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The Arctic region has warmed nearly four times faster than the global average since 1979, with far-reaching global implications. However, model projections of Arctic warming rates are uncertain and one key component is the ocean heat transport (OHT) into the Arctic Ocean. Here we use high-resolution historical and future climate simulations to show that the OHT through the Bering Strait exerts a more substantial influence on Arctic warming than previously recognized. The high-resolution ensemble exhibits a 20% larger warming rate for 2006–2100 compared with standard low-resolution model simulations. The enhanced Arctic warming in the high-resolution simulations is primarily attributable to an increased OHT through the narrow and shallow Bering Strait that is nearly four times larger than in the low-resolution simulations. Consequently, the projected rate of Arctic warming by low-resolution climate simulations is likely to be underestimated due to the model resolution being insufficient to capture future changes in Bering Strait OHT.

Abstract:This essay describes a plan for Indigenizing medieval studies that has two elements. The first is an area of research inquiry, “The Global Far North, 500–1500 CE,” which moves past the written records of the Vinland sagas to privilege alternative forms of evidence about cultural contact in the defined period, particularly the oral traditional evidence of Indigenous communities. The project’s investigations will apply the emerging protocols for research ethics and for reciprocity with Indigenous communities, and they will aim to historicize and challenge settler notions of legality that rely on written documents. The essay concludes by arguing that teaching, service, and community outreach must be prioritized over publication as modes of professional activity more conducive to Indigenization’s political goals. Decolonizing medieval studies will require not only that we engage with Indigenous communities but also that we actively center their concerns and contributions at every step.

This essay describes a plan for Indigenizing medieval studies that has two elements. The first is an area of research inquiry, “The Global Far North, 500–1500 CE,” which moves past the written records of the Vinland sagas to privilege alternative forms of evidence about cultural contact in the defined period, particularly the oral traditional evidence of Indigenous communities. The project’s investigations will apply the emerging protocols for research ethics and for reciprocity with Indigenous communities, and they will aim to historicize and challenge settler notions of legality that rely on written documents. The essay concludes by arguing that teaching, service, and community outreach must be prioritized over publication as modes of professional activity more conducive to Indigenization’s political goals. Decolonizing medieval studies will require not only that we engage with Indigenous communities but also that we actively center their concerns and contributions at every step.

The characteristics of the North Atlantic jet stream play a key role in the weather and climate of western Europe. Although much of the year-to-year variability in the jet stream arises from internal atmospheric processes that are inherently unpredictable on timescales beyond a few days to weeks, any low-frequency variability or long-term trends that can be considered forced by slowly varying boundary conditions offer the potential for extended range predictability of climatological conditions in western Europe. Here we demonstrate that station-based precipitation observations have displayed pronounced multidecadal variability over the past century in western Europe during the late winter. We then use these precipitation observations as an independent verification of the multidecadal Atlantic jet stream variability found in reanalysis products. Both signals are highly correlated with sea surface temperature variability in the North Atlantic that is well predicted in initialized decadal prediction experiments with a coupled general circulation model. Combining the model-based predictions of the sea surface temperature with the observed relationship between precipitation and sea surface temperature, we show that there is great potential for skilful predictions of the forthcoming decadal average of March precipitation in western Europe, with hindcasts for the UK and Portugal yielding anomaly correlation coefficients of 0.82 and 0.69, respectively. Decadal averages of March precipitation in western Europe can be predicted by exploiting links with the jet stream and ocean along with skilful predictions of sea surface temperatures, according to an analysis of observations and reanalysis products.

