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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Numerical case study of the aerosol–cloud interactions in warm boundary layer clouds over the eastern North Atlantic with an interactive chemistry module

The presence of warm boundary layer stratiform clouds over the eastern North Atlantic (ENA) region is commonly influenced by the Azores High, especially during the summer season. To investigate comprehensive aerosol–cloud interactions, this study employs the Weather Research and Forecasting model coupled with a chemistry component (WRF-Chem), incorporating aerosol chemical components that are relevant to the formation of cloud condensation nuclei (CCN) and accounting for aerosol spatiotemporal variation. This study focuses on aerosol indirect effects, particularly the long-range transport of aerosols, in the ENA region under three different weather regimes: a ridge with a surface high-pressure system, a post-trough with a surface high-pressure system, and a weak trough. The WRF-Chem simulations conducted at a near-large-eddy scale offer valuable insights into the model's performance, especially in terms of its ability to use high spatial resolution to capture mesoscale cloud features across various weather regimes. Our result shows that introducing 5 times more aerosols to either non-precipitating or precipitating clouds significantly increases ambient CCN numbers, resulting in, to varying degrees, higher liquid water path (LWP) values. The substantial aerosol–cloud interaction especially occurs in the precipitating clouds and demonstrates the susceptibility of the LWP to changes in CCN under different regimes. Conversely, thin, non-rain clouds at the edges of a cloud system are prone to evaporation, exhibiting an aerosol drying effect. The aerosols released during this process transition back to the accumulation mode, facilitating future activation. This dynamic behavior is not adequately represented in prescribed-aerosol simulations.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Subpolar North Atlantic Mean State Affects the Response of the Atlantic Meridional Overturning Circulation to the North Atlantic Oscillation in CMIP6 Models

Abstract The Atlantic meridional overturning circulation (AMOC) plays an important role in climate, transporting heat and salt to the subpolar North Atlantic. The AMOC’s variability is sensitive to atmospheric forcing, especially the North Atlantic Oscillation (NAO). Because AMOC observations are short, climate models are a valuable tool to study the AMOC’s variability. Yet, there are known issues with climate models, like uncertainties and systematic biases. To investigate this, preindustrial control experiments from models participating in the phase 6 of Coupled Model Intercomparison Project (CMIP6) are evaluated. There is a large, but correlated, spread in the models’ subpolar gyre mean surface temperature and salinity. By splitting models into groups of either a warm–salty or cold–fresh subpolar gyre, it is shown that warm–salty models have a lower sea ice cover in the Labrador Sea and, hence, enable a larger heat loss during a positive NAO. Stratification in the Labrador Sea is also weaker in warm–salty models, such that the larger NAO-related heat loss can also affect greater depths. As a result, subsurface density anomalies are much stronger in the warm–salty models than in those that tend to be cold and fresh. As these anomalies propagate southward along the western boundary, they establish a zonal density gradient anomaly that promotes a stronger delayed AMOC response to the NAO in the warm–salty models. These findings demonstrate how model mean state errors are linked across variables and affect variability, emphasizing the need for improvement of the subpolar North Atlantic mean states in models.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

An Overview of Atmospheric Features Over the Western North Atlantic Ocean and North American East Coast—Part 2: Circulation, Boundary Layer, and Clouds

The Western North Atlantic Ocean (WNAO) is a complex land-ocean-atmosphere system that experiences a wide range of atmospheric phenomena, which in turn drive unique aerosol transport pathways, cloud morphologies, and boundary layer variability. This work, Part 2 of a 2-part paper series, provides an overview of the atmospheric circulation, boundary layer variability, three-dimensional cloud structure, and precipitation over the WNAO; the companion paper (Part 1) focused on chemical characterization of aerosol, gas, and wet deposition. Seasonal changes in the circulation and sea surface temperature explain a clear transition in cloud morphologies from small shallow cumulus clouds, convective clouds, and tropical storms in summer, to stratus/stratocumulus and multi-layer cloud systems associated with winter storms. Synoptic variability in cloud fields is estimated using satellite-based weather states, and the role of postfrontal conditions (cold-air outbreaks) in the development of stratiform clouds is further analyzed. Precipitation is persistent over the ocean, with a regional peak over the Gulf Stream path, where offshore sea surface temperature gradients are large and surface fluxes reach a regional peak. Satellite data show a clear annual cycle in cloud droplet number concentration along the coast in winter, and minima in summer, suggesting a marked annual cycle in aerosol-cloud interactions. The performance of four climate models are assessed against satellite cloud retrievals. The models qualitatively reproduce the annual cycle in cloud cover and liquid water path, but with large discrepancies across models, especially in the extra-tropics. The paper concludes with a summary of outstanding issues and recommendations for future work.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA)

