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Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter, but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (i) the zonal-mean zonal wind, (ii) the eddy kinetic energy (EKE), and (iii) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in the poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE, and thus the observed weakening might have contributed to more persistent heat waves in recent summers.

The Heat Flow and Physical Properties Package HP 3 for the InSight mission will attempt the first measurement of the planetary heat flow of Mars. The data will be taken at the InSight landing site in Elysium planitia (136  ∘ E, 5  ∘ N) and the uncertainty of the measurement aimed for shall be better than ±5 mW m −2 . The package consists of a mechanical hammering device called the “Mole” for penetrating into the regolith, an instrumented tether which the Mole pulls into the ground, a fixed radiometer to determine the surface brightness temperature and an electronic box. The Mole and the tether are housed in a support structure before being deployed. The tether is equipped with 14 platinum resistance temperature sensors to measure temperature differences with a 1- σ uncertainty of 6.5 mK. Depth is determined by a tether length measurement device that monitors the amount of tether extracted from the support structure and a tiltmeter that measures the angle of the Mole axis to the local gravity vector. The Mole includes temperature sensors and heaters to measure the regolith thermal conductivity to better than 3.5% (1- σ ) using the Mole as a modified line heat source. The Mole is planned to advance at least 3 m—sufficiently deep to reduce errors from daily surface temperature forcings—and up to 5 m into the martian regolith. After landing, HP 3 will be deployed onto the martian surface by a robotic arm after choosing an instrument placement site that minimizes disturbances from shadows caused by the lander and the seismometer. The Mole will then execute hammering cycles, advancing 50 cm into the subsurface at a time, followed by a cooldown period of at least 48 h to allow heat built up during hammering to dissipate. After an equilibrated thermal state has been reached, a thermal conductivity measurement is executed for 24 h. This cycle is repeated until the final depth of 5 m is reached or further progress becomes impossible. The subsequent monitoring phase consists of hourly temperature measurements and lasts until the end of the mission. Model calculations show that the duration of temperature measurement required to sufficiently reduce the error introduced by annual surface temperature forcings is 0.6 martian years for a final depth of 3 m and 0.1 martian years for the target depth of 5 m.

The complex thermal history of the parts manufactured by selective laser melting (SLM) leads to complex residual stress, having a significant impact on the quality of SLM part. The origin of residual stress was investigated in terms of temperature gradient mechanism. Then, stresses along the height and horizontal directions were measured by X-ray diffraction, and effects of processing parameters on the stress distribution were studied. Results showed that residual stress distribution and evolution along the height direction are affected by the subsequent thermal cycling (STC) significantly. In the horizontal direction, higher energy input and longer track length induce larger residual stress. The stress parallel to the scanning direction is much larger than that perpendicular to the scanning direction, and the peak values of residual stress always occurs at the onset of scanning tracks. Based on this study, corresponding measures can be taken to reduce the residual stress or avoid stress concentration, thereby improving the process stability of SLM.

Dust aerosols play key roles in affecting regional and global climate through their direct, indirect, and semi-direct effects. Dust events have decreased rapidly since the 1980s in East Asia, particularly over northern China, primarily because of changes in meteorological parameters (e.g. surface wind speed and precipitation). In this study, we found that winter (December–January–February) Arctic amplification associated with weakened temperature gradients along with decreased zonal winds is primarily responsible for the large decline in following spring (March–April–May) dust event occurrences over northern China since the mid-1980s. A dust index was developed for northern China by combining the daily frequency of three types of dust event (dust storm, blowing dust, and floating dust). Using the empirical orthogonal function (EOF) analysis, the first pattern of dust events was obtained for spring dust index anomalies, which accounts for 56.2% of the variability during 1961–2014. Moreover, the enhanced Arctic amplification and stronger Northern Hemisphere annular mode (NAM) in winter can result in the anticyclonic anomalies over Siberia and Mongolia, while cyclonic anomalies over East Europe in spring. These results are significantly correlated with the weakened temperature gradients, increased precipitation and soil moisture, and decreased snow cover extent in the mid-latitude over Northern Hemisphere. Based on the future predictions obtained from the Fifth Climate Models Intercomparison Project (CMIP5), we found that the dust event occurrences may continually decrease over northern China due to the enhanced Arctic amplification in future climate.