By synthesizing recent studies employing a wide range of approaches (modern observations, paleo reconstructions, and climate model simulations), this paper provides a comprehensive review of the linkage between multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability and Atlantic Multidecadal Variability (AMV) and associated climate impacts. There is strong observational and modeling evidence that multidecadal AMOC variability is a crucial driver of the observed AMV and associated climate impacts and an important source of enhanced decadal predictability and prediction skill. The AMOC‐AMV linkage is consistent with observed key elements of AMV. Furthermore, this synthesis also points to a leading role of the AMOC in a range of AMV‐related climate phenomena having enormous societal and economic implications, for example, Intertropical Convergence Zone shifts; Sahel and Indian monsoons; Atlantic hurricanes; El Niño–Southern Oscillation; Pacific Decadal Variability; North Atlantic Oscillation; climate over Europe, North America, and Asia; Arctic sea ice and surface air temperature; and hemispheric‐scale surface temperature. Paleoclimate evidence indicates that a similar linkage between multidecadal AMOC variability and AMV and many associated climate impacts may also have existed in the preindustrial era, that AMV has enhanced multidecadal power significantly above a red noise background, and that AMV is not primarily driven by external forcing. The role of the AMOC in AMV and associated climate impacts has been underestimated in most state‐of‐the‐art climate models, posing significant challenges but also great opportunities for substantial future improvements in understanding and predicting AMV and associated climate impacts. There is strong evidence that multidecadal variability of the AMOC is a key driver of AMV and associated climate impactsThe AMOC‐AMV linkage is consistent with observed key elements of AMV, and it is important to use multivariate metrics to understand AMVAMOC‐induced Atlantic meridional heat transport and net surface heat flux anomalies are crucial for many AMV‐related climate impacts

Our current understanding of the global meridional overturning circulation (GMOC) is revisited using a surface‐forced ocean model simulation constrained by global hydrographic data. The derived GMOC is qualitatively consistent with previous observation‐based studies and further provides enhanced spatial details in the sources, transformations, and transports of major global water masses including in poorly observed regions. Several important but relatively underexplored aspects of the GMOC are highlighted, including complex but vigorous heavy‐to‐light water mass transformation that occurs in the Indo‐Pacific and Southern Oceans, and the role of the equatorial Pacific upwelling in closing the GMOC circuit. These and other key aspects of the GMOC are poorly captured in a surface‐forced ocean model simulation without the temperature and salinity corrections, suggesting that current climate models do not realistically simulate the GMOC and the associated global heat, salt, and carbon balances. Ocean tracers such as heat, salt, and carbon are perpetually carried by the global meridional overturning circulation (GMOC) and redistributed between hemispheres and across ocean basins from their source regions. The GMOC is therefore a crucial component of the global heat, salt, and carbon balances. In order to better understand the GMOC, here we carry out a global ocean model simulation with its temperature and salinity corrected toward observations. The derived GMOC is presented, validated against observations, and summarized in a schematic, which highlights several important but relatively underexplored aspects of the GMOC, including the pathway through which the heaviest water mass formed around the Antarctica is brought to the surface. This and other key aspects of the GMOC are poorly captured in a model run without the temperature and salinity corrections, suggesting that current climate models do not reproduce realistic paths of the GMOC and the associated global heat, salt, and carbon balances. The global meridional overturning circulation is revisited using an ocean model with its T&S corrected and validated against observationsSeveral underexplored aspects are highlighted, including complex but vigorous heavy‐to‐light water conversion in the Indo‐Pacific OceanKey processes including deep water formation & heavy‐to‐light water conversion are poorly captured in a model without T&S corrections

Ocean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high‐ (HR) and low‐resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation‐based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960–2020 are more realistic in CESM‐HR than ‐LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM‐HR and ‐LR. However, CESM‐HR has a larger increase in OHC in the upper 250 m compared to CESM‐LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy‐induced vertical heat transport between CESM‐HR and ‐LR in the historical period. Moreover, our results suggest that with the same heat input, upper‐ocean warming is likely to be underestimated by most non‐eddy‐resolving climate models. With the rise in anthropogenic emissions, the ocean has absorbed over 90% of greenhouse gas‐related heat, resulting in well‐known ocean warming. This warming is the main cause of severe deoxygenation, coral bleaching, and sea‐level rise, among others. Furthermore, the upper ocean experiences more warming than the deep ocean, which leads to strengthened ocean stratification. On a global scale, the heat into the ocean is vertically redistributed by a downward mean‐flow‐induced heat transport, upward eddy‐induced heat transport, and vertical turbulent mixing. In this study, we compare vertical structures of ocean heat uptake in response to anthropogenic forcing between high (∼10 km) and low horizontal resolution (∼100 km) future climate simulations. We find that, with similar heat input at the ocean surface, the high‐resolution simulations exhibit more warming in the upper 250 m of the ocean than their low‐resolution counterparts. This difference is caused by the different representations of vertical heat transport by mesoscale ocean eddies in high‐ and low‐resolution models. Simulated upper‐ocean ocean heat content anomalies are more realistic in high‐resolution simulations than in low‐resolution counterpartsWith similar ocean surface heating, high‐resolution simulations project stronger upper‐ocean warming than low‐resolution simulationsFuture changes in vertical heat transport depend on the representation of the mean ocean states at present day Simulated upper‐ocean ocean heat content anomalies are more realistic in high‐resolution simulations than in low‐resolution counterparts With similar ocean surface heating, high‐resolution simulations project stronger upper‐ocean warming than low‐resolution simulations Future changes in vertical heat transport depend on the representation of the mean ocean states at present day

