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Wave-mean flow interaction is usually regarded as accounting for the origin of the Arctic Oscillation/Northern Hemisphere Annular Mode (AO/NAM). It is inferred that the combination of the local wave-mean flow interactions at the AO/NAM's three regional centers of action on three important time scales contributes to the main behavior of the AO/NAM index. To discuss the variations of the AO/NAM indices on the three prominent time scales, we take the 2007/08 and 2009/10 winters as two comparative examples to analyze the local wave-mean flow interactions at the AO/NAM's three centers. The following three facets are identified: (1) Synoptic-scale wave breakings in the North Atlantic can explain the variances of the AO/NAM index on a time scale of 10–20 days. In the 2007/08 winter, there were both cyclonic and anticyclonic synoptic wave breakings, while in the 2009/10 winter, cyclonic synoptic wave breaking was dominant, and the flow characteristics were strikingly similar to the blocking. (2) In the 2007/08 and 2009/10 winters, the signals of the AO/NAM indices on the time scale of 30–60 days are mainly from the interactions between the upward propagating quasi-stationary waves and the polar vortex in the stratosphere. (3) This work also demonstrates that the AO/NAM is linked to the El Niño–Southern Oscillation (ENSO) by the Pacific–North American pattern (PNA) on the winter mean time scale. In the 2007/08 (2009/10) winter, La Niña (El Niño) forced the Pacific jet to shift poleward (equatorward), in favor of weakening (enhancing) the polar waveguide; thus, the polar vortex became stronger (weaker), corresponding to the positive (negative) winter mean AO/NAM index. [ABSTRACT FROM AUTHOR]

An appropriate record period is necessary for calculating outdoor design conditions precisely, for example, the meteorological conditions of a particular location for heating, ventilation and air‐conditioning (HVAC) design purposes. A method for calculating the optimal period of record (OPR) for air‐conditioning outdoor design conditions is given in this study. The combination of standard deviation method (avoiding the OPR being too short) and optimal climate normal (OCN) model (avoiding the OPR being too long) was used to obtain the appropriate record period for the air‐conditioning outdoor design conditions. Especially, the method was examined in five different climate zones to determine its application. The results show that the OPR is in the range of 5–21 years depending on the climates, much less than the current 30‐year record period used for calculating outdoor design conditions. This indicates the 30‐year record period is no longer suitable for calculating air‐conditioning design conditions. The relatively short record period can well reflect the past climate change and support precise meteorological information for the design of air‐conditioning systems. By combining standard deviation and optimal climate normal model methods, a novel method was proposed to calculate the optimal period of record for air‐conditioning outdoor design conditions. The feasibility of the method was proved in five different climate zones in China. The previous study confirmed that the 30‐year record is no longer suitable for calculating the air‐conditioning design conditions and relative shorter record ranging from five to 21 years should be used in different climate zones. [ABSTRACT FROM AUTHOR]

Studies on the nonlinear natures of the spatiotemporal structure of the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) in the context of nonlinear interaction among the North Atlantic Oscillation (NAO), the Pacific-North American pattern (PNA), and the stratospheric polar vortex (SPV) are performed. The non-Gaussianity of the multivariate probability density function (PDF) in the phase space spanned by their indices is examined first. Five local maxima potentially related to circulation regimes are identified from the so-called angular PDF. One opposite pair of these regimes is found to correspond to the positive and negative phases of the AO/NAM. Since the authors are not sure that, due to uncertainty as suggested by statistical tests, some of the above regimes are non-Gaussian, the nonlinearity of phase-space tendency is employed as an assistant measure to identify them as nonlinear modes. It seems phase-space tendency traditionally estimated from time difference failed to be effective because of its dependence on Δ t. To overcome this drawback a low-order stochastic dynamical model is established empirically from the indices. The investigation on the basic deterministic dynamics of this model suggests that the existence of regimes, such as those associated with the AO/NAM, can primarily be explained by its nonlinear deterministic part. However, two problems still remain unsolved: 1) one of the local maxima was almost not identified and 2) life cycles of the basic deterministic dynamics are too long to be related to the low-frequency variability. By introducing a multilevel approach of modeling, further insight into the residual noise of the above stochastic model can address these two issues quite well. [ABSTRACT FROM AUTHOR]

The Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM) is attributed to wave-mean flow interaction over the extratropical region of the Northern Hemisphere. This wave-mean flow interaction is closely related to three atmospheric centers of action, corresponding to three regional oscillations: the NAO, the PNA, and the stratosphere polar vortex (SPV), respectively. It is then natural to infer that local wave-mean flow interactions at these three centers of action are dynamically coupled to each other and can thus explain the main aspects of the three-dimensional coherent structure of the annular mode, which also provides a possible way to understand how the local NAO-PNA-SPV perspective and the hemispheric AO/NAM perspective are interrelated. By using a linear stochastic model of coupled oscillators, this study suggests that two coupling modes among the PNA, NAO, and SPV are related to the two-dimensional pattern in sea level pressure of the AO. Although both of them may contribute to the AO/NAM, only one is related to the three-dimensional equivalent barotropic structure of the NAM, while the other one is mainly restricted to the troposphere. So the equivalent barotropic structure of the NAM, as usually revealed by the regression of the zonal wind against the AO index, is the manifestation of just one coupling mode. Another coupled mode is a baroclinic mode that resembles the NAM only in the troposphere. However, this similarity in spatial structures does not imply that the total variability of the AO/NAM index can be explained by those of the NAO-PNA-SPV or their coupling modes, because of the existence of the variability that may contribute to the AO/NAM, produced outside of these three regions. It is estimated that the coupling modes can jointly explain 44%% of the variance of the AO/NAM index. [ABSTRACT FROM AUTHOR]

