The variability of isopycnal stirring and diapycnal mixing associated with mesoscale and microscale turbulence, respectively, is assessed using Argo float observations in the eastern subtropical North Atlantic. A new method is introduced combining a finescale parameterization with a triple decomposition framework to separate isopycnal stirring and diapycnal mixing. Our approach is validated by comparing with microstructure measurements from an independent North Atlantic tracer release experiment in the 1990s. Both sets of diagnostics reveal that diapycnal mixing dominates in the upper thermocline and that isopycnal stirring prevails in deeper, intermediate layers, influenced by Mediterranean Outflow Water. The analysis of Argo data for years 2014-24 reveals substantial interannual variations in stirring and mixing rates and in the relative contributions of mesoscale and microscale processes to thermohaline variance dissipation, which persisted in time (;3 yr) and across isopycnal layers (26.75-27.5 kg m23). A shift in the relative contributions to the variance dissipation occurred over the period, with more prevalent isopycnal stirring in the thermocline after 2018. This enhancement of isopycnal stirring, which concurred with lower isopycnal diffusivities, is attributed to increased property gradient along-isopycnals induced by a large-scale cooling and freshening of the eastern subtropical North Atlantic. Our results highlight how property contrasts in the thermocline, induced originally by ocean ventilation, are modified by isopycnal stirring and diapycnal mixing. This application of finescale parameterizations constitutes a powerful tool that has the potential to capture and quantify the temporal evolution of isopycnal stirring and diapycnal mixing rates from Argo observations across the global ocean. SIGNIFICANCE STATEMENT: Ocean ventilation is vital in the climate system, transferring fluid and properties from the surface to the ocean interior. Yet, the ways in which stirring and mixing modulate ventilation are poorly understood, due to the sparseness of small-scale ocean observations. By applying a novel approach to Argo float data, we diagnose stirring and mixing processes in the subtropical North Atlantic over a recent decade. Our results reveal a shift after 2018, with mesoscale stirring becoming more dominant in response to a major cooling and freshening event. The enhanced stirring dampens temperature and salinity contrasts, thus distributing the effects of externally forced, large-scale changes across the North Atlantic. Our study opens new opportunities to diagnose stirring and mixing across the global ocean.

Small-scale diapycnal mixing and mesoscale isopycnal stirring redistribute heat and freshwater and other solutes in the ocean. These processes shape regional and global circulation patterns and impact the climate system. Due to the scarcity of mixing observations, appraising such a critical role remains a major challenge. Here, we revisit and expand a recent reformulation of the water-mass transformation framework to derive global thermohaline transformation rates from estimates of variance dissipation rates of temperature XQ and salinity XS in the ocean interior. To estimate XQ and XS, we leverage new global, vertically resolved maps of diapycnal and isopycnal diffusivity. Global diathermal and diahaline water-mass transformations by interior mixing show respective double circulation cells transporting 600 TW of heat and 20 3 106 kg s21 of salt to waters cooler than 20 degrees C and fresher than 35.1 g kg21. Diathermal transformations are dominated by diapycnal mixing, involving the formation of 36 Sv (1 Sv ; 106 m3 s21) of subtropical thermocline waters (10 degrees-25 degrees C) from warmer tropical waters and colder, deeper waters. Isopycnal mixing is the main driver of global diahaline transformations, forming 34 Sv of waters with intermediate salinity (34.4-35.7 g kg21), and of diathermal transformations in cold waters (,5 degrees C). The Antarctic Circumpolar Current is the global hotspot for isopycnal mixing, controlling the redistribution of heat and freshwater between the Southern Ocean and the rest of the global ocean. Climatological estimations were validated against regional observational microstructure datasets, demonstrating that even a modest number of microstructure profiles can yield meaningful regional estimates of water-mass transformations. SIGNIFICANCE STATEMENT: Oceanic turbulence mixes water masses of varying temperature and salinity, shaping global circulation and redistributing heat and freshwater critical for climate. Quantifying turbulent fluxes at large scales is challenging because direct measurements require centimeter-scale observations, which are costly and scarce. Here, we combine recent global estimates of turbulent diffusivity from hydrographic data and theory with a new water-mass transformation framework to quantify based on observations the roles of diapycnal and isopycnal mixing in ocean circulation. We find diapycnal mixing sustains shallow low-latitude circulations, while isopycnal mixing in high latitudes is crucial for deep overturning. This challenges the traditional emphasis on diapycnal mixing. Applying the framework to localized direct microscale mixing observations shows that even sparse data can yield valuable regional insights.

