Enhanced predictability of rapidly intensifying tropical cyclones over the western North Pacific associated with snow depth changes over the Tibetan Plateau

This study finds an enhanced relationship in recent years between January–March eastern Tibetan Plateau snow depth (TPSD) and the frequency of rapidly intensifying tropical cyclones (RITCs) over the western Northern Pacific (WNP) during the following peak TC season (July–November). The correlation between TPSD and RITCs is significant during 2000–2014 but was insignificant from 1979–1999. During 2000–2014, when TPSD increases, there is an enhanced low-level anomalous anticyclone over the subtropical eastern North Pacific mainly due to the combined effect of advection and dynamics of the climatological prevailing westerly jet. Northeasterly wind anomalies are observed on the flank of the anticyclonic circulation anomaly, favoring anomalously cool sea surface temperature (SST). These anomalies lead to an anomalous pattern similar to the Pacific meridional mode (PMM), via a wind-evaporation feedback and cold advection. A Gill-type Rossby response to the PMM-like negative phase results in an anticyclonic circulation anomaly over the WNP, suppressing RITCs during 2000–2014. A nearly opposite circulation anomaly occurred when TPSD was lower during 2000–2014. There is a weak relationship between TPSD and RITCs, due to the lack of a link between TPSD and the PMM-like pattern from 1979–1999. Decadal changes in the relationship between TPSD and RITCs are mainly due to the meridional displacement of the prevailing westerly jet which may be in response to decadal-to-multi-decadal variability of SST anomalies. These changes then result in changes in the relationship between January–March TPSD and the PMM-like pattern.

Two Distinct Types of 10-30-day Persistent Heavy Rainfall Events over the Yangtze River Valley

Large-scale circulation anomalies associated with 10-30-day filtered persistent heavy rainfall events (PHREs) over the middle and lower reaches of the Yangtze River Valley (MLYV) in boreal summer for the period of 1961-2017 were investigated. Two distinct types of PHREs were identified based on configurations of anomalies in western Pacific subtropical high (WPSH) and South Asian High (SAH) during the peak wet phase. One type named as PSAH is characterized by eastward extension of the SAH while the other named as NSAH is featured by westward retreat of the SAH, and they both exhibit westward extension of the WPSH. Both types of PHREs are dominated by Mei-yu frontal systems. The lower-level circulation anomalies play a crucial role in initiating rainfall but through different processes. Prior to rainfall occurrence, a strong anticyclonic circulation anomaly is over the western North Pacific (WNP) for the PSAH events and the related southwesterly wind anomaly prevails over the south-eastern China, which advects moisture into the MLYV, moistens the boundary layer, and induces atmospheric convective instability. For the NSAH events, the WNP anticyclonic circulation is weak while a strong northerly wind is observed north of the MLYV. It brings cold air mass southward, favoring initiating frontal rainfall over the MLYV. The formation of upper-level circulation anomalies over the MLYV is primarily due to the shift of anomalous circulations from mid-high latitudes. After the rainfall generation, the precipitation would influence the lower- and upper-level circulation anomalies.

Temporospatial distribution and trends of thunderstorm, hail, gale and heavy precipitation events over the Tibetan Plateau and associated mechanisms

Temporospatial distribution and trends of thunderstorm, hail, gale and heavy precipitation events over the Tibetan Plateau (TP), as well as the associated mechanisms with observational data from 1979-2016 are investigated, which have not been fully studied under a changing climate. The results indicate that thunderstorm, hail and gale events over the whole TP show significant decreasing trends, while heavy precipitation events have an insignificant increasing trend. The southeast (SE) and central south (SC) subregions have obvious significant decreasing trends in thunderstorm, hail and gale events, while the northeast (NE) subregion has a significant increasing trend in heavy precipitation events. It is found that the atmospheric circulation anomaly caused by the northwestern Atlantic Sea Surface Temperature (SST) anomaly associated with the North Atlantic Oscillation (NAO) should be responsible for these changes. A strong wave train triggered by the northwestern Atlantic SST anomaly propagates from the northern Atlantic to East Asia through Europe, and induces a more upper-level warming over the TP and an anomalous anticyclonic circulation near the Lake Baikal, resulting in more stable atmosphere and blocking effect, which forces the mid-latitude westerlies and associated cold air to shift poleward. The weakened cold air advection over the TP decreases the baroclinic instability and convection initiation, and finally causes the significant decreasing trends in severe weather events. On the other hand, the enhanced easterly winds in the southern flank of the anticyclonic circulation can significantly increase the water vapor flux from the eastern boundary of the TP and heavy precipitation events in the NE subregion.

