ALBERT

Description: Tropical sea surface temperature (SST) biases can cause atmospheric biases on global scales, hence SST needs to be represented well in climate models. A major source of uncertainties is the representation of turbulent mixing in the oceanic boundary layer, or mixed layer (ML). In the present study we focus on near-inertial wave (NIW) induced mixing. The performance of two mixing schemes, Turbulent Kinetic Energy and K-profile parameterization (KPP), is assessed at two sites (11.5°N, 23°W and 15°N, 38°W) in the tropical Atlantic. At 11.5°N, turbulence observations (eddy diffusivities, shear and stratification) are available for comparison. We find that the schemes differ in their representation of NIWs, but both under-represent the observed enhanced diffusivities below the observed ML. However, we find that the models do mix below the ML at 15°N when a storm passes nearby. The near-inertial oscillations remain below the ML for the following 10 days. Near-inertial kinetic energy (NIKE) biases in the models are not directly correlated with the wind speed, the MLD biases, or the stratification at the ML base. Instead, NIKE biases are sensitive to the vertical mixing scheme parameterization. NIKE biases are lowest when the KPP scheme is used. Key Points: - Observations of inertial oscillations are used to evaluate the performance of two vertical mixing schemes in two high-resolution models - Both the K-profile parameterization and the Turbulent Kinetic Energy closure underestimate the NIW-induced mixing - Near-inertial kinetic energy biases are sensitive to the vertical mixing parameterization

Description: Global coupled climate models are in continuous need for evaluation against independent observations to reveal systematic model deficits and uncertainties. Changes in terrestrial water storage (TWS) as measured by satellite gravimetry missions GRACE and GRACE-FO provide valuable information on wetting and drying trends over the continents. Challenges arising from a comparison of observed and modelled water storage trends are related to gravity observations including non-water related variations such as, for example, glacial isostatic adjustment (GIA). Therefore, correcting secular changes in the Earth's gravity field caused by ongoing GIA is important for the monitoring of long-term changes in terrestrial water from GRACE in particular in former ice-covered regions. By utilizing a new ensemble of 56 individual realizations of GIA signals based on perturbations of mantle viscosities and ice history, we find that many of those alternative GIA corrections change the direction of GRACE-derived water storage trends, for example, from gaining mass into drying conditions, in particular in Eastern Canada. The change in the sign of the TWS trends subsequently impacts the conclusions drawn from using GRACE as observational basis for the evaluation of climate models as it influences the dis-/agreement between observed and modelled wetting/drying trends. A modified GIA correction, a combined GRACE/GRACE-FO data record extending over two decades, and a new generation of climate model experiments leads to substantially larger continental areas where wetting/drying trends currently observed by satellite missions coincide with long-term predictions obtained from climate model experiments.

Description: The Cabo Verde Archipelago is related to a mantle plume located close to the rotational pole of the African Plate. It consists of islands and seamounts arranged in a horseshoe‐shaped pattern open to the west, thus forming two volcanic chains, each with a weak east‐west age progression. High‐resolution swath bathymetry of 12 Cabo Verde seamounts is used here to assign each seamount to its pre‐shield, shield or post‐shield evolutionary stage, respectively. The eastern seamounts exhibit degraded and partially eroded morphologies, and are mainly in their post‐shield stage. A new 40 Ar‐ 39 Ar date for Senghor Seamount at 14.872 ± 0.027 Ma supports old ages for the eastern seamounts. The western seamounts generally exhibit younger volcanic‐edifice‐construction morphologies, showing fresh effusive and explosive volcanics, including rarely observed deep‐water explosive volcanism in the Charles Darwin Volcanic Field. Furthermore, the two previously unknown seamounts Sodade and Tavares in the westernmost termini of both volcanic chains exhibit pristine volcanic morphologies, in agreement with present‐day volcanism and seismic activity recorded from the western seamounts. The islands and seamounts rest on three submarine platforms to the east, northwest and southwest, respectively. Taken together, the seamount and island data suggest a shift in igneous activity from the eastern to the other platforms at about 8–6 Ma. However, the complex evolution pattern for both volcanic chains includes the simultaneous occurrence of pre‐shield or shield edifices at any time, followed by erosional and rejuvenation stages. The new seamount data still demonstrate ongoing westward submarine‐growth in both volcanic chains. Plain Language Summary The Cabo Verde volcanic islands and seamounts are located in the central Atlantic Ocean, ∼570 km off the west coast of Africa. They form a horseshoe‐shaped archipelago with two volcanic chains, which were formed by the African plate moving very slowly over a mantle hotspot (the Cabo Verde Plume). Both the northern and southern volcanic chains show weak east‐to‐west age progressions from ∼26 million years to the present day. This study uses underwater topographic data and observations/rock sampling via remotely operated vehicles from 12 submarine volcanic seamounts, including two previously unknown seamounts, collected during four research cruises in the Cabo Verde Archipelago. Geomorphology is used to classify each seamount as being in its pre‐shield, shield or post‐shield evolutionary stage, respectively. Cabo Verde islands and seamounts rest on three submarine morphological platforms, reflecting westward jumps of the main igneous activity, and also confirming the westward migration of the Cabo Verde hotspot beneath both volcanic chains. Key Points We present bathymetrical maps of 12, in part previously uncharted Cabo Verde seamounts Geomorphology reflects various evolutionary seamount stages and relative ages. Four older seamounts indicate late Quaternary sea level lowstands Islands and seamounts rest on three morphological platforms, indicating westward jumps of the main igneous activity