The annual cycle of the Somali Current is studied using a forced strongly eddy‐active global ocean general circulation model. The annual cycle and timing of the reversal of the Somali Current differ depending on the location along the coast of northeastern Africa. A momentum budget analysis calculated from the model output shows that both annual Rossby waves and the life cycle of the Great Whirl impact the northern part of the Somali Current (roughly 5°N to 10°N). In particular, the nonlinear term is of leading order importance when the Great Whirl is present. This is in contrast to the leading‐order geostrophic balance between 2°N and 5°N. South of 2°N, equatorial dynamics prevail, and the nonlinear term is again important, and its reversal is strongly affected by the northward flowing East African Coastal Current. Alongshore wind forcing is influential at all latitudes, but only the transition in flow direction during boreal spring between 2°N and 5°N directly follows that of the wind. The Somali Current flows along the western boundary of northwest Africa in the Arabian Sea. Unlike other strong western boundary currents, the Somali Current reverses seasonally; its reversal has been attributed to the seasonally reversing Indian monsoon wind. The Somali Current is important for both regional climate and ecology and can potentially impact monsoon precipitation. Due to the difficulties in collecting observations due to piracy in the western Arabian Sea over the past decade, the scientific community has relied on numerical ocean models to enhance understanding of the behavior of this current. In this study, we use a global state‐of‐the‐art numerical ocean model that resolves “weather‐scale” features in the ocean, that is, mesoscale eddies, to understand Somali Current drivers and what controls its seasonal reversal. We find that in addition to the local wind, the Somali Current system is influenced by winds and oceanic phenomena distant from the region, which leads to different reversal times in different parts of the current. As a result, we identify three distinct regimes along the Somali Current and analyze their respective dynamics. Three dynamical regimes are identified in the Somali Current based on their different annual cycles and leading‐order momentum balancesNonlinearity and remote forcing are important north of 5°N (Great Whirl and Rossby wave) and close to the equator (cross‐equatorial current)Local wind is important in all three regimes, but only the reversal of surface flow between 2° and 5°N directly follows that of the wind

