Meridional Overturning Research Articles

Precisely constrained 134-ka strong monsoon event in the penultimate deglaciation by an annually laminated speleothem from the Asian monsoon domain

AbstractThe penultimate deglaciation was characterized by a sub-millennial-scale warm event in the Heinrich Stadial 11(HS11), termed the 134-ka event. However, its precise timing and structure remain poorly constrained due to the lack of high-resolution and precisely dated records. We present an oxygen isotope record of a speleothem with well-developed annual lamina from Zhangjia Cave, located on the north margin of the Sichuan Basin, characterizing Asian summer monsoon (ASM) changes in the 134-ka event, which included an increase excursion of ca. 149 years and decrease excursion of ca. 200 years, inferred from 3.3‰ δ18O variations. This event also divided the weak ASM interval-II (WMI-II), corresponding to HS11, into two stages, the WMI-IIa 132.8–134.1 ka and WMI-IIb 134.4–136.4 ka. With a comparable climatic pattern globally, the 134-ka event is essentially similar to the millennial-scale events in last glacial–deglacial period. Particularly, the observed weak-strong-weak ASM sequence (138.8–132.8 ka) is largely controlled by changes in the Atlantic Meridional Overturning Circulation (AMOC) forced by the meltwater of northern high-latitude ice sheets. Moreover, our results underpin that AMOC, rather than the global ice volume, is more critical to ASM variations during the last two deglaciations.

The role of internal feedbacks in sustaining multi-centennial variability of the Atlantic Meridional Overturning Circulation revealed by EC-Earth3-LR simulations

A significant multi-centennial climate variability with a distinct peak at approximately 200 years is observed in a pre-industrial (PI) control simulation using the EC-Earth3-LR climate model. This oscillation originates predominately from the North Atlantic and displays a strong association with the Atlantic Meridional Overturning Circulation (AMOC). Our study identifies the interplay between salinity advection feedback and vertical mixing in the subpolar North Atlantic as key roles in providing the continues internal energy source to maintain this multi-centennial oscillation. The perturbation flow of mean subtropical-subpolar salinity gradients serves as positive feedback to sustain the AMOC anomaly, while the mean advection of salinity anomalies and the vertical mixing or convection acts as negative feedback, constraining the AMOC anomaly. Notably, this low-frequency variability persists even in a warmer climate with weakened AMOC, emphasizing the robustness of the salinity advection feedback mechanism.

Geostrophic and Mesoscale Eddy Contributions to the Atlantic Meridional Overturning Circulation Decline under CO2 Increase in the GFDL CM2-O Model Suite

Abstract The pattern and magnitude of the Atlantic meridional overturning circulation (AMOC) in response to an increase in atmospheric carbon dioxide (CO2) concentration greatly differ across climate models in particular due to differences in the representation of oceanic processes. Here, we investigate the response of the AMOC to an idealized climate change scenario, along with the drivers of this response, in the three configurations of a coupled climate model suite with varying resolutions in the ocean (1°, 0.25°, 0.10°). In response to the CO2 increase, the AMOC shows a reduction of similar magnitude in the low and high resolutions, while a muted response is found in the medium resolution. A decomposition of the AMOC into its geostrophic and residual components reveals that most of the AMOC reduction is due to a weakening of the geostrophic streamfunction driven by temperature anomalies, partly opposed by a strengthening of the geostrophic streamfunction driven by salinity anomalies. Changes in the AMOC due to the mesoscale eddy streamfunction contribute to 13% and 17% of the AMOC decline in the low and high resolutions, respectively, but induce very little change in the medium resolution. The similar response of the AMOC strength in the low and high resolutions hides important differences in the contribution and pattern of the geostrophic and eddy streamfunctions. The lack of sensitivity of the medium resolution to the CO2 forcing is due to a weak connection between the deep water formation regions in the northern subpolar gyre and the Deep Western Boundary Current. Significance Statement The Atlantic meridional overturning circulation (AMOC) is a major system of ocean currents in the Atlantic that contributes to shaping the climate at regional and global scales, notably through the transport of heat from the low to the high latitudes. A major slowdown of the AMOC over the twenty-first century is predicted by current climate models in response to increasing greenhouse gases. Yet, the magnitude and timing of this slowdown are uncertain. The purpose of this study is to investigate the expected weakening of the AMOC using state-of-the-art numerical climate models that include higher resolutions than typically used in climate change assessments. Our results provide insights into the mechanisms driving the weakening of the AMOC and into differences arising from model resolutions.

