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This study aims to investigate variability of the deep South China Sea (SCS) circulation using the Hybrid Coordinate Ocean Model (HYCOM) global reanalysis product. The results reveal that annual cycle is a dominant component in the deep SCS circulation. Meanwhile, the boundary circulation strength is the weakest in January and peaks between June and September. The eastern and southern boundary currents strengthen and weaken one to three months earlier than that of the western and northern boundaries. Vector Empirical Orthogonal Functions (VEOF) analysis results reveal that semiannual and intraseasonal fluctuations are significant components, of which the spatial patterns are mainly confined in the northern and western boundary areas as well as the southwestern sub-basin. Wavelet analysis results show the strength of significant fluctuation varies year to year. Trend analysis results indicate a decadal weakening in the deep SCS circulation. An anomalous anticyclonic circulation, 50–70 km apart from the slope break, tends to weaken the cyclonic boundary circulation in the western and northern boundaries as well as the southwestern sub-basin. This trend is similar to the observed decadal weakening in the Northern Atlantic deep circulation. Thus, the findings of this study reveal that the variability of the deep SCS circulation has a remarkable response to the climate change. The mechanisms responsible for the variability are worth pursuing if more observations are available.

The influence of El Niño Southern Oscillation (ENSO) and the Stratospheric Polar Vortex (SPV) on the zonal asymmetries in the Southern Hemisphere atmospheric circulation during spring and summer is examined. The main objective is to explore if the SPV can modulate the ENSO teleconnections in the extratropics. We use a large ensemble of seasonal hindcasts from the European Centre for Medium-Range Weather Forecasts Integrated Forecast System to provide a much larger sample size than is possible from the observations alone.We find a small but statistically significant relationship between ENSO and the SPV, with El Niño events occurring with weak SPV and La Niña events occurring with strong SPV more often than expected by chance, in agreement with previous works. We show that the zonally asymmetric response to ENSO and SPV can be mainly explained by a linear combination of the response to both forcings, and that they can combine constructively or destructively. From this perspective, we find that the tropospheric asymmetries in response to ENSO are more intense when El Niño events occur with weak SPV and La Niña events occur with strong SPV, at least from September through December. In the stratosphere, the ENSO teleconnections are mostly confounded by the SPV signal. The analysis of Rossby Wave Source and of wave activity shows that both are stronger when El Niño events occur together with weak SPV, and when La Niña events occur together with strong SPV.

Dynamical mechanisms for the summer Eurasian circulation trend pattern are investigated by analyzing reanalysis data and conducting numerical model simulations. The daily circulations that resemble the Eurasian circulation trend pattern are identified and categorized into two groups based on surface warming signal over central and eastern Europe. In the group with large warm anomaly, the upper-level circulation takes on a wave packet form over Eurasia, and there are enhanced latent heating anomalies centered over the North Sea and suppressed latent heating anomalies over the Caspian Sea. The numerical model calculations indicate that these latent heating anomalies can excite an upper-level circulation response that resembles the Eurasian circulation trend pattern. Additional analysis indicates that trends of these two latent heating centers contribute to the long-term circulation trend. In the weak warm anomaly group, the circulation pattern takes on a circumglobal teleconnection (CGT) pattern, and there is no heating signal that reinforces the circulation. These results indicate that not all CGT-like patterns excite temperature anomalies that are persistent and in phase with the trend pattern, and that quasi-stationary forcings, such as the latent heating anomalies, play an important role in driving the boreal summer circulation anomaly that accompanies the strong and persistent surface temperature signal.

The topographic dynamical effect from Eurasia (EA_Topo) and North America (NA_Topo) on the winter isentropic meridional mass circulation (IMMC) is investigated using the WACCM. The independent effect of EA_Topo and that of NA_Topo, with the former much stronger, are both to strengthen the IMMC that is composed of the lower equatorward cold air branch (CB) and the upper poleward warm air branch in the extratropical tropopshere (WB_TR) and stratosphere (WB_ST). Further investigation of the individual contributions from changes in stationary vs. transient and zonal-mean flow vs. waves reveals that, due to the topography-forced mass redistribution, changes in the low-level meridional pressure gradient force a zonal-mean counter-clockwise/ clockwise meridional cell in the southern/northern side of topography. This weakens/strengthens the IMMC south/north of 30°N from the troposphere to lower stratosphere, acting as a dominant contributor to the IMMC changes south of 50°N. Meanwhile, the EA/NA_Topo-forced amplification of stationary waves constructively interacts with those determined by land-sea contrast, making the dominant/minor contributions to the strengthening of CB and WB_TR north of 50°N. The related increase in the upward wave propagation further dominates the WB_ST strengthening in the subpolar region. Meanwhile, transient eddy activities are depressed by EA/NA_Topo along with the weakened background westerly, which partly-offset/dominate-over the contribution from stationary flow in midlatitudes and subpolar region. The coexistence of the other topography (NA/EA_Topo) yields destructive mutual interferrence, which can weaken/offset the independent-EA/NA_Topo-forced meridional mass transport mainly via changing the zonal-mean as well as the downstream wave pattern of mass and meridional wind.

This study investigates the role of convection–circulation coupling on the simulated eastward propagation of the Madden–Julian Oscillation (MJO) over the Maritime Continent (MC). Experiments are conducted with the European Centre Hamburg Model Version 5 (ECHAM5) coupled with the one-column ocean model—Snow-Ice-Thermocline (SIT) and two different cumulus schemes, Nordeng-Tiedtke (E5SIT-Nord) and Tiedtke (E5SIT-Tied). During the early phase of MJO composites, the E5SIT-Nord simulation reveals stronger intraseasonal anomalies in the apparent heat source (Q1) over the convective center, however, the E5SIT-Tied produces a stronger background Q1, suggesting that deep convection prevails over the MC but does not couple with the MJO circulation. Similarly, in the E5SIT-Tied simulation, in-column moisture is kept mostly by local deep convection over the MC, which is in contrast to the well-correlated relationship between moisture anomaly and MJO circulation in E5SIT-Nord. A case study based on an observational MJO reveals similar biases concerning of convection–circulation coupling emerges within a few days of simulations. The E5SIT-Tied simulation produces weaker heating at the convective center of the MJO than the E5SIT-Nord a few days after model initiation, resulting weaker subsidence to the east and less favorable for propagation. The present findings highlight the instantaneous responses of cumulus parameterization schemes to MJO-related environmental changes can further affect intraseasonal variability through altering convection–circulation coupling over the MC. Physical schemes of moist convection are essential to realistically represent this coupling and thereby improve the simulation of the eastward propagation of the MJO.