Small-scale turbulent mixing in the ocean interior is vital in governing ocean circulation and tracer distributions, and hence global climate. However, the planetary extent of this role and its dependence on the microphysics of mixing remain inadequately understood. Here, we emphasize the variety of spatio-temporal scales on which such interior turbulent mixing can shape the climate system. In addition to its well-established role in facilitating the equilibration of deep branches of ocean circulation on centennial-to-millennial timescales, interior turbulent mixing is a leading determinant of oceanic tracer budgets on timescales as short as sub-annual. We highlight the importance of the co-dependence of vertical (diapycnal) mixing and lateral (isopycnal) stirring in establishing the large-scale impacts of oceanic turbulence. We conclude with a summary of theoretical, observational and computational bottlenecks in the way of a sufficiently accurate representation of mixing in Earth System Models, and discuss emerging opportunities for making progress in these areas.

The Gulf Stream is important for the climate system through its transport and air-sea exchange of heat. What is less well accepted is the role of the Gulf Stream in the carbon cycle. Here we examine how the Gulf Stream provides a "biogeochemical stream", a sub-surface horizontal flux carrying waters with high concentrations of nutrients and low concentrations of anthropogenic carbon. Model experiments reveal particles released in dense layers at the start of the Gulf Stream follow trajectories extending into the subpolar gyre, while particles released at the surface are confined to the subtropics. Following a pathway to the subpolar gyre, the biogeochemical stream carries older, nutrient-rich and anthropogenically carbon-depleted waters along density layers and, when those dense layers outcrop into the mixed layer, enhances the subpolar drawdown of atmospheric carbon. This connectivity is supported by model sensitivity experiments revealing the subpolar upper ocean carbon content and upstream dense waters in the Gulf Stream connecting on timescales of 4 to 8 years. The likely effect of climate change on the biogeochemical stream is a decrease in the delivery of these older waters, both high in concentrations of nutrients and depleted in anthropogenic carbon, to the subpolar mixed layer, so weakening future North Atlantic carbon uptake from the atmosphere.

Oceanic tracer distributions are shaped by turbulent mixing, which may be understood via a traver variance budget. There is a tracer variance lifecycle: variance production by turbulent flows stirring large-scale gradients, redistribution by currents, and variance dissipation by molecular diffusion. A comprehensive tracer variance budget, including variance transport, is diagnosed from North Atlantic hydrographic observations and reveals three different regimes. In the subtropical thermocline, there is a local balance between diapycnal production by microscale turbulence and molecular dissipation. In intermediate waters and the subpolar gyre, the local balance is between isopycnal production by mesoscale stirring and dissipation. Near gyre boundaries, a three-way balance emerges between isopycnal production by mesoscale stirring, variance transport by currents, and dissipation downstream. Transitions between mixing regimes are determined by water-mass contrasts and circulation strength. Our expectation is that these different mixing regimes apply to the global ocean and affect how the ocean sequesters and redistributes anthropogenic heat.

Mode waters provide an important role within the climate system, sequestering large amounts of heat and anthropogenic carbon and play a key role in the transport of these properties around the globe. Our aim is to assess the roles of local versus remote surface forcing in controlling the properties of mode waters over the northern Atlantic and Pacific basins and the Southern Ocean. A set of adjoint sensitivity experiments are conducted using the ECCOv4r4 state estimate to assess the impacts of surface heat flux, freshwater flux, and wind stresses on the volume and heat content of mode waters in density space. Mode waters are identified using areas of deep winter mixed layers and their characteristic temperature, stratification, and neutral density properties. The adjoint modelling approach calculates time-evolving sensitivity maps that identify where and when specific surface forcing impacts properties in the mode water formation sites. The sensitivity analysis reveals the dominance of local forcing from surface heat fluxes with surface cooling initially increasing volume. On longer time scales, the sensitivities have differing responses to surface forcing including surface heat loss leading to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped. The responses of the mode waters to surface forcing are then compared across their formation sites, in the northern basins involving western boundary currents and gyre interiors and in the Southern Ocean involving the Antarctic Circumpolar Current.