Bifurcation analysis of a North Atlantic Ocean box model with two deep-water formation sites

The tipping of the Atlantic Meridional Overturning Circulation (AMOC) to a ‘shutdown’ state due to changes in the freshwater forcing of the ocean is of particular interest and concern due to its widespread ramifications, including a dramatic climatic shift for much of Europe. A clear understanding of how such a shutdown would unfold requires analyses of models from across the complexity spectrum. For example, detailed simulations of sophisticated Earth System Models have identified scenarios in which deep-water formation first ceases in the Labrador Sea before ceasing in the Nordic Seas, en route to a complete circulation shutdown. Here, we study a simple ocean box model with two polar boxes designed to represent deep-water formation at these two distinct sites. A bifurcation analysis reveals how, depending on the differences of freshwater and thermal forcing between the two polar boxes, transitions to ‘partial shutdown’ states are possible. Our results shed light on the nature of the tipping of AMOC and clarify dynamical features observed in more sophisticated models.

The effect of initialisation on 20 year multi-decadal climate predictions

This study investigates initialised decadal predictions of 2-m air temperature over lead times of up to 20 years and compares them against uninitialised simulations in the time period 1960–2019. We demonstrate that in the North and South Atlantic, as well as in the Northeast Pacific, the effect of initialisation within the prediction persists for longer than 10 lead years. In these regions, the skill of the initialised decadal predictions does not necessarily regress back to the skill of the uninitialised simulations, which is indeed the case for other regions. We analyse the Atlantic Meridional Overturning Circulation (AMOC) and show that within the first 10 years after initialisation, it drifts towards a state, which is different from both the initial state and the state of the uninitialised simulations. We show that the AMOC stays within this new state for at least another 10 years. We find that in our decadal predictions, the correct determination of future external forcings plays an important role on the global scale, while correct initialisation increases prediction skill on the regional scale.

Deglacial Temperature and Carbonate Saturation State Variability in the Tropical Atlantic at Antarctic Intermediate Water Depths

AbstractVariations in the Atlantic Meridional Overturning Circulation (AMOC) redistribute heat and nutrients, causing pronounced anomalies of temperature and nutrient concentrations in the subsurface ocean. However, exactly how millennial‐scale deglacial AMOC variability influenced the subsurface is debated, and the role of other deglacial forcings of subsurface temperature change is unclear. Here, we present a new deglacial temperature reconstruction, which, with published records, helps assess competing hypotheses for deglacial warming in the upper tropical North Atlantic. Our record provides new evidence of regional subsurface warming in the western tropical North Atlantic within the core of modern Antarctic Intermediate Water (AAIW) during Heinrich Stadial 1 (HS1), an early deglacial interval of iceberg discharge into the North Atlantic. Our results are consistent with model simulations that suggest subsurface heat accumulates in the northern high‐latitude convection regions and along the upper AMOC return path when the AMOC weakens, and with warming due to rising greenhouse gases. Warming of AAIW may have also contributed to warming in the tropics at modern AAIW depths during late HS1. Nutrient and reconstructions from the same site suggest a link between AMOC intensity and the northward extent of AAIW in the northern tropics across the deglaciation and on millennial time scales. However, the timing of the initial deglacial increase in AAIW to the northern tropics is ambiguous. Deglacial trends and variability of in the upper North Atlantic have likely biased temperature reconstructions based on the elemental composition of calcitic benthic foraminifera.