With their extensive coverage, marine low clouds greatly impact global climate. Presently, marine low clouds are poorly represented in global climate models, and the response of marine low clouds to changes in atmospheric greenhouse gases and aerosols remains the major source of uncertainty in climate simulations. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In addition, the ENA is periodically impacted by continental aerosols, making it an excellent location to study the cloud condensation nuclei (CCN) budget in a remote marine region periodically perturbed by anthropogenic emissions, and to investigate the impacts of long-range transport of aerosols on remote marine clouds. The Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign was motivated by the need of comprehensive in-situ measurements for improving the understanding of marine boundary layer CCN budget, cloud and drizzle microphysics, and the impact of aerosol on marine low cloud and precipitation. The airborne deployments took place from June 21 to July 20, 2017 and January 15 to February 18, 2018 in the Azores. The flights were designed to maximize the synergy between in-situ airborne measurements and ongoing long-term observations at a ground site. Here we present measurements, observation strategy, meteorological conditions during the campaign, and preliminary findings. Lastly, we discuss future analyses and modeling studies that improve the understanding and representation of marine boundary layer aerosols, clouds, precipitation, and the interactions among them.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

An Overview of Atmospheric Features Over the Western North Atlantic Ocean and North American East Coast - Part 1: Analysis of Aerosols, Gases, and Wet Deposition Chemistry

The Western North Atlantic Ocean (WNAO) and adjoining East Coast of North America are of great importance for atmospheric research and have been extensively studied for several decades. This broad region exhibits complex meteorological features, diverse cloud types, and a wide range of conditions associated with gas and particulate species that originate from many sources regionally and from other continents. As Part 1 of a 2-part paper series, this work provides a characterization of quantities associated with atmospheric chemistry, including gases, aerosols, and wet deposition, based on satellite observations, ground-based data, model simulations, and reanalysis products. Key results include spatial and seasonal differences in composition along the North American East Coast and over the WNAO associated with varying sources of smoke and dust and meteorological drivers such as temperature, moisture, and precipitation. Spatial and seasonal variations of tropospheric carbon monoxide and ozone highlight different pathways towards accumulation of these species in the troposphere. Spatial distributions of speciated aerosol optical depth and vertical profiles of aerosol mass mixing ratios show a clear seasonal cycle highlighting the influence of both natural and anthropogenic sources in addition to the impact of intercontinental transport on their vertical profiles. Analysis of long-term climate model simulations of aerosol species and satellite observations of carbon monoxide confirm that there has been a significant decline in recent decades among anthropogenic constituents owing to regulatory activities. The results can inform current and future lines of research for the region.

Corral, Andrea↗
DOE OSTI2020

Atmospheric Research Over the Extended Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

Decades of atmospheric research have focused on the extended Western North Atlantic Ocean (WNAO) region, including the East Coast of North America and the island of Bermuda, because of its unique location that offers accessibility, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews decades of scientific investigations for the extended WNAO region. Approximately 40 combined field campaigns and long-term monitoring programs, in addition to 563 peer-reviewed publications between 1950 and 2019 have provided a firm foundation of knowledge for the extended WNAO region. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO research into eight major categories, with some studies fitting into multiple categories (relative %): Aerosols (26%), Gases (23%), Development/Validation of Techniques, Models, and Retrievals (17%), Meteorology and Transport (10%), Air-Sea Interactions (9%), Wet Deposition (6%), Clouds/Storms (6%), and Aerosol-Cloud Interactions (3%). These extensive works have revealed a series of major knowledge gaps. For instance, a disproportionately low number of studies have been devoted to aerosol-cloud interactions, which is identified here as one of the most pressing research needs for the extended WNAO region. Recommendations for future research are provided in the categories highlighted above. Part 2 of this paper series will summarize major spatial and temporal features for the extended WNAO region.