Description: Mineral dust is one of the most abundant atmospheric aerosol species and has various far-reaching effects on the climate system and adverse impacts on air quality. Satellite observations can provide spatio-temporal information on dust emission and transport pathways. However, satellite observations of dust plumes are frequently obscured by clouds. We use a method based on established, machine-learning-based image in-painting techniques to restore the spatial extent of dust plumes for the first time. We train an artificial neural net (ANN) on modern reanalysis data paired with satellite-derived cloud masks. The trained ANN is applied to cloud-masked, gray-scaled images, which were derived from false color images indicating elevated dust plumes in bright magenta. The images were obtained from the Spinning Enhanced Visible and Infrared Imager instrument onboard the Meteosat Second Generation satellite. We find up to 15% of summertime observations in West Africa and 10% of summertime observations in Nubia by satellite images miss dust plumes due to cloud cover. We use the new dust-plume data to demonstrate a novel approach for validating spatial patterns of the operational forecasts provided by the World Meteorological Organization Dust Regional Center in Barcelona. The comparison elucidates often similar dust plume patterns in the forecasts and the satellite-based reconstruction, but once trained, the reconstruction is computationally inexpensive. Our proposed reconstruction provides a new opportunity for validating dust aerosol transport in numerical weather models and Earth system models. It can be adapted to other aerosol species and trace gases. Key Points: - We present the first fast reconstruction of cloud-obscured Saharan dust plumes through novel machine learning applied to satellite images - The reconstruction algorithm utilizes partial convolutions to restore cloud-induced gaps in gray-scaled Meteosat Second Generation-Spinning Enhanced Visible and Infrared Imager Dust RGB images - World Meteorological Organization dust forecasts for North Africa mostly agree with the satellite-based reconstruction of the dust plume extent

Description: Observation‐based quantification of ocean carbon dioxide (CO 2 ) uptake relies on synthesis data sets such as the Surface Ocean CO 2 ATlas (SOCAT). However, the data collection effort has dramatically declined and the number of annual data sets in SOCATv2023 decreased by ∼35% from 2017 to 2021. This decline has led to a 65% increase (from 0.15 to 0.25 Pg C yr −1 ) in the standard deviation of seven SOCAT‐based air‐sea CO 2 flux estimates. Reducing the availability of the annual data to that in the year 2000 creates substantial bias (50%) in the long‐term flux trend. The annual mean CO 2 flux is insensitive to the seasonal skew of the SOCAT data and to the addition of the lower accuracy data set available in SOCAT. Our study highlights the need for sustained data collection and synthesis, to inform the Global Carbon Budget assessment, the UN‐led climate negotiations, and measurement, reporting, and verification of ocean‐based CO 2 removal projects. Plain Language Summary The Surface Ocean CO 2 ATlas (SOCAT) data set plays a crucial role in estimating the ocean carbon sink component of the Global Carbon Budget. However, the number of data sets available in SOCAT each year has drastically decreased since 2017. This study shows that the uncertainty in the data‐based ocean CO 2 flux estimate has increased by 65% due to this decline in data availability. The estimated fluxes, especially the long‐term flux trend, are remarkably affected by the data availability in SOCAT, reducing the reliability of ocean CO 2 uptake estimates in years and regions with sparse observations. Key Points Lower surface ocean f CO 2 data availability leads to higher uncertainty in data‐based estimates of ocean CO 2 uptake The long‐term trend in the ocean CO 2 flux increases by 1.5 times for subsequent years if the data availability is reduced to that in 2000 The annual mean CO 2 flux is not sensitive to the seasonal skew in the data and to the addition of low accuracy data