Mechanism on the Short-Term Variability of the Atlantic Meridional Overturning Circulation in the Subtropical and Tropical Regions

Abstract The continuous, moored observation revealed significant variability in the strength of the Atlantic meridional overturning circulation (AMOC). The cause of such AMOC variability is an extensively studied subject. This study focuses on the short-term variability, which ranges up to seasonal and interannual time scales. A mechanism is proposed from the perspective of ocean water redistribution by layers. By offering explanations for four phenomena of AMOC variability in the subtropical and tropical oceans (seasonality, meridional coherence, layered-transport compensation as observed at 26.5°N, and the 2009/10 downturn that occurred at 26.5°N), this mechanism suggests that the short-term AMOC variabilities in the entire subtropical and tropical regions are governed by a basinwide adiabatic water redistribution process, or the so-called sloshing dynamics, rather than diapycnal processes. Significance Statement The Atlantic meridional overturning circulation (AMOC) is a key component in the global climate system due to its immense power in redistributing heat meridionally, which contributes to the hospitable climate of the United Kingdom and western Europe. Therefore, any changes in AMOC can have significant impacts on both global and local climate variability. Here I propose a mechanism to explain the short-term (up to interannual) AMOC variability in the subtropical and tropical regions from the perspective of ocean water redistribution. This mechanism suggests that the short-term variability of AMOC strength is dominated by an adiabatic process, and thus, its large-amplitude variation is mostly a reversible process. In other words, AMOC may be more resilient to short-term variability than previously believed, and it could recover autonomously from the abrupt changes.

Meridional Connectivity of a 25‐Year Observational AMOC Record at 47°N

AbstractSince climate model studies project a decline of the Atlantic Meridional Overturning Circulation (AMOC) in the 21st century, monitoring AMOC changes remains essential. While AMOC variability is expected to be coherent across latitudes on longer than decadal timescales, connectivity on inter‐annual and seasonal timescales is less clear. Model studies and observational estimates disagree on the regions and timescales of meridional connectivity and AMOC observations at multiple latitudes are needed to study its connectivity. We calculate basin‐wide AMOC volume transports (1993–2018) from measurements of the North Atlantic Changes (NOAC) array at 47°N, combining data from moored instruments with hydrography and satellite altimetry. The mean NOAC AMOC is 17.2 Sv exhibiting no long‐term trend. Both the unfiltered and low‐pass filtered NOAC AMOC show a significant correlation with the RAPID‐MOCHA‐WBTS AMOC at 26°N when the NOAC AMOC leads by about one year.

Atlantic meridional overturning circulation increases flood risk along the United States southeast coast

The system of oceanic flows constituting the Atlantic Meridional Overturning Circulation (AMOC) moves heat and other properties to the subpolar North Atlantic, controlling regional climate, weather, sea levels, and ecosystems. Climate models suggest a potential AMOC slowdown towards the end of this century due to anthropogenic forcing, accelerating coastal sea level rise along the western boundary and dramatically increasing flood risk. While direct observations of the AMOC are still too short to infer long-term trends, we show here that the AMOC-induced changes in gyre-scale heat content, superimposed on the global mean sea level rise, are already influencing the frequency of floods along the United States southeastern seaboard. We find that ocean heat convergence, being the primary driver for interannual sea level changes in the subtropical North Atlantic, has led to an exceptional gyre-scale warming and associated dynamic sea level rise since 2010, accounting for 30-50% of flood days in 2015-2020.

The magnitude and source of meltwater forcing of the 8.2 ka climate event constrained by relative sea-level data from eastern Scotland

The 8.2 ka climate event is the most significant North Atlantic cooling event during the Holocene. Freshwater pulses from the melting Laurentide Ice Sheet draining into the North Atlantic Ocean are commonly thought to be its cause by perturbing the Atlantic Meridional Overturning Circulation. The timing, magnitude and number of freshwater pulses, however, remain uncertain. This is problematic for predicting future climate scenarios because it prevents rigorous testing of coupled ocean–atmosphere climate models against an otherwise excellent test case of climate effects of meltwater inputs into the North Atlantic. To address this knowledge gap, we present a high-resolution relative sea-level record from the Ythan Estuary, Scotland, spanning the centuries leading into the 8.2 ka climate event. The results show a ‘sea-level event’ with two distinct stages between 8,530 and 8,240 cal yr BP when rates of sea-level rise departed from the background rates of around 2 mm yr-1 and reached around 13 mm yr-1 and 4 mm yr-1, respectively. The maximum probable magnitude of local sea-level rise during the stages was 1.67 and 0.41 m, which equate to barystatic magnitudes of 2.39 and 0.58 m respectively after considering the geographic location relative to the source.For the first time, we demonstrate that Lake Agassiz-Ojibway drainage alone is insufficient to explain the large volumes of North Atlantic freshwater input, and that the collapse of the Hudson Bay Ice Saddle appears to have been the main source of meltwater in to the North Atlantic. By comparing the Ythan sea-level record with other sources of evidence, we hypothesise that an initial thinning of the Laurentide Ice Sheet enabled subglacial drainage of Lake Agassiz and subsequent collapse of the Hudson Bay Ice Saddle. This was followed by the terminal drainage of Lake Agassiz completing a sequence of events that likely forced the shift in the Atlantic Meridional Overturning Circulation and hence the 8.2 ka climate event.