High cooling rates within the selective laser melting (SLM) process can generate large residual stresses within fabricated components. Understanding residual stress development in the process and devising methods for in-situ reduction continues to be a challenge for industrial users of this technology. Computationally efficient FEA models representative of the process dynamics (temperature evolution and associated solidification behaviour) are necessary for understanding the effect of SLM process parameters on the underlying phenomenon of residual stress build-up. The objective of this work is to present a new modelling approach to simulate the temperature distribution during SLM of Ti6Al4V, as well as the resulting melt-pool size, solidification process, associated cooling rates and temperature gradients leading to the residual stress build-up. This work details an isotropic enhanced thermal conductivity model with the SLM laser modelled as a penetrating volumetric heat source. An enhanced laser penetration approach is used to account for heat transfer in the melt-pool due to Marangoni convection. Results show that the developed model was capable of predicting the temperature distribution in the laser/powder interaction zone, solidification behaviour, the associated cooling rates, melt-pool width (with 14.5% error) and melt-pool depth (with 3% error) for SLM Ti6Al4V. The model was capable of predicting the differential solidification behaviour responsible for residual stress build-up in SLM components. The model-predicted trends in cooling rates and temperature gradients for varying SLM parameters correlated with experimentally measured residual stress trends. Thus, the model was capable of accurately predicting the trends in residual stress for varying SLM parameters. This is the first work based on the enhanced penetrating volumetric heat source, combined with an isotropic enhanced thermal conductivity approach. The developed model was validated by comparing FEA melt-pool dimensions with experimental melt-pool dimensions. Secondly, the model was validated by comparing the temperature evolution along the laser scan path with experimentally measured temperatures from published literature.

Summary Reproductive stages of life cycle are important for the explanation of distribution patterns of plant species at different scales, due to their extreme vulnerability to environmental conditions. Despite reported evidences that seed germination is related to habitat macroclimatic characteristics such as mean annual temperature (MAT) and precipitation, the role of this trait in controlling plant species distribution is not systematically and quantitatively evaluated yet. Using the data on seed germination along a temperature gradient for 49 species originating from contrasting climatic conditions, we test here whether initial temperature of seed germination (Tmin) is a direct correlate for predicting species distribution ranges along the temperature gradient. Our study reveals that Tmin is strongly negatively correlated with habitat temperature; among the studied species, Tmin clearly increased with decreasing MAT (r2 = 0·57, P < 0·001). Considering phylogenetic biases, co‐evolution of seed traits as well as precipitation along with microclimatic factors did not affect the strength of this relationship. The results suggest that the Tmin–MAT relation can provide insights particularly into species distribution patterns, vegetation dynamics and community assembly rules along altitudinal and latitudinal gradients. We argue that including the Tmin in species distribution models may help to improve the accuracy and specificity of predictions of vegetation shifts under global change scenarios. Lay Summary

Interannual relationship between the Pacific Meridional Mode (PMM) and landfalling tropical cyclone (TC) frequency in China (LTCFC) exhibits an interdecadal enhancement since the late-1980s, Epochs I (E1, 1951–1985) and II (E2, 1987–2019) are therefore defined to unravel possible mechanisms. Strengthened low-level vorticity and weakened vertical wind shear always favor TC genesis in the western North Pacific (WNP) main development region (MDR) in positive PMM (PPMM) years of two epochs. During PPMM years of E1, many TCs recurve northeastward due to anomalous westerly steering flow over the MDR while only handful TCs move northwestward and land over China due to climatological mean southeasterly steering flow. In PPMM years of E2, prevailing easterly steering flow anomalies on the MDR’s northern edge cause more TCs to move westward/northwestward and make landfall over China. Therefore, LTCFC is insignificantly (significantly) correlated with PMM during E1 (E2) due to the interdecadal change of steering flow, which is linked to different sea surface temperature (SST) patterns of PMM during two epochs. An anomalous WNP cyclone can be induced during PPMM years of both epochs. A northwestward shift of PPMM-associated positive SST anomalies causes a large zonal SST gradient in the off-equatorial WNP which induces westerlies during E1 while the zonal SST gradient is weak during E2. Thus, westerlies on the southern edge of the anomalous cyclone extend northward, leading to a northward shift of the anomalous cyclone during E1. The distinct responses of two PMM patterns can be well reproduced by the Coupled Model Intercomparison Project Phase 6 models.

