Description: The present study classifies regional persistent extreme precipitation events (PEPEs) in North China into two types in accordance with variance contributions of different timescale rainfall variability in boreal summer. For Type 1, PEPEs are dominated by 10-20-day periodicity, and for Type 2 PEPEs are mainly influenced by a 30-60-day mode. Atmospheric circulation anomalies associated with the two types of PEPEs are characterized by a zonal wave train (the EU pattern) in the mid-high latitudes in Type 1 but a meridional wave train (the EAP pattern) in East Asia in Type 2. The common feature of the two types is anomalous southerly on the west edge of the West Pacific Subtropical High (WPSH), which favors anomalous moisture transport into the key region. Additional moisture source for Type 2 is linked to anomalous cross-equatorial flow. Both types of PEPEs result from the combined effect of intraseasonal oscillations in both the mid-high latitudes and the tropics. The impact of ENSO on the two types of PEPEs is investigated. While a La Niña and a neutral SST condition in the preceding winter favor for the occurrence of PEPEs, their subsequent transition in central and eastern equatorial Pacific will determine which of the two types of PEPEs is pronounced.

Description: The effect of tropical precipitation on the inverse change in the entrance and exit of the tropical easterly jet (TEJ) is studied. The TEJ's entrance and exit are over the northwest Pacific Ocean and equatorial Africa, inversely varying from July to September. When the entrance of the TEJ is weaker-than-normal, the exit is stronger-than-normal, and vice versa. Observed data show that a west-east dipole precipitation pattern located in the Maritime Continent (MC) and tropical Pacific (TP) has a distinct influence on such inverse variation of the TEJ. A series of numerical experiments are carried out based on the linear baroclinic model, qualitatively exploring the independent effects of MC and TP precipitation anomalies. The results show that the heat source over the MC can induce a Kelvin wave propagating eastward in the upper troposphere, resulting in the weakening of the TEJ entrance. The heat sink over the TP generates a Rossby wave response, forming a pair of cyclones in the troposphere over the equatorial western Pacific, and the westerly anomaly in the middle weakens the TEJ's entrance; meanwhile, a Kelvin wave propagating eastward in the upper troposphere between 120°W and 70°E, which is caused by the heat sink over the TP strengthens the exit of the TEJ. There is a reverse phase change in the inlet and exit regions of the jet stream as a result. It’s shows that the west-east dipole precipitation pattern in the MC and TP may be related to La Niña-like SST anomalies in synchronous summer.

Description: As is known to all that the North China Plain (NCP) has been plagued by severe water shortages and land subsidence. It is of great practical importance to investigate whether the recharge water provided by the South-North Water Diversion Project (SNWDP) for NCP can effectively alleviate the problems such as land subsidence in the area. In our study, we analyzed the equivalent water height and land deformation of NCP based on GRACE and GRACE-FO data for the past decade and GNSS observations for the same period, and found that water recharge well mitigated the deficiency of land water storage, but the vertical motion images reflected by GNSS data showed that the rate of land subsidence was not slowed down by the increase of water storage. The results imply that the release of water from the weakly permeable layer is delayed due to previous over-exploitation of groundwater, while the short-term supply of surface water has little effect on replenishing groundwater.

Description: Arctic temperature is raising at a rate of two to four times faster than the global average. The Arctic cryosphere has been undergoing rapid melting over the past few decades. Previous studies indicated that short-lived climate forcers (SLCFs) have substantial impacts on Arctic warming. Black carbon (BC) is one of the SLCFs, which can absorb solar radiation efficiently warming the atmosphere. The deposition of BC on snow and ice accelerates snow and ice melting by reducing surface albedo. In this study, we investigated the distribution, transportation, and radiation effects of BC in the Arctic from June 2015 to May 2017 by using the meteorology–chemistry model (Weather Research and Forecasting model couple with Chemistry, WRF-Chem). The results showed that near-surface BC concentrations in the Arctic presented higher values during winter-spring, which can be largely contributed by the stronger near-surface northward transport of aerosols from northern Eurasia during this period. The northward transport can be found in the higher troposphere during summer-autumn, while it was probably inefficient due to enhanced wet scavenging of aerosols. BC-induced near-surface temperature changes were stronger in the Arctic in winter and autumn, and the downward longwave radiation related to cloudiness changes played an important role for driving near-surface temperature. In summer and spring, the relatively less changes in near-surface temperature may be the result of the mutual offset between the surface longwave and shortwave radiation changes.

