Northward Heat Transport Research Articles

Simulated changes of the Southern Ocean air-sea heat flux feedback in a warmer climate

In this paper, we have evaluated the Southern Ocean (SO) heat flux feedback in a fully coupled model and for the first time examined how this feedback evolves in response to global warming. The model broadly captures the observed characteristics of heat flux feedback over the SO. The heat flux tends to damp SST anomalies over the SO and thus the feedback is negative. In a warmer climate, the negative heat flux feedback in the SO, contributed mainly from turbulent component, becomes stronger. The turbulent feedback in the present day is primarily balanced by the upper boundary that strongly depends on background SST and wind and the thermal adjustment of boundary layer to SST anomalies. It is found that this balance shifts a little bit under global warming scenario. The upper limit increases in a warmer climate due to warm SST responses. The thermal adjustment of boundary layer becomes weaker in a warmer climate because of decreased atmospheric background heat convergence. The mean Deacon Cell transports anomalous heat caused by the greenhouse gas effect northward, leading to a heat convergence along the northern flank of the Antarctic Circumpolar Current. Constrained by the energy, the atmospheric northward heat transport has a corresponding divergence north of 55°S. This anomalous heat transport divergence favors air heat leaving away from 55°S–35°S regions to the polar region, leads to smaller air temperature tendencies in the local compared to the present day and therefore leads to a weakened thermal adjustment of boundary layer. Therefore, both changes in the upper limit and thermal adjustment of boundary layer contribute positively to the enhanced turbulent feedback in a warmer climate. The dynamic component due to changes in wind tends to compensate these two positive contributors, but its magnitude is too small to become a dominant factor.

Variability in the global energy budget and transports 1985\u20132017

The study of energy flows in the Earth system is essential for understanding current climate change. To understand how energy is accumulating and being distributed within the climate system, an updated reconstruction of energy fluxes at the top of atmosphere, surface and within the atmosphere derived from observations is presented. New satellite and ocean data are combined with an improved methodology to quantify recent variability in meridional and ocean to land heat transports since 1985. A global top of atmosphere net imbalance is found to increase from 0.10 ± 0.61 W m−2 over 1985–1999 to 0.62 ± 0.1 W m−2 over 2000–2016, and the uncertainty of ± 0.61 W m−2 is related to the Argo ocean heat content changes (± 0.1 W m−2) and an additional uncertainty applying prior to 2000 relating to homogeneity adjustments. The net top of atmosphere radiative flux imbalance is dominated by the southern hemisphere (0.36 ± 0.04 PW, about 1.41 ± 0.16 W m−2) with an even larger surface net flux into the southern hemisphere ocean (0.79 ± 0.16 PW, about 3.1 ± 0.6 W m−2) over 2006–2013. In the northern hemisphere the surface net flux is of opposite sign and directed from the ocean toward the atmosphere (0.44 ± 0.16 PW, about 1.7 ± 0.6 W m−2). The sea ice melting and freezing are accounted for in the estimation of surface heat flux into the ocean. The northward oceanic heat transports are inferred from the derived surface fluxes and estimates of ocean heat accumulation. The derived cross-equatorial oceanic heat transport of 0.50 PW is higher than most previous studies, and the derived mean meridional transport of 1.23 PW at 26° N is very close to 1.22 PW from RAPID observation. The surface flux contribution dominates the magnitude of the oceanic transport, but the integrated ocean heat storage controls the interannual variability. Poleward heat transport by the atmosphere at 30° N is found to increase after 2000 (0.17 PW decade−1). The multiannual mean (2006–2013) transport of energy by the atmosphere from ocean to land is estimated as 2.65 PW, and is closely related to the ENSO variability.

