Anomalous Cyclonic Circulation Research Articles

Role of the Indian Ocean basin mode in driving the interdecadal variations of summer precipitation over the East Asian monsoon boundary zone

Abstract. Based on long-term observational and reanalysis datasets from 1901 through 2014, this study investigates the characteristics and physical causes of the interdecadal variations in the summer precipitation over the East Asian monsoon boundary zone (EAMBZ), which is a peculiar domain defined from the perspective of the interplay between climatic systems (i.e., mid-latitude westerly and East Asian summer monsoon). Observational evidence reveals that, similarly to previous studies, the EAMBZ precipitation featured prominent interdecadal fluctuations, e.g., with dry summers during the periods preceding 1927, 1939–1945, 1968–1982, and 1998–2010 and wet summers during the periods of 1928–1938, 1946–1967, and 2011 onwards. Further analyses identify that, amongst the major interdecadal oceanic forcings (e.g., Atlantic multidecadal oscillation and Pacific decadal oscillation), the Indian Ocean basin mode (IOBM) is a significant oceanic forcing responsible for the interdecadal variations of the EAMBZ precipitation, playing an independent and critical modulation role. When the cold phase of the IOBM occurs, an anomalous cyclonic circulation is excited around the northeast corner of the tropical Indian Ocean, which further induces a north-low–south-high meridional seesaw pattern over the Northeast China–subtropical western Pacific (SWP) sector. Such seesaw pattern is conducive to the enhanced EAMBZ precipitation by linking favorable environments for the transportation of water vapor from the SWP and the convergence over the EAMBZ at interdecadal timescales. For this reason, a physical–empirical model for the EAMBZ precipitation is developed in terms of the IOBM cooling. Despite the fact that the extreme summer EAMBZ precipitation cannot be captured by this model, it can still well capture its interdecadal fluctuations and reflect their steady relationship. The key physical pathway connecting the IOBM cooling with the interdecadal variations of the summer EAMBZ precipitation is supported by the numerical results based on the large ensemble experiment and the Indian Ocean pacemaker experiment. Our findings may provide new insights into the understanding of the causes of the interdecadal variations in the summer EAMBZ precipitation, which may favor the long-term policy decision-making for the local hydrometeorological planning.

Unraveling the Influence of Equatorial Waves on Post-Monsoon Sea Surface Salinity Anomalies in the Bay of Bengal

In this study, we investigate the connection between planetary equatorial waves, modulated by the Indian Ocean dipole (IOD) and El Niño Southern Oscillation (ENSO), and the interannual variabilities of the salinity distribution in the Bay of Bengal (BoB) in October–December (OND), along with its associated dynamics, using satellite and reanalysis datasets. In OND 2010 and 2016 (1994, 1997, 2006, and 2019), positive (negative) sea surface salinity anomalies (SSSAs) were distributed in the eastern equatorial Indian Ocean (EIO) and Andaman Sea. Moreover, the southward movement of negative (positive) SSSAs along the eastern Indian coast was observed. This phenomenon was caused by large-scale anomalous currents associated with zonal wind over the EIO. During OND 2010 and 2016 (1994, 1997, 2006, and 2019), due to anomalous westerlies (easterlies) over the EIO and anomalous downwelling (upwelling) Kelvin waves, the strengthened (weakened) Wyrtki jet and the basin-scale anomalous cyclonic (anticyclonic) circulation in the BoB gave rise to positive (negative) SSSAs within the eastern EIO and Andaman Sea. In addition, the intensified (weakened) eastern Indian coastal currents led to the southward movement of negative (positive) SSSAs. It is worth noting that downwelling Kelvin waves reached the western coast of India during OND 2010 and 2016, while upwelling Kelvin waves were only confined to the eastern coast of India during OND 1994, 1997, 2006, and 2019. Furthermore, westward salinity signals associated with reflected westward Rossby waves could modulate the spatial pattern of salinity. The distribution of salinity anomalies could potentially influence the formation of the barrier layer, thereby impacting the sea surface temperature variability and local convection.