Due to its large northward heat transport, the Atlantic meridional overturning circulation influences both weather and climate at the mid-latitude Northern Hemisphere. Here we use a state-of-the-art global weather/climate modeling system with high resolution (GFDL CM4C192) to quantify this influence focusing on the U.S. extreme cold weather during winter. We perform a control simulation and the water-hosing experiment to obtain two climate states with and without a vigorous Atlantic meridional overturning circulation. We find that in the control simulation with an overturning circulation, the U.S. east of the Rockies is a region characterized by intense north-south heat exchange in the atmosphere during winter. Without the northward heat transport by the overturning circulation in the hosing experiment, this channel of atmospheric heat exchange becomes even more active through the Bjerknes compensation mechanism. Over the U.S., extreme cold weather intensifies disproportionately compared with the mean climate response after the shutdown of the overturning circulation. Our results suggest that an active overturning circulation in the present-day climate likely makes the U.S. winter less harsh and extreme. A shutdown of the Atlantic Meridional Overturning Circulation could lead to a disproportionate increase in the occurrence of extremely cold weather in the U.S. via enhanced north-south heat exchange in the atmosphere, suggest simulations with and without a vigorous overturning circulation.

This study investigates the stratosphere-troposphere coupling associated with the Scandinavian (SCA) pattern in boreal winter. The results indicate that the SCA impacts stratospheric circulation but that its positive and negative phases have different effects. The positive phase of the SCA (SCA+) pattern is restricted to the troposphere, but the negative phase (SCA−) extends to the upper stratosphere. The asymmetry between phases is also visible in the lead-lag evolution of the stratosphere and troposphere. Prominent stratospheric anomalies are found to be intensified following SCA+ events, but prior to SCA− events. Further analysis reveals that the responses are associated with upward propagation of planetary waves, especially wavenumber 1 which is asymmetric between SCA phases. The wave amplitudes in the stratosphere, originating from the troposphere, are enhanced after the SCA+ events and before the SCA− events. Furthermore, the anomalous planetary wave activity can be understood through its interference with climatological stationary waves. Constructive wave interference is accompanied by clear upward propagation in the SCA+ events, while destructive interference suppresses stratospheric waves in the SCA− events. Our results also reveal that the SCA+ events are more likely to be followed by sudden stratospheric warming (SSW) events, because of the deceleration of stratospheric westerlies following the SCA+ events.

Reliable simulations of historical and future climate are critical to assessing ecological and hydrological responses over the Third Pole (TP). In this study, we evaluate the historical and future temperature and precipitation simulations of 18 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) in southeastern TP (SETP) and the upstream of the Amu Darya and Syr Darya (UAS) regions, two typical TP subregions dominated by the Indian summer monsoon system and westerlies, respectively. Comparison against station observations suggests that CMIP6 models generally capture the intra-annual variability and spatial pattern of historical climate over both subregions. However, the wetting and cold biases observed in CMIP5 still persist in CMIP6; annual temperature is underestimated by most models and annual precipitation is overestimated by all models. Multi-model average cold biases in SETP and UAS are 1.18°C and 0.32°C, respectively, and wet biases in SETP and UAS are 119% and 46%, respectively. We further analyze climate projections under SSP1-2.6, SSP2-4.5, and SSP5-8.5 scenarios. Both SETP and UAS subregions are projected to experience significant warming in 2015–2100, with warming trends 34%–42% and 40%–50% higher than the global trend, respectively. Model projections suggest that the warming trend will slow down under SSP1-2.6 and SSP2-4.5 but further intensify under SSP5-8.5 in 2050–2100. Monsoon-dominated SETP is projected to experience a significant wetting trend stronger than UAS over the entire future period, especially in summer (cf. winter in westerlies-dominated UAS). Concurrently, a significant drying trend in summer is found in UAS during 2050–2100 under SSP5-8.5, suggesting the intensified uneven distributions of seasonal precipitation based on projections.

This paper examines the potential of reconstructing the Atlantic Meridional Overturning Circulation (AMOC) using surface data and linear machine learning algorithms. The algorithms are trained on p...

Persistent hot–dry or cold–wet summer weather can have significant impacts on agriculture, health, and the environment. For northwestern Europe, these weather regimes are typically linked to, respectively, blocked or zonal jet stream states. The fundamental dynamics underlying these circulation states are still poorly understood. Edward Lorenz postulated that summer circulation may be either fully or almost intransitive, implying that part of the phase space (capturing circulation variability) cannot be reached within one specific summer. If true, this would have major implications for the predictability of summer weather and our understanding of the drivers of interannual variability of summer weather. Here, we test the two Lorenz hypotheses (i.e., fully or almost intransitive) for European summer circulation, capitalizing on a newly available very large ensemble (2000 years) of present-day climate data in the fully coupled global climate model EC-Earth. Using self-organizing maps, we quantify the phase space of summer circulation and the trajectories through phase space in unprecedented detail. We show that, based on Markov assumptions, the summer circulation is strongly dependent on its initial state in early summer with the atmospheric memory ranging from 28 days up to ~45 days. The memory is particularly long if the initial state is either a blocked or a zonal flow state. Furthermore, we identify two groups of summers that are characterized by distinctly different trajectories through phase space, and that prefer either a blocked or zonal circulation state, respectively. These results suggest that intransitivity is indeed a fundamental property of the atmosphere and an important driver of interannual variability.

A long simulation over the period 1901–2010 with an eddy-permitting ocean circulation model is used to study the variability of the upper layer circulation in the North Ionian Gyre (NIG) in the Eastern Mediterranean Sea (EMed). The model is driven by the atmospheric forcing from the twentieth century reanalysis data set ERA-20C, ensuring a consistent performance of the model over the entire simulation period. The main modes of variability known in the EMed, in particular the decadal reversals of the NIG upper layer circulation observed since the late 1980s are well reproduced. We find that the simulated NIG upper layer circulation prior to the observational period is characterized by long-lasting cyclonic phases with weak variability during years 1910–1940 and 1960–1985, while in the in-between period (1940–1960) quasi-decadal NIG circulation reversals occur with similar characteristics to those observed in the recent decades. Our simulation indicates that the NIG upper layer circulation is rather prone to the cyclonic mode with occasional kicks to the anticyclonic mode. The coherent variability of the NIG upper layer circulation mode and of the Adriatic Deep Water (AdDW) outflow implies that atmospheric forcing triggering strong AdDW formation is required to kick the NIG into an anticyclonic circulation 1–2 years later. A sensitivity experiment mimicking a cold winter event over the Adriatic Sea supports this hypothesis. Our simulation shows that it is the multi-decadal variability of the salinity in the Adriatic Sea that leads to periods where low salinity prevents strong AdDW formation events. This explains the absence of quasi-decadal NIG reversals during 1910–1940 and 1960–1985.