The decadal variability in the subpolar North Atlantic Ocean heat content is significantly influenced by the atmosphere. The impact of seasonal-annual atmospheric perturbations lasts for many years in the oceans due to the ocean's long memory. The anomalous air-sea heat fluxes and winds associated with atmospheric perturbations first rapidly modify upper ocean temperatures, initiating a short-term or local ocean response. Subsequently, these modifications can alter meridional heat transport rates, leading to anomalous heat convergence persisting for several years—a long-term or far-field ocean response—in the subpolar ocean (Khatri et al., 2022, Geophys Res Lett). We propose a novel technique that incorporates these two ocean responses to evaluate ocean memory and examine its role in driving decadal ocean variability. Here, we combine heat budget analysis with linear response theory to examine how the North Atlantic Oscillation (NAO), which captures about 40% of atmospheric variability, controls the decadal variability in upper ocean temperatures and quantify the associated ocean memory. Utilising CMIP6 climate model outputs and observations, our estimations suggest ocean memory for the subpolar North Atlantic to be between 10 to 20 years. Furthermore, we find that the NAO strongly influences long-term ocean variability, explaining 30% to 40% of subpolar ocean heat content variability on decadal timescales. Specifically, the impact of seasonal atmospheric events on the ocean persists for more than a decade through a combination of local and far-field ocean responses. The proposed ocean memory-based framework, integrating local and far-field ocean effects into a single metric, can be utilised to analyse how relatively short-timescale atmospheric variability drives changes in the ocean state over decadal timescales.

Sub‐Antarctic Mode Waters (SAMWs) form to the north of the Antarctic Circumpolar Current in the Indo‐Pacific Ocean, whence they ventilate the ocean's lower pycnocline and play an important role in the climate system. With a backward Lagrangian particle‐tracking experiment in a data‐assimilative model of the Southern Ocean (B‐SOSE), we address the long‐standing question of the extent to which SAMWs originate from densification of southward‐flowing subtropical waters versus lightening of northward‐flowing Antarctic waters sourced by Circumpolar Deep Water (CDW) upwelling. Our analysis evidences the co‐occurrence of both sources in all SAMW formation areas, and strong inter‐basin contrasts in their relative contributions. Subtropical waters are the main precursor of Indian Ocean SAMWs (70%–75% of particles) but contribute a smaller amount (40% of particles) to Pacific SAMWs, which are mainly sourced from the upwelled CDW. By tracking property changes along particle trajectories, we show that SAMW formation from northern and southern sources involves contrasting drivers: subtropical source waters are cooled and densified by surface heat fluxes, and freshened by ocean mixing. Southern source waters are warmed and lightened by surface heat and freshwater fluxes, and they are made either saltier by mixing in the case of Indian SAMWs, or fresher by surface fluxes in the case of Pacific SAMWs. Our results underscore the distinct climatic impact of Indian and Pacific SAMWs formation, involving net release of atmospheric heat and uptake of atmospheric freshwater, respectively; a role that is conferred by the relative contributions of subtropical and Antarctic sources to their formation.

Thermal variability in the subpolar North Atlantic Ocean may be understood in terms of opposing fast and slow responses to atmospheric events, such as involving the response to the North Atlantic Oscillation (NAO). What is unclear is the associated ocean carbon response to atmospheric events, and how that response differs from the thermal response? Here, we diagnose the output from a full Earth system model, UKESM1 piControl simulation integrated over 1100 years, and analyse the transient response to a composite NAO event, derived from combining 270 NAO+ and 246 NAO- individual events. The carbon response is then separated into a fast and slow response to the onset of a single NAO event. During a NAO+ event, there is an initial local response extending over the first one to two years involving anomalous surface cooling and air-sea uptake of carbon in the subpolar gyre. Consequently, there is a reduction in heat storage and an increase in ocean dissolved inorganic carbon (DIC), together with enhanced mixed-layer entrainment of nutrients leading to an increase in biological export of carbon. There is then a delayed response extending for a further 10 years, involving an influx of warm and salty waters through ocean advection, which also carries an increase in both alkalinity and dissolved inorganic carbon. Hence, the ocean thermal and carbon responses involve a combination of fast, local responses to atmospheric forcing (involving air-sea exchange, entrainment and biological export) plus a slow, far-field response to prior atmospheric events (involving ocean redistribution of heat, salt, alkalinity and carbon together with continued air-sea exchange). The thermal and carbon responses differ in that the thermal response involves opposing signs in the fast and slow contributions, while the carbon response involves reinforcing fast and slow contributions. This asymmetry is primarily due to opposing signs in the fast contributions with surface cooling leading to a reduction in heat storage, but an increase in carbon storage. Hence, the ocean memory of an atmospheric event is greater for carbon than for heat.