A Potential Seasonal Predictor for Summer Rainfall over Eastern China: Spring Eurasian Snowmelt

Abstract The hydrological effect of snow over Eurasia is important for regulating regional and global climate through affecting land–atmosphere energy exchange. Based on observational and reanalysis datasets, this study investigates the effect of spring Eurasian snowmelt on the following summer rainfall over eastern China during the period of 1979–2018. The results show that a substantial meridional dipole pattern of summer rainfall anomalies over eastern China is closely associated with the preceding spring snowmelt anomalies over Eurasia, especially over remote Siberia. Excessive snowmelt anomalies over Siberia in spring could result in a wetter local soil condition from spring until the following summer, thereby increasing latent heat fluxes and reducing local surface temperature, and vice versa. Then, the anomalous summer surface cooling over Siberia increases the meridional gradient of temperature between the Eurasian midlatitudes and high latitudes, which intensifies the Eurasian atmospheric baroclinicity and motivates the eddy‐induced geopotential height responses along with the significant wave propagations spreading from the Eurasian high latitudes to Lake Baikal. As a result, excessive spring snowmelt anomalies over Siberia tend to be accompanied with an anomalous anticyclone circulation to the east of Lake Baikal and an anomalous cyclonic circulation over southeastern China in the following summer. This could lead to a meridional dipole pattern of summer rainfall anomalies over eastern China, with deficient rainfall over northern China and slightly excessive rainfall over southern China. The present findings highlight the lagged effect of spring Eurasian snowmelt on summer climate over eastern China, with implications for the regional seasonal climate prediction. Significance Statement Frequent summer droughts and floods over eastern China cause serious damage to the regional economy and society. This study reveals an important effect of spring Eurasian snowmelt anomalies on the following summer rainfall anomalies over eastern China. Through affecting the local land–atmosphere energy exchange and regulating the remote atmospheric circulation, excessive spring Eurasian snowmelt anomalies could induce a substantial meridional dipole pattern of summer rainfall anomalies over eastern China, with deficient rainfall over northern China and excessive rainfall over southern China, and vice versa. Therefore, this work identifies spring Eurasian snowmelt as a potential seasonal predictor for summer rainfall over eastern China, which is home to more than a billion people. This is important for the local livelihood, including agriculture, water resources, ecosystems, human health, and economies.

Characteristics of precipitation in East Asia associated with Okhotsk Sea blocking in June and July

AbstractThis study investigates the influence of Okhotsk Sea blocking (OKB) on summer precipitation in East Asia in terms of the movement, enhancement and extension of the monsoon rainband. For this purpose, a composite analysis is conducted for nine OKB events that occur in June and July from 2000 to 2022. The results show that the OKB events that occur in June are associated with decreased precipitation in East Asia, whereas those that occur in July are related to increased precipitation in the same region. The long‐lasting OKB can continuously maintain and intensify anomalous cyclonic circulations in northeastern Asia, leading to enhanced vertically integrated moisture flux convergence (VIMFC) along the northern edge of the Western North Pacific subtropical high. However, there are differences in spatial distribution and displacement of anomalous cyclonic circulations between June and July. The OKB occurring in June is related to simultaneously developing and progressively strengthening anticyclonic circulation in the Okhotsk Sea as well as cyclonic circulation in northeastern Asia. Under the displacement of an intensified and southwardly extended cyclonic circulation, the VIMFC shifts southward and into a southeast‐to‐northwest orientation, compared to climatology, resulting in decreased precipitation in East Asia, particularly in southern China and southern Japan. On the other hand, when the OKB occur in July, anticyclonic circulation in the Okhotsk Sea is developed initially followed by cyclonic circulation in northeastern Asia, amplifying several days after onset. Anomalous cyclonic circulation is much weaker and located more northward than in June, which is favourable for stagnating and enhancing the VIMFC in East Asia. As a result, precipitation can increase in the region, particularly on the Korean Peninsula and in Japan.

The Principal Modes of Morning Extreme Precipitation over Inland Guangdong, China during Pre-Summer Rainy Season

The study explores the characteristics of morning extreme precipitation (MEP) during the pre-summer in inland Guangdong. Based on the principal modes, MEP events can be classified into four groups. The first group of MEP (G1) is a typical southeastward-propagating rainfall system originating from the northwestern mountains. This is caused by the strongest accelerated southwesterly winds at night, which bring abundant moist and warm air from the South China Sea (SCS) along with the shear line and the highest convective available potential energy (CAPE). The second group of MEP (G2) is warm-sector heavy rainfall with large-scale warming and higher CAPE. This local rainfall system originates in the south of Nanling mountains at night and reaches its mature stage in the morning. The rainfall system of the third group (G3) originates in central Guangxi and propagates to the southern inland region. The southeasterly winds in Guangxi intensify at night due to the anomalous cyclonic circulation. However, in the morning, the easterly winds shift to the westerlies, favoring eastward propagation. After SCS monsoon onset, cold air intrudes southward, colliding with moist warm air from the SCS, leading to heavy frontal precipitation in the inland region, classified as the fourth group MEP (G4).

