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Changing atmospheric circulations shift global weather patterns and their extremes, profoundly affecting human societies and ecosystems. Studies using atmospheric reanalysis and climate model data indicate diverse circulation changes in recent decades but show discrepancies in magnitude and even direction, underscoring the urgent need for validation with independent, climate-quality measurements. Here we show statistically significant changes in tropospheric circulation over the past two decades using satellite-observed, height-resolved cloud motion vectors from the Multi-angle Imaging SpectroRadiometer (MISR). Upper tropospheric cloud motion speeds in the mid-latitudes have increased by up to about 4 m s−1 decade−1. This acceleration is primarily because of the strengthening of meridional flow, potentially indicating more poleward storm tracks or intensified extratropical cyclones. The Northern and Southern Hemisphere tropics shifted poleward at rates of 0.42 ± 0.22 and 0.02 ± 0.14° latitude decade−1 (95% confidence interval), respectively, whereas the corresponding polar fronts shifted at rates of 0.37 ± 0.31 and 0.31 ± 0.21° latitude decade−1. We also show that the widely used ERA5 reanalysis winds subsampled to the MISR are in good agreement with the climatological values and trends of the MISR but indicate probable ERA5 biases in the upper troposphere. These MISR-based observations provide critical benchmarks for refining reanalysis and climate models to advance our understanding of climate change impacts on cloud and atmospheric circulations. 

Abstract Following a previous study examining the influence of an upper-tropospheric cold low (CL) on the track of a nearby tropical cyclone (TC), this study investigates the impacts of a CL on TC intensity. The results suggest that the relative position and separation distance between the CL and the TC are the key factors affecting TC intensity. When located outside the CL’s radius of maximum winds (RMW) but within its circulation, TCs initially in the northwest quadrant of the CL intensify faster than those in the other quadrants. Theβeffect causes the CL to move northwestward toward the TC and enhances eddy angular momentum flux convergence. Meanwhile, the upper-level TC outflow erodes the CL and reduces the associated vertical wind shear, promoting TC intensification. In contrast, for TCs initially located southeast of the CL, the attraction of the Fujiwhara effect between the two entities counteracts the CL’sβdrift and helps to maintain their separation distance. Moreover, Rossby wave energy dispersion induces an anticyclone southeast of the CL, which transports lower-θ_eair toward the TC and hinders the TC development. Furthermore, TCs within the CL’s RMW reach a similar intensity due to their PV superposition, irrespective of their relative positions to the CL. For TCs located outside the CL circulation, the CL’s impacts are largely negligible for TCs located northwest of the CL, but TCs located southeast of the CL may still be affected by the CL-induced anticyclone. Significance StatementThis study examines the influence of an upper-tropospheric cold low on tropical cyclone intensity. The results illustrate that the relative position and separation distance between the cold low and tropical cyclone are crucial factors in determining tropical cyclone intensity. Tropical cyclones initially northwest of a cold low intensify faster than those to the southeast when located outside the cold low’s radius of maximum winds but still within its circulation. The main mechanisms are how theβsteering and interactions between the two entities act together. The midlevel intrusion of cold, dry air and Rossby energy dispersion also contribute to their complex interaction. These insights provide a guide for forecasting the tropical cyclone intensity when influenced by a nearby upper-level cold low. 

Abstract The interannual variability of summertime subtropical stationary waves, the forcing mechanisms, and their connections to regional tropical cyclone (TC) variability are investigated in this study. Two indices are identified to characterize the interannual variability of subtropical stationary waves: the longitudinal displacement of the zonal wavenumber-1 component (WN1) and the intensity change of the zonal wavenumber-2 component (WN2). These two indices are strongly anticorrelated and offer simple metrics to depict the interannual variability of subtropical stationary waves. Furthermore, the longitudinal displacement of the WN1 is significantly correlated with the variability of TC activity over the North Pacific and North Atlantic, and its influences on regional TC activity can be explained by variations in vertical wind shear, tropospheric humidity, and the frequency of Rossby wave breaking. The subtropical stationary waves are strongly related to precipitation anomalies over different oceanic regions, implying the possible impacts of low-frequency climate modes. Semi-idealized experiments using the Community Earth System Model version 2 (CESM2) show that the longitude of the WN1 is strongly modulated by ENSO, as well as SST anomalies over the Atlantic main development region and the central North Pacific. Further diagnosis using a baroclinic stationary wave model demonstrates the dominant role of diabatic heating in driving the interannual variability of stationary waves and confirms the impacts of different air–sea coupled modes on subtropical stationary waves. Overall, subtropical stationary waves provide a unified framework to understand the impacts of various forcing agents, such as ENSO, the Atlantic meridional mode, and extratropical Rossby wave breaking, on TC activity over the North Atlantic and North Pacific.