental feasibility and low institutional or social opposition but show comparably small CO2 removal potentials. In contrast, some BECCS options that show high CDR potentials face significant techno-economic, societal and institutional hurdles when it comes to the geological storage of CO2. While a combination of CDR options is likely required to meet the net-zero target in Germany, the current climate protection law includes a limited set of options. Our analysis aims to provide comprehensive information on CDR hurdles and possibilities for Germany for use in further research on CDR options, climate, and energy scenario development, as well as an effective decision support basis for various actors. Key Points: - More context-specific assessments of carbon dioxide removal (CDR) options are needed to guide national net-zero decision making - Ecosystem-based CDR options with comparably low implementation hurdles in Germany show relatively small CO2 removal potentials - High CDR potential options in Germany face high institutional, technological and societal hurdles linked in many ways to geological storage

Description: The Northwest Tropical Atlantic (NWTA) is a region of complex surface ocean circulation. The most prominent feature is the North Brazil Current (NBC) and its retroflection at 8°N, which leads to the formation of numerous mesoscale eddies known as NBC rings. The NWTA also receives the outflow of the Amazon River, generating freshwater plumes that can extend up to 100,000 km2. We show that these two processes influence the spatial variability of the region's surface latent heat flux (LHF). On the one hand, the presence of surface freshwater modifies the vertical stratification of the ocean, the mixed layer heat budget, and thus the air-sea heat exchanges. On the other hand, NBC rings create a highly heterogeneous mesoscale sea surface temperature (SST) field that directly influences the near-surface atmospheric circulation. These effects are illustrated by observations from the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte - Ocean Atmosphere (EUREC4A-OA) and Atlantic Tradewind Ocean-Atmosphere Interaction Campaign (ATOMIC) experiments, satellite and reanalysis data. We decompose the LHF budget into several terms controlled by different atmospheric and oceanic processes to identify the mechanisms leading to LHF changes. We find LHF variations of up to 160 W m2, of which 100 W m2 are associated with wind speed changes and 40 W m2 with SST variations. Surface currents or heat release associated with stratification changes remain as second-order contributions with LHF variations of less than 10 W m2 each. This study highlights the importance of considering these three components to properly characterize LHF variability at different spatial scales, although it is limited by the scarcity of collocated observations. Key Points: - Latent heat flux (LHF) presents strong spatial variations in the northwest tropical Atlantic (NWTA), which has a complex ocean circulation - Surface winds and sea surface temperature are the major drivers of LHF changes. The Amazon plume remains as a second-order contributor - It is necessary to distinguish between spatial scales (mesoscale and below vs. large-scale) when assessing the ocean's influence on LHF