Effects of Potential Large-Scale Irrigation on Regional Precipitation in Northwest China

Understanding the impact of irrigation on the spatiotemporal characteristics of precipitation is important for the ecological stability of the arid region of Northwest China (NWC). In this study, the global climate model MPI-ESM-MR is utilized to provide the initial and lateral boundary conditions for the regional climate model RegCM4, enabling the simulation of the long-term climate of the arid NWC region under two Representative Concentration Pathways (RCPs). The study focuses on analyzing the differences in the spatiotemporal distribution, intensity, and frequency of precipitation before and after irrigation. Furthermore, the study explores the primary factors influencing changes in the spatiotemporal distribution characteristics of precipitation in the irrigation district and its downwind region. The results indicate that RegCM4 performs well in simulating the climatology and diurnal cycle of precipitation in the NWC, particularly excelling during the summer. Large-scale irrigation significantly impacts the structure of summer precipitation, leading to a notable increase in convective precipitation near the irrigation district and surrounding mountain ranges. Anomalous cyclonic circulation and enhanced vertical velocity enhancement provide dynamic conditions for increased precipitation in the irrigation district and surrounding mountainous regions. Furthermore, the evaporation of water vapor resulting from large-scale irrigation serves as an additional source of moisture, contributing to increased precipitation in the irrigation district and its downwind region. Due to the difference in climatic conditions between the western and eastern regions around the irrigated areas, the summer extreme precipitation in the west predominantly increases. At the same time, in the east, it predominantly decreases due to irrigation. The findings of this study contribute to a deeper understanding of the physical mechanisms through which agricultural irrigation influences precipitation in the arid region of the NWC, thereby providing scientific evidence for the sustainable utilization of water resources in the region.

Opposite Skills of ENGPI and DGPI in Depicting Decadal Variability of Tropical Cyclone Genesis over the Western North Pacific

Abstract The genesis potential index (GPI) has been used widely to estimate the influence of large-scale conditions on tropical cyclone (TC) genesis. Here we find that two GPIs, the Emanuel–Nolan GPI (ENGPI) and the dynamic GPI (DGPI), show opposite skills in quantifying decadal variability of TC genesis in the western North Pacific (WNP). During 1979–2020, ENGPI shows a reverse decadal variation to the WNP TC genesis with a significant negative correlation of −0.61, while DGPI can reasonably reproduce the decadal variation of the WNP TC genesis with a significant correlation of 0.66. The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis induced by a WNP anomalous cyclonic circulation that controls the decadal variation of TC genesis. On the one hand, the cyclonic circulation leads to favorable dynamical conditions including ascending motion, cyclonic vorticity, and weakened vertical shear, and thus tends to increase the DGPI. On the other hand, the cyclonic circulation leads to unfavorable thermodynamical conditions (decreased maximum potential intensity and midlevel humidity) that tends to decrease the ENGPI. As a result, the DGPI and ENGPI are reversely evolved and eventually lead to their opposite correlation between TC genesis. The significant positive correlation between DGPI and TC genesis suggests a critical role in the large-scale dynamical control of the decadal variability of the WNP TC genesis. Significance Statement Tropical cyclones (TCs) account for one-third of the deaths and economic losses from weather-, climate-, and water-related disasters. Understanding variations in TC activity from the perspective of large-scale conditions is of great importance to seasonal forecasting and disaster mitigation. Here we find that two genesis potential indexes (GPIs), the Emanuel–Nolan GPI (ENGPI) and dynamic GPI (DGPI), show opposite skill in quantifying decadal variability of TC genesis in the western North Pacific (WNP). The opposite skills of the two indices arise from the opposed effects of dynamic and thermodynamic parameters on TC genesis. The result suggests a critical role of large-scale dynamic control in the decadal variability of TC genesis in the WNP.