Sensitivity of the Atlantic Meridional Overturning Circulation to Model Resolution in CMIP6 HighResMIP Simulations and Implications for Future Changes

AbstractA multimodel, multiresolution ensemble using Coupled Model Intercomparison Project Phase 6 (CMIP6) High Resolution Model Intercomparison Project (HighResMIP) coupled experiments is used to assess the performance of key aspects of the North Atlantic circulation. The Atlantic Meridional Overturning Circulation (AMOC), and related heat transport, tends to become stronger as ocean model resolution is enhanced, better agreeing with observations at 26.5°N. However, for most models the circulation remains too shallow compared to observations and has a smaller temperature contrast between the northward and southward limbs of the AMOC. These biases cause the northward heat transport to be systematically too low for a given overturning strength. The higher‐resolution models also tend to have too much deep mixing in the subpolar gyre. In the period 2015–2050 the overturning circulation tends to decline more rapidly in the higher‐resolution models, which is related to both the mean state and to the subpolar gyre contribution to deep water formation. The main part of the decline comes from the Florida Current component of the circulation. Such large declines in AMOC are not seen in the models with resolutions more typically used for climate studies, suggesting an enhanced risk for Northern Hemisphere climate change. However, only a small number of different ocean models are included in the study.

Extracting the Buoyancy-Driven Atlantic Meridional Overturning Circulation

AbstractVariations in the Atlantic meridional overturning circulation (AMOC) driven by buoyancy forcing are typically characterized as having a low-frequency time scale, interhemispheric structure, cross-equatorial heat transport, and linkages to the strength of Northern Hemisphere gyre circulations and the Gulf Stream. This study first tests whether these attributes ascribed to the AMOC are reproduced in a coupled model that is mechanically decoupled and, hence, is only buoyancy coupled. Overall, the mechanically decoupled model reproduces these attributes, with the exception that in the subpolar gyre, buoyancy drives AMOC variations on interannual to multidecadal time scales, yet only the multidecadal variations penetrate into the subtropics. A stronger AMOC is associated with a strengthening of the Northern Hemisphere gyre circulations, Gulf Stream, and northward oceanic heat transport throughout the basin. We then determine whether the characteristics in the mechanically decoupled model can be recovered by low-pass filtering the AMOC in a fully coupled version of the same model, a common approach used to isolate the buoyancy-driven AMOC. A major conclusion is that low-pass filtering the AMOC in the fully coupled model reproduces the buoyancy-driven AMOC pattern and most of the associated attributes, but not the statistics of the temporal variability. The strength of the AMOC–Gulf Stream connection is also not reproduced. The analyses reveal caveats that must be considered when choosing indexes and filtering techniques to estimate the buoyancy-driven AMOC. Results also provide insight on the latitudinal dependence of time scales and drivers of ocean circulation variability in coupled models, with potential implications for measurement and detection of the buoyancy-driven AMOC in the real world.

Topological Constraints by the Greenland–Scotland Ridge on AMOC and Climate

AbstractChanges in the geometry of ocean basins have been influential in driving climate change throughout Earth’s history. Here, we focus on the emergence of the Greenland–Scotland Ridge (GSR) and its influence on the ocean state, including large-scale circulation, heat transport, water mass properties, and global climate. Using a coupled atmosphere–ocean–sea ice model, we consider the impact of introducing the GSR in an idealized Earth-like geometry, comprising a narrow Atlantic-like basin and a wide Pacific-like basin. Without the GSR, deep-water formation occurs near the North Pole in the Atlantic basin, associated with a deep meridional overturning circulation (MOC). By introducing the GSR, the volume transport across the sill decreases by 64% and deep convection shifts south of the GSR, dramatically altering the structure of the high-latitude MOC. Due to compensation by the subpolar gyre, the northward ocean heat transport across the GSR only decreases by ~30%. As in the modern Atlantic Ocean, a bidirectional circulation regime is established with warm Atlantic water inflow and a cold dense overflow across the GSR. In sharp contrast to the large changes north of the GSR, the strength of the Atlantic MOC south of the GSR is unaffected. Outside the high latitudes of the Atlantic basin, the surface climate response is surprisingly small, suggesting that the GSR has little impact on global climate. Our results suggest that caution is required when interpreting paleoproxy and ocean records, which may record large local changes, as indicators of basin-scale changes in the overturning circulation and global climate.