Description: Freshwater input from Greenland ice sheet melt has been increasing in the past decades from warming temperatures. To identify the impacts from enhanced meltwater input into the subpolar North Atlantic from 1997 to 2021, we use output from two nearly identical simulations in the eddy-rich model VIKING20X (1/20°) only differing in the freshwater input from Greenland: one with realistic interannually varying runoff increasing in the early 2000s and the other with climatologically (1961–2000) continued runoff. The majority of the additional freshwater remains within the boundary current enhancing the density gradient toward the warm and salty interior waters yielding increased current velocities. The accelerated boundary current shows a tendency to enhanced, upstream shifted eddy shedding into the Labrador Sea interior. Further, the experiments allow to attribute higher stratification and shallower mixed layers southwest of Greenland and deeper mixed layers in the Irminger Sea, particularly in 2015–2018, to the runoff increase in the early 2000s. Key Points The West Greenland Current (WGC) freshens and cools with the observed recent increase in meltwater runoff from Greenland The density gradient across the boundary current intensifies, strengthening the WGC and increasing local eddy formation Enhanced meltwater runoff contributed to an eastward shift in deep convection towards the Irminger Sea (2015–2018) Plain Language Summary Global warming has accelerated the melting of the Greenland ice sheet over the past few decades resulting in enhanced freshwater input into the North Atlantic. The additional freshwater can potentially inhibit deep water formation and have future implications on ocean circulation. To determine the influence from Greenland melt, we compare two high-resolution model experiments all with the same forcing but differing input of Greenland freshwater fluxes from 1997 to 2021. We find that in the experiment with realistically increasing Greenland meltwater, the water becomes fresher and cooler along the continental shelf and boundary of the subpolar gyre. The density difference between the shelf and interior increases with more freshwater, resulting in faster West Greenland Current speeds and enhanced eddy formation. Deeper mixed layers are found in the eastern Irminger Sea, particularly in 2015–2018. From 2009 to 2013, there were shallower mixed layers in the Labrador Sea where less Greenland meltwater was mixed downwards and spread eastward, causing mixed layers to deepen in the Irminger Sea.

Description: We report the first catalog of low‐frequency earthquakes in the Hikurangi subduction zone, located beneath the Kaimanawa Range of the North Island at 50 km depth, downdip of regularly recurring (every 4–5 years) deep M7 slow slip events. To systematically detect low‐frequency earthquakes within the regional continuous seismic data, we utilized a matched‐filter approach with template waveforms derived from previous observations of tectonic tremor. We built our catalog of 36 low‐frequency earthquake sources, that produced almost 21,000 events over more than a decade, with two matched‐filter search iterations. In each iteration, the detections were gathered into families and their coherent waveforms processed and stacked to extract high‐quality waveforms, allowing us to pick seismic phase arrivals to locate the low‐frequency earthquakes. We highlight three characteristic features to validate that our detected events are indeed low‐frequency earthquakes: the eponymous deficit of high frequencies in their seismic waveforms, the episodic swarms of activity that define their activity through time, and their location at the plate boundary with a double‐couple source mechanism and geometry consistent with the subduction interface. Considering the observed low‐frequency earthquakes' relationship to neighboring slow slip, we observe the event swarms to occur much more frequently than the M7 slow slip events located just updip. Similar to other deep low‐frequency earthquakes in other subduction zones, we suggest that this characteristic clustering in time is driven by more frequent, smaller slow slip events that are not clearly observable at the surface. Plain Language Summary Slow slip is episodic fault slip that lasts days, weeks or months, rather than the rapid ruptures of regular earthquakes. Geodetic observations of the surface displacement produced by slow slip suggest that their timing and location influence the seismic cycle of nearby faults and may even trigger large earthquakes. Although slow slip does not produce seismic radiation itself, slow slip is often accompanied by tiny repetitive seismic signals. These tiny seismic events, called low‐frequency earthquakes, can act as a powerful indicator of when and where slow slip is happening. In this study, we develop a new approach to detect low‐frequency earthquakes within continuous seismic waveforms, revealing the first observations of low‐frequency earthquakes in the Hikurangi subduction zone beneath the North Island of New Zealand. Our catalog of low‐frequency earthquakes suggests a complex pattern of slow fault slip at depth, with more frequent activity than geodetic data alone would suggest. The observed low‐frequency earthquake activity in the Hikurangi subduction zone thus represents a unique opportunity to study the slip history at depth beneath the North Island of New Zealand. Key Points 36 low‐frequency earthquake sources are extracted from continuous waveforms through template matching, deblurring, and unsupervised learning Low‐frequency earthquake sources locate close to the plate boundary with source mechanisms consistent with the subduction interface Detected low‐frequency earthquakes are likely triggered by small, frequent, and deep slow slip not geodetically observable at the surface

Description: The potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting. Plain Language Summary The potential for future earthquakes on faults is often inferred from velocities of the ground surface derived from satellite geodesy, but this approach can be challenging in complex deforming zones with many closely spaced faults. We present a new methodology for estimating the rate at which energy is accumulating on faults using measurements of surface strain rates. The method is applied to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. We show that 70%–80% of the total deformation field can be attributed to energy accumulation on known active faults while the source of the remaining 20%–30% remains unknown. Along some of the major faults in New Zealand we find some important differences in rates of energy accumulation from what is expected from geologic data. Estimated rates are significantly lower than even the lowest geologic estimates on some faults in the fault system near highly‐populated Wellington. Key Points We develop a method to invert geodetically derived strain rates for slip deficit rates on faults We find small but systematic differences between slip deficit rates and geologic slip rates About 70%–80% of the surface strain can be attributed to elastic strain due to coupling on faults

Description: Oceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges.

Description: Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N 2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N 2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region. Key Points Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edge Enhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumption No substantial total phytoplankton biomass increase was found within the eddy core