The classification of torrential rain is crucial to the flood mitigation and water resources, but the relevant research in the Haihe River Basin (HRB) is lacking. In this study, 156 regional torrential rain events from 1972 to 2022 were classified into two types using cluster analysis: eastern type and southern type. The eastern type is the typical torrential rain in the HRB, accounting for 61.5% of the total days, whereas the southern type mainly occurs in the piedmont plain with larger intensity and scope, which would pose more severe hazards. Corresponding large‐scale circulation patterns of the two types reveal a clear relationship between the HRB torrential rain and ascending motion, associated with the anomalies of low‐level moisture convergence and upper‐level wind divergence. The anomalous upper‐level divergence is accompanied by the northward expansion of South Asian High and intensified westerly jets. In the eastern type, strong low‐pressure anomalies north of Lake Baikal at mid‐level produce anomalous northwesterlies. The cold air from mid–high latitudes encounters enhanced southwesterly moist air accompanied by the northward expansion of Western Pacific Subtropical High (WPSH), resulting in the low‐level moisture convergence. In the southern type, the low‐level anomalous convergence is caused by a relatively weak cyclonic anomaly and significantly strengthened southeasterly moisture from the Pacific. The enhancement of southwesterlies results from the cooperative effect of the remarkably northwesterly WPSH and low‐pressure anomalies in South China Sea. As a whole, the eastern type is the result of convergence of cold air and southwesterly moisture driven by a dipole pattern at mid–high latitudes, whereas the southern type is mainly attributed to the accumulation of abundant southeasterly water vapour. This classification can identify high‐risk areas for torrential rain in the HRB and contribute to the forecast skills of extreme precipitation. [ABSTRACT FROM AUTHOR]

The variations in summer precipitation according to different grades and their effects on summer drought/flooding in the Haihe River basin were analyzed using the daily precipitation data from 161 meteorological stations from 1972 to 2021. The results showed that the number of rainy days (NRD) in summer in the Haihe River basin significantly declined in the past 50 years, mainly due to the reduction in the number of light-rain days. The precipitation amount (PA) exhibited prominent interdecadal characteristics, showing an upward tendency in the past 20 years accompanied by a remarkable increase in the proportion of torrential rain. The NRD in the northern part of the basin significantly decreased, while the PA in the southeast showed an increasing trend. Summer drought/flooding was strongly linked to the changes in the NRD and was predominantly affected by intense precipitation, with contribution rates of 5.5%, 16.8%, 31.2%, and 46.5% from light, moderate, heavy, and torrential rain, respectively. The effects of torrential rain increased in recent decades, particularly in the flooding scenarios. In addition, July was the critical period for summer drought/flooding, with the major influence of heavy and torrential rain. The most intense summer rainfall event in the Haihe River basin could contribute from 15% to 29% of total precipitation, resulting in changes in the severity and state of summer drought/flooding, which indicated that the precipitation process had a decisive impact on seasonal drought/flooding. Therefore, when predicting summer precipitation in the Haihe River basin, it is necessary to pay attention to the intense rainfall events during critical periods. [ABSTRACT FROM AUTHOR]

Major nuclear bomb tests and nuclear power plant incidents release large amounts of radionuclides. This study investigates beta (β) activities of radionuclides from four ice cores in the Third Pole (TP) to understand the transport routes and related atmospheric processes affecting the radionuclides deposition in glaciers of the region. All the ice cores show three major β activity peaks in the ice layers corresponding to 1963, 1986, and 2011. The β activity peak in the 1963 ice layer is referred to as the well-known 1962 Nuclear Bomb Test. Beta activity peaks in 1986 and 2011 ice layers from the Chernobyl and Fukushima Nuclear Incidents (CNI, FNI). Hysplit forward and backward trajectory analyses suggest that the radionuclides were transported by the westerly into the stratosphere and then to the high elevation TP glaciers. In the FNI case, the radionuclides traveled over Japan, the Pacific Ocean, Europe, and central Asia before being deposited in the TP glaciers. Investigations of the atmospheric circulation confirm that the stronger northern branch of westerly is responsible for high radionuclides during the FNI in the TP. Less precipitation with water vapor flux component divergence after the FNI also contributed to the enriched radionuclides. • We report 3 nuclear events, FNI, CNI, and NBT from β activity peak in TP ice cores. • We found more radionuclides in ice core records after FNI than that after CNI. • Atmosphere transports are responsible for the difference between FNI and CNI. • Strong northern westerly after FNI is responsible for more radionuclides. • Less precipitation and water vapor divergence after FNI led to more radionuclides. [ABSTRACT FROM AUTHOR]