Hemisphere differences in response of sea surface temperature and sea ice to precession and obliquity

The response of the climate system to astronomical parameters is an important scientific issue, but the internal processes and feedbacks need to be better understood. This study investigates the differences of the climate response to the astronomical forcing between the Northern (NH) and Southern (SH) hemispheres based on a more than 90,000-year long transient simulation using the model LOVECLIM. The astronomical parameters of the period 511–417 ka BP covering MIS-13, MIS-12 and MIS-11 are used, and greenhouse gases (GHG) concentrations and ice sheets are fixed, in order to investigate the role of insolation alone. Our results show that the response of sea ice and sea surface temperature (SST) to precession and obliquity is different between the two hemispheres. Precession plays a dominant role on the Arctic sea ice. This is mainly due to its response to the local summer insolation and also, to a less degree, the influence of the northward oceanic heat transport. However, obliquity plays a dominant role on the Southern Ocean sea ice through its influence on local solar radiation and also on the westerly winds. As far as the SST is concerned, it shows a strong precessional signal at low latitudes in both hemispheres. For the SST in the mid and high latitudes, obliquity plays a dominant role in the SH whereas precession is more important in the NH. This is largely due to the different response to insolation and feedbacks related to the different land-ocean distribution in the two hemispheres. Near the Equator, besides the precessional signal, the SST also shows strong half-precessional signal, which can be explained by the unique characteristics of the insolation variations at the Equator. Our results also show that during the period of low eccentricity, obliquity is more important than precession and the half-precessional signal vanishes due to reduced impact of precession, but precession is always more important in the NH than in the SH. • Precession and obliquity have different weight on sea ice of the two hemispheres and on sea surface temperature in different latitudes. • Precession is generally more important than obliquity in NH whereas obliquity is more important in SH. Strong half-precessional signal is found near Equator. • During low-eccentricity period, obliquity is more important than precession and the half-precessional signal vanishes.

Linking Ocean Forcing and Atmospheric Interactions to Atlantic Multidecadal Variability in MPI‐ESM1.2

AbstractWe investigate how ocean‐driven multidecadal sea surface temperature (SST) variations force the atmosphere to jointly set the pace of Atlantic multidecadal variability (AMV). We generate periodic low‐frequency Atlantic Meridional Overturning Circulation oscillations by implementing time‐dependent deep‐ocean‐density restoring in MPI‐ESM1.2 to explicitly identify variations driven by Atlantic Meridional Overturning Circulation without any perturbation at the ocean‐atmosphere interface. We show in a coupled experiment that ocean heat convergence variations generate positive SST anomalies, turbulent heat release, and low sea level pressure in the subpolar North Atlantic (NA) and vice versa. The SST signal is communicated to the tropical NA by wind‐evaporative‐SST feedbacks and to the North‐East Atlantic by enhanced northward atmospheric heat transport. Such atmospheric feedbacks and the characteristic AMV‐SST pattern are synchronized to the multidecadal time scale of ocean circulation changes by air‐sea heat exchange. This coupled ocean‐atmosphere mechanism is consistent with observed features of AMV and thus supports a key role of ocean dynamics in driving the AMV.

The role of Amazon river runoff on the multidecadal variability of the Atlantic ITCZ

Climate model projections for the 21st century predict an elongated dry season in the Amazon basin, potentially reducing the discharge into the equatorial Atlantic Ocean. In order to understand the climatic role of Amazon runoff into the ocean, sensitivity experiments were carried out using the Community Earth System Model (CESM). Without Amazon runoff, the Atlantic Meridional Overturning Circulation (AMOC) strengthens and the associated increase in northward heat transport induces a positive temperature anomaly in the North Atlantic Ocean with a spatial structure similar to the positive phase of the Atlantic Multidecadal Variability (AMV). A positive phase of AMV developed in the absence of Amazon runoff triggers a bipolar seesaw in SST across the thermal equator with warming to the north and cooling to the south. The boreal summer rainfall in the tropical Atlantic Ocean sector responds to this change in SST by displacing the intertropical convergence zone (ITCZ) to the north of its mean position. An alternate experiment by doubling the Amazon runoff shows a weakening of AMOC and AMV and a southward shift in the summer–time ITCZ. In both the experiments, we find that the largest change in rainfall is exhibited over the region where the AMV–induced decadal variability in rainfall is prominent, confirming the source of rainfall variability. Based on sensitivity experiments by varying the runoff, we propose that the Amazon discharge can affect the multidecadal variabilities, the AMOC and AMV, and thereby the low–frequency variability of rainfall over the tropical North Atlantic Ocean and northwest Africa. We conclude that the freshwater input from the Amazon plays a significant role in the sustained wet and dry climatic phases of rainfall events over the tropical North Atlantic Ocean and West African nations and thus have an impact on the regional hydrological cycle and economy.