Impacts of model horizontal resolution on sea surface temperature (SST) biases are studied using high‐resolution (HR) and low‐resolution (LR) simulations with the Community Earth System Model (CESM) where the nominal resolutions are 0.1° for ocean and sea‐ice and 0.25° for atmosphere and land in HR, and 1° for all component models in LR, respectively. Results show that, except within eastern boundary upwelling systems, SST is warmer in HR than LR. Globally averaged surface ocean heat budget analysis indicates that 1°C warmer global‐mean SST in HR is mainly attributable to stronger nonlocal vertical mixing and shortwave heat flux, with the former prevailing over the latter in eddy‐active regions. In the tropics, nonlocal vertical mixing is slightly more important than shortwave heat flux for the warmer SST in HR. Further analysis shows that the stronger nonlocal mixing in HR can be attributed to differences in both the surface heat flux and shape function strength used in the parameterization. In addition, the shape function shows a nonlinear relationship with surface heat flux in HR and LR, modulated by the eddy‐induced vertical heat transport. The stronger shortwave heat flux in HR, on the other hand, is mainly caused by fewer clouds in the tropics. Finally, investigation of ocean advection reveals that the improved western boundary currents in HR also contribute to the reduction of SST biases in eddy‐active regions. Sea surface temperature (SST) is a key climate variable, through which the atmosphere and ocean are coupled. However, current generation climate models that have nominal horizontal resolutions of ∼1° generally produce colder‐than‐observation SST over much of the tropics and midlatitudes. Increasing model resolution to 0.25° or finer can help reduce this cold bias, but the underlying physics is not well understood. By analyzing high‐resolution (HR) and low‐resolution (LR) Community Earth System Model (CESM) simulations, we find that vertical turbulent processes, particularly the convective component, show dominant contributions to the improved SST bias in the tropics and midlatitudes. The difference in the convective flux between HR and LR CESM is partially attributable to the difference in the surface heat flux, and the other important contributing factor is the warmer subsurface temperature in HR CESM driven by the vertical eddy heat transport. Overall, this study highlights the importance of improving representation of ocean eddy and turbulent processes in climate models. Sea surface temperature (SST) biases are considerably reduced in high‐resolution models compared to low‐resolution modelsNonlocal K‐profile parameterization KPP mixing is an important factor in SST bias reductionEddy vertical heat transport can modulate nonlocal KPP mixing in eddy‐active regions Sea surface temperature (SST) biases are considerably reduced in high‐resolution models compared to low‐resolution models Nonlocal K‐profile parameterization KPP mixing is an important factor in SST bias reduction Eddy vertical heat transport can modulate nonlocal KPP mixing in eddy‐active regions

The Blob was the early manifestation of the Northeast Pacific marine heat wave from 2013 to 2016. While the upper ocean temperature in the Blob has been well described, the impacts on marine biogeochemistry have not been fully studied. Here, we characterize and develop understanding of Eastern North Pacific upper ocean biogeochemical properties during the Winter of 2013–2014 using in situ observations, an observation‐based product, and reconstructions from a collection of ocean models. We find that the Blob is associated with significant upper ocean biogeochemical anomalies: A 5% increase in aragonite saturation state (temporary reprieve of ocean acidification) and a 3% decrease in oxygen concentration (enhanced deoxygenation). Anomalous advection and mixing drive the aragonite saturation anomaly, while anomalous heating and air‐sea gas exchange drive the oxygen anomaly. Marine heatwaves do not necessarily serve as an analog for future change as they may enhance or mitigate long‐term trends. The global ocean is experiencing major changes due to human‐made carbon emissions and climate change, leading to a warming ocean with increasing acidity and declining oxygen. On top of these long‐term changes in the ocean are short‐term extreme events, such as marine heatwaves. These extreme events quickly change the ocean state and can stress marine ecosystems in multiple ways. The Northeast Pacific marine heat wave (2013–2016) was one such marine heatwave. Here we focus on the early portion of this marine heatwave, called the Blob. While the ocean temperature changes during the event are well understood, the effects on ocean biogeochemistry have not been fully examined. In this study, we use an earth system model that accurately simulates the Blob to examine short‐term changes in oxygen and acidity. We find that the warming signal leads to a decline in the effects of ocean acidification, mainly due to changes in the movement of carbon, and lowers the amount of oxygen, due primarily to temperature‐driven effects. These results suggest that some effects of climate change may be exacerbated (warming) or mitigated (ocean acidification) by marine heatwaves. The North Pacific Blob had a distinct biogeochemical signature that is captured by observations and multiple ocean modelsThe Blob was characterized by anomalously high aragonite saturation states and anomalously low oxygen concentrationsThe biogeochemical signature of the Blob was driven by changes in temperature and physical ocean circulation processes The North Pacific Blob had a distinct biogeochemical signature that is captured by observations and multiple ocean models The Blob was characterized by anomalously high aragonite saturation states and anomalously low oxygen concentrations The biogeochemical signature of the Blob was driven by changes in temperature and physical ocean circulation processes