Sensitivity of summer precipitation simulation to the physical parameterizations in WRF over the Tibetan Plateau: A case study of 2018

The Tibetan Plateau (TP) holds a pivotal position in regional and global climate, whereas the atmospheric characteristics vary greatly in the simulation with different physical parameterization schemes at the gray-zone scale (around 9 km). In this study, 24 sets of experiments were set up with different cumulus parameterization schemes (CPSs), microphysics parameterization schemes (MPSs) and planetary boundary layer parameterization schemes (PBLSs) using the WRF model driven by the ERA5 reanalysis to explore the sensitivity and try to summarize a combination of parameterization schemes suitable for the simulation over TP. Based on comparisons against in-situ observations, most CPS experiments showed large wet bias and overestimated occurrence of precipitation events, and CTL without CPS and CU6 with Tiedtke CPS show better performance at gray-zone scale. The influence of MPS and PBLS is less than CPS on the simulation, and Thompson MPS and UW PBLS showed a better performance. Most of the CPS experiments reach an earlier and more intensive peak compared to the Integrated Multi-satellite Retrievals for GPM (IMERG) version 6 satellite precipitation product, except CTL, CU11, and CU6, which show closer but weaker peaks. CPS experiments simulate more precipitation associated with water vapor transporting into TP and anomalous cyclonic circulation against CTL. The less water vapor transport from the south edge of TP in the CTL against CU14, CU16, and CU2, leads to more local precipitation. Graupel particles and the warm-rain process play a crucial role in the precipitation simulation with different MPSs. Although with less water vapor in the low atmosphere, MP2 simulates more precipitation due to more graupel particles and warm-rain processes against CTL. PBLSs have a substantial impact on latent heat, then influence evaporation and precipitation. The lower PBL height usually leads to more latent heat and more precipitation.

40-Year Statistics of Warm-Season Extreme Hourly Precipitation over Southwest China

Abstract Southwest China (SWC) possesses complicated topography with frequent geological activities, where heavy precipitation occurs frequently in warm seasons. Few previous studies on extreme precipitation were carried out at hourly scales. In this study, spatiotemporal variations of the extreme hourly precipitation (EHP) over SWC during the warm season of 1981–2020 and the involved mechanisms are investigated. Results show that the threshold and intensity of EHP present similar spatial distribution—lower (higher) in the west (east) part of SWC, while the EHP frequency is opposite. The long-term trend of EHP amount shows a more significant positive tendency than that of hourly precipitation (HP) amount due to synchronous increases in intensity and frequency. The significant increasing trend of EHP occurs in areas above 500-m terrain height, with a weak increasing trend below 500 m (e.g., Chongqing and eastern Sichuan). EHP appears mainly from June to August and exhibits a bimodal distribution in diurnal variation. The mechanism analysis demonstrates that occurrences of EHP are generally accompanied by positive anomalies of temperature, humidity, and geopotential height. Anomalous cyclonic circulation can also be found in the low-level wind field. The westward and northward extension of the western North Pacific subtropical high (WNPSH) as well as temperature rise may be the primary reason for the increase of EHP. For Chongqing and eastern Sichuan, the anticyclone circulation in low-level and the significantly weakened water vapor flux convergence cause poor moisture and dynamic conditions, inhibiting the growth of EHP. Significance Statement Heavy precipitation occurs frequently during the warm season in Southwest China (SWC), often causing severe impacts on human safety and economic property. This study analyses spatiotemporal variations of the extreme hourly precipitation (EHP) over SWC during the warm season of 1981–2020 and the involved mechanisms. The increasing trend of EHP far exceeds that of hourly precipitation (HP), especially in areas above 500 m. The westward and northward extension of the western North Pacific subtropical high (WNPSH) and temperature rise may be the main reason for the increase of EHP. For areas below 500 m (e.g., Chongqing and eastern Sichuan), poor moisture and dynamic conditions inhibited the growth of EHP.