This study examines the role of the latent heating in exciting the upper-level circulation anomaly, which destructively interferes with the climatological stationary wave in the Western Hemisphere during boreal summer. This destructive interference pattern closely resembles the circulation trend that is known to be responsible for surface heat extreme trends. To investigate the mechanism behind this circulation anomaly, daily stationary–transient wave interference and related meteorological variables are analyzed using reanalysis data for the period of 1979–2017. Numerical model simulations forced by reanalysis heating anomalies indicate that the destructive interference pattern is most effectively excited by latent heating anomalies over the North Pacific Ocean and eastern Canada. The North Pacific heating anomaly drives circulation anomalies that not only resemble the destructive interference pattern, but also transport moisture into eastern Canada. The resulting latent heating over eastern Canada drives circulation that further reinforces the destructive interference pattern, which includes a prominent high pressure system over Greenland. Tropical heating also plays a role in driving the destructive interference pattern. On intraseasonal time scales, the destructive interference pattern is preceded by suppressed Indo–western Pacific heating and enhanced North American monsoon heating. On decadal time scales, both heating centers have strengthened, but the trend of the North American monsoon heating was greater than that of the Indo–western Pacific heating. These uneven heating trends help to explain the resemblance between the destructive interference pattern and the circulation trend over the Western Hemisphere.

Dynamics of the East Asian spring rainband are investigated with a reanalysis dataset and station observations. Here, it is revealed that the rainband is anchored by external forcings. The midtropospheric jet core stays quasi-stationary around Japan. It has two branches in its entry region, which originate from the south and north flanks of the Tibetan Plateau and then run northeastward and southeastward, respectively. The southern branch advects warm air from the Tibetan–Hengduan Plateau northeastward, forming a rainband over southern China through causing adiabatic ascent motion and triggering diabatic feedback. The rainband is much stronger in spring than in autumn due to the stronger diabatic heating over the Tibetan–Hengduan Plateau, a more southward-displaced midtropospheric jet, and the resulting stronger warm advection over southern China. The northern jet branch forms a zonally elongated cold advection belt, which reaches a maximum around northern China, and then weakens and extends eastward to east of Japan. The westerly jet also steers strong disturbance activities roughly collocated with the cold advection belt via baroclinic instability. The high disturbance activities belt causes large cumulative warm advection (CWA) through drastically increasing extremely warm advection days on its eastern and south flanks, where weak cold advection prevails. CWA is more essential for monthly/seasonally rainfall than conventionally used time-average temperature advection because it is shown that strengthened warm advection can increase rainfall through positive diabatic feedback, while cold advection cannot cause negative rainfall. Thus, the rainband is collocated with the large CWA belt instead of the warm advection south of it. This rainband is jointed to the rainband over southern China, forming the long southwest–northeast-oriented East Asian spring rainband. Increasing moisture slightly displaces the rainband southeastward.

Recent enhanced warming and sea ice depletion in the Arctic have been put forward as potential drivers of severe weather in the midlatitudes. Evidence of a link between Arctic warming and midlatitude atmospheric circulation is growing, but the role of Arctic processes relative to other drivers remains unknown. Arctic–midlatitude connections in the North Atlantic region are particularly complex but important due to the frequent occurrence of severe winters in recent decades. Here, dynamic Bayesian networks with hidden variables are introduced to the field to assess their suitability for teleconnection analyses. Climate networks are constructed to analyze North Atlantic circulation variability at 5-day to monthly time scales during the winter months of the years 1981–2018. The inclusion of a number of Arctic, midlatitude, and tropical variables allows for an investigation into the relative role of Arctic influence compared to internal atmospheric variability and other remote drivers. A robust covariability between regions of amplified Arctic warming and two definitions of midlatitude circulation is found to occur entirely within winter at submonthly time scales. Hidden variables incorporated in networks represent two distinct modes of stratospheric polar vortex variability, capturing a periodic shift between average conditions and slower anomalous flow. The influence of the Barents–Kara Seas region on the North Atlantic Oscillation is found to be the strongest link at 5- and 10-day averages, while the stratospheric polar vortex strongly influences jet variability on monthly time scales.

Zonally averaged characteristics of the North Atlantic (NA) thermohaline circulation are investigated in different phases of the Atlantic Multidecadal Oscillation (AMO) index over the past 70 years using ocean objective analyses EN4 and WOA13 and reanalyses GFDL, ESTOC, ORA-S4, and GECCO2. It is shown that similar thermohaline patterns appear in zonally averaged climatic trends and in differences in the composites of anomalies of the potential temperature and salinity fields for all data sets. To find out how the variability in the thermohaline state of waters in the NA is accompanied by the variability in its thermohaline circulation, we have analyzed the variability of the Atlantic Meridional Overturning Circulation (AMOC) stream function. The average AMOC stream functions calculated from ocean objective analyses and reanalyses, reflecting the NA thermohaline circulation, are generally similar to each other. Nonetheless, they can be divided according to their spatial structure into two groups: (EN4, WOA13, and GFDL) and (ESTOC, ORAS4, and GECCO2). In their trends and anomaly composite differences, the AMOC stream functions differ significantly among themselves, as well as from EN4 and WOA13. Moreover, the main distinction is in the sign of climatic trends and composite differences, which determines changes in the direction of the zonally averaged thermohaline circulation. It is found that positive and negative temperature (salinity) anomalies in alternating successive phases of the AMO index circulate with a period of roughly 60 years along the pathway of water in the AMOC, sinking into deep ocean layers at about 60° N and rising toward the surface at 25° N. This process is most pronounced for the EN4 and WOA13 objective analyses and ORAS4 reanalysis. It can be assumed that this mechanism contributes substantially to the formation of the AMO index phases.

We study the impact of horizontal resolution in setting the North Atlantic gyre circulation and representing the ocean–atmosphere interactions that modulate the low-frequency variability in the region. Simulations from five state-of-the-art climate models performed at standard and high-resolution as part of the High-Resolution Model Inter-comparison Project (HighResMIP) were analysed. In some models, the resolution is enhanced in the atmospheric and oceanic components whereas, in some other models, the resolution is increased only in the atmosphere. Enhancing the horizontal resolution from non-eddy to eddy-permitting ocean produces stronger barotropic mass transports inside the subpolar and subtropical gyres. The first mode of inter-annual variability is associated with the North Atlantic Oscillation (NAO) in all the cases. The rapid ocean response to it consists of a shift in the position of the inter-gyre zone and it is better captured by the non-eddy models. The delayed ocean response consists of an intensification of the subpolar gyre (SPG) after around 3 years of a positive phase of NAO and it is better represented by the eddy-permitting oceans. A lagged relationship between the intensity of the SPG and the Atlantic Meridional Overturning Circulation (AMOC) is stronger in the cases of the non-eddy ocean. Then, the SPG is more tightly coupled to the AMOC in low-resolution models.