In the South Pacific Subantarctic mode water (SAMW) formation region, central and eastern pools of SAMW have been found to be linked to winter mixed-layer thicknesses that vary strongly interannually and out of phase across the basin. This mixed-layer variability is associated with peaks in sea level pressure variability at a quasi-stationary anomaly situated between the two pools. To investigate how surface forcing, as well as the propagation of upstream anomalies, affects the formation of these SAMW pools, a set of adjoint sensitivity experiments with a density-following feature are conducted. Adjoint sensitivities reveal that local cooling can lead to an increase in the SAMW pool volume through mixed-layer-depth changes and the lateral movement of the northern boundary of the pool. In addition, upstream warming along the Antarctic Circumpolar Current can lead to an increase in the SAMW pool volume through lateral density surface movement shifting the southern boundary polewards. The density properties are advected from upstream to the downstream pool over 1 year. Optimal conditions for SAMW formation involve a combination of local cooling and upstream warming of SAMW formation sites. Hence, South Pacific SAMW variability is particularly sensitive to atmospheric modes which lead to a dipole in heating across the formation sites.

Thermal and biogeochemical states of the North Atlantic Ocean are affected on seasonal to decadal timescales by atmospheric forcing associated with the North Atlantic Oscillation (NAO). An NAO–based composite approach is applied to an Earth system model to reveal the fast and slow responses of the ocean to atmospheric impulse forcing. Over the seasonal boundary layer, the atmosphere induces a “fast”, seasonal ocean response driven by anomalies in the air–sea flux, vertical entrainment and Ekman transport. This fast response to an NAO anomaly results in negative temperature and positive carbon and nutrient anomalies over the subpolar gyre, and positive temperature and negative carbon and nutrient anomalies over the subtropical gyre. The “slow” response on inter‐annual timescales involves changes in meridional overturning, tracer transport and entrainment. The slow response leads to a redistribution of anomalies between subpolar and subtropical regions, generating opposing–signed tracer anomalies in the subpolar and subtropical North Atlantic Oceans. These ocean responses also involve changes in the mixed‐layer depth, as well as the associated changes in entrainment and turbulent mixing rates, which are particularly important for the carbon and nutrient responses given their large vertical gradients. Modifications in nutrient concentrations subsequently influence biological activity and biomass production. These thermal, carbon and nutrient responses to atmospheric events linked to the NAO can persist for up to a decade, often characterized by opposing–signed temperature and carbon anomalies, along with contrasting changes in the subtropical and subpolar gyres.

The Zero Emissions Commitment (ZEC) measures the transient climate response after carbon emissions cease, defined by whether there is a continued rise or decrease in global surface temperature. This delayed climate response affects the maximum cumulative carbon emission to avoid exceeding a warming target. In a set of 9 Earth system models following an idealised atmospheric CO2 scenario with a cumulative emission of 1000 Pg C, the ZEC after 50 years ranges from −0.3 to 0.28 °C with a model mean of −0.11 °C and standard deviation of 0.19 °C. In order to understand these different climate responses, a normalised framework is introduced that quantifies the relative importance of carbon, radiative and thermal drivers of the ZEC. Inter-model differences in the ZEC are primarily due to differences in the radiative response, planetary heat uptake and the land carbon sink, with more minor contributions from differences in the ocean carbon sink and climate feedback. The ZEC response is controlled by opposing-signed contributions: (i) cooling from a decrease in radiative forcing from a carbon contribution due to increasing land and ocean carbon uptake, versus (ii) surface warming from a thermal contribution involving a decline in the fraction of radiative forcing used for planetary heat uptake plus possible amplification by climate feedback. The carbon contribution to the ZEC depends on the increase in the ocean carbon sink and whether the land carbon sink either increases or saturates in time. The thermal contribution to the ZEC depends upon how radiative forcing is partitioned between planetary heat uptake and radiative response with the radiative response either declining in time or remaining constant. These inferences as to the controls of the ZEC broadly carry over for diagnostics for a large ensemble, observationally-constrained, efficient Earth system model using two different emission scenarios to reach net zero. The large set of ensembles reveal a partial compensation between the changes in landborne and oceanborne fractions, as well as including ensembles with a greater range in amplification of warming by climate feedbacks.

