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The El Niño wind and Luzon Strait transport are important factors modulating the interannual variability of the southern and northern South China Sea (SCS) winter circulation, respectively. The joint effect of El Niño wind and westward Luzon Strait transport drives a dipolar gyre with an anticyclonic (cyclonic) circulation anomaly in the southern (northern) SCS, which enhances the cross-basin current that connects the SCS western boundary current with the eastern boundary of the SCS around the Mindoro Strait, and then effectively modulates the interannual variability of Mindoro Strait transport. An extreme El Niño and large Luzon Strait intrusion occurred in winter 2015/16, and mooring observations and remote sensing data confirmed the presence of an intense dipolar gyre. This resulted in an extreme eastward cross-basin current and outward SCS Mindoro Strait transport anomaly in 2015/16.

A set of atmosphere-only timeslice experiments are described, designed to examine the processes that cause regional climate change and inter-model uncertainty in coupled climate model responses to C O 2 forcing. The timeslice experiments are able to reproduce the pattern of regional climate change in the coupled models, and are applied here to two cases where inter-model uncertainty in future projections is large: the tropical hydrological cycle, and European winter circulation. In tropical forest regions, the plant physiological effect is the largest cause of hydrological cycle change in the two models that represent this process. This suggests that the CMIP5 ensemble mean may be underestimating the magnitude of water cycle change in these regions, due to the inclusion of models without the plant effect. SST pattern change is the dominant cause of precipitation and circulation change over the tropical oceans, and also appears to contribute to inter-model uncertainty in precipitation change over tropical land regions. Over Europe and the North Atlantic, uniform SST increases drive a poleward shift of the storm-track. However this does not consistently translate into an overall polewards storm-track shift, due to large circulation responses to SST pattern change, which varies across the models. Coupled model SST biases influence regional rainfall projections in regions such as the Maritime Continent, and so projections in these regions should be treated with caution.

In recent years, the Northern Hemisphere midlatitudes have suffered from severe winters like the extreme 2012/13 winter in the eastern United States. These cold spells were linked to a meandering upper-tropospheric jet stream pattern and a negative Arctic Oscillation index (AO). However, the nature of the drivers behind these circulation patterns remains controversial. Various studies have proposed different mechanisms related to changes in the Arctic, most of them related to a reduction in sea ice concentrations or increasing Eurasian snow cover. Here, a novel type of time series analysis, called causal effect networks (CEN), based on graphical models is introduced to assess causal relationships and their time delays between different processes. The effect of different Arctic actors on winter circulation on weekly to monthly time scales is studied, and robust network patterns are found. Barents and Kara sea ice concentrations are detected to be important external drivers of the midlatitude circulation, influencing winter AO via tropospheric mechanisms and through processes involving the stratosphere. Eurasia snow cover is also detected to have a causal effect on sea level pressure in Asia, but its exact role on AO remains unclear. The CEN approach presented in this study overcomes some difficulties in interpreting correlation analyses, complements model experiments for testing hypotheses involving teleconnections, and can be used to assess their validity. The findings confirm that sea ice concentrations in autumn in the Barents and Kara Seas are an important driver of winter circulation in the midlatitudes.

The impact of projected Arctic sea ice loss on the atmospheric circulation is investigated using the Whole Atmosphere Community Climate Model (WACCM), a model with a well-resolved stratosphere. Two 160-yr simulations are conducted: one with surface boundary conditions fixed at late twentieth-century values and the other with identical conditions except for Arctic sea ice, which is prescribed at late twenty-first-century values. Their difference isolates the impact of future Arctic sea ice loss upon the atmosphere. The tropospheric circulation response to the imposed ice loss resembles the negative phase of the northern annular mode, with the largest amplitude in winter, while the less well-known stratospheric response transitions from a slight weakening of the polar vortex in winter to a strengthening of the vortex in spring. The lack of a significant winter stratospheric circulation response is shown to be a consequence of largely cancelling effects from sea ice loss in the Atlantic and Pacific sectors, which drive opposite-signed changes in upward wave propagation from the troposphere to the stratosphere. Identical experiments conducted with Community Atmosphere Model, version 4, WACCM’s low-top counterpart, show a weaker tropospheric response and a different stratospheric response compared to WACCM. An additional WACCM experiment in which the imposed ice loss is limited to August–November reveals that autumn ice loss weakens the stratospheric polar vortex in January, followed by a small but significant tropospheric response in late winter and early spring that resembles the negative phase of the North Atlantic Oscillation, with attendant surface climate impacts.