Asymmetric response of the subtropical western South Atlantic thermocline to the Dansgaard-Oeschger events of Marine Isotope Stages 5 and 3

The last glacial period (116–11.7 ka BP) was an interval characterized by a sequence of abrupt millennial-scale events well documented in Greenland and Antarctica ice-cores. Throughout this period, Greenland cold stadials were accompanied by warm conditions in the thermocline to intermediate waters of the Atlantic Ocean that may have played a role in both the basal melting of ice shelves and the rapid atmospheric warming during the onset of warm interstadials. Climate model simulations indicated an accentuated response of the subtropical western South Atlantic thermocline to the disturbances in the Atlantic circulation. Such works encourage investigations upon thermocline/deep-dwelling planktic foraminifera in this region; however, a study with this aim was not performed. Here we present a paleoceanographic reconstruction from the subtropical western South Atlantic based on the thermocline planktic foraminifera Globorotalia inflata. Our high-resolution δ18O record for the last glacial period presents a millennial-scale variability that strongly resembles the structure of the Greenland Dansgaard-Oeschger cycles during Marine Isotope Stage (MIS) 5. During MIS 3, this millennial-scale variability is absent or considerably dampened. Mg/Ca-derived temperature and seawater δ18O corrected for ice-volume for the MIS 5 interval demonstrate that the region was warmer and saltier (colder and fresher) during early-glacial stadials (interstadials). Our data suggest a reorganization of the northward heat transport throughout the last glacial, in which regions as far south as 24 °S acted as heat reservoirs in periods of weakened Atlantic Meridional Overturning Circulation during MIS 5 but not necessarily (or only marginally) during MIS 3.

The Atlantic Meridional Overturning Circulation in High‐Resolution Models

AbstractThe Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N—a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

Portraying the Impact of the Tibetan Plateau on Global Climate

AbstractAs the most extensive highland in the world, the Tibetan Plateau (TP) plays an important role in shaping the global climate. Quantifying the effect of the TP on global climate is the first step for a full understanding of the TP’s standing on planet Earth. Through coupled model sensitivity experiments, we draw a panorama of the TP’s global impact in this paper. Our model results show that the absence of the TP would result in a 4°C colder and 10% drier climate in the Northern Hemisphere (NH). The TP has a striking remote effect on the North Atlantic. Removing the TP would enhance the westerlies in the mid- to high latitudes of the NH and weaken the easterlies over the tropical Pacific. More moisture would be relocated from the tropical Pacific to the North Atlantic, shutting down the Atlantic thermohaline circulation, which would eventually result in more than 15°C colder and 20% drier climate over the North Atlantic. Our model results suggest that the presence of the TP may have contributed greatly to the hospitable modern climate in the NH, by promoting the establishment of the thermohaline circulation in the Atlantic, and therefore enhancing the northward ocean heat transport and atmosphere moisture transport across the equator.