Remarkably stable excitations known as skyrmions have recently garnered significant attention in condensed-matter systems. It is now shown that skyrmions in thin films of MnSi and Cu 2 OSeO 3 can be made to rotate as a result of thermal fluctuations. Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences 1 . It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium 2 . Here we report our finding that a topologically nontrivial spin texture known as a skyrmion—a particle-like object in which spins point in all directions to wrap a sphere 3 —constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micrometre-sized crystals of skyrmions in thin films of Cu 2 OSeO 3 and MnSi exhibit a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau–Lifshitz–Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr–van Leeuwen theorem 4 , 5 . We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

The intertropical convergence zone (ITCZ) accounts for more than 30% of the global precipitation and its variability has a great effect on the people living in the tropical area. It is the manifestation of the Hadley circulation, tropical dynamic and thermodynamic coupling and the air-sea interaction. Therefore, it is essential to understand the changes and variability of the ITCZ, including its position and intensity. Using observed precipitation from GPCP and TRMM 3B43, as well as the ERA5 reanalysis data, we examine the ITCZ variations over the global and seven regions from 1998-2018. These data sets show consistent ITCZ climatology and inter-annual variability. Except over Atlantic and eastern Pacific where ITCZ stays in the northern hemisphere, the ITCZ crosses the equator after equinoxes over other sections. There is no overall significant shift of annual mean ITCZ position over the globe and seven regions, except over the America section where the GPCP data show a significant increase trend of 0.8° decade−1 and over the western Pacific section where ERA5 data show a significant decrease trend of −1.2° decade−1. The ITCZ positions over the globe and Africa are related to both ENSO and NAO, while ITCZ position over eastern Pacific is significantly affected by ENSO and the Atlantic ITCZ is mainly related to NAO. Except for the western Pacific, all other sections are significantly related to local meridional surface temperature gradient, particularly over America, Atlantic and eastern Pacific. The meridional gradient variation of the vertically integrated moist static energy has generally good agreement with the shift of the intertropical convergence zone, particularly for the seasonal climatology over the Africa region. The relationship between ITCZ position and intensity shows complicated patterns, with positive correlation over the globe, but different correlations over different sections. The two observed data sets are more or less consistent, but ERA5 shows discrepancies over some sections. It is also found that the local meridional temperature gradient has more influences on the ITCZ positions than the global one.

Observational studies suggest that the stratospheric quasi-biennial oscillation (QBO) can modulate tropical deep convection. The authors use a cloud-resolving model with a limited domain, representing a convective column in the tropics, to study the mechanisms of this modulation. The large-scale circulation is parameterized using the weak temperature gradient (WTG) approximation, under which the parameterized large-scale vertical motion acts to relax the horizontal-mean temperature toward a specified reference profile. Temperature variations typically seen in easterly and westerly phases are imposed in the upper troposphere and lower stratosphere of this reference profile. The responses of convection are studied over different sea surface temperatures, holding the reference temperature profile fixed. This can be thought of as studying the response of convection to the QBO over different “relative SSTs” and also corresponds to different equilibrium precipitation rates in the control simulation. The equilibrium precipitation rate shows slight increases in response to a QBO easterly phase temperature perturbation over small SST anomalies and strong decreases over large SST anomalies, and vice versa for the QBO westerly phase perturbation. A column moist static energy budget analysis reveals that the QBO modulates the convective precipitation through two pathways: it changes the high-cloud properties and thus the column radiative cooling, and it alters the shape of the large-scale vertical motion and thus the efficiency of energy transport by the large-scale flow. The nonmonotonicity of the precipitation response with respect to relative SST results from the competition of these two effects.