Historical simulations from 14 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) are evaluated for their ability to reproduce observed precipitation in the northeastern United States and its associated circulation, with particular emphasis on extreme (top 1%) precipitation. The models are compared to observations in terms of the spatial variations of extreme precipitation, seasonal cycles of precipitation and extreme precipitation frequency and intensity, and extreme precipitation circulation regimes. The circulation regimes are identified using k-means clustering of 500-hPa geopotential heights on extreme precipitation days, in both observations and in the models. While all models capture an observed northwest-to-southeast gradient of precipitation intensity (reflected in the top 1% threshold), there are substantial differences from observations in the magnitude of the gradient. These differences tend to be more substantial for lower-resolution models. However, regardless of resolution, and despite a bias toward too-frequent precipitation, many of the models capture the seasonality of observed daily precipitation intensity, and the approximate magnitude and seasonality of observed extreme precipitation intensity. Many of the simulated extreme precipitation circulation patterns are visually similar to the set of observed patterns. However, the location and magnitude of specific troughs and ridges within the patterns, as well as the seasonality of the patterns, may differ substantially from the observed corresponding patterns. A series of metrics is developed based on the observed regional characteristics to facilitate comparison between models.

Sixteen historical simulations (1950–2014) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) are compared to Northeast U.S. observed precipitation and extreme precipitation–related synoptic circulation. A set of metrics based on the regional climate is used to assess how realistically the models simulate the observed distribution and seasonality of extreme precipitation, as well as the synoptic patterns associated with extreme precipitation. These patterns are determined byk-means typing of 500-hPa geopotential heights on extreme precipitation days (top 1% of days with precipitation). The metrics are formulated to evaluate the models’ extreme precipitation spatial variations, seasonal frequency, and intensity; and for circulation, the fit to observed patterns, pattern seasonality, and pattern location of extreme precipitation. Based on the metrics, the models vary considerably in their ability to simulate different aspects of regional precipitation, and a realistic simulation of the seasonality and distribution of precipitation does not necessarily correspond to a realistic simulation of the circulation patterns (reflecting the underlying dynamics of the precipitation), and vice versa. This highlights the importance of assessing both precipitation and its associated circulation. While the models vary in their ability to reproduce observed results, in general the higher-resolution models score higher in terms of the metrics. Most models produce more frequent precipitation than that for observations, but capture the seasonality of precipitation intensity well, and capture at least several of the key characteristics of extreme precipitation–related circulation. These results do not appear to reflect a substantial improvement over a similar analysis of selected CMIP5 models.

The globally integrated subseasonal variability associated with the two main atmospheric circulation regimes, the balanced (or Rossby) and unbalanced (or inertia–gravity) regimes, is evaluated for the four reanalysis datasets: ERA-Interim, JRA-55, MERRA, and ERA5. The results quantify amplitudes and trends in midlatitude traveling and quasi-stationary Rossby wave patterns as well as in the equatorial wave activity across scales. A statistically significant reduction of subseasonal variability is found in Rossby waves with zonal wavenumber k = 6 along with an increase in variability in wavenumbers k = 3–5 in the summer seasons of both hemispheres. The four reanalyses also agree regarding increased variability in the large-scale Kelvin waves, mixed Rossby–gravity waves, and westward-propagating inertio-gravity waves with the lowest meridional mode. The amplitude and sign of trends in inertia–gravity modes with smaller zonal scales and greater meridional modes differ between the ERA-Interim and JRA-55 datasets on the one hand and the ERA5 and MERRA data on the other. An increased variability in the ERA-Interim and JRA-55 accounts for positive trends in their total subseasonal variability.

In winter 2018/19, southeastern coastal China experienced extreme warm temperatures that were due to a weak East Asian winter monsoon. On the basis of observations from 10 meteorological stations and reanalysis data, the large-scale circulation patterns associated with this extreme warm winter and the possible driving mechanism of its related sea surface temperature (SST) anomalies are investigated in this study. During this winter, many places in this region reached their highest winter mean temperature record and had more extreme warm days and fewer extreme cold days relative to climatology. According to the circulation patterns during winter 2018/19, several large-scale circulation conditions associated mainly with the weak East Asian winter monsoon are identified: the eastward shift of the Siberian high and a shallower East Asian trough, which is related to the low blocking frequency over the Aleutian region, are both unfavorable for cold-air intrusion southward. Meanwhile, strong low-level southerly wind anomalies over southeastern China are related mainly to the 2018/19 El Niño event. Furthermore, the possible role of SST anomalies over the North Atlantic and tropical western Pacific Oceans is examined by using an atmospheric general circulation model, suggesting that both the “tripole pattern” of North Atlantic SST and tropical western Pacific SST anomalies in winter 2018/19 played a role in influencing the East Asian trough. The combined effect of all of these factors seems to be responsible for this extreme warm winter over southeastern coastal China.

In this study, long-term structural changes in the intertropical convergence zone (ITCZ) and associated precipitation–radiation–circulation feedback processes are examined using multiple sources of reanalysis data for temperature, winds, moisture, and observed precipitation and outgoing longwave radiation (OLR) during 1980–2014. Consistent with CMIP5 climate model projections of the “deep tropical squeeze” under greenhouse warming, this period witnessed a warming and wetting (increased specific humidity) global trend, characterized by a narrowing of the ITCZ core with increased precipitation, coupled to widespread tropospheric drying (deficient relative humidity), increased OLR in the subtropics and midlatitudes, a widening of the descending branches of the Hadley circulation, and a poleward shift of the jet streams in both hemispheres. The widespread tropospheric drying stems from 1) a faster rate of increased saturated water vapor with warming, relative to the increase in ambient moisture due to convective and large-scale transport, and 2) enhanced anomalous subsidence, and low-level moisture divergence in the subtropics and midlatitudes. The long-term trend in enhanced precipitation (latent heating) in the ITCZ core region is strongly coupled to increasing OLR (radiative cooling to space) in the expanding dry zones, particularly over land regions in the subtropics and midlatitudes, arguably as a necessary condition for global thermodynamic energy balance. Analyses of the trend patterns in vertical profiles of p velocity, temperature, and relative humidity with respect to ITCZ precipitation rate and OLR reveal that the contrast between the wet and dry regions in the troposphere has been increasing globally, with the ITCZ core getting wetter and contracting, while the marginal convective and dry zones are getting drier and expanding.