Relationship between Summer Synoptic Circulation Patterns and Extreme Precipitation in Northern China

Synoptic circulation patterns over the midlatitudes play a pivotal role in regional precipitation changes; however, the synoptic circulation patterns over eastern Asia (35°–60° N, 105°–145° E) and their effects on extreme precipitation events in the North China Plain (NCP) and northeastern China (NEC) remain unclear. The summer daily 500 hPa geopotential height anomaly fields for 1979–2021 are classified into six synoptic circulation patterns using self-organizing map (SOM) cluster analysis. The SOM1 pattern, characterized by a high-pressure ridge over the north of eastern Asia and a trough near the Korean Peninsula, yields decreased precipitation in NEC. The SOM2 pattern reveals a robust high ridge over eastern Asia, resulting in a higher incidence of regional extreme precipitation events (REPEs) of approximately 24% in the NCP. Under the SOM3 pattern, the anomalous cyclonic circulation over eastern Asia leads to above-average precipitation in the NCP. The SOM4 pattern yields the highest incidence of REPEs in NEC, with the lowest incidence of REPEs in the NCP, as the anomalous cyclonic circulation over eastern Asia moves southeastward compared to the SOM3 pattern. The SOM5 pattern presenting an anticyclone–cyclone dipole reduces precipitation in the NCP and NEC, and the anticyclonic circulation near eastern China associated with the SOM6 pattern causes above-average precipitation in the NCP. On interannual time scales, the SOM2 pattern occurrence with an increasing trend tends to induce an increasing summer precipitation trend in the NCP. The SOM3 pattern occurrence is negatively correlated with the summer precipitation in NEC. Overall, classifying the synoptic circulation patterns helps to improve precipitation forecasting and provides insights into the synoptic circulation patterns dominating the occurrences of REPEs.

Quasi-Biweekly Oscillation of Surface Sensible Heating over the Central-Eastern Tibetan Plateau and Its Relationship with Spring Rainfall in China

Abstract This study examines the characteristics and phase evolution of the quasi-biweekly oscillation of surface sensible heating (SH) over the central-eastern Tibetan Plateau (CETP) during spring. The mechanism connecting CETP SH to spring rainfall in China on the quasi-biweekly time scale is further investigated. Results show that the dominant mode of quasi-biweekly CETP SH presents a monopole pattern, in which the peak leads the maximum of the quasi-biweekly rainfall in the middle and lower reaches of the Yangtze River (MLYR) and South China by approximately 5 and 7 days, respectively. As an upper-level Rossby wave train propagates eastward, an anomalous center of convergence moves to the CETP, which leads to a strong downdraft and reduced cloud cover. The resultant elevated shortwave radiation input and drier soil conditions are favorable for the CETP SH quasi-biweekly oscillation to enter a positive phase. When reaching its peak, the CETP SH efficiently heats the lower atmosphere, resulting in a local updraft. Due to the “SH-driven air pump” effect, abundant water vapor is transported from the oceans to China. A lower-layer southerly anomaly on the east side of the TP develops into an anomalous cyclonic circulation via the effect of topographic friction, which leads to the expansion of the positive potential vorticity anomaly and the maximum of the quasi-biweekly rainfall in the MLYR. Further southeastward propagation of the wave train leads to a shift in the rainfall anomaly center to South China. These findings suggest that the CETP monopole SH warming could be a good indicator for predicting intraseasonal variations in spring rainfall over China.

Dominant modes and mechanisms of the interannual variability of atmospheric water vapor content during winter over Central Asia

Atmospheric water vapor content (WVC)—a key component of the water cycle—plays a crucial role as an indicator of precipitation changes and directly influences precipitation in the arid areas of Central Asia. These areas are extremely sensitive to global climate change because of their dry climates and fragile water resource systems, especially during winter. This study analyzes the main interannual variation patterns of the atmospheric WVC during winter over Central Asia and reveals the possible physical mechanism behind the changes in the dominant modes (EOF1 and EOF2) of water vapor. The results show that EOF1 (49.4%)—responsible for the interannual variation of the winter atmospheric WVC in Central Asia—has a uniform monopole mode, while EOF2 (23.7%) shows a reverse north–south dipole distribution (with the boundary being at approximately 44° N). The negative geopotential height anomaly over the high latitudes of Europe and the positive geopotential height anomaly over Central Asia are the major impact factors of EOF1, whereas Central Asia has a prevalence of westerly airflow during winter. By influencing the geopotential height anomaly in Central Asia, the winter Niño 3.4 index intensifies the convergence of Atlantic Ocean moisture with tropical Indian Ocean moisture and their eastward transport. In contrast, EOF2 is mainly affected by the positive geopotential height anomaly over the high latitudes of Europe and the negative geopotential height anomaly over the middle latitudes of this region. In particular, the negative anomaly center of the geopotential height on the eastern side of Central Asia extends westward into Central Asia, resulting in reverse moisture transport by westerly and easterly airflows in the southern and northern parts of Central Asia, respectively. This anomaly is correlated with the formation of anomalous cyclonic circulation over North Africa and Europe because of the negative phase of the Arctic Oscillation, leading to prevailing southerly winds in the south of Central Asia, which correspond to abnormal vertical upward movement; however, the situation in the northern part of Central Asia is the opposite. Therefore, the distribution of winter precipitation in Central Asia shows the following pattern: more in the south and less in the north. This study provides scientific and technological support for the formation of research mechanisms and the prediction of winter precipitation in Central Asia.