High-northern-latitude forcing modulated fluvial‒aeolian interplay during the Last Glacial Period in the Mu Us Desert, northern central China

Substantial stratigraphic evidence from the Late Quaternary at orbital- and millennial‒scales indicates that the formation and evolution of aeolian‒sand landscapes results from the interaction between regional aeolian and fluvial‒lacustrine activity. In many deserts (sandy lands) across the planet, fluvial‒aeolian interplay at the glacial‒interglacial scale since the Late Quaternary has been responsive to monsoonal precipitation, and linked to atmospheric CO2 and periodic variations in insolation. However, the patterns of fluvial‒aeolian interplay and their relationship with climate change are not always consistent over a given timescale during the Late Pleistocene. The paucity of studies on fluvial‒aeolian interplay in the Mu Us Desert (MUD) during the Last Glacial Period (LGP) hinders our understanding of the relationships between the dynamics of aeolian and fluvial‒lacustrine landscapes and their drivers. Here, based on a comprehensive field survey and stratigraphic dating of a typical fluvial‒aeolian sequence in the MUD, we synthesized the chronology of fluvial‒lacustrine and aeolian strata during the LGP. Our results show that extensive development of a fluvial‒lacustrine environment in the MUD during the LG mainly occurred during the late Last Glacial Maximum (LGMa, 20.1–18.4 ka BP) and Bølling/Allerød interstadial (14.8–12.6 ka BP), while thicker aeolian sands extensively accumulated during the Heinrich Stadial 2, Heinrich Stadial 1, and Younger Dryas periods. However, the development of fluvial‒lacustrine environments showed considerable spatial heterogeneity, mainly occurring on the southern and eastern margins of the desert, while they were extremely rare in the central and western parts of the desert owing to strong wind erosion. Comparisons with other high-resolution geological records from the Asian monsoon region reveal that the fluvial‒aeolian interplay in the MUD during the LGP was highly consistent with the evolution pattern of the East Asian summer monsoon (EASM) on the millennial‒centennial scale, and indicate that the development of fluvial‒lacustrine environments was mainly controlled by an increase in monsoonal precipitation during strengthening of the EASM. Consistency with other broad changes at high latitudes in the Northern Hemisphere suggests that fluvial‒aeolian interplay during the LGP in northern Chinese deserts, as represented by the MUD, was mainly controlled by changes in high-northern-latitude ice volume and the Atlantic Meridional Overturning Circulation.

Decadal changes in Atlantic overturning due to the excessive 1990s Labrador Sea convection

Changes in the Atlantic Meridional Overturning Circulation (AMOC) represent a crucial component of Northern Hemisphere climate variability. In modelling studies decadal overturning variability has been attributed to the intensity of deep winter convection in the Labrador Sea. This linkage is challenged by transport observations at sections across the subpolar gyre. Here we report simulations with an eddy-rich ocean model which captures the observed concentration of downwelling in the northeastern Atlantic and the negligible impact of interannual variations in Labrador Sea convection during the last decade. However, the exceptionally cold winters in the Labrador Sea during the first half of the 1990s induced a positive AMOC anomaly of more than 20%, mainly by augmenting the downwelling in the northeastern North Atlantic. The remote effect of excessive Labrador Sea buoyancy forcing is related to rapid spreading of mid-depth density anomalies into the Irminger Sea and their entrainment into the deep boundary current off Greenland.