Previous studies showed that global cloud-radiative changes contribute half or more to the midlatitude atmospheric circulation response to global warming. Here, we investigate the relative importance of tropical, midlatitude, and polar cloud-radiative changes for the annual-mean, wintertime, and summertime circulation response across regions in AMIP-like simulations. To this end, we study global warming simulations from the ICON model run with the cloud-locking method and prescribed sea surface temperatures, which isolate the impact of changes in atmospheric cloud-radiative heating. Tropical cloud changes dominate the global cloud impact on the 850 hPa zonal wind, jet strength, and storm track responses across most seasons and regions. For the jet shift, a more diverse picture is found. In the annual mean and DJF, tropical and midlatitude cloud changes contribute substantially to the poleward jet shift in all regions. The poleward jet shift is further supported by polar cloud changes across the Northern Hemisphere but not in the Southern Hemisphere. In JJA, the impact of regional cloud changes on the jet position is small, consistent with an overall small jet shift during this season. The jet shift can be largely understood via the anomalous atmospheric cloud-radiative heating in the tropical and midlatitude upper troposphere. The circulation changes are broadly consistent with the influence of cloud-radiative changes on upper-tropospheric baroclinicity and thus the mean potential energy available for conversion into eddy kinetic energy. Our results help to explain the jet response to global warming and highlight the importance of tropical and midlatitude cloud-radiative changes for this response.

In this work, results of modeling the intra-annual variability of water and ice circulation in the Arctic Ocean by means of the INMIO4.1 three-dimensional model of ocean dynamics and the CICE5.1 sea-ice model are considered. The coupled ocean–ice model is implemented on massively parallel computers under the control of the CMF2.0 cosimulation software package. Results of a numerical experiment carried out according to the CORE-I protocol are presented. Geographical distributions, mean values, and intra-annual variability of the model solution characteristics describing the flow of water through key straits at the boundaries of the Arctic Ocean, propagation paths and vertical structure of the warm Atlantic water layer, and the area and volume of sea ice are analyzed. Qualitative agreement with observational and other high-resolution modeling data is obtained.

The Southern Ocean overturning circulation is driven by winds, heat fluxes, and freshwater sources. Among these sources of freshwater, Antarctic sea ice formation and melting play the dominant role. Even though ice-shelf melt is relatively small in magnitude, it is located close to regions of convection, where it may influence dense water formation. Here, we explore the impacts of ice-shelf melting on Southern Ocean water-mass transformation (WMT) using simulations from the Energy Exascale Earth System Model (E3SM) both with and without the explicit representation of melt fluxes from beneath Antarctic ice shelves. We find that ice-shelf melting enhances transformation of Upper Circumpolar Deep Water, converting it to lower density values. While the overall differences in Southern Ocean WMT between the two simulations are moderate, freshwater fluxes produced by ice-shelf melting have a further, indirect impact on the Southern Ocean overturning circulation through their interaction with sea ice formation and melting, which also cause considerable upwelling. We further find that surface freshening and cooling by ice-shelf melting cause increased Antarctic sea ice production and stronger density stratification near the Antarctic coast. In addition, ice-shelf melting causes decreasing air temperature, which may be directly related to sea ice expansion. The increased stratification reduces vertical heat transport from the deeper ocean. Although the addition of ice-shelf melting processes leads to no significant changes in Southern Ocean WMT, the simulations and analysis conducted here point to a relationship between increased Antarctic ice-shelf melting and the increased role of sea ice in Southern Ocean overturning.

This paper studies the changes in winter air temperatures in the Siberian sector of the northern hemisphere (60–120° E) over a long-term period of 1947–2017. A positive trend in air temperature (0.3°C/10 years) was revealed. An analysis of the magnitude and frequency of temperature anomalies also confirms an increase in surface temperature. The distribution of large positive and negative temperature anomalies was investigated. Centers of the largest recurrence of large negative anomalies were located in the south and south-west of the study area. A maximum number of negative anomalies were observed in December. Large positive anomalies (the highest frequency in January) were observed throughout the territory under consideration, and extremely large anomalies were found only at 20% of the stations. The largest number of anomalies in the period of 1947–2017 was observed at the Irkutsk station. Based on the analysis of elementary circulation mechanisms (classification by B.L. Dzerdzeevskii), a relationship was established between the atmospheric circulation and the distribution of air temperature anomalies. It was concluded that the temperature regime in the winter months was formed under the influence of 26 elementary circulation mechanisms, combined into seven groups. The meridional circulation prevailed (75% of cases). The elementary circulation mechanisms of the meridional northern circulation group were all characterized by the presence of blockings and intrusions of southern cyclones into the study area. In general, over the past two decades, an increase in duration of macrocirculation processes resulting in the inter-latitudinal air mass exchange has been revealed.

Interannual variabilities of sea level and upper-ocean gyre circulation of the western tropical Pacific Ocean (WTPO) have been predominantly attributed to El Niño–Southern Oscillation (ENSO). The results of the present study put forward important modulation effects by the Indian Ocean dipole (IOD) mode. The observed sea level in the WTPO shows significant instantaneous and lagged correlations (around −0.60 and 0.40, respectively) with the IOD mode index (DMI). A composite of 14 “independent” IOD events for 1958–2017 shows negative sea level anomalies (SLAs) of 4–7 cm in the WTPO during positive IOD events and positive SLAs of 6–8 cm in the following year that are opposite in sign to the El Niño effect. The IOD impacts are reproduced by large-ensemble simulations of a climate model that generate respectively 430 and 519 positive and negative independent IOD events. A positive IOD induces westerly winds over the western and central tropical Pacific and causes negative SLAs through Ekman upwelling, and it facilitates the establishment of a La Niña condition in the following year that involves enhanced Pacific trade winds and causes positive SLAs in the WTPO. Ocean model experiments confirm that the IOD affects the WTPO sea level mainly through modulating the tropical Pacific winds. Variability of the Indonesian Throughflow (ITF) induced by IOD winds has a relatively weak effect on the WTPO. The IOD’s impacts on the major upper-ocean currents are also considerable, causing anomalies of 1–4 Sv (1 Sv ≡ 106 m3 s−1) in the South Equatorial Current (SEC) and North Equatorial Countercurrent (NECC) volume transports.