A 500‐year‐long high‐resolution Community Earth System Model simulation under preindustrial radiative forcing produces Maud Rise Polynyas (MRPs) and Weddell Sea Polynya (WSP) events intermittently from decadal (MRP) to multidecadal (WSP) timescales. This periodicity is correlated with the variation of a regional Southern Annular Mode (SAM) index (evaluated regionally instead of circum‐Antarctic) with corresponding variations of precipitation and winds over the Weddell Sea. A negative index causes the upper‐ocean salinity to increase over multiple decades. Ultimately, brine rejection during seasonal sea‐ice formation superimposed on the multidecadal increase raises the upper‐ocean salinity beyond the tipping point for triggering deep convection that leads to polynyas. The initiation of polynya events is thus controlled by surface properties while the location of initiation is determined by bathymetric features. The persistent Taylor column effect, which is well represented by the high‐resolution model topography of the Maud Rise seamount, preconditions this region for MRP initiation. Therefore, MRPs form more frequently than WSPs in the simulation. When the upper‐ocean salinity is high, deep convection in the MRP region tends to be stronger, in which case MRPs are also more likely to grow into WSPs. Once WSPs emerge, they affect the regional atmospheric circulation and associated variables. We propose a regional coupled ocean‐atmosphere mechanism to explain both the periodic emergence of polynyas and the periodic variation of the regional SAM index. Although the temperature and salinity of Weddell Deep Water show upward trends due to model drift, these density‐compensating changes do not affect the frequency of polynya formation. Regions of open water surrounded by frozen sea water (sea ice) in winter are generally referred to as open‐ocean polynyas. A large version of such polynya has last been observed in the Weddell Sea in the mid 1970s. Such large polynyas play an important role for the global ocean circulation because they allow for vigorous exchange of heat between the atmosphere and the ocean. This process is associated with deep vertical overturning of the ocean, thus contributing significantly to the ventilation of the deep ocean. Many low‐resolution Earth System Models simulate open‐ocean polynyas in the Weddell Sea, but presumably for the wrong physical reasons. Based on a 500‐year‐long high‐resolution model simulation run under preindustrial climate conditions, we find that the recurrence of Weddell Sea polynyas (WSPs) is mainly linked to periodic shifts of the precipitation‐rich Southern Hemisphere westerly wind belt. It is ultimately the high‐resolution ocean floor topography of the model that adds realism to the sequence of events that lead to WSPs while low‐resolution versions of the same model do not produce any open‐ocean polynyas. In this study, we propose a coupled ocean‐atmosphere mechanism to explain the periodic occurrence of polynyas. Weddell Sea polynyas occur intermittently in a 500‐year‐long high‐resolution preindustrial Community Earth System Model simulationPeriodic changes of a regional Southern Annular Mode (SAM)‐like index control the periodic changes of upper‐ocean salinity and the opening of polynyasA coupled ocean‐atmosphere mechanism is proposed to explain the periodic change of both polynya occurrence and the regional SAM‐like index Weddell Sea polynyas occur intermittently in a 500‐year‐long high‐resolution preindustrial Community Earth System Model simulation Periodic changes of a regional Southern Annular Mode (SAM)‐like index control the periodic changes of upper‐ocean salinity and the opening of polynyas A coupled ocean‐atmosphere mechanism is proposed to explain the periodic change of both polynya occurrence and the regional SAM‐like index

The Middle English verse “Life of St. Egwine” is one of the many hagiographic poems affiliated with the so-called South English Legendaryor Legendaries (SEL), a widely copied collection of vernacular devotional texts whose earliest compilation has been dated to the thirteenth century, and whose latest manuscripts date to the first half of the fifteenth. A minor saint, Ecgwine was the third bishop of Worcester and the founder of the monastic community at Evesham Abbey. One of the most striking features of his early hagiography is that the earliest version of his vitacontains the only surviving account of a dispute between a monastery and a tenant to be dated to the Anglo-Saxon period. This is indicative of his cult's close association with the endowed properties of Evesham, an aspect of his hagiographic tradition that is also discernible in the SELlegend.

Many scholars believe that the anti-fraternal and overall anticlerical thrust of Mum and the Sothseggerindicates sympathies with Lollardy or Wycliffism, but this article argues that the poem’s anticlericalism is best explained with reference to alliterative Anglo-Saxon homilies and the Anglo-Saxon charters of ancient Benedictine monastic houses. Mum and the Sothseggernostalgically draws from the Anglo-Saxon anticlerical critique in order to argue both for a return to older institutional arrangements, and for the rights of (Anglo-Saxon) kings to confer properties and privileges.