Sorooshian, Armin↗
DOE OSTI2024

Wide range of possible trajectories of North Atlantic climate in a warming world

Decadal variability in the North Atlantic Ocean impacts regional and global climate, yet changes in internal decadal variability under anthropogenic radiative forcing remain largely unexplored. Here we use the Community Earth System Model 2 Large Ensemble under historical and the Shared Socioeconomic Pathway 3-7.0 future radiative forcing scenarios and show that the ensemble spread in northern North Atlantic sea surface temperature (SST) more than doubles during the mid-twenty-first century, highlighting an exceptionally wide range of possible climate states. Furthermore, there are strikingly distinct trajectories in these SSTs, arising from differences in the North Atlantic deep convection among ensemble members starting by 2030. We propose that these are stochastically triggered and subsequently amplified by positive feedbacks involving coupled ocean-atmosphere-sea ice interactions. Freshwater forcing associated with global warming seems necessary for activating these feedbacks, accentuating the impact of external forcing on internal variability. Further investigation on seven additional large ensembles affirms the robustness of our findings. By monitoring these mechanisms in real time and extending dynamical model predictions after positive feedbacks activate, we may achieve skillful long-lead North Atlantic decadal predictions that are effective for multiple decades.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Atmospheric Transport of North African Dust-Bearing Supermicron Freshwater Diatoms to South America: Implications for Iron Transport to the Equatorial North Atlantic Ocean

The equatorial North Atlantic Ocean (NAO) is a nutrient-limited ecosystem that relies on the deposition of long-range transported iron (Fe)-containing aerosols to stimulate primary productivity. Using microscopy, we characterized supermicron and supercoarse mode African aerosols transported to the western NAO in boreal winter/spring. In this work, we detected three particle types including African dust, primary biological aerosol particles, and freshwater diatoms (FDs). FDs contained 4% Fe by weight due to surficial dust inclusions that may be susceptible to chemical processing and dissolution. FDs were typically larger than dust particles and comprised 38% of particles between 10 and 18 μm in diameter. The low density, high surface-area-to-volume ratio, and large aspect ratios of FD particles suggest a mechanism by which they can be carried great distances aloft. These same properties likely increase the residence time of FDs in surface waters thereby increasing the time for Fe dissolution and their potential impact on marine biogeochemical cycles.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Impact of US SO 2 Emission Reductions Between 1970 and 2010 on Seasonal Sulfate Aerosol Burden and Radiative Forcing Over the North Atlantic

Sulfate burden over the North Atlantic Ocean (NATL) exhibits strong seasonality despite no seasonality in anthropogenic sulfur dioxide (SO 2 ) emissions. However, the seasonality of sulfate aerosols over NATL has decreased since 1970, likely due to a reduction in the United States (US) SO 2 emissions following the Clean Air Act of 1970. We performed atmospheric chemistry and transport simulations to assess the impact of changing US SO 2 emissions between 1970 and 2010 on NATL sulfate burden and radiative forcing. United States SO 2 emission reductions weakened the seasonality in NATL sulfate burden by ∼17%, primarily due to a decrease in chemical production and transport in summer. These emission reductions caused a summertime radiative forcing (∼2 W m −2 ) twice as large as the wintertime forcing. Our findings highlight the complex, season-dependent responses of sulfate burden and radiative effects to regional emission changes.

North Atlantic↗
DOE OSTI2024

Roles of the atmosphere and ocean in the projected north atlantic warming hole

There exists a warming deficit in sea surface temperatures (SST) over the subpolar North Atlantic in response to quadrupled CO 2 , referred to as the projected North Atlantic warming hole (WH). This study employs a partial coupling technique to accurately verify the relative roles of oceanic and atmospheric processes in the formation of the projected WH within an atmosphere-ocean coupled framework. By decomposing the SST anomalies in the subpolar North Atlantic into two components: those induced by atmospheric processes (i.e., the atmosphere-forced component) and those driven by changes in ocean circulation (i.e., the ocean-driven component), we find that the projected WH is primarily driven by changes in ocean circulation, with almost no contribution from atmospheric processes. Specifically, the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) results in a cooling of SST in the WH region due to reduced northward ocean heat transport into this region. This study further quantifies the influence of a positive coupled feedback through surface heat flux (SHF) on the AMOC response under greenhouse gas forcing within this self-consistent framework. It is found that the AMOC slowdown leads to a negative SST anomaly in the subpolar North Atlantic and subsequently a positive ocean-driven SHF anomaly, which in turn further weakens the AMOC. In conclusion, this positive feedback through the SHF contributes about 50% to the total AMOC slowdown in response to quadrupled CO 2 .