The variation in the strength and structure of the overturning circulation under varying Southern Ocean buoyancy forcing, corresponding to present day, a cooler (glacial) state, and a possible future warmer state is analyzed in an idealized two-basin general circulation model connected by a southern circumpolar channel. A connection between the North Atlantic Deep Water (NADW) cell in the Atlantic basin and the Pacific Deep Water (PDW) cell in the Pacific basin occurs with a direct flow of NADW into the channel’s lower cell, while PDW upwelled in the Pacific basin can flow directly into the upper wind-driven cell in the channel. The intersection of these cells along with direct zonal flows between the basins completes the interbasin circulation. The present-day Atlantic meridional overturning circulation (AMOC) in the model is upwelled both by wind-driven upwelling in the Southern Ocean and by diffusion in the Pacific and Atlantic. In a cooler climate with enhanced sea ice, the NADW cell shoals, which can then no longer flow directly into the channel’s lower cell, reducing the Pacific pathway of NADW. This leads to a substantial weakening of the AMOC, suggesting buoyancy forcing changes can play a substantial role in the transition of the AMOC to a glacial state. In contrast, in a warmer equilibrium climate state with reduced AABW formation, the NADW cell strengthens and deepens. NADW is increasingly directed along the Pacific pathway, while the direct upwelling in the channel’s wind-driven upper cell plays a smaller role.

The eastward propagating Madden–Julian oscillation (MJO) events exhibit various speeds ranging from 1 to 9 m s−1, but what controls the propagation speed remains elusive. This study attempts to address this issue. It reveals that the Kelvin wave response (KWR) induced by the MJO convection is a major circulation factor controlling the observed propagation speed of the MJO, with a stronger KWR corresponding to faster eastward propagation. A stronger KWR can accelerate the MJO eastward propagation by enhancing the low-level premoistening and preconditioning to the east of the MJO deep convection. The strength of the KWR is affected by the background sea surface temperature (SST). When the equatorial central Pacific SST warms, the zonal scale of the Indo-Pacific warm pool expands, which increases the zonal scale of the MJO, favoring enhancing the KWR. This effect of warm-pool zonal scale has been verified by idealized experiments using a theoretical model. The findings here shed light on the propagation mechanism of the MJO and provide a set of potential predictors for forecasting the MJO propagation.

Using various observations, reanalysis datasets, and a general circulation model (CESM-WACCM4), the relationship between the Arctic total column ozone (TCO) and the tropospheric circulation and sea surface temperatures (SSTs) over the western North Pacific (30°–45°N, 130°E–170°W) was investigated. We find that anomalies in the circulation and SSTs over the western North Pacific in June are closely related to anomalies in the Arctic TCO in March; that is, when the Arctic TCO in March decreases, the anomalous tropospheric cyclone and negative SST anomalies (SSTAs) will occur over the western North Pacific in June. Further analysis indicates that the decreased Arctic TCO in March tends to result in positive Victoria mode (VM)-like SSTAs over the North Pacific in April, which persist and develop an anomalous cyclone over the eastern North Pacific in May via atmosphere–ocean coupling. This anomalous cyclone over the eastern North Pacific subsequently induces an anomalous cyclone over the western North Pacific in June via westward-propagating Rossby waves in the lower troposphere. Furthermore, the negative SSTAs over the western North Pacific are enhanced by the anomalous northerly wind related to the anomalous cyclone in June. The effects of increased Arctic TCO in March on the tropospheric circulation and SSTs are almost opposite to those of decreased Arctic TCO. These results are also supported by our numerical simulations. Moreover, 10%–20% of the anomalies in the tropospheric circulation and SSTs over the western North Pacific in June are affected by the anomalies in the Arctic TCO in March.

El Niño stimulates an anomalous cyclone over the North Pacific during its developing phase. Using 30 CGCMs and 11 AGCMs from CMIP5, we find a weakly strengthened anomalous North Pacific cyclone (NPC) in a warmer climate in CGCMs, and intermodel uncertainty exists. A similar change of the anomalous NPC is found in AGCMs with increased mean state SST but with a stronger amplitude of enhancement. Based on a simple Gill model, the diabatic heating anomaly, mean state static stability, and meridional gradient of relative vorticity are identified to be responsible for the change of the anomalous NPC. Analyses of the CMIP5 models suggest that the change of the anomalous NPC is largely determined by the competition between the enhanced diabatic heating anomaly and the enhanced mean state static stability. The amplitude of enhancement of the anomalous NPC is strongly modulated by the change of precipitation anomaly over the equatorial central-eastern Pacific, which depends on the changes of mean state SST and the El Niño–related SST anomaly. Compared with a uniform warming, an El Niño–like mean state SST warming favors a much stronger enhancement of the anomalous NPC, by enhancing the mean state precipitation and latent heating anomaly associated with the precipitation anomaly over the equatorial Pacific. However, the air–sea coupling acts to weaken the SST anomaly associated with El Niño in the CGCMs, which further reduces the enhancement of the anomalous NPC.

Forecasting extreme events is important for having more time to prepare and mitigate high-impact events because those are expected to become more frequent, intense, and persistent around the globe in the future under the warming atmosphere. This study evaluates the probabilistic predictability of the heat wave index (HWI) associated with large-scale circulation patterns for predicting heat waves over South Korea. The HWI, reflecting heat waves over South Korea, was defined as the vorticity difference at 200 hPa between the South China Sea and northeast Asia. The forecast of up to 15 days from five ensemble prediction systems and the multimodel ensemble has been used to predict the probabilistic HWI during the summers of 2011–15. The ensemble prediction systems consist of different five operational centers, and the forecast skill of the probability of heat waves occurrence was assessed using the Brier skill score (BSS), relative operating characteristics (ROC), and reliability diagram. It was found that the multimodel ensemble is capable of better predicting the large-scale circulation patterns leading to heat waves over South Korea than any other single ensemble system through all forecast lead times. We concluded that the probabilistic forecast of the HWI has promise as a tool to take appropriate and timely actions to minimize the loss of lives and properties from imminent heat waves.