Propagation and Transformation of Upper North Atlantic Deep Water From the Subpolar Gyre to 26.5°N

AbstractBecause new observations have revealed that the Labrador Sea is not the primary source for waters in the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) during the Overturning in the Subpolar North Atlantic Programme (OSNAP) period, it seems timely to re‐examine the traditional interpretation of pathways and property variability for the AMOC lower limb from the subpolar gyre to 26.5°N. In order to better understand these connections, Lagrangian experiments were conducted within an eddy‐rich ocean model to track upper North Atlantic Deep Water (uNADW), defined by density, between the OSNAP line and 26.5°N as well as within the Labrador Sea. The experiments reveal that 77% of uNADW at 26.5°N is directly advected from the OSNAP West section along the boundary current and interior pathways west of the Mid‐Atlantic Ridge. More precisely, the Labrador Sea is a main gateway for uNADW sourced from the Irminger Sea, while particles connecting OSNAP East to 26.5°N are exclusively advected from the Iceland Basin and Rockall Trough along the eastern flank of the Mid‐Atlantic Ridge. Although the pathways between OSNAP West and 26.5°N are only associated with a net formation of 1.1 Sv into the uNADW layer, they show large density changes within the layer. Similarly, as the particles transit through the Labrador Sea, they undergo substantial freshening and cooling that contributes to further densification within the uNADW layer.

Response of ocean climate to different heat-flux perturbations over the North Atlantic in FAFMIP

The diversity of surface-flux perturbations, especially for heat-flux perturbations, notably leads to uncertainties surrounding the responses of ocean climate under global warming scenarios projected by climate/Earth system models. However, when imposing heat-flux perturbations on the models, strong feedback persists between the atmosphere and the ocean, resulting in nearly doubled heat-flux perturbation over the North Atlantic (NA). Herein, quantitative evaluation of the influences of magnitude change of heat-flux perturbations over the NA on the changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat uptake (OHU) and dynamic sea level (DSL) has been conducted by analysis of eight coupled model responses to the heat-flux perturbation experiments in the Flux-Anomaly-Forced Model Inter-comparison Project. It has been demonstrated that the magnitude of the AMOC change is extremely sensitive to the magnitude change of imposed NA heat-flux perturbation, and the weakening amplitude of the AMOC is nearly halved as the imposed heat-flux perturbation F is halved over the NA. The most remarkable responses of both DSL and OHU to the magnitude changes of NA heat-flux perturbation have been primarily found in the Atlantic and Arctic basins, especially for the NA region. Both the added ocean heat uptake (OHUa) and redistributed ocean heat uptake (OHUr) play key roles in OHU changes among the various NA heat-flux perturbation experiments. The magnitude change of NA-mean OHUa is almost linearly related to the imposed NA heat-flux perturbation, while the magnitude change of NA-mean OHUr, which is primarily caused by AMOC change and redistributed heat flux, is not proportional to the imposed NA heat-flux perturbation. There is a nearly linear relationship between the magnitude of AMOC change and the OHUr in tropical regions, including the regions in the low-latitude South Atlantic, the tropical Pacific Ocean and the Indian Ocean.

The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

AbstractThe specifics of the simulated injection choices in the case of stratospheric aerosol injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle‐of‐the‐road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5°C above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0°C and 0.5°C above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy‐relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.

Diurnal variations of the meridional overturning circulations over West Africa during the premonsoon and monsoon seasons