Higher precipitation is expected over most of the world’s continents under climate change, except for a few specific regions where models project robust declines. Among these, the Mediterranean stands out as a result of the magnitude and significance of its winter precipitation decline. Locally, up to 40% of winter precipitation could be lost, setting strong limits on water resources that will constrain the ability of the region to develop and grow food, affecting millions of already water-stressed people and threatening the stability of this tense and complex area. To this day, however, a theory explaining the special nature of this region as a climate change hot spot is still lacking. Regional circulation changes, dominated by the development of a strong anomalous ridge, are thought to drive the winter precipitation decline, but their origins and potential contributions to regional hydroclimate change remain elusive. Here, we show how wintertime Mediterranean circulation trends can be seen as the combined response to two independent forcings: robust changes in large-scale, upper-tropospheric flow and the reduction in the regional land–sea temperature gradient that is characteristic of this region. In addition, we discuss how the circulation change can account for the magnitude and spatial structure of the drying. Our findings pave the way for better understanding and improved modeling of the future Mediterranean hydroclimate.

The high‐resolution Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM‐X) is used to study the impacts of gravity waves (GWs) on the thermospheric circulation and composition. The resolved GWs are found to propagate anisotropically with stronger eastward components at most altitudes. The dissipation of these waves in the thermosphere produces a net eastward forcing that reaches peak values between 200 and 250 km at mid‐high latitudes in both hemispheres. Consequently, the mean circulation is weakened in the winter hemisphere and enhanced in the summer, which in turn impacts the thermospheric composition. Most notably, the column integrated O/N2 in both hemispheres is reduced and agrees better with observations. The mean thermospheric GW forcing in the meridional direction has comparable amplitude and acts to modify the gradient‐wind relationship. Plain Language Summary Small‐scale waves originate from the lower atmosphere have been shown to propagate into the thermosphere. To study their effects a high‐resolution whole atmosphere model has been employed. Using this high‐resolution model, which can partially resolve the small‐scale waves, we can directly quantify the force exerted by these waves on the general circulation in the thermosphere. We found that such force is strong, and affects the thermospheric circulation in both winter and summer hemisphere. This consequently changes the distribution of important thermospheric species. One measure of the thermospheric composition is the ratio of atomic oxygen and molecular nitrogen, which is an indicator of the relative abundance of atomic and molecular species. This ratio has been grossly over‐estimated in previous modeling studies. It is reduced as a result of the circulation change, and is much better agreement with observations. Key Points Gravity waves (GWs) resolved by high‐resolution WACCM‐X displays anisotropic propagation GW forcing alters thermospheric circulation The circulation change leads to a much improved thermospheric O/N2

Arctic sea ice has declined rapidly over the past four decades and climate models project a seasonally ice-free Arctic Ocean by the middle of this century, with attendant consequences for regional climate. However, modeling studies lack consensus on how the large-scale atmospheric circulation will respond to Arctic sea ice loss. In this study, the authors conduct a series of 200-member ensemble experiments with the Community Atmosphere Model version 6 (CAM6) to isolate the atmospheric response to past and future sea ice loss following the Polar Amplification Model Intercomparison Project (PAMIP) protocol. They find that the stratospheric polar vortex response is small compared to internal variability, which in turn influences the signal-to-noise ratio of the wintertime tropospheric circulation response to ice loss. In particular, a strong (weak) stratospheric polar vortex induces a positive (negative) tropospheric northern annular mode (and North Atlantic Oscillation), obscuring the forced component of the tropospheric response, even in 100-member averages. Stratospheric internal variability is closely tied to upward wave propagation from the troposphere and can be explained by linear wave interference between the anomalous and climatological planetary waves. Implications for the detection of recent observed trends and model realism are also presented. These results highlight the inherent uncertainty of the large-scale tropospheric circulation response to Arctic sea ice loss arising from stratospheric internal variability.