Performance in simulating atmospheric rivers (ARs) over western North America based on AR frequency and landfall latitude is evaluated for 10 models from phase 5 of the Coupled Model Intercomparison Project among which the CanESM2 model performs well. ARs are classified into southern, northern, and middle types using self-organizing maps in the ERA-Interim reanalysis and CanESM2. The southern type is associated with the development and eastward movement of anomalous lower pressure over the subtropical eastern Pacific, while the northern type is linked with the eastward movement of anomalous cyclonic circulation stimulated by warm sea surface temperatures over the subtropical western Pacific. The middle type is connected with the negative phase of North Pacific Oscillation–west Pacific teleconnection pattern. CanESM2 is further used to investigate projected AR changes at the end of the twenty-first century under the representative concentration pathway 8.5 scenario. AR definitions usually reference fixed integrated water vapor or integrated water vapor transport thresholds. AR changes under such definitions reflect both thermodynamic and dynamic influences. We therefore also use a modified AR definition that isolates change from dynamic influences only. The total AR frequency doubles compared to the historical period, with the middle AR type contributing the largest increases along the coasts of Vancouver Island and California. Atmospheric circulation (dynamic) changes decrease northern AR type frequency while increasing middle AR type frequency, indicating that future changes of circulation patterns modify the direct effect of warming on AR frequency, which would increase ARs (relative to fixed thresholds) almost everywhere along the North American coastline.

The Southern Ocean (SO) surface wind stress is a major atmospheric forcing for driving the Antarctic Circumpolar Current and the global overturning circulation. Here the effects of wind fluctuations at different time scales on SO wind stress in 18 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are investigated. It is found that including wind fluctuations, especially on time scales associated with synoptic storms, in the stress calculation strongly enhances the mean strength, modulates the seasonal cycle, and significantly amplifies the trends of SO wind stress. In 11 out of the 18 CMIP5 models, the SO wind stress has strengthened significantly over the period of 1960–2005. Among them, the strengthening trend of SO wind stress in one CMIP5 model is due to the increase in the intensity of wind fluctuations, while in all the other 10 models the strengthening trend is due to the increasing strength of the mean westerly wind. These discrepancies in SO wind stress trend in CMIP5 models may explain some of the diverging behaviors in the model-simulated SO circulation. Our results suggest that to reduce the uncertainty in SO responses to wind stress changes in the coupled models, both the mean wind and wind fluctuations need to be better simulated.

The predictability and forecast skill of the models participating in the Climate Historical Forecast Project (CHFP) database is assessed through evaluating the representation of the upper-tropospheric extratropical circulation in the Southern Hemisphere (SH) in winter and summer and its main modes of variability. In summer, the predictability of 200-hPa geopotential height anomalies mainly comes from the ability of the multimodel ensemble mean (MMEM) to forecast the first three modes of interannual variability with high fidelity. The MMEM can reproduce not only the spatial patterns of these modes but also their temporal evolution. On the other hand, in JJA only the second and fourth modes of variability are predictable by the MMEM. These seasonal differences in the performance of the MMEM seem to be related to the role that the sea surface temperature (SST) anomalies have in influencing the variability of each mode. Accordingly, modes that are strongly linked to tropical SST anomalies are better forecast by the MMEM and show less spread among models. The analysis of both 2-m temperature and precipitation anomalies in the SH associated with the predictable modes reveals that DJF predictable modes are accompanied by significant temperature anomalies. In particular, temperatures at polar (tropical) latitudes are significantly correlated with the first (second) mode. Furthermore, these links obtained with observations are also well forecast by the MMEM and can help to improve seasonal forecast of climate anomalies in those regions with low skill. Fil: Osman, Marisol. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Vera, Carolina Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina

The interannual variability of wintertime North American surface temperature extremes and its generation and maintenance are analyzed in this study. The leading mode of the temperature extreme anomalies, revealed by empirical orthogonal function (EOF) analyses of December–February mean temperature extreme indices over North America, is characterized by an anomalous center of action over western-central Canada. In association with the leading mode of temperature extreme variability, the large-scale atmospheric circulation features an anomalous Pacific–North American (PNA)-like pattern from the preceding fall to winter, which has important implications for seasonal prediction of North American temperature extremes. A positive PNA pattern leads to more warm and fewer cold extremes over western-central Canada. The anomalous circulation over the PNA sector drives thermal advection that contributes to temperature anomalies over North America, as well as a Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomaly pattern in the midlatitude North Pacific. The PNA-like circulation anomaly tends to be supported by SST warming in the tropical central-eastern Pacific and a positive synoptic-scale eddy vorticity forcing feedback on the large-scale circulation over the PNA sector. The leading extreme mode–associated atmospheric circulation patterns obtained from the observational and reanalysis data, together with the anomalous SST and synoptic eddy activities, are reasonably well simulated in most CMIP5 models and in the multimodel mean. For most models considered, the simulated patterns of atmospheric circulation, SST, and synoptic eddy activities have lower spatial variances than the corresponding observational and reanalysis patterns over the PNA sector, especially over the North Pacific.

The thermodynamic processes and synoptic circulation features driving lower-tropospheric temperature extremes in the high Arctic (>80°N) are investigated. Based on 10-day kinematic backward trajectories from the 5% most intense potential temperature anomalies, the contributions of horizontal and vertical transport, subsidence-induced warming, and diabatic processes to the generation of the Arctic temperature anomaly are quantified. Cold extremes are mainly the result of sustained radiative cooling due to a sheltering of the Arctic from meridional airmass exchanges. This is linked to a strengthening of the tropospheric polar vortex, a reduced frequency of high-latitude blocking, and in winter also a southward shift of the North Atlantic storm track. The temperature anomaly of 60% of wintertime extremely warm air masses (90% in summer) is due to transport from a potentially warmer region. Subsidence from the Arctic midtroposphere in blocking anticyclones is the most important warming process with the largest contribution in summer (70% of extremely warm air masses). In both seasons, poleward transport of already warm air masses contributes around 20% and is favored by a poleward shift of the North Atlantic storm track. Finally, about 40% of the air masses in winter are of an Arctic origin and experience diabatic heating by surface heat fluxes in marine cold air outbreaks. Our study emphasizes the importance of processes in the Arctic and the relevance of anomalous blocking—in winter in the Barents, Kara, and Laptev Seas and in summer in the high Arctic—for the formation of warm extremes.

Previous studies indicate that the temperature changes from a day to the following day are not normally distributed either in winter or in summer. However, this study is the first to investigate the causes of the asymmetry in these temperature changes. We hypothesised that the asymmetries are driven by the passages of atmospheric fronts and by the radiative processes during anticyclonic circulation conditions. We used the maximum and minimum temperatures observed at the Praha-Karlov station, the records of front passages, and 7 different catalogues of circulation types over central Europe. We demonstrated that the asymmetries in the large temperature changes (5 °C and larger) were related to the passages of all three front types in winter and to the passages of cold fronts in summer. The asymmetries in the slight temperature changes (up to 3 °C) were mainly caused by cold advection from the north and the northeast in winter and by warm advection from the south, the southeast, and the east in summer. We suppose that our results can be generalized; that is, the passages of atmospheric fronts and cold/warm advection are the likely causes of the asymmetries of the day-to-day temperature changes elsewhere in Europe. The presented findings can be applied as a tool for the validation of climate model outputs.