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Aircraft measurements of aerosol and trace gas chemistry in the eastern North Atlantic

The Aerosol and Cloud Experiment in the Eastern North Atlantic (ACE-ENA) investigated properties of aerosols and subtropical marine boundary layer (MBL) clouds. Low subtropical marine clouds can have a large effect on Earth's radiative budget, but they are poorly represented in global climate models. In order to understand their radiative effects, it is imperative to understand the composition and sources of the MBL cloud condensation nuclei (CCN). The campaign consisted of two intensive operation periods (IOPs) (June–July 2017 and January–February 2018) during which an instrumented G-1 aircraft was deployed from Lajes Field on Terceira Island in the Azores, Portugal. The G-1 conducted research flights in the vicinity of the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) atmospheric observatory on Graciosa Island. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and Ionicon proton-transfer-reaction mass spectrometer (PTR-MS) were deployed aboard the aircraft, characterizing chemistry of non-refractory aerosol and trace gases, respectively. The eastern North Atlantic region was found to be very clean, with an average non-refractory submicrometer aerosol mass loading of 0.6 µg m −3 in the summer and 0.1 µg m −3 in the winter, measured by the AMS. Average concentrations of the trace reactive gases methanol and acetone were 1–2 ppb; benzene, toluene and isoprene were even lower, <1 ppb. Mass fractions of sulfate, organics, ammonium and nitrate in the boundary layer were 69 %, 23 %, 7 % and 1 % and remained largely similar between seasons. The aerosol chemical composition was dominated by sulfate and highly processed organics. Particulate methanesulfonic acid (MSA), a well-known secondary biogenic marine species, was detected, with an average boundary layer concentration of 0.021 µg m −3 , along with its gas-phase precursor, dimethyl sulfide (DMS). MSA accounted for no more than 3 % of the submicron, non-refractory aerosol in the boundary layer. Examination of vertical profiles of aerosol and gas chemistry during ACE-ENA reveals an interplay of local marine emissions and long-range-transported aged aerosol. A case of transport of biomass burning emissions from North American fires has been identified using back-trajectory analysis. In the summer, the non-refractory portion of the background CCN budget was heavily influenced by aerosol associated with ocean productivity, in particular sulfate formed from DMS oxidation. Episodic transport from the continents, particularly of biomass burning aerosol, periodically increased CCN concentrations in the free troposphere. In the winter, with ocean productivity lower, CCN concentrations were overall much lower and dominated by remote transport. These results show that anthropogenic emissions perturb CCN concentrations in remote regions that are sensitive to changes in CCN number and illustrate that accurate predictions of both transport and regional aerosol formation from the oceans are critical to accurately modeling clouds in these regions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Observation-Based Constraints on the Condensational Growth of Marine Aerosol Particles to Cloud Condensation Nuclei in the Eastern North Atlantic

This report describes the results from a field project in the Eastern North Atlantic (Aerosol Growth in the Eastern North Atlantic; AGENA) that was designed to determine the source of condensable material that contributes to the growth of Aitken-mode aerosol particles to CCN in the remote marine boundary layer at the ARM Eastern North Atlantic (ENA) location.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2025

Atmospheric and oceanic energy transport during North Atlantic freshening events: influences of moisture transport and hydrologic cycle feedbacks

Analogs of present-day rapid ice melt can be found in episodic discharges of icebergs that occurred during glacial periods called Heinrich events. This introduces excess meltwater into the North Atlantic and weakens the Atlantic thermohaline circulation (AMOC), triggering a hydrologic cycle–AMOC collapse feedback as the atmospheric energy transport compensates for reduced northward heat transport. Here we employ a novel series of 100-year North Atlantic “hosing” simulations to investigate atmospheric and oceanic energy transport response from freshwater forcing, focusing in particular on the role of atmospheric rivers (ARs) within atmospheric energy transport. Importantly, we use an “overwriting” methodology that allow us to attribute AMOC weakening to added North Atlantic meltwater and subsequent hydrologic cycle responses, respectively. In contrast to far-reaching response of transient eddies, our results show a substantial increase in moisture convergence from ARs that is geographically constrained to the North Atlantic midlatitudes. Such AR changes nevertheless comprise an important component of net precipitation changes over the Euro-Atlantic sector, with the amount being comparable to that from transient eddies over the subpolar Atlantic. Over the course of the century-long simulations, we demonstrate that hydrologic cycle responses to North Atlantic freshening and subsequent feedbacks, including those from ARs, account for approximately half of the simulated AMOC collapse. Furthermore, our work highlights the dynamics of atmospheric moisture transport response to North Atlantic freshening events and elucidates how intensifying moisture transport may accelerate AMOC collapse in the future.