Atmospheric–Oceanic Processes over the Pacific Involved in the Effects of the Indian Summer Monsoon on ENSO

Abstract This study investigates the physical mechanisms responsible for the impacts of the Indian summer monsoon (ISM) on the evolution of El Niño–Southern Oscillation (ENSO), with a focus on understanding the monsoon-induced Pacific air–sea interactive processes. An observational analysis displays that a weaker-than-normal ISM can strengthen an ongoing El Niño event and weaken a La Niña event, and conversely for a stronger-than-normal ISM. A 1000-yr output from the Community Earth System Model version 2 is capable of reproducing this observed feature and is therefore used to explore the responsible ocean–atmosphere interactive processes involved. Results show that a weak ISM can cause a cyclonic circulation over the western North Pacific via stimulating atmospheric cold Kelvin waves, and conversely for a strong ISM. The westerly (easterly) wind anomalies on the southern flank of the anomalous cyclone (anticyclone) generate eastward (westward) current anomalies in the mixed layer and thus induce anomalous warm (cold) zonal advection. Furthermore, the wind anomalies excite oceanic downwelling (upwelling) Kelvin waves, which deepen (shoal) the thermocline in the equatorial eastern Pacific and result in anomalous warm (cold) vertical advection. A quantitative mixed layer heat budget analysis demonstrates that the influence of the monsoon-induced Pacific wind anomalies on ENSO is mainly achieved by changing the zonal advective feedback and thermocline feedback. This result is confirmed by model sensitivity experiments in which additional monsoon heating or cooling anomalies are imposed over the Indian region during the developing summer of an ENSO event. Significance Statement While previous studies have reported that the Indian summer monsoon (ISM) could influence the evolution of El Niño–Southern Oscillation (ENSO), this study is aimed at understanding the physical mechanisms for the Pacific air–sea interactive processes induced by the monsoon and their impact on ENSO. It is found that a weak and strong ISM can respectively induce an anomalous cyclonic and anticyclonic circulation over the western North Pacific. The wind anomalies on the southern flank of the anomalous circulation mainly change the zonal advective feedback and the thermocline feedback to affect ENSO development. This study provides a detailed physical explanation of how the ISM influences ENSO evolution.

Influencing factors associated with the second dominant pattern of the Indian summer monsoon

AbstractThe interannual variation of the Indian summer monsoon (ISM) affects millions of people in India and the global weather and climate. The teleconnections that affect this variation are not stable. The recent four decades of the second dominant mode of ISM rainfall show a unique north–south tripole pattern, with above‐normal rainfall in the north and peninsular India sandwiching suppressed rainfall in central‐east India. The pattern relates to extending the Indo‐Pacific warm‐pool's warmer sea‐surface temperature (SST) towards the south of the equatorial eastern Indian Ocean. Most of the time, this warming and the extension of the warm‐pool's warmer SST are associated with La Niña events, which activate more in situ vigorous convection. The Rossby‐gyres generated west of the equatorial heating increase the tropospheric height over north India, shifting and strengthening the Tibetan High northwards, facilitating heavy rainfall in the north. Meanwhile, the more vigorous convection south of the equatorial eastern Indian Ocean produces compensatory subsidence over central‐east India, suppressing rainfall. The northern hemispheric Rossby‐gyres brings anomalous cyclonic circulation over peninsular India, producing excess rainfall. Also, the dipole pressure anomaly between the northwest Pacific and south tropical Indian Ocean generates anomalous lower‐level easterly winds over the Bay of Bengal. It supplies excess moisture to the north India convections. The co‐occurrence of the active Atlantic intertropical convergence zone supports this tripole rainfall pattern. This teleconnection could further be examined in climate models.