South Asian monsoon response to weakening of Atlantic meridional overturning circulation in a warming climate

Observational records and climate model projections reveal a considerable decline in the Atlantic Meridional Overturning Circulation (AMOC). Changes in the AMOC can have a significant impact on the global climate. Sustained warming due to increased greenhouse gas emissions is projected to weaken the AMOC, which in turn can lead to changes in the location of Inter-tropical convergence zone (ITCZ), oceanic and atmospheric large-scale circulation, tropical precipitation and regional monsoons. Using proxy records, observations and CMIP6 simulations of IITM Earth System Model (IITM-ESM), we investigate the changes in the AMOC and associated changes in the large-scale circulation and precipitation patterns over the South Asian monsoon region. Transient CO2 simulation and additional model sensitivity experiments with realistic surface heat and freshwater perturbation anomalies under the experimental protocol of Flux Anomaly Forcing Model Intercomparison Project (FAFMIP) performed with IITM-ESM reveal a decline in the strength of AMOC. The weakening of AMOC is associated with enhanced heat and freshwater forcing in the North Atlantic resulting in the reduction of northward oceanic heat transport and an enhanced northward atmospheric heat transport. Changes in AMOC lead to weakening of large-scale north–south temperature gradient and regional land-sea thermal gradient, which in turn weaken the regional Hadley circulation and, monsoon circulation over the South Asian region. Both the FAFMIP and transient CO2 experiments reveal consistent results of weakening South Asian Monsoon circulation with a decline of AMOC, while precipitation exhibits contrasting responses as precipitation changes are dominated by the thermodynamic response. The suite of observational and numerical analysis provides a mechanistic hypothesis for the weakening of South Asian monsoon circulation concomitant with a weakening of AMOC in a warming climate.

Hypersensitivity of glacial summer temperatures in Siberia

Abstract. Climate change in Siberia is currently receiving a lot of attention because large permafrost-covered areas could provide a strong positive feedback to global warming through the release of carbon that has been sequestered there on glacial–interglacial timescales. Geological evidence and climate model experiments show that the Siberian region also played an exceptional role during glacial periods. The region that is currently known for its harsh cold climate did not experience major glaciations during the last ice age, including its severest stages around the Last Glacial Maximum (LGM). On the contrary, it is thought that glacial summer temperatures were comparable to the present day. However, evidence of glaciation has been found for several older glacial periods. We combine LGM experiments from the second and third phases of the Paleoclimate Modelling Intercomparison Project (PMIP2 and PMIP3) with sensitivity experiments using the Community Earth System Model (CESM). Together, these climate model experiments reveal that the intermodel spread in LGM summer temperatures in Siberia is much larger than in any other region of the globe and suggest that temperatures in Siberia are highly susceptible to changes in the imposed glacial boundary conditions, the included feedbacks and processes, and to the model physics of the different components of the climate model. We find that changes in the circumpolar atmospheric stationary wave pattern and associated northward heat transport drive strong local snow and vegetation feedbacks and that this combination explains the susceptibility of LGM summer temperatures in Siberia. This suggests that a small difference between two glacial periods in terms of climate, ice buildup or their respective evolution towards maximum glacial conditions can lead to strongly divergent summer temperatures in Siberia, allowing for the buildup of an ice sheet during some glacial periods, while during others, above-freezing summer temperatures preclude a multi-year snowpack from forming.

Natural drivers of multidecadal Arctic sea ice variability over the last millennium

The climate varies due to human activity, natural climate cycles, and natural events external to the climate system. Understanding the different roles played by these drivers of variability is fundamental to predicting near-term climate change and changing extremes, and to attributing observed change to anthropogenic or natural factors. Natural drivers such as large explosive volcanic eruptions or multidecadal cycles in ocean circulation occur infrequently and are therefore poorly represented within the observational record. Here we turn to the first high-latitude annually-resolved and absolutely dated marine record spanning the last millennium, and the Paleoclimate Modelling Intercomparison Project (PMIP) Phase 3 Last Millennium climate model ensemble spanning the same time period, to examine the influence of natural climate drivers on Arctic sea ice. We show that bivalve oxygen isotope data are recording multidecadal Arctic sea ice variability and through the climate model ensemble demonstrate that external natural drivers explain up to third of this variability. Natural external forcing causes changes in sea-ice mediated export of freshwater into areas of active deep convection, affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC) and thereby northward heat transport to the Arctic. This in turn leads to sustained anomalies in sea ice extent. The models capture these positive feedbacks, giving us improved confidence in their ability to simulate future sea ice in in a rapidly evolving Arctic.