Atmospheric Science↗
DOE OSTI2020

Decadal predictability of North Atlantic blocking and the NAO

Can multi-annual variations in the frequency of North Atlantic atmospheric blocking and mid-latitude circulation regimes be skilfully predicted? Recent advances in seasonal forecasting have shown that mid-latitude climate variability does exhibit significant predictability. However, atmospheric predictability has generally been found to be quite limited on multi-annual timescales. New decadal prediction experiments from NCAR are found to exhibit remarkable skill in reproducing the observed multi-annual variations of wintertime blocking frequency over the North Atlantic and of the North Atlantic Oscillation (NAO) itself. This is partly due to the large ensemble size that allows the predictable component of the atmospheric variability to emerge from the background chaotic component. The predictable atmospheric anomalies represent a forced response to oceanic low-frequency variability that strongly resembles the Atlantic Multi-decadal Variability (AMV), correctly reproduced in the decadal hindcasts thanks to realistic ocean initialization and ocean dynamics. The occurrence of blocking in certain areas of the Euro-Atlantic domain determines the concurrent circulation regime and the phase of known teleconnections, such as the NAO, consequently affecting the stormtrack and the frequency and intensity of extreme weather events. Therefore, skilfully predicting the decadal fluctuations of blocking frequency and the NAO may be used in statistical predictions of near-term climate anomalies, and it provides a strong indication that impactful climate anomalies may also be predictable with improved dynamical models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

The Remarkable 2024 North Atlantic Mid‐Season Hurricane Lull

Abstract The 2024 North Atlantic (hereafter Atlantic) hurricane season started quickly, with the earliest Category 5 on record (Beryl) and three hurricanes forming through 14 August. Following Ernesto's dissipation on 20 August, the Atlantic hurricane season became extremely quiet during the climatological peak of hurricane season, with only one Category 2 hurricane (Francine) and one tropical storm through 23 September. Several environmental factors likely contributed to this unexpected, prolonged lull. During mid‐to‐late August, subseasonal conditions were broadly favorable for Atlantic hurricanes, but a northward shift in African easterly wave emergence latitude yielded fewer tropical cyclone seed disturbances that also traversed unfavorably cool ocean water. During early‐to‐mid September, subseasonal variability driven by the Madden‐Julian oscillation was less conducive to hurricane activity, with several bouts of increased vertical wind shear across the central Atlantic. Throughout most of the lull, the tropical Atlantic was anomalously dry and subsident, suppressing hurricane formation chances. Plain Language Summary The 2024 North Atlantic (hereafter Atlantic) hurricane season was anticipated to be extremely active, with seasonal forecasts calling for a potentially historic season. Following a busy start including the earliest Category 5 Atlantic hurricane on record (Beryl), the season became very quiet between 20 August–23 September, with only one Category 2 hurricane (Francine) and one tropical storm forming. Following the lull, the Atlantic became extremely busy again, with seven hurricanes forming from 25 September through the end of the season–the most on record for this period. This paper focuses on the surprising lull and attributes it to several different factors, including a late‐August northward shift in African easterly wave tracks (organized thunderstorm complexes spanning thousands of kilometers that often serve as Atlantic hurricane seeds). This northward shift brought these systems out over colder water and into a drier airmass, suppressing their hurricane formation chances. During the first 3 weeks of September, a large‐scale tropical phenomenon known as the Madden‐Julian oscillation, resulted in increased sinking motion, drier mid‐levels and increased vertical wind shear–all of which typically reduce Atlantic hurricane activity. Overall, the lull demonstrates how seed disturbances and other environmental factors can strongly modulate aggregate seasonal tropical cyclone activity. Key Points Following an active start, the 2024 Atlantic hurricane was quiet during the climatological peak with only 1 tropical storm and 1 hurricane The lull was not anticipated by seasonal forecasting agencies, which called for an extremely active 2024 Atlantic hurricane season Factors reducing storm count included a northward shift in African easterly waves, broad‐scale subsidence, and reduced mid‐level moisture

Klotzbach, P J↗
DOE OSTI2022

Environmental effects on aerosol–cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