Description: The isotopic and hydrochemical signatures measured from various waters in a catchment have been widely used to separate flow components, identify recharge sources, and integrate such tracers into hydrological models to help model evaluation. However, the reliability of using auxiliary data for strengthening hydrological functioning heavily depends on if the sampling is representative of hydro-chemical dynamic behaviors in a catchment. Herein, we illustrate the necessary sampling resolution or frequency to facilitate our understanding and illustrating the complex hydrological functioning in two catchments respectively characterizing the high aquifer heterogeneity in karst landform and the multi-recharge sources in the cryosphere environment. Our comprehensive analysis and modeling show that the sampling intervals of stable isotope should be shorter than hours for aiding hydrological models in capturing the sharp rise and decline of hydrograph in the cockpit karst catchment of southwest China. The daily sampling of stable isotope and chemistry can identify turning points of dominant recharge sources (rainwater, melt water from glacier and snowpack, and shallow and deep groundwater) and their contributions to streamflow in the glacierized catchment of the Tibet Plateau in China.

Description: The vertical structure of Arctic warming is of great importance and attracts increasing attention. This study defines two types of Arctic warming events (viz., deep versus shallow) according to their temperature profiles averaged over the Barents-Kara Seas (BKS), and thereupon compares their characteristics and examines their difference in generation through thermodynamic diagnoses. The deep Arctic warming event—characterized by significant bottom-heavy warming extending from the surface into the middle-to-upper troposphere—emanates from the east of Greenland and then moves downstream towards the BKS primarily through zonal temperature advection. The peak day of deep warming event lags that of the precipitation and resultant diabatic heating over Southeast Greenland by about four days, suggesting that the middle-to-high tropospheric BKS warming is likely triggered by the enhanced upstream convection at the North Atlantic high latitudes. In contrast, the shallow warming event—manifested by warming confined within the lower troposphere—is preceded by the meridional advection of warm air from inland Eurasia. These anomalous southerlies over Eurasian lands during shallow warming events are related to the eastward extension of deepened Icelandic Low. Whereas during deep warming events, the insitu reinforcement of Icelandic Low favors abundant moisture transport interplaying with the Southeast Greenland terrain, leading to intense precipitation and latent heat release there. Both deep and shallow warming events are accompanied by Eurasian cooling, but the corresponding cooling of deep warming event is profoundly stronger. Further, intraseasonal deep Arctic warming events could explain nearly half of the winter-mean change in warm Arctic-cold Eurasia anomaly.

Description: Air–sea coupling system in the southwestern Indian Ocean (SWIO) exhibits predominant multidecadal variability that is the strongest during austral summer. It is characterized by an equivalent barotropic atmospheric high (low) pressure over warm (cold) sea surface temperature (SST) anomalies and a poleward (equatorward) shift of the westerlies. In this study, physical processes of this multidecadal variability are investigated by using observations/reanalysis and CMIP6 model simulations. Results suggest that the multidecadal fluctuation can be explained by the modulation of the Atlantic meridional overturning circulation (AMOC) and the local air–sea positive feedback in the SWIO. In both observation/reanalysis and CMIP6 model simulations, the AMOC presents a significant negative correlation with the multidecadal SST variation in the SWIO when the AMOC is leading by about a decade. The mechanisms are that the preceding AMOC variation can cause an interhemispheric dipolar pattern of SST anomalies in the Atlantic Ocean. Subsequently, the SST anomalies in the midlatitudes of the South Atlantic can propagate to the SWIO by the oceanic Rossby wave influenced by the Antarctic Circumpolar Current (ACC). Once the SST anomalies reach the SWIO, these SST anomalies in the oceanic front can affect the baroclinicity in the lower troposphere to influence the synoptic transient eddy and then intensify the atmospheric circulation anomaly via the eddy–mean flow interaction. Subsequently, the anomalous pressure over the SWIO can significantly strengthen the SST anomalies through modifying the surface heat flux and radiation as well as the oceanic meridional advection.