Atlantic Ocean Heat Transport Enabled by Indo‐Pacific Heat Uptake and Mixing

AbstractThe ocean transports vast amounts of heat around the planet, helping to regulate regional climate. One important component of this heat transport is the movement of warm water from equatorial regions toward the poles, with colder water flowing in return. Here, we introduce a framework relating meridional heat transport to the diabatic processes of surface forcing and turbulent mixing that move heat across temperature classes. Applied to a (1/4)° global ocean model the framework highlights the role of the tropical Indo‐Pacific in the global ocean heat transport. A large fraction of the northward heat transport in the Atlantic is ultimately sourced from heat uptake in the eastern tropical Pacific. Turbulent mixing moves heat from the warm, shallow Indo‐Pacific circulation to the cold deeper‐reaching Atlantic circulation. Our results underscore a renewed focus on the tropical oceans and their role in global circulation pathways.

Black Carbon Increases Frequency of Extreme ENSO Events

ABSTRACTEl Niño–Southern Oscillation (ENSO) is the leading mode of Earth’s climate variability at interannual time scales with profound ecological and societal impacts, and it is projected to intensify in many climate models as the climate warms under the forcing of increasing CO2 concentration. Since the preindustrial era, black carbon (BC) emissions have substantially increased in the Northern Hemisphere. But how BC aerosol forcing may influence the occurrence of the extreme ENSO events has rarely been investigated. In this study, using simulations of a global climate model, we show that increases in BC emissions from both the midlatitudes and Arctic weaken latitudinal temperature gradients and northward heat transport, decrease tropical energy divergence, and increase sea surface temperature over the tropical oceans, with a surprising consequential increase in the frequency of extreme ENSO events. A corollary of this study is that reducing BC emissions might serve to mitigate the possible increasing frequency of extreme ENSO events under greenhouse warming, if the modeling result can be translated into the climate in reality.

Tropical temperature in the Maastrichtian Danish Basin: Data from coccolith Δ47 and δ18O

AbstractThe Cretaceous Earth, with its greenhouse climate and absence of major ice caps in the polar regions, represents an extreme scenario for modeling future warming. Despite considerable efforts, we are just at the verge of fully understanding the conditions of a warm Earth, and better, more extensive proxy evidence is needed to solve existing discrepancies between the applied temperature proxies. In particular, the Maastrichtian temperature trends are controversial, since data indicate cooling in the South Atlantic and contemporary warming of the North Atlantic. The “heat piracy” hypothesis involves northward heat transport to midlatitudes via oceanic currents and is used to explain the contrasting polar cooling/warming patterns. Here, we present Δ47 and δ18O data from nine coccolith-enriched samples from a shallow core taken from the Danish Basin (Chalk Sea), representing a key location at the northern mid-latitudes. Based on Δ47 data of coccolith-enriched material, sea-surface temperatures for the late Campanian–Maastrichtian ranged from 24 °C to 30 °C, with an average of 25.9 °C ± 2 °C. This is 4–6 °C higher than estimates based on Δ47 of bulk samples and 8–10 °C higher than reported temperatures based on bulk δ18O data from the same core. However, these higher temperature estimates are lower, but overall in line with estimates of Late Cretaceous tropical sea-surface temperatures from TEX86 (tetraether index of 86 carbons), when considering latitudinal differences. The study highlights the potential of clumped isotope paleothermometry on coccoliths as a valid, reliable proxy with which to reconstruct sea-surface temperatures.