Circulation Pathways and Exports of Arctic River Runoff Influenced by Atmospheric Circulation Regimes

River runoff supplies the Arctic Ocean with a large amount of freshwater and land-derived material, so it is important for both the physical and biogeochemical marine environment. In this study, we used wind perturbation simulations to elucidate the response of the circulation pathways and exports of Arctic river runoff to different atmospheric circulation regimes. Specifically, wind perturbations representing the negative and positive phases of the Arctic Oscillation and Beaufort High modes were imposed over the Arctic Ocean separately in different sensitivity experiments. In addition, some combinations of the two modes were also considered in sensitivity experiments. By comparing these experiments with a control simulation, we revealed the impact of different wind perturbations. The atmospheric circulation regimes influence the Arctic surface geostrophic currents through changing the halosteric height, which is associated with the changes in spatial distribution of surface freshwater. The circulation pathways of river runoff, and Pacific and Atlantic derived surface waters are mainly determined by the surface geostrophic currents. The positive (negative) Arctic Oscillation reduces (increases) freshwater storage and sea surface height in the Makarov and Eurasian basins, thus strengthening (weakening) the cyclonic circulation and weakening (strengthening) the anticyclonic circulation; Accordingly, the Eurasian runoff leaves the Siberian shelf at more eastern (western) locations, and has an enhanced export through the Fram Strait (Canadian Arctic Archipelago). The positive (negative) Beaufort High increases (reduces) freshwater storage and sea surface height in the Amerasian Basin, thus strengthening (weakening) the anticyclonic circulation; Accordingly, the Eurasian runoff export through the Fram Strait and the Mackenzie River runoff export through the Canadian Arctic Archipelago are reduced (increased). The positive Arctic Oscillation increases freshwater available to the Beaufort Gyre, which can be efficiently accumulated there in the presence of a positive Beaufort High forcing. The impact of the Beaufort High mode on the location of the Transpolar Drift Stream and runoff circulation pathways is stronger with a positive Arctic Oscillation than with a neutral Arctic Oscillation state. Our results also showed that Eurasian runoff can only have a relatively small contribution to freshwater accumulation in the Beaufort Gyre region.

Tropical Cyclone Footprints in Long-Term Mean State and Multiscale Climate Variability in the Western North Pacific as Seen in the JRA-55 Reanalysis

The monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of TCs in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity.Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

Effect of rainfall-induced diabatic heating over southern China on the formation of wintertime haze on the North China Plain

During the winters (December–February) between 1985 and 2015, the North China Plain (NCP) suffered many periods of heavy haze, and these episodes were contemporaneous with extreme rainfall over southern China; i.e., South Rainfall−North Haze events. The formation of such haze events depends on meteorological conditions, which are controlled by the atmospheric circulation associated with rainfall over southern China, but the underlying physical mechanism remains unclear. This study uses observations and model simulations to demonstrate that haze over the NCP is modulated by anomalous anticyclonic circulation caused by the Rossby wave train, in conjunction with the north−south circulation system (NSC), which ascends over southern China, moves north into northern China near 200–250 hPa, and then descends in the study area. Moreover, in response to rainfall heating, southern China is an obvious Rossby wave source, supporting waves along the subtropical westerly jet waveguide and finally strengthening anticyclonic circulation over the NCP. Composite analysis indicates that these changes lead to a stronger descending motion, higher relative humidity, and a weaker northerly wind, which favors the production and accumulation of haze over the NCP. A linear baroclinic model (LBM) simulation reproduced the observed NSC reasonably well and supports the diagnostic analysis. Quasi-geostrophic (QG) vertical pressure velocity (ω) diagnostics were used to quantify the contributions to the NSC made by large-scale adiabatic forcing and diabatic heating. The results indicated that the NSC is induced mainly by diabatic heating related to precipitation over southern China, and the effect of large-scale circulation is negligible. These results provide the basis for a more comprehensive understanding of the mechanisms that drive the formation of haze over the NCP.

Impact of the radiative forcing on the winter North Atlantic-European atmospheric circulation

<p><span>The accelerated warming linked to climate change has become a topic of great interest due to its projected impact on ecosystems. In this work, we assess the causes and impacts of the anthropogenic radiative forcing on the North Atlantic-European atmospheric circulation in boreal winter (DJF). To isolate the response to radiative forcing, we have used two approaches, whose simulations follow the historical/scenario concentrations from CMIP6. The first approach consists of three 240-year simulations with the European Consortium – Earth System model version 3.3 (EC-EARTH v3.3) keeping fixed the radiative forcing at 1950, characterizing the Past climate, at 2000, representative of Present-day conditions, and at 2050, projecting the near-Future climate. The second approach makes use of the Large Ensemble (i.e. 24 members) of transient simulations performed with the Institut Pierre-Simon Laplace Coupled Model version 6 (IPSL-CM6), where three 10-year periods have been considered, namely 1949-1959, 1999-2009, and 2049-2059, assuming that the radiative forcing remains relatively constant in each of them. Results show that both approaches yield a consistent forced response, and that it scales linearly with radiative forcing, increasing in amplitude from Present-minus-Past to Future-minus-Present. At low latitudes, in the tropical Atlantic, the forced atmospheric response is characterized by a Gill-type baroclinic structure, where the anomalous anticyclonic circulation at upper levels reinforces the westerly wind at the equatorward flank of the North Atlantic jet. At high latitudes, the forced response is reminiscent of the ‘Arctic Amplification’ linked to sea-ice reduction, and the thermally-driven baroclinic structure can be seen over the Labrador Sea-Hudson Bay region. At mid-latitudes, the forced response shows a barotropic pattern, with a cyclonic (anticyclonic) circulation in the North Atlantic (Euro-Mediterranean) sector, pointing out a role for non-radiative, eddy-related effects.  </span></p>