The "warming stripes" are an iconic climate data visualization, adopted globally as a symbol of our warming world. We discuss their origin and uses for communication, including understanding long-term changes in the climate and consequences of future emission choices. We also extend the stripes concept to explore observed temperature variations throughout the climate system, revealing coherent warming for the troposphere and upper ocean and cooling in the stratosphere, consistent with our understanding of human influences on our climate.

An ocean memory framework is proposed to reveal the atmosphere's influence on ocean temperatures. Anomalous atmospheric forcing alters the ocean state through two mechanisms: short-term, local effects involving air- ${-}$sea heat fluxes and Ekman circulation, and long-term, far-field effects involving changes from overturning and gyre circulations. The framework employs the Green function's method to incorporate both effects, enabling the quantification of ocean memory and the contribution of atmospheric forcing to ocean thermal variability. The framework is employed to examine the North Atlantic Oscillation's (NAO) influence on the North Atlantic Ocean variability, including the Atlantic Multidecadal Variability, with its memory estimated to be 10-20 $10\mbox{--}20$ years. The NAO and variability in the North Atlantic jet speed explain up to 30% of ocean decadal variability, primarily driven by temporal changes in ocean heat transport. Therefore, decadal fluctuations in ocean temperatures cannot be accurately modeled solely as a passive response to stochastic atmospheric forcing. The atmosphere and ocean are intricately linked, with the ever-changing atmosphere driving fluctuations in ocean temperatures. However, the atmosphere changes much faster than the ocean due to their different timescales and the ocean's ability to store excess heat for many years commonly referred to as ocean memory. As a consequence, isolating and understanding how short-term atmospheric variations influence long-term fluctuations in ocean temperatures, particularly on decadal timescales, can be difficult. To address this challenge, we introduce an ocean memory framework that allows us to analyze and quantify the atmosphere's contribution to ocean variability. We apply this framework to study how the North Atlantic Oscillation, a major atmospheric pattern in the northern hemisphere, influences decadal fluctuations in the subpolar North Atlantic Ocean temperatures. We estimate the ocean memory timescale in the subpolar North Atlantic to be about 10-20 $10\mbox{--}20$ years, which is how long temperature anomalies persist, while being redistributed, in the ocean before dissipating. New ocean memory framework accounts for local response due to air-sea fluxes and far-field response due to ocean circulation The framework is employed to quantify subpolar North Atlantic Ocean thermal variability due to the North Atlantic Oscillation (NAO) The framework captures the sign reversal in the correlation between the NAO and ocean temperatures across seasonal to decadal timescales

The ocean carbon sink plays a critical role in climate, absorbing anthropogenic carbon from the atmosphere and mitigating climate change. The sink shows significant variability on decadal timescales, but estimates from models and observations disagree with one another, raising uncertainty over the magnitude of the sink, its variability, and its driving mechanisms. There is a need to reconcile observation-based estimates of air–sea CO2 fluxes with those of the changing ocean carbon inventory in order to improve our understanding of the sink, and doing so requires knowledge of how carbon is transported within the interior by the ocean circulation. Here we employ a recently developed optimal transformation method (OTM) that uses water-mass theory to relate interior changes in tracer distributions to transports and mixing and boundary forcings, and we extend its application to include carbon using synthetic data. We validate the method using model outputs from a biogeochemical state estimate, and we test its ability to recover boundary carbon fluxes and interior transports consistent with changes in heat, salt, and carbon. Our results show that the OTM effectively reconciles boundary carbon fluxes with interior carbon distributions when given a range of prior fluxes. The OTM shows considerable skill in its reconstructions, reducing root-mean-squared errors from biased priors between model “truth” and reconstructed boundary carbon fluxes by up to 71 %, with the bias of the reconstructions consistently ≤0.06 molCm-2yr-1 globally. Inter-basin transports of carbon also compare well with the model truth, with residuals <0.25 Pg C yr−1 for reconstructions produced using a range of priors. The OTM has significant potential for application to reconcile observational estimates of air–sea CO2 fluxes with the interior accumulation of anthropogenic carbon.

