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The Baltic Sea is one of the fastest-warming semi-enclosed seas in the world over the last decades, yielding critical consequences on physical and biogeochemical conditions and on marine ecosystems. Although long-term trends in sea surface temperature (SST) have long been attributed to trends in air temperature, there are however, strong seasonal and sub-basin scale heterogeneities of similar magnitude than the average trend which are not fully explained. Here, using reconstructed atmospheric forcing fields for the period 1850–2008, oceanic climate simulations were performed and analyzed to identify areas of homogenous SST trends using spatial clustering. Our results show that the Baltic Sea can be divided into five different areas of homogeneous SST trends: the Bothnian Bay, the Bothnian Sea, the eastern and western Baltic proper, and the southwestern Baltic Sea. A classification tree and sensitivity experiments were carried out to analyze the main drivers behind the trends. While ice cover explains the seasonal north/south warming contrast, the changes in surface winds and air-sea temperature anomalies (along with changes in upwelling frequencies and heat fluxes) explain the SST trends differences between the sub-basins of the southern part of the Baltic Sea. To investigate future warming trends climate simulations were performed for the period 1976–2099 using two RCP scenarios. It was found that the seasonal north/south gradient of SST trends should be reduced in the future due to the vanishing of sea ice, while changes in the frequency of upwelling and heat fluxes explained the lower future east/west gradient of SST trend in fall. Finally, an ensemble of 48 climate change simulations has revealed that for a given RCP scenario the atmospheric forcing is the main source of uncertainty. Our results are useful to better understand the historical and future changes of SST in the Baltic Sea, but also in terms of marine ecosystem and public management, and could thus be used for planning sustainable coastal development.

Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990 A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980 This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening

Over the past decades, Beijing, the capital city of China, has encountered increasingly frequent persistent haze events (PHE). While the increased pollutant emissions are considered as the most important reason, changes in regional atmospheric circulations associated with large-scale climate warming also play a role. In this study, we find a significant positive trend of PHE in Beijing for the winters from 1980 to 2016 based on updated daily observations. This trend is closely related to an increasing frequency of extreme anomalous southerly episodes in North China, a weakened East Asian trough in the mid-troposphere and a northward shift of the East Asian jet stream in the upper troposphere. These conditions together depict a weakened East Asian winter monsoon (EAWM) system, which is then found to be associated with an anomalous warm, high-pressure system in the middle–lower troposphere over the northwestern Pacific. A practical EAWM index is defined as the seasonal meridional wind anomaly at 850 hPa in winter over North China. Over the period 1900–2016, this EAWM index is positively correlated with the sea surface temperature anomalies over the northwestern Pacific, which indicates a wavy positive trend, with an enhanced positive phase since the mid-1980s. Our results suggest an observation-based mechanism linking the increase in PHE in Beijing with large-scale climatic warming through changes in the typical regional atmospheric circulation.

Climate models generally overestimate observed Southern Ocean surface warming trends over the past three decades. This discrepancy could be due to biased surface freshwater fluxes in climate models, which underestimate observed precipitation increases and do not account for Antarctic Ice Sheet and shelf mass loss. Though past modeling experiments show surface cooling in response to freshwater perturbations, sea surface temperature (SST) responses vary widely across models. To address these ambiguities, we compute linear SST response functions for standardized freshwater flux increases across a subset of CMIP6 models. For 1990–2021, underestimated freshwater fluxes can explain up to 60% of the model‐observation SST trend difference. The response functions reveal that Southern Ocean SST trends are more sensitive to freshwater fluxes concentrated along the Antarctic margin versus more spatially distributed fluxes. Our results quantify, for the first time, the impact of missing freshwater forcing on Southern Ocean SST trends across a multi‐model ensemble. Plain Language Summary While most of Earth's surface has warmed over the satellite era, the Southern Ocean sea surface experienced a prolonged cooling trend. One proposed driver of this anomalous cooling is the increasing flux of freshwater into the Southern Ocean, which restricts the exchange of surface waters with warmer waters at depth. Climate model simulations underestimate the Southern Ocean freshening trend and fail to simulate the observed surface cooling. Here, we quantify the impact of missing freshwater fluxes on sea surface temperatures across a range of climate models and assess their importance for historical climate trends. We focus on freshwater contributions from the melting Antarctic Ice Sheet and changes in precipitation, which play an important role in the Southern Ocean. A key conclusion is that underestimated freshwater fluxes into the Southern Ocean help explain climate models' inability to reproduce observed sea surface temperature trends. Key Points In recent decades, Earth system models have underestimated Southern Ocean freshening from increased precipitation and Antarctic meltwater The magnitude of missing freshwater produces significant Southern Ocean sea surface cooling, with comparable contributions from each source In a multi‐model ensemble, these sources explain up to 60 percent of the bias in simulated historical sea surface temperature trends