Interdecadal change in the genesis activity of the first tropical cyclone of each year over the western North Pacific

This study analyzed time series of the genesis latitude, longitude, and date of the first tropical cyclone (TC) each year over the 38 years between 1979 and 2016. Statistical change-point analysis applied to these three variables showed that a shift in climate regime occurred around 1998. More specifically, recent TCs have shown a strong tendency to occur more northwest in the western North Pacific (WNP), and day of TC genesis tend to be delayed. Also, we compared differences between the periods 1998 to 2016 (post-1998) and 1979–1997 (pre-1998) in terms of outgoing longwave radiation (OLR), total cloud cover, precipitable water, precipitation, vertical wind shear, 850 hPa relative vorticity, and sea surface temperature (SST). Our results showed that a favorable environment for TC genesis was formed near the South China Sea (SCS) and the Philippines and an unfavorable environment for TC genesis was formed in the southeastern part of the WNP. Analysis of stream flow showed that an anomalous cyclonic circulation at 850 hPa was formed in the SCS and an anomalous large anticyclonic circulation was formed in the North Pacific. From these circulations, a ridge extended to the east sea of the Philippines, and consequently, anomalous trade winds were strengthened in the equatorial Pacific. Such anomalous atmospheric circulation seems to be associated with the cold Pacific Decadal Oscillation (PDO) phase. At 200 hPa, the anomalous anticyclonic circulation was strengthened in the SCS, and an anomalous cyclonic circulation formed in the east sea of the Philippines, which strengthened anomalous westerlies in the equatorial Pacific. Furthermore, this circulation pattern is found to be related with a strengthening of Walker circulation. Therefore, during the post-1998, when trade winds were strengthened by the development of Walker circulation, the cold PDO phase was strengthened, the location of TC genesis moved toward the northwestern WNP, and TC genesis day tended to be delayed.

Modeling the Influence of Upstream Land-atmosphere Coupling on the 2017 Persistent Drought over Northeast China

Persistent drought events that cause serious damages to economy and environment are usually intensified by the feedback between land surface and atmosphere. Therefore, reasonably modeling land-atmosphere coupling is critical for skillful prediction of persistent droughts. However, most high-resolution regional climate modeling focused on the amplification effect of land-atmosphere coupling on local anticyclonic circulation anomaly, while less attention was paid to the non-local influence through altering large-scale atmospheric circulation. Here we investigate how the antecedent land-atmosphere coupling over the area south to Lake Baikal (ASLB) influences the drought events occurred over its downstream region (Northeast China; NEC) by using Weather Research and Forecasting (WRF) model and linear baroclinic model (LBM). When the ASLB is artificially forced to be wet in the WRF simulations during March-May, the surface sensible heating is weakened and results in a cooling anomaly in low level atmosphere during May-July. Consequently, the anticyclonic circulation anomalies over ASLB and NEC are weakened, and the severity of NEC drought during May-July cannot be captured due to the upstream wetting in March-May. In the LBM experiments, idealized atmospheric heating anomaly that mimics the diabatic heating associated with surface wetness is imposed over ASLB, and the quasi-steady response pattern of 500-hPa geopotential height to the upstream wetting is highly consistent with that in the WRF simulation. In addition, the lower level heating instead of the upper level cooling makes a major contribution to the high pressure anomaly over NEC. This study implies the critical role of modeling upstream land-atmosphere coupling in capturing downstream persistent droughts.

Large Day-to-Day Variability of Extreme Air Temperatures in Poland and Its Dependency on Atmospheric Circulation

The primary purpose of the study was the determination of the spatial day-to-day variability of extreme air temperatures in Poland and the dependency of large temperature changes on atmospheric circulation in accordance with the Grosswetterlagen (GWL) classification. The goal was achieved based on data from 1966 to 2015, made available by the Institute of Meteorology and Water Management—National Research Institute. Day-to-day changes in maximum and minimum air temperatures were designated with a rate of ≥6 °C (large) and with a rate of ≥12 °C (very large) and their spatial distribution was presented. Finally, the analysis of the dependency of considerable temperature changes on atmospheric circulation in accordance with the Grosswetterlagen (GWL) classification was conducted. The obtained results showed that in Poland in the period 1966–2015 there was a statistically significant increase in the number of large changes in both Tmax and Tmin. The number of large changes in Tmax increases from north to south, and the number of large changes in Tmin from north to east and south of Poland indicate the range of the effect of marine and continental properties on the climate of Poland and the influence of local relief as well as the atmospheric circulation impact. Large changes in Tmax occur more frequently in spring and in Tmin in winter. Large changes in Tmax and Tmin are mainly recorded during cyclonic circulation, however, the anticyclonic circulation types favour especially large decreases in Tmin.