The Southern Ocean provides dominant contributions to global ocean heat and carbon uptake, which is widely interpreted as resulting from its unique upwelling and circulation. Here we show a large asymmetry in these contributions, with the Southern Ocean accounting for 83 +/- 33% of global heat uptake versus 43 +/- 3% of global ocean carbon uptake over the historical period in state-of-the-art climate models. Using single radiative forcing experiments, we demonstrate that this historical asymmetry is due to suppressed heat uptake by northern oceans from enhanced aerosol forcing. In future projections, such as SSP2-4.5 where greenhouse gases increasingly dominate radiative forcing, the Southern Ocean contributions to global heat and carbon uptake become more comparable, 52 +/- 5% and 47 +/- 4%, respectively. Hence, the past is not a reliable indicator of the future, with the northern oceans becoming important for heat uptake while the Southern Ocean remains important for both heat and carbon uptake. The Southern Ocean takes up substantial amounts of heat and carbon. Here the authors show that it has historically accounted for a much greater proportion of global ocean heat uptake-and link this to aerosols depressing uptake in northern oceans-but that future heat and carbon uptake will become more comparable.

Subantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual-average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely-signed advective haline contribution dominates. This two-phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current. The Southern Ocean, surrounding the Antarctic continent, plays an important role in the uptake and transport of heat and carbon. Subantarctic mode waters, which are characterized by their low stratification, play an important role in this uptake of heat and carbon, and therefore the factors impacting their properties need to be properly understood. To understand how surface forcing affects Subantarctic mode waters, sensitivity studies are conducted in an ocean state estimate, which consider the relative importance of surface heat flux, freshwater flux, and wind stresses on the stratification of mode waters. Surface heat flux has the largest impact on mode water formation both on seasonal and longer interannual timescales. Initially, surface heat loss leads to a decrease in stratification in the mode waters. However, there is a delayed response where the surface temperature response is effectively damped by the atmosphere and there is an opposing-signed salinity response advected into the region, leading to a subsequent increase in stratification in the mode waters. The sensitivity of Southern Ocean mode water stratification to surface heat flux changes sign over time Surface heat loss leads to an initial decrease in stratification in the mode waters Surface heat loss leads to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped

Over the last century Earth's surface temperatures have warmed by order 1 K as a global average, but with significant variation in latitude: there has been most surface warming at high Northern latitudes, around 3 times more than in low latitude regions (termed Arctic Amplification), while there has been least warming over the Southern Ocean. Many contributing processes have been suggested to explain this asymmetrical latitudinal warming pattern, but quantification of the contributing factors responsible remains elusive. Complex general circulation climate models can reproduce similar asymmetrical patterns of warming, but it can be difficult to interpret the contributing processes. Meanwhile, idealised conceptual energy balance climate models have been able to reproduce a general polar amplification of warming whose origins can be interpreted, but this warming is often symmetrical across both hemispheres and may not be responsible for the real-world pattern. Here, we use a conceptual Energy Balance Model, with imposed closures for initial horizontal diffusivity and cloudiness drawing upon observational constraints and including temperature-dependent diffusivity and a sub-surface ocean heat reservoir, to show that the magnitude of present-day Arctic Amplification may arise through relatively simple thermodynamic (Clausius-Clapeyron) and radiative (climate feedback) processes. The current asymmetry between hemispheric warming may arise due to the transient heat transport up through the base of the surface ocean mixed layer from the slow-responding deep ocean to the fast responding surface ocean being dominated by upwelling in the Southern Ocean. It should be noted that the processes identified here are not a unique in offering a potential solution, and so significant, or dominant, roles for dynamical processes remain plausible explanations for Arctic Amplification.& COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

The climate feedback determines how Earth’s climate responds to anthropogenic forcing. It is thought to have been more negative in recent decades due to a sea surface temperature “pattern effect,” whereby warming is concentrated in the western tropical Pacific, where nonlocal radiative feedbacks are very negative. This phenomenon has however primarily been studied within climate models. We diagnose a pattern effect from historical records as an evolution of the climate feedback over the past five decades. Our analysis assumes a constant rate of change of the climate feedback, which is justified post hoc. We find a decrease in climate feedback by 0.8 ± 0.5 W m −2 K −1 over the past 50 years, corresponding to a reduction in climate sensitivity. Earth system models’ climate feedbacks instead increase over this period. Understanding and simulating this historical trend and its future evolution are critical for reliable climate projections.