The El Niño–Southern Oscillation (ENSO) teleconnection to Europe is projected to strengthen under global warming in most climate model simulations. However, given the current difference between recent observations and historical model simulations of tropical Pacific sea surface temperature trends, with models simulating an El Niño‐like warming in recent decades which is in disagreement with observations, it is important to understand the relative contributions of changes to the teleconnection forcing and background state to the overall teleconnection change. Using idealized climate model experiments, we show that both the eastward shift of El Niño precipitation and background state changes make contributions to the overall teleconnection change. These results suggest that the ENSO–Europe teleconnection can be expected to strengthen under global warming, even if ENSO precipitation anomalies do not shift eastwards as currently projected. However, the magnitude of the strengthening may depend on how much of an eastward shift does occur. Plain Language Summary El Niño–Southern Oscillation (ENSO) can influence the weather and climate in Europe, and this connection is expected to become stronger under global warming according to most climate models. However, there is a difference between recent observations of sea surface temperatures in the Pacific Ocean and what climate models simulate. These differences could have important implications for future projections of ENSO's impact on Europe. To understand how likely the projected future strengthening of this link is, we looked at how two factors contribute to this connection change: (a) changes in rainfall associated with El Niño in the tropical Pacific and (b) overall changes in global climate elsewhere. Using a simplified climate model, we found that both these factors play a role in making the connection between El Niño and Europe stronger. This suggests that even if the specific patterns of El Niño rainfall don't change as projected, the link between El Niño and Europe will still get stronger due to global warming. However, how much stronger it gets might depend on the actual changes in El Niño rainfall, so it's important to figure out why there's a difference between observations and the model projections of Pacific Ocean temperature trends. Key Points The El Niño–Southern Oscillation teleconnection to Europe is projected to strengthen under global warming This change is driven by an eastward shift of El Niño precipitation anomalies in the tropical Pacific and background state changes globally Background state changes have a greater impact on European circulation than precipitation changes, particularly over central‐eastern Europe

We show that the wintertime (December‐January‐February) North Pacific jet in ERA5 has shifted northwards over the satellite‐era (1979–2023) at a faster rate than any of the state‐of‐the‐art coupled climate models used in this study. Differences in tropical sea surface temperature (SST) trends can only partially explain the discrepancy in jet trends between models and observations and a small minority of simulations forced with observed SSTs match the magnitude of the observed jet trend. However, analysis of longer‐term jet variability in reanalysis suggests that the jet trend has not clearly emerged from multi‐decadal internal climate variability. Consequently, it is unclear whether the difference in observed and modeled jet trends arises due to differing responses to anthropogenic forcing or overly weak long‐term internal variability in models. These results have important implications for future climate projections for North America and motivate further research into the underlying causes of long‐term jet trends. Plain Language Summary The North Pacific jet stream has a large effect on precipitation and temperatures over North America. Climate model simulations suggest that the jet is likely to move northwards with climate change which could lead to a shift in rainfall patterns. We show that the observed jet has shifted northwards at a faster rate over the past 45 years than in the vast majority of model simulations. However, we cannot rule out the possibility that the observed shift may largely be the result of natural climate variability. Our findings suggest that models either underplay natural jet stream variability or the Pacific jet response to climate change, both of which increase the real uncertainty in future climate change for North America. Key Points The North Pacific jet has shifted northward over the satellite‐era but no models show a trend of the same magnitude over this period This trend is consistent with variability associated with the Interdecadal Pacific Oscillation and is not necessarily externally forced Model‐observation differences in tropical Pacific temperature trends can only partially explain differences in North Pacific jet trends

Precipitation projections in transient climate change scenarios have been extensively studied over multiple climate model generations. Although these simulations have also been used to make projections at specific Global Warming Levels (GWLs), dedicated simulations are more appropriate to study changes in a stabilizing climate. Here, we analyze precipitation projections in six multi‐century experiments with fixed atmospheric concentrations of greenhouse gases, conducted with the UK Earth System Model and which span a range of GWLs between 1.5 and 5°C of warming. Regions are identified where the sign of precipitation trends in high‐emission transient projections is reversed in the stabilization experiments. For example, stabilization reverses a summertime precipitation decline across Europe. This precipitation recovery occurs concurrently with changes in the pattern of Atlantic sea surface temperature trends due to a slow recovery of the Atlantic Meridional Overturning Circulation in the stabilization experiments, along with changes in humidity and atmospheric circulation. Plain Language Summary Climate model projections consistently predict that summer precipitation over Europe is expected to decline in the future as global temperatures rise under continued global warming. In our study, we use new climate model simulations that simulate a world where atmospheric concentrations of greenhouse gases are no longer increasing and the rise in global temperatures has slowed down. We show that the summer rainfall decline across Europe can, to some extent, be reversed if global temperatures were to stabilize. This has important implications for adaptation and planning decisions, particularly in so‐called climate change “hot‐spots” such as the Mediterranean. Key Points Climate stabilization experiments show significant differences in projected precipitation compared to high‐emission transient scenarios Northern European and Mediterranean projected summer drying is partially reversed European summer precipitation changes are consistent with the atmospheric response to Atlantic SST changes