Decadal Atlantic Meridional Overturning Circulation slowing events in a climate model

There is a controversy about the origin of the recent decadal Atlantic Meridional Overturning Circulation (AMOC) slowing observed at 26.5°N and concurrent sea surface temperature cooling in the central and eastern mid-latitude North Atlantic. We investigate decadal AMOC slowing events simulated in a multi-millennial preindustrial control integration of the Kiel Climate Model (KCM), providing an estimate of internal AMOC variability. Preindustrial control integrations of 15 models participating in the Coupled Model Intercomparison Project phase 5 also are investigated, as well as historical simulations with them providing estimates of AMOC variability during 1856–2005. It is shown that the recent decadal AMOC decline is still within the range of the models’ internal AMOC variability and thus could be of natural origin. In this case, the decline would represent an extreme realization of internal variability provided the climate models yield realistic levels of AMOC variability. The model results suggest that internal decadal AMOC variability is large, requiring multi-decadal observational records to detect an anthropogenic AMOC signal with high confidence. When analyzing the strongest decadal AMOC slowing events in the KCM, which have amplitudes similar to or larger than the recently observed decadal AMOC decline, the following composite picture emerges: a very strong decadal AMOC decline is preceded by a decadal rise in atmospheric surface pressure over large parts of the mid-latitude North Atlantic. The change in low-level atmospheric circulation drives reduced oceanic heat loss over and diminished upper-ocean salt content in the Labrador Sea. In response, oceanic deep convection and subsequently the AMOC and northward oceanic heat transport weaken, and anomalously cold sea surface temperatures develop in the central and eastern mid-latitude North Atlantic.

On the causes of Arctic sea ice in the warm Early Pliocene

Scattered and indirect evidence suggests that sea ice occurred as far south as the Iceland Sea during the Early Pliocene, when the global climate was warmer than present. However, conclusive evidence as well as potential mechanisms governing sea ice occurrence outside the Arctic Ocean during a time with elevated greenhouse gas concentrations are still elusive. Here we present a suite of organic biomarkers and palynological records from the Iceland Sea and Yermak Plateau. We show that sea ice appeared as early as ~4.5 Ma in the Iceland Sea. The sea ice either occurred seasonally or was transported southward with the East Greenland Current. The Yermak Plateau mostly remained free of sea ice and was influenced dominantly by Atlantic water. From ~4.0 Ma, occurrence of extended sea ice conditions at both the Yermak Plateau and Iceland Sea document a substantial expansion of sea ice in the Arctic. The expansion occurred contemporaneous with increased northward heat and moisture transport in the North Atlantic region, which likely led to a fresher Arctic Ocean that favors sea ice formation. This extensive sea ice cover along the pathway of the East Greenland Current gradually isolated Greenland from warmer Atlantic water in the Late Pliocene, providing a positive feedback for ice sheet expansion in Greenland.

Evidence for a volcanic underpinning of the Atlantic multidecadal oscillation

The Atlantic multidecadal oscillation (AMO) is a 60–70 year pattern of sea-surface temperature (SST) variability in the North Atlantic commonly ascribed to internal ocean dynamics and changes in northward heat transport. Recent modeling studies, however, suggest that SSTs fluctuate primarily in response to major volcanic eruptions and changes in atmospheric circulation. Here, we utilize historical SST, atmospheric reanalysis, and stratospheric aerosol optical depth data to examine the basic evidence supporting a volcanic link. We find that cool intervals across the North Atlantic coincide with two distinct episodes of explosive volcanic activity (1880s–1920s and 1960s–1990s), where key eruptions include 1883 Krakatau, 1902 Santa María, 1912 Novarupta, 1963 Agung, 1982 El Chichón, and 1991 Pinatubo. Cool SST patterns develop in association with an increased prevalence of North Atlantic Oscillation (NAO)+ atmospheric patterns caused by stratospheric aerosol loading and a steepened poleward temperature gradient. NAO+ patterns promote wind-driven advection, evaporative cooling, and increased albedo from enhanced Saharan dust transport and anthropogenic aerosols. SSTs across the subpolar gyre are regulated by strength of low pressure near Iceland and the associated wind-driven advection of cold surface water from the Labrador Sea. This is contrary to an interpretation that subpolar SSTs are driven by changes in ocean overturning circulation. We also find that North Pacific and global mean SST declines can be readily associated with the same volcanic triggers that affect the North Atlantic. Thus, external forcing from volcanic aerosols appears to underpin multi-decade SST variability observed in the historical record.