AbstractDiurnal variations in the shallow and deep meridional overturning over West Africa during the premonsoon (April–May) and monsoon (July–September) seasons were analyzed using ECMWF Reanalysis 5th Generation (ERA5) reanalysis and Tropical Rainfall Measuring Mission (TRMM)/Global Precipitation Measurement (GPM) satellite radar data from 1998 to 2019. We found that a daytime sea breeze and two nocturnal low‐level jets have varying impacts on the diurnal cycle of convection and the meridional circulations over West Africa, depending on the season. During the premonsoon, the zonal rain belt is centered over the Gulf of Guinea and the sea breeze initiates intense deep convection (and thus deep overturning) over the coastal region in the afternoon that propagates northward into the early evening. The nocturnal Sahel/Sahara low‐level jet (SLLJ) then drives low‐level convergence into the West African heat low, producing a strong, dry shallow overturning over land at night. During the monsoon, the rain belt is centered over land (N) and the sea breeze is forced by weaker temperature gradients, producing less intense convection that does not propagate as far inland as during the premonsoon. However, organized convection inland maintains deep daytime overturning. At night, a coastal low‐level jet strengthens northward moisture transport from the Gulf of Guinea into the active monsoon region after the sea breeze dissipates, helping maintain convective systems with large stratiform components and continued deep meridional overturning over land. The nocturnal SLLJ still drives low‐level convergence into the West African heat low, but the dry shallow meridional overturning is farther north and weaker than in the premonsoon. While ERA5 winds and moisture‐flux convergence are generally consistent with diurnal variations in the TRMM/GPM precipitation, ERA5 precipitation is not. The reanalysis indicates significant biases in the timing and magnitude of rainfall over the Gulf of Guinea and West Africa in each season, with the convective and large‐scale rain fields both contributing to the disparate precipitation patterns.

An interdisciplinary framework for navigating social–climatic tipping points

Abstract To effectively navigate out of the climate crisis, a new interdisciplinary approach is needed to guide and facilitate research that integrates diverse understandings of how transitions evolve in intertwined social–environmental systems. The concept of tipping points, frequently used in the natural sciences and increasingly in the social sciences, can help elucidate processes underlying major social–environmental transitions. We develop the notion of interlinked ‘social–climatic tipping points’ in which desirability and intentionality are key constitutive features alongside stable states, feedbacks, reversibility and abruptness. We demonstrate the new insights that our interdisciplinary framework can provide by analysing the slowdown of the Atlantic Meridional Overturning Circulation and associated flooding of the Ahr Valley in Germany as a social–climatic tipping point. This framework can enable more sustainable and equitable futures by prioritising social–climatic tipping points for interdisciplinary research, identifying opportunities for action, and evaluating the nuanced desirability and acceptability of proposed solutions. Read the free Plain Language Summary for this article on the Journal blog.

Investigating Winter Temperatures in Sweden and Norway: Potential Relationships with Climatic Indices and Effects on Electrical Power and Energy Systems

This paper presents a comprehensive study of winter temperatures in Norway and northern Sweden, covering a period of 50 to 70 years. The analysis utilizes Singular Spectrum Analysis (SSA) to investigate temperature trends at six selected locations. The results demonstrate an overall long-term rise in temperatures, which can be attributed to global warming. However, when investigating variations in highest, lowest, and average temperatures for December, January, and February, 50% of the cases exhibit a significant decrease in recent years, indicating colder winters, especially in December. The study also explores the variations in Atlantic Meridional Overturning Circulation (AMOC) variations as a crucial climate factor over the last 15 years, estimating a possible 20% decrease/slowdown within the first half of the 21st century. Subsequently, the study investigates potential similarities between winter AMOC and winter temperatures in the mid to high latitudes over the chosen locations. Additionally, the study examines another important climatic index, the North Atlantic Oscillation (NAO), and explores possible similarities between the winter NAO index and winter temperatures. The findings reveal a moderate observed lagged correlation for AMOC-smoothed temperatures, particularly in December, along the coastal areas of Norway. Conversely, a stronger lagged correlation is observed between the winter NAO index and temperatures in northwest Sweden and coastal areas of Norway. Thus, NAO may influence both AMOC and winter temperatures (NAO drives both AMOC and temperatures). Furthermore, the paper investigates the impact of colder winters, whether caused by AMOC, NAO, or other factors like winds or sea ice changes, on electrical power and energy systems, highlighting potential challenges such as reduced electricity generation, increased electricity consumption, and the vulnerability of power grids to winter storms. The study concludes by emphasizing the importance of enhancing the knowledge of electrical engineering researchers regarding important climate indices, AMOC and NAO, the possible associations between them and winter temperatures, and addressing the challenges posed by the likelihood of colder winters in power systems.