In parts of the global ocean, large internal variability continues to mask the detection of externally forced sea surface temperature (SST) trends in observations and climate models. Such regions of large internal variability are typically where wind driven ocean dynamical processes contribute heavily to SST variability. Through analysis of two climate model ensembles, we find that internal wind driven ocean circulation variability delays the time of emergence of SST signals nearly everywhere, but the delay is longest (>10 years) in dynamically active regions like the tropical oceans. We also find that internal wind driven ocean circulation variability is the dominant contributor to changes in the amplitude of internal SST variability over the historical period. Results suggest that inter‐model differences in wind driven SST variability may be a key contributor to inter‐model differences in the time of emergence of externally forced SST signals in climate change scenarios. Plain Language Summary Global warming trends in sea surface temperature have emerged in some regions of the ocean but not in others. Where the warming trends have emerged is, in part, dependent on the amplitude of sea surface temperature variability unrelated to global warming, often referred to as the internal variability. The amplitude of internal variability varies widely by region and is larger where ocean dynamics, including currents and waves, driven by wind changes are active. This study compares when sea surface temperature trends emerge in climate models with and without internal variability due to wind driven ocean dynamics. We find that internal variability due to wind driven ocean dynamics delays when sea surface temperature trends emerge almost everywhere. Results suggest that models with differences in simulating wind driven ocean dynamics are likely to have differences in when global warming trends emerge in sea surface temperature. Key Points Internal wind driven ocean circulation variability delays the time of emergence of externally forced sea surface temperature (SST) trends globally Regional differences in the delayed time of emergence of externally forced SST trends are linked to ocean dynamics Wind driven ocean circulation variability is the dominant contributor to non‐stationarity in the amplitude of internal SST variability

We investigate the linear trends in meridional atmospheric heat transport (AHT) since 1980 in atmospheric reanalysis datasets, coupled climate models, and atmosphere-only climate models forced with historical sea surface temperatures. Trends in AHT are decomposed into contributions from three components of circulation: (i) transient eddies, (ii) stationary eddies, and (iii) the mean meridional circulation. All reanalyses and models agree on the pattern of AHT trends in the Southern Ocean, providing confidence in the trends in this region. There are robust increases in transient-eddy AHT magnitude in the Southern Ocean in the reanalyses, which are well replicated by the atmosphere-only models, while coupled models show smaller magnitude trends. This suggests that the pattern of sea surface temperature trends contributes to the transient-eddy AHT trends in this region. In the tropics, we find large differences between mean-meridional circulation AHT trends in models and the reanalyses, which we connect to discrepancies in tropical precipitation trends. In the Northern Hemisphere, we find less evidence of large-scale trends and more uncertainty, but note several regions with mismatches between models and the reanalyses that have dynamical explanations. Throughout this work we find strong compensation between the different components of AHT, most notably in the Southern Ocean where transient-eddy AHT trends are well compensated by trends in the mean-meridional circulation AHT, resulting in relatively small total AHT trends. This highlights the importance of considering AHT changes holistically, rather than each AHT component individually.

Climate models struggle to produce sea surface temperature (SST) gradient trends in the tropical Pacific comparable to those seen recently in nature. Here, we find that the magnitude of the cloud‐SST feedback in the subtropical Southeast Pacific is correlated across models with the magnitude of Eastern Pacific multi‐decadal SST variability. A heat‐budget analysis reveals coupling between cloud‐radiative effects, circulation, and SST gradients in driving multi‐decadal variability in the Eastern Pacific. Using this relationship and observed feedback estimates, we find that internal Eastern Pacific SST variability is underestimated in most models. Adjusting for model bias increases the likelihood of generating a cooling trend at least as large as observations in preindustrial control simulations by ∼${\\sim} $ 56% on average. If models underestimate climate “noise,” as our results suggest, this bias should be accounted for when attributing the relative importance of forced versus unforced changes in the climate. Plain Language Summary In recent decades, observed sea surface temperatures (SSTs) have cooled in the eastern tropical Pacific and warmed in the western tropical Pacific. Historical simulations using state‐of‐the‐art climate models fail to reproduce this pattern. We find that the feedback between low‐lying clouds and SSTs is related to the magnitude of naturally occurring SST variability in the Southeast Pacific. On average, climate models that have too weak a cloud‐SST feedback in the Southeast Pacific underestimate the likelihood of multi‐decadal cooling in the eastern Pacific in preindustrial simulations. Our results suggest that biases in cloud feedbacks may be causing models to underestimate internal SST variability. Key Points Multi‐decadal Southeast Pacific sea surface temperature trends are related to the strength of the subtropical cloud feedback Cloud radiative effects amplify multi‐decadal Pacific sea surface temperature trends by impacting circulation and surface energy fluxes Correcting model biases in cloud feedback raises the likelihood of internally producing Southeast Pacific cooling as large as observations