Arctic sea ice is declining at an increasing rate with potentially important repercussions. To understand better the atmospheric changes that may have occurred in response to Arctic sea ice loss, this study presents results from atmospheric general circulation model (AGCM) experiments in which the only time-varying forcings prescribed were observed variations in Arctic sea ice and accompanying changes in Arctic sea surface temperatures from 1979 to 2009. Two independent AGCMs are utilized in order to assess the robustness of the response across different models. The results suggest that the atmospheric impacts of Arctic sea ice loss have been manifested most strongly within the maritime and coastal Arctic and in the lowermost atmosphere. Sea ice loss has driven increased energy transfer from the ocean to the atmosphere, enhanced warming and moistening of the lower troposphere, decreased the strength of the surface temperature inversion, and increased lower-tropospheric thickness; all of these changes are most pronounced in autumn and early winter (September–December). The early winter (November–December) atmospheric circulation response resembles the negative phase of the North Atlantic Oscillation (NAO); however, the NAO-type response is quite weak and is often masked by intrinsic (unforced) atmospheric variability. Some evidence of a late winter (March–April) polar stratospheric cooling response to sea ice loss is also found, which may have important implications for polar stratospheric ozone concentrations. The attribution and quantification of other aspects of the possible atmospheric response are hindered by model sensitivities and large intrinsic variability. The potential remote responses to Arctic sea ice change are currently hard to confirm and remain uncertain.

We analyze the characteristics of atmospheric variations over tropical South America using the pattern recognition framework of weather typing or atmospheric circulation patterns (CPs). During 1979–2020, nine CPs are defined in the region, using a 𝑘-means algorithm based on daily unfiltered 850-hPa winds over 10°N–30°S, 90°–30°W. CPs are primarily interpreted as stages of the annual cycle of the low-level circulation. We identified three \"winter\" CPs (CP7, CP8, and CP9), three \"summer\" CPs (CP3, CP4, and CP5), and three \"transitional\" CPs (CP1, CP2, and CP6). Significant long-term changes are detected during the dry-to-wet transition season (July–October) over southern tropical South America (STSA). One of the wintertime patterns (CP9) increases from 20% in the 1980s to 35% in the last decade while the \"transitional\" CP2 decreases from 13% to 7%. CP9 is characterized by enhancement of the South American low-level jet and increasing atmospheric subsidence over STSA. CP2 is characterized by southerly cold-air incursions and anomalous convective activity over STSA. The years characterized by high frequency of CP9 and low frequency of CP2 during the dry-to-wet transition season are associated with a delayed South American monsoon onset and anomalous dry conditions over STSA. Consistently, a higher frequency of CP9 intensifies the fire season over STSA (1999–2020). Over the Brazilian states of Maranhão, Tocantins, Goiás, and São Paulo, the seasonal frequency of CP9 explains around 35%–44% of the interannual variations of fire counts.

Modern observations reveal that large‐scale ocean‐atmosphere circulation (OAC) is drifting toward higher latitudes under global warming. Paleoclimate proxies indicate that similar OAC drifts occurred on orbital timescale as well. However, the characteristics and underlying mechanisms remain unclear. Here, by conducting simulations with different Earth's orbits, we investigate how changes in Earth‐Sun distance affect the OAC. We find that a closer Earth‐Sun distance (perihelion) causes a poleward drift of OAC. This drift in circulation is dynamically consistent with displacement of meridional temperature gradient. Precession alters the perihelion season on orbital timescales, leading to a seasonal poleward drift in OAC. This drift is amplified during the hemispheric summer, reaching magnitudes of ∼${\\sim} $ 10° under high eccentricity. The identified OAC drifts reshape the seasonality of precipitation and temperature over land, as well as ocean upwelling and downwelling, ultimately affecting the distribution of Earth's terrestrial and marine ecosystems. Plain Language Summary The distance between the Earth and the Sun keeps changing due to movement and gravitational attraction of different planets in the solar system. This may have an impact on the global circulation pattern. Understanding the characteristics and mechanism of this impact is important for understanding and predicting the evolution of Earth's climate. Using climate model simulations mimicking different Earth's orbits, this study reports that a shorter distance between the Earth and the Sun leads to a poleward shift of the oceanic and atmospheric circulation (OAC). Conversely, the OAC contracts toward the equator when the Earth‐Sun distance enlarges. Currently, the closest Earth‐Sun distance occurs during boreal winter, and the corresponding boreal winter circulation pattern is relatively poleward. Due to the precession of the Earth's rotation axis, the season when the Earth is closest to the Sun changes throughout the year, drifting the OAC toward higher latitude in different seasons. This seasonal drift can reach a magnitude of ∼${\\sim} $ 10° when the Earth's orbit around the sun is extremely elliptic. Key Points A shorter Earth‐Sun distance drifts the ocean‐atmosphere circulation pattern poleward Drifting circulation occurs in different seasons due to the seasonal shift in perihelion Hemispheric summer perihelion potentially causes substantial circulation drift by ∼${\\sim} $ 10°