Extending climate prediction from seasonal to decadal timescales requires realistic initialization of not only upper ocean heat content but also the Atlantic meridional overturning circulation (AMOC). However, it remains a major challenge to realistically initialize AMOC in a coupled system while also maintaining a balanced atmosphere‐ocean initial state. This study demonstrates the feasibility of generating fully coupled historical states with realistic AMOC variability. Employing a forced ocean—sea‐ice (FOSI) model simulation as the “truth,” we show reproducibility of key features of historical AMOC decadal variability in a fully coupled model by restoring sea‐surface salinity, in addition to sea‐surface temperature restoring widely used in seasonal prediction. The atmospheric state of the restored coupled model solution is much closer to that of the free coupled simulation than to the observations used in FOSI, pointing to potential advantages of using this approach for initializing decadal predictions with reduced ocean initialization shock. Realistic simulations of the Atlantic meridional overturning circulation (AMOC) at decadal time scales in coupled climate models are vital to decadal climate predictions. Because of the sparseness of deep ocean observations, forced ocean—sea‐ice (FOSI) model simulations have been used to initialize decadal climate predictions in a fully coupled model. However, such an approach suffers from imbalance between atmosphere and ocean states because of systematic differences in the atmospheric state used for FOSI and that of the coupled climate model, causing ocean initial shocks that may lower prediction skill. Here, using a FOSI simulation as synthetic observations, we demonstrate that key features of historical AMOC decadal variability can be realistically replicated in a fully coupled simulation by nudging both sea‐surface temperature (SST) and salinity toward those of FOSI. Because the atmospheric state in the restored simulation is generated by the same coupled model, it is much closer to that in the free coupled simulation than in FOSI. This approach may be considered as an extension of the SST‐restoring practice widely used for seasonal climate prediction, opening an alternative way to initialize decadal climate prediction to reduce the imbalance and initialization shocks, thereby improving prediction skill. Realistic Atlantic meridional overturning circulation (AMOC) decadal variations can be simulated in a coupled climate model by sea‐surface temperature (SST) and sea‐surface salinity (SSS) restoringSea‐surface salinity restoring plays a critical role in simulating realistic AMOC decadal variationsSimultaneously restoring SST and SSS in coupled model simulations may reduce initialization shock in initialized decadal climate predictions Realistic Atlantic meridional overturning circulation (AMOC) decadal variations can be simulated in a coupled climate model by sea‐surface temperature (SST) and sea‐surface salinity (SSS) restoring Sea‐surface salinity restoring plays a critical role in simulating realistic AMOC decadal variations Simultaneously restoring SST and SSS in coupled model simulations may reduce initialization shock in initialized decadal climate predictions

A coordinated set of large ensemble atmosphere‐only simulations is used to investigate the impacts of observed Arctic sea ice‐driven variability (SIDV) on the atmospheric circulation during 1979–2014. The experimental protocol permits separating Arctic SIDV from internal variability and variability driven by other forcings including sea surface temperature and greenhouse gases. The geographic pattern of SIDV is consistent across seven participating models, but its magnitude strongly depends on ensemble size. Based on 130 members, winter SIDV is ~0.18 hPa2for Arctic‐averaged sea level pressure (~1.5% of the total variance), and ~0.35 K2for surface air temperature (~21%) at interannual and longer timescales. The results suggest that more than 100 (40) members are needed to separate Arctic SIDV from other components for dynamical (thermodynamical) variables, and insufficient ensemble size always leads to overestimation of SIDV. Nevertheless, SIDV is 0.75–1.5 times as large as the variability driven by other forcings over northern Eurasia and Arctic. Changing Arctic sea ice conditions since the late 1970s have exerted profound impacts on environment and ecosystem at the high latitudes and have been suggested to affect midlatitude weather and climate, although this topic has been controversial. In order to improve our understanding on how Arctic sea ice changes influence local and remote weather and climate, a coordinated set of experiments has been performed using various state‐of‐the‐art atmosphere‐only models to study the linkages between the Arctic climate change and lower latitudes. This study uses seven models following a common experimental protocol to investigate the atmospheric circulation changes forced by Arctic sea ice variability. The protocol allows the Arctic sea ice‐driven variability (SIDV) to be singled out. In boreal winter, the Arctic SIDV is ~0.18 hPa2and accounts for only ~1.5% of the total variance for sea level pressure, while it is ~0.35 K2and accounts for ~21% for surface air temperature. The results also suggest that using insufficient ensembles always leads to an overestimation of SIDV, and more than 100 and 40 ensemble members are needed for sea level pressure and surface air temperature within the Arctic, respectively, to separate the SIDV from the variability due to other factors, primarily the atmospheric internal variability. Arctic sea ice variations drive ~1.5% of boreal winter Arctic SLP variance during 1979–2014 in coordinated 130‐member AGCM experimentsAn accurate quantification of the impacts of Arctic sea ice‐driven SLP variability requires an ensemble size greater than 100 membersInsufficient ensemble size always leads to an overestimation of the Arctic sea ice‐driven variance within and outside Arctic

