Abstract. The North Atlantic Oscillation (NAO) has been confirmed to be closely related to the weather and climate in many regions of the Northern Hemisphere; however, its effect and mechanism upon the formation of dust events (DEs) in China have rarely been discussed. By using the station observation dataset and multi-reanalysis datasets, it is found that the spring dust aerosols (DAs) in North China (30–40∘ N, 105–120∘ E), a non-dust source region, show high values with a strong interannual variability, and the spring DAs in North China are significantly correlated with the previous winter's NAO. According to the nine spring DEs affected significantly by the negative phase of the preceding winter's NAO in North China during 1980–2020, it is shown that before the outbreak of DEs, due to the transient eddy momentum (heat) convergence (divergence) over the DA source regions, the zonal wind speed increases in the upper-level troposphere, strengthening the zonal wind in the middle–lower levels through momentum downward transmission. Simultaneously, there is transient eddy momentum (heat) divergence (convergence) around the Ural Mountains, which is favorable for the establishment and maintenance of the Ural ridge, as well as the development of the air temperature and vorticity advections. The combined effects of temperature and vorticity advections result in the Siberian Highs and Mongolian cyclone to be established, strengthen, and move southward near the surface, guiding the cold air from high latitudes southward, and is favorable for the uplift and transmission of DAs to North China downstream. Simultaneously, the changes in upstream transient eddy flux transport can cause both energy and mass divergence in North China, resulting in diminishing winds during DEs, which would facilitate the maintenance of dust aerosols here and promote the outbreak of DEs. This study reveals the impact of transient eddy flux transport on the dusty weather anomalies modulated by the NAO negative signal in North China, which deepens the understanding of the formation mechanism of DEs in China.

Accurately obtaining the spatial distribution of soil moisture and its variability are the basis for the land-atmosphere interaction study. We investigated the fidelity of four satellite-based soil moisture products (AMSR2, CCI, SMAP, and SMOS) using in situ observation during the period 2019–2020. The spatial distribution and variability of different soil moisture products in northern China were analyzed for different seasons and climate zones. The satellite products showed the best performance of summer soil moisture with the bias and uncertainty of the three products (CCI, SMAP, and SMOS) being less than 0.041 and 0.097, whereas soil moisture showed a large bias in winter. For all seasons, AMSR2 and CCI demonstrated a positive bias whereas SMAP and SMOS showed a negative bias. CCI product had little bias in spring, summer, and fall in northern China, while SMAP and SMOS had the smallest bias in winter. For different climate zones, CCI product performed better in describing the temporal variability of soil moisture in arid climate zones with the correlation coefficients > 0.50 for most areas, while AMSR2 product provided a similar spatial distribution. In the eastern monsoon region, the soil moisture from SMAP and SMOS was found to have a large bias, whereas the bias in CCI product was small. Four products failed to reproduce the observed soil moisture characteristics in the transitional zones affected by the summer monsoon, with a positive bias found in AMSR2 and CCI and the largest biases in SMAP and SMOS products. We also suggest several reasons for the bias and error in the satellite soil moisture products. These results have important implications for soil moisture studies over midlatitude regions.

Abstract. The North Atlantic Oscillation (NAO) has been confirmed to be closely related to the weather-climate in many regions of the Northern Hemisphere, however, its effect and mechanism upon the formation of regional dust events (DEs) have rarely been involved in China. By using the station observation data, and multi reanalysis datasets, the influence of NAO on the dust aerosols (DAs) in China, as well as the corresponding mechanism of synoptic cause are explored in perspective of transient eddy fluxes. It is found that the DAs in the non-dust source areas show high values with a strong annual variability in north of the Yangtze River (30–40° N, 105–120° E), which is referred to as the North China hereafter. A significant negative relationship is indicated between the boreal winter NAO index and the late spring DAs in the North China with the correlation coefficient of −0.39. According to the 9 spring DEs affected significantly by negative phase of the preceding winter NAO in the North China during 1980–2020, it is shown that before the outbreak of DEs, due to the transient eddy momentum (heat) convergence (divergence) over the dust aerosol (DA) source regions, the zonal wind speed increases in upper-level troposphere, strengthening the zonal wind in the middle-lower levels through momentum downward transmission. Simultaneously, there is transient eddy momentum (heat) divergence (convergence) around the Ural Mountains, which is favorable for establishment and maintenance of the Ural ridge, as well as development of the air temperature and vorticity advections. The combined action of temperature and vorticity advections results in the Siberian Highs and Mongolian cyclone to establish, strengthen, and move southward near the surface, guiding the cold air from high latitudes southward, and is favorable to the uplift and transmission of DAs to the downstream North China. After the outbreak of DEs, change of transient eddy fluxes in the DA source regions and the Urals regions, leads to both energy and mass divergence and reduction of the zonal winds over the North China. Accompanying with the prevailing southerly airflow in south of the North China, a stable high value of DA concentration is maintained for 1–2 days. This study reveals the impact of transient eddy fluxes transport on the dusty weather anomalies modulated by the NAO negative signal in the North China, which can deepen the understanding of formation mechanism of DEs in China.

The variability of the Hadley Circulation (HC) is greatly impacted by the meridional structure of sea surface temperature (SST), which has varied effects depending on its symmetrical or asymmetrical pattern relative to the equator. By using the Coupled Comparison Program International Project Phase 6 (CMIP6) model outputs and reanalysis datasets, this study assesses the capacity of CMIP6 models to simulate the relationship between the HC and tropical SST under different meridional structures, as well as investigates the possible causes for simulation biases. It is shown that the CMIP6 models can successfully reproduce climatological HC, tropical SST, and their spatial patterns of first leading modes under different meridional structures, where the correlation coefficient between simulations and observations reaches 0.8 or above. By comparison, the CMIP6 model outputs exhibit substantial differences in simulating the HC to SST response over the different meridional structures, with obvious inter-model differences. Considering the capability in simulating the HC to tropical SST response, the CMIP6 models are divided into two types, Type I model and Type II model. Models of Type I are those whose simulation results are basically close to the reanalysis data, with the biases being less than 20%. The models of Type II are those whose simulated response ratios are much stronger than those of the reanalysis. It is found that the models of Type II overestimate the intensity of El Niño-Southern Oscillation (ENSO) events, and remarkably underestimate the HC and SST correlation in the equatorial symmetric part, resulting in the inability of the models of Type II to capture the connection of the HC and tropical SST. The results indicate that, the component of the CMIP6 models in reproducing the ENSO events has a considerable impact on the simulation of the HC and tropical SST relationship, which offers recommendations for enhancing the capability of models to simulate large-scale tropical air-sea interactions.