Description: Under the combined effect of climate change and urbanization, the occurrence and intensity of heat wave events and extreme precipitation events are increasing in the Guangdong-Hong Kong-Macao Greater Bay Area (GBA). Whether and how the changes of meteorological factors brought by urbanization have observable effects on the characteristics of high temperature events and extreme precipitation events has become a research hotspot. At the same time, the two events are also interacting. The link between temperature and precipitation extremes is often characterized by the Clausius-Clapeyron (C-C) relationship, but more and more studies show that the basic C-C equation does not necessarily reveal the relationship. This paper will study the mechanism of the impact of urbanization on temperature and precipitation in the GBA, quantifying the contribution of urbanization to temperature and precipitation changes by comparing the differences in spatial and temporal distribution structure and evolution characteristics of temperature and precipitation between urban and rural areas. In addition, this paper will base on binning scaling function to explore the thermodynamic response structure of extreme precipitation in the GBA, reveal the causes of the changes in the frequency of extreme precipitation by combining the relationship between temperature and precipitation duration, and explain the possible reasons for the bimodal structure of the thermodynamic response structure of extreme precipitation in the GBA by combining relevant meteorological factors.

Description: The present study investigates the characteristics and possible origins of the synoptic and intraseasonal variability of the extreme heavy rainfall (EHR) event in Henan Province, China in July 2021. Two dominant modes with periods about 8 days and 10–40 days feature the Henan EHR event and explain more than 80% of total rainfall. The synoptic variability of the Henan EHR event can be attributed to the northwestward-propagating synoptic-scale wave train modulated by the monsoon trough over the western North Pacific. A northernmost-displacement monsoon trough in July 2021 (~23.2°N) would facilitate the synoptic-scale wave train to propagate farther northwestward, which reaches to the subtropical region and thus influences the Henan EHR event. The intraseasonal variability of the Henan EHR event is tightly related to the meridional dipole pattern with an anticyclonic circulation over North China and Sea of Japan and a cyclonic circulation over the western North Pacific. Both tropical and extratropical signals are responsible for the intraseasonal variability of the Henan rainfall, including the northward-propagating Boreal Summer Intraseasonal Oscillation, mid-latitude wave train over the Eurasian continent and air-sea interaction over the Kuroshio Extension region. The synoptic and intraseasonal atmospheric circulations favor abundant moisture transporting to Henan and strong ascending flow, jointly leading to the rainfall extreme therein. The results implicate that these tropical and extratropical forcings may be the potential precursors for heavy rainfalls in Henan Province or even the northern Central China.

Description: Tropical sea surface temperature (SST) and associated precipitation, acting as diabatic heat forcing, has far- reaching climatic impacts across the globe through exciting poleward-propagating Rossby waves. It is found that the leading mode of tropical Pacific forcing in austral autumn experiences a significant interdecadal shift from an eastern Pacific (EP) to a central Pacific (CP) type around the late 1990s. More specifically, the EP-type precipitation anomaly mode before 1998 drives a quadrupole-like teleconnection pathway emanating from the tropical Pacific to the Ross Sea and Amundsen–Bellingshausen Seas (ABS) region, whereas the CP-type mode after 1999 excites a Pacific–South American (PSA)-like teleconnection orienting along a great circle. Divergent flows induced by different precipitation anomaly modes primarily determine the generation of Rossby waves by means of the vortex stretching and vorticity advection processes. Furthermore, the synoptic high-frequency transient eddy activity along with its dynamic forcing effect differs greatly before and after 1998/99, contributing to different locations of the teleconnection lobes at mid- to high latitudes. In contrast, the subseasonal low-frequency transient eddy activity exerts a limited influence. Our findings also indicate that the EP-type (CP-type) tropical forcing mode could significantly modulate the zonal displacement (strength) of the Amundsen Sea low, which could lead to distinct climate responses of West Antarctica and the Antarctic Peninsula in austral autumn.