This study presents multiple lines of evidence from observations and model simulations that support a key role for ocean dynamics, rather than external forcings, in Atlantic multidecadal variability (AMV) during the last half century. Observed AMV fingerprints considered here include the low‐frequency spatiotemporal evolution of sea surface temperature, surface heat fluxes, and deep ocean hydrography. While largely absent in the forced response of a large ensemble historical simulations (LENSs), these fingerprints are clearly discernible in a long control simulation where the variability is purely internal. Further evidence derives from initialized decadal prediction simulations, which exhibit much higher skill at predicting the observed AMV of the past 50 years than LENS. The high correlation between the observed AMV and the externally forced version derived from LENS, which has been invoked as evidence for externally driven AMV, is shown to be largely an artifact of concurrent warming since the 1990s. The basin‐wide sea surface temperature variability in the North Atlantic on multidecadal timescales, often called Atlantic multidecadal variability (AMV), has profound impacts on surface climate in the Northern Hemisphere. This work sheds light on the ongoing debate regarding the driving mechanisms of the AMV: whether it is primarily driven by factors associated with natural fluctuations of the coupled ocean‐atmosphere system (such as slow changes in the strength of the ocean circulation) or forced by factors that are external to that system (such as large volcanic eruptions). In this study, we present multiple lines of evidence, from ocean observations as well as climate model simulations, that suggest that AMV is largely attributable to the former and is thus primarily reflecting a natural mode of variability in the climate system. The observed AMV shows distinctive spatiotemporal patterns with concomitant variability in related fieldsWhile absent in the forced signals in historical simulations, this observed evidence is well reproduced in a preindustrial simulationA high correlation between the observed and externally forced, simulated AMV arises by coincidence from concurrent warming after the 1990s

Historical observations show that one in two La Niña events last for two consecutive years. Despite their outsized impacts on drought, these 2 year La Niña are not predicted on a routine basis. Here we assess their predictability using retrospective forecasts performed with a climate model that simulates realistic multiyear events, as well as with an empirical model based on observed predictors. The skill of the retrospective forecasts allows us to make predictions for the upcoming 2017–2018 boreal winter starting from conditions in November 2015. These 2 year forecasts indicate that the return of La Niña is more likely than not, with a 60% probability based on the climate model and an 80% probability based on the empirical model; the likelihood of El Niño is less than 8% in both cases. These results demonstrate the feasibility of predictions of the duration of La Niña. Historical observations show that cold La Niña events in the tropical Pacific often reintensify for a second year. Despite their outsized climate impacts throughout the world, these 2 year La Niña events are not routinely predicted. Our study demonstrates that long‐term forecasts of these events are feasible. Moreover, we show increased likelihood of returning La Niña for next boreal winter, a result that is directly relevant for assessing climate risks throughout the world, for instance, over the southern tier of the U.S., where La Niña events create extreme seasonal heat and drought during winter and spring, and the Maritime Continent and Northern Australia, where it causes excess rainfall and flooding. Thermocline discharge driven by strong El Niño requires several years of La Niña conditions to return to a neutral stateForecast initialization at the peak of a strong El Niño event leads to skillful predictions of subsequent 2 year La Niña eventReturning La Nina conditions in 2018 have 60% probability based on a dynamical model and 80% probability based on an empirical model