Based on a single-model 100-member ensemble of climate change simulations performed with the MPI-ESM, this paper investigates how the probability density function (PDF) of atmospheric general circulation (AGC)—a marginal distribution of the full climate PDF—responds to increasing $${\hbox {CO}}_2$$ concentration. The investigation provides a first assessment of $${\hbox {CO}}_2$$-induced changes in AGC-PDF derived from ensemble statistics (as oppose to time statistics in usual practice) that arise solely from internal variability consistent with the imposed $${\hbox {CO}}_2$$-forcing. By focusing on the first two moments of the AGC-PDF related to the Hadley circulation and the subtropical jets, we find that the major $${\hbox {CO}}_2$$-induced changes in the PDF are related to structure changes in the mean circulation and in the circulation variability. As $${\hbox {CO}}_2$$ concentration increases, different components of the AGC can co-evolve with changing forcing. In particular, the mean Hadley cell co-evolves with the mean subtropical jets. The co-evolution is characterized by the concurrent Hadley cell widening and poleward shifting of the subtropical jets (particularly strong in January), by a strengthening of both the Hadley cell and the subtropical jets in October, and by a weakening of the Hadley cell and a strengthening of the northern and southern jets in January. The changes in circulation variability are more subtle, and reveal only large and spatially coherent changes in places, where the variability is already strong without increasing $${\hbox {CO}}_2$$ concentration, such as at the northern and southern flanks of the summer southern subtropical jet, and at the northern flank of the southern polar night jet. There are some significant cross-variabilities between different components of the AGC. Overall, the $${\hbox {CO}}_2$$-induced AGC-changes can be described by linear trends in the first and second moments, with the trends in the means being much stronger than those in the variances and co-variances.

Based on a set of pilot atmosphere-only experiments from the Cloud Feedback Model Intercomparison Project Phase 3 (CFMIP-3), in this study, the winter surface air temperature (SAT) and monsoonal circulation changes in East Asia as a response to the 4 × CO2 forcing in coupled model are decomposed into the four parts in terms of the responses to the uniform SST warming, 4 × CO2 radiative effect, SST pattern changes, and plant physiological effect. The uniform SST warming presents the most significant influence on the increase of SAT change, which strengthens the East Asian winter monsoon (EAWM) circulation. The CO2 radiative effect can also induce the SAT increase over East Asia but with a magnitude smaller compared to the uniform SST warming, in which more warming is in land than ocean and the EAWM circulation could be weakened consequently due to the decreased land–sea thermal contrast in response to the CO2 radiative effect. The SAT changes in response to the SST pattern change show inconsistencies over the eastern and southern parts of East Asia between the two models, associated with the large difference for EAWM circulation changes, indicating that the SST pattern change could be the primary source of inter-model uncertainties in the East-Asian SAT change. As for the influence of plant physiological effect, it could generate a SAT rise in many highly vegetated regions. Further analyses for different areas show that both the uniform SST warming and CO2 radiative effects could induce more intense SAT increase in northern East Asia, while the plant physiological effect has a more significant influence on that in southern/eastern part of East Asia.

The Atlantic Meridional Overturning Circulation, a key constituent of the climate system, is projected to slow down in the twenty-first century due to a weakening of the Labrador Sea convection, itself a response to greenhouse gas warming and/or enhanced freshwater flux from the Arctic. However, the first observations from the Overturning in the Subpolar North Atlantic Program reveal a minimal response of the Meridional Overturning Circulation to the strong Labrador Sea convection during the winters of 2015–2016. From an analysis of the observational and reanalysis data, we show here that this weak response can be explained by a strong density compensation in the Labrador Sea. Although convection induces important changes of temperature and salinity in the basin interior, the export of the thermal and haline anomalies to the boundary current largely takes place along density surfaces. As a result, the transformation across density surfaces, that is, the imprint on the overturning circulation, is relatively small. This finding highlights the critical relationship between temperature and salinity in determining the overturning strength in the Labrador Sea and underlines the necessity of accurate freshwater flux estimates for improved Meridional Overturning Circulation predictions. Compensation of the effects of salt and temperature changes on water density in the Labrador Sea can explain the minimal response of the Atlantic overturning circulation to convection, according to analyses of observations and reanalysis data.

Atmospheric circulation anomalies over the Ural Mountains are crucial indicators of the anomalous downstream weather and climate over East Asia. Here, we provide a new perspective on the mechanism of Ural circulation anomalies. We use a simple theoretical model to determine that the relationship between the solar forcing and three Ural circulation patterns, namely, neutral type, trough anomaly and ridge anomaly, is a nonlinear relationship following the supercritical pitchfork bifurcation theory. The theory predicts that when the total solar irradiance (TSI) is below a critical value, trough and ridge anomalies represent duplex equilibria and are equally likely to occur at the same TSI. Based on 180 winter months record, we have estimated the bidimensional probability density of TSI and the monthly mean geopotential height at 500 hPa or zonal wind at 850 hPa over the Ural Mountains. Results show that Sc = 1360.9 W m−2 is a critical value of TSI, the neutral type pattern is the single circulation regime when TSI > Sc, whereas trough and ridge anomaly patterns are duplex circulation regimes when TSI

This study gives a comprehensive picture of how atmospheric large-scale circulation is related to moisture transport and to distributions of moisture, clouds, and surface downward longwave radiation in the Arctic in winter. Anomaly distributions of the abovementioned variables are compared in 30 characteristic wintertime atmospheric circulation regimes, which are allocated from 15 years (2003–17) of mean sea level pressure data of ERA-Interim reanalysis applying the self-organizing map method. The characteristic circulation regimes are further related to known climate indices—the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO), and Greenland blocking index—as well as to a frequent high pressure pattern across the Arctic Ocean from Siberia to North America, herein called the Arctic bridge. Effects of large-scale circulation on moisture, cloud, and longwave radiation are to a large extent occurring through the impact of horizontal moisture transport. Evaporation is typically not efficient enough to shape those distributions, and much of the moisture evaporated in the Arctic is transported southward. The positive phase of the NAO and AO increases moisture and clouds in northern Europe and the eastern North Atlantic Ocean, and a strong Greenland blocking typically increases those in the southwest of Greenland. When the Arctic bridge is lacking, the amount of moisture, clouds, and downward longwave radiation is anomalously high near the North Pole. Our results reveal a strong dependence of moisture, clouds, and longwave radiation on atmospheric pressure fields, which also appears to be important from a climate change perspective.