Deconstructing Future AMOC Decline at 26.5°N

AbstractThe Atlantic Meridional Overturning Circulation (AMOC) is frequently used to diagnose the state of the North Atlantic circulation, but as an integrated quantity the AMOC strength does not necessarily mirror changes in the individual circulation components. Here, we investigate future circulation changes in the subtropical North Atlantic (26.5°N) in CMIP6 models, diagnosing the relationship between the Gulf Stream, Deep Western Boundary Current (DWBC), gyre recirculation, and the integrated AMOC response. Under continued high emissions, we find a multi‐model mean Gulf Stream weakening of 29% (11.2 Sv) and a DWBC weakening of 47% (8.5 Sv) by the end of the century. However, 33% (3.7 Sv) of the Gulf Stream weakening is due to changes in wind stress and therefore not simply a compensating effect for reduced high‐latitude water mass transformation and a weaker DWBC. Our findings have implications for how we understand the dynamics of future North Atlantic circulation changes.

Millennial hydrological variability in the continental northern Neotropics during Marine Isotope Stages (MISs) 3–2 (59–15 cal ka BP) inferred from sediments of Lake Petén Itzá, Guatemala

Abstract. Lake Petén Itzá (Guatemala) possesses one of the longest lacustrine sediment records in the northern Neotropics, which enabled study of paleoclimate variability in the region during the last ∼400 000 years. We used geochemical (Ti, Ca/(Ti+Fe) and Mn/Fe) and mineralogical (carbonates, gypsum, quartz, clay) data from sediment core PI-2 to infer past changes in runoff, lake evaporation, organic matter sources and redox conditions in the water column, caused by hydrological changes in the northern Neotropics during Marine Isotope Stages (MISs) 3–2. From 59 to 39 cal ka BP climate conditions were relatively wet, and the lake was marked by higher primary productivity and anoxic bottom waters. This wet environment was interrupted for two periods of possible low water level at 52 and 46 cal ka BP, when our data suggest higher evaporation, high terrestrial organic matter input and persistent oxic conditions. Between 39 and 23 cal ka BP, evaporation and input of terrestrial organic matter increased considerably, lake level declined, and lake bottom waters generally became oxic. These conditions reversed during the Last Glacial Maximum (23.5–18.0 cal ka BP), when runoff and lake productivity increased, and rising lake level caused bottom waters to again become anoxic. Comparison of our hydrologic proxy data with sea surface temperature anomalies between the eastern Pacific and the Caribbean suggests that changes in the intensity of the Caribbean Low-Level Jet (CLLJ) may have influenced long-term changes in runoff during MISs 3–2. Higher intensity of the CLLJ during the onset of MIS 3 and the LGM might have led to greater runoff into the lake, whereas the MIS 3–2 transition experienced a weaker CLLJ and consequently less runoff. A refined, high-resolution age–depth model for the PI-2 sediment core enabled us to identify millennial-scale Greenland interstadials (GIs) 14–2, Greenland stadials (GSs) 14–2 and Heinrich stadials (HSs) 5–1. In general, HSs and GSs were characterized by drier conditions. In contrast to GSs and HSs, GIs were characterized by greater runoff and overall wetter conditions, with the most pronounced GI peaks between 40 and 30 cal ka BP. Whereas GSs 9, 8, 7 and 6 began with abrupt increases in evaporation and ended with gradual increases in humidity, GSs 11 and 10 showed reversed patterns. The Lake Petén Itzá paleohydrology record, along with other regional paleoclimate records, led us to conclude that shifts in the position of the Intertropical Convergence Zone (ITCZ) altered moisture delivery to the lake on millennial timescales. During GSs and HSs, high evaporation from Petén Itzá (dry climate conditions) was associated with a more southerly position of the ITCZ, whereas wetter GIs prevailed during a more northerly ITCZ position. Although abrupt millennial-scale shifts in ITCZ and hydroclimate between GSs/HSs and GIs can be linked to instabilities in the Atlantic Meridional Overturning Circulation (AMOC), longer-term changes were additionally influenced by changes in atmospheric convection linked to modulations of the CLLJ in response to ΔSST between the equatorial Pacific and tropical Atlantic.