Abstract. Over the eastern North Atlantic (ENA) ocean, a total of 20 non-precipitating single-layer marine boundary layer (MBL) stratus and stratocumulus cloud cases are selected to investigate the impacts of the environmental variables on the aerosol–cloud interaction (ACIr) using the ground-based measurements from the Department of Energy Atmospheric Radiation Measurement (ARM) facility at the ENA site during 2016–2018. The ACIr represents the relative change in cloud droplet effective radius re with respect to the relative change in cloud condensation nuclei (CCN) number concentration at 0.2 % supersaturation (NCCN,0.2 %) in the stratified water vapor environment. The ACIr values vary from −0.01 to 0.22 with increasing sub-cloud boundary layer precipitable water vapor (PWVBL) conditions, indicating that re is more sensitive to the CCN loading under sufficient water vapor supply, owing to the combined effect of enhanced condensational growth and coalescence processes associated with higher Nc and PWVBL. The principal component analysis shows that the most pronounced pattern during the selected cases is the co-variations in the MBL conditions characterized by the vertical component of turbulence kinetic energy (TKEw), the decoupling index (Di), and PWVBL. The environmental effects on ACIr emerge after the data are stratified into different TKEw regimes. The ACIr values, under both lower and higher PWVBL conditions, more than double from the low-TKEw to high-TKEw regime. This can be explained by the fact that stronger boundary layer turbulence maintains a well-mixed MBL, strengthening the connection between cloud microphysical properties and the below-cloud CCN and moisture sources. With sufficient water vapor and low CCN loading, the active coalescence process broadens the cloud droplet size spectra and consequently results in an enlargement of re. The enhanced activation of CCN and the cloud droplet condensational growth induced by the higher below-cloud CCN loading can effectively decrease re, which jointly presents as the increased ACIr. This study examines the importance of environmental effects on the ACIr assessments and provides observational constraints to future model evaluations of aerosol–cloud interactions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

Fine Aerosol Acidity and Water during Summer in the Eastern North Atlantic

Aerosol pH governs many important atmospheric processes that occur in the marine boundary layer, including regulating halogen and sulfur chemistries, and nutrient fertilization of surface ocean waters. In this study, we investigated the acidity of PM1 over the eastern North Atlantic during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) aircraft campaign. The ISORROPIA-II thermodynamic model was used to predict PM1 pH and water. We first investigated the sensitivities of PM1 pH and water predictions to gas-phase NH3 and HNO3 concentrations. Our sensitivity analysis indicated that even though NH3 and HNO3 were present at very low concentrations in the eastern North Atlantic during the campaign, PM1 pH calculations can still be sensitive to NH3 concentrations. Specifically, NH3 was needed to constrain the pH of populations of PM1 that had low mass concentrations of NH4+ and non-volatile cations (NVCs). We next assumed that gas-phase NH3 and HNO3 concentrations during the campaign were 0.15 and 0.09 µg m−3, respectively, based on previous measurements conducted in the eastern North Atlantic. Using the assumption that PM1 were internally mixed (i.e., bulk PM1), we determined that PM1 pH ranged from 0.3–8.6, with a mean pH of 5.0 ± 2.3. The pH depended on both Hair+ and Wi. Hair+ was controlled primarily by the NVCs/SO42− molar ratio, while Wi was controlled by the SO42− mass concentration and RH. Changes in pH with altitude were driven primarily by changes in SO42−. Since aerosols in marine atmospheres are rarely internally mixed, the scenario where non-sea salt species and sea-salt species were present in two separate aerosol modes in the PM1 (i.e., completely externally mixed) was also considered. Smaller pH values were predicted for the aerosol mode comprised only of non-sea salt species compared to the bulk PM1 (difference of around 1 unit on average). This was due to the exclusion of sea-salt species (especially hygroscopic alkaline NVCs) in this aerosol mode, which led to increases in Hair+ values and decreases in Wi values. This result demonstrated that assumptions of aerosol mixing states can impact aerosol pH predictions substantially, which will have important implications for evaluating the nature and magnitude of pH-dependent atmospheric processes that occur in the marine boundary layer.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2023

Physical Insights From the Multidecadal Prediction of North Atlantic Sea Surface Temperature Variability Using Explainable Neural Networks

Abstract North Atlantic sea surface temperatures (NASST), particularly in the subpolar region, are among the most predictable in the world's oceans. However, the relative importance of atmospheric and oceanic controls on their variability at multidecadal timescales remain uncertain. Neural networks (NNs) are trained to examine the relative importance of oceanic and atmospheric predictors in predicting the NASST state in the Community Earth System Model 1 (CESM1). In the presence of external forcings, oceanic predictors outperform atmospheric predictors, persistence, and random chance baselines out to 25‐year leadtimes. Layer‐wise relevance propagation is used to unveil the sources of predictability, and reveal that NNs consistently rely upon the Gulf Stream‐North Atlantic Current region for accurate predictions. Additionally, CESM1‐trained NNs successfully predict the phasing of multidecadal variability in an observational data set, suggesting consistency in physical processes driving NASST variability between CESM1 and observations.

Geology↗