Interdecadal variation and possible causes of summer extreme precipitation over northern Xinjiang province, northwestern China

Using daily precipitation data from 25 stations, NCEP/NCAR reanalysis data, and 12 climate indices for boreal summers from 1963 to 2017, we investigate the interdecadal variation and causes of extreme precipitation in northern Xinjiang Province (NXJ), northwestern China. The summer frequency of daily precipitation extremes (DPEs), summer precipitation from DPEs, and contribution of precipitation from DPEs to total summer precipitation have increased significantly since the early 1990s, and this increase in extreme precipitation is more intense at stations in the eastern part of NXJ. Comparing the periods 1963–1988 and 1992–2017, the anomalous circulation over the Mongolian Plateau changed from cyclonic to anti-cyclonic, and the center of the anomalous cyclonic circulation over Central Asia shifted northward from the Iranian Plateau to the Aral Sea. These changes in anomalous circulation pattern induced changes in the paths of water vapor transport and the distribution of moisture convergence, which explain well the interdecadal increase in extreme precipitation and its spatial variability. Further analysis confirms evident impacts of the Indian Ocean basin mode (IOBM) and Scandinavia teleconnection pattern on the development of anomalous circulation related to the interdecadal increase in extreme precipitation. A positive IOBM favors the northward transport of moisture from the northern Indian Ocean to Xinjiang Province and the development of anomalous cyclonic circulation over Central Asia in the middle and upper troposphere. The negative phase of the Scandinavia pattern, which has prevailed since the late 1970s, and the enhanced out of phase connection between the anomalous circulation over Scandinavia Peninsula and the West Siberia during 1992–2017 favors the development of anomalous anti-cyclonic circulation over the Mongolian Plateau.

Linkage between the Asian-Pacific Oscillation and winter precipitation over southern China: CMIP6 simulation and projection

Based on the simulations of 30 CMIP6 models, this paper evaluates their performance in simulating the linkage between the winter Asian-Pacific Oscillation (APO) and precipitation over southern China (SC). Results show that 12 out of the 30 models can reproduce well the observed inverse relationship featuring a positive APO phase corresponding to a decrease in SC precipitation. Associated with the positive APO phase, an anomalous anticyclonic circulation dominates the southern part of Asia in the upper troposphere, and an anomalous cyclonic circulation prevails particularly in the lower troposphere of the South China Sea and the Malay Archipelago. Accordingly, the East Asian westerly jet (EAWJ) shifts northward, and low-level northeasterly anomalies appear over SC, which yield anomalous descending motion and water vapor flux divergence in SC, respectively, hence decreasing the in-situ precipitation. Using the ensemble of the 12 models, the future relationship between the winter APO and SC precipitation under the SSP5-8.5 scenario was further projected. The projection indicates that the APO connection with SC precipitation will still be significant, but weakened slightly, during the second half of the 21st century as compared to the present. Such a weakening may result from the weaker linkage between SC precipitation and the meridional displacement of the EAWJ.

Synergistic effect of El Niño Southern oscillation and subtropical Indian Ocean Dipole on Southern China winter precipitation

Both the El Niño Southern Oscillation (ENSO) and subtropical Indian Ocean Dipole (SIOD) have significant impacts on the winter precipitation over southern China (SC). In this study, we investigate the synergistic effect of ENSO and SIOD on southern China winter precipitation (SCWP). Our findings indicate that SCWP is closely associated with the out-of-phase configurations of ENSO and SIOD, with precipitation tending to increase (decrease) during El Niño and negative SIOD (La Niña and positive SIOD). During El Niño and negative SIOD, the El Niño leads to an anomalous anticyclone over the northwest Pacific through Walker circulation and Gill-type tropical response. The associated southwesterly winds of the anticyclone transport ample water vapor towards SC. The El Niño and negative SIOD both trigger a wave-like pattern from Europe to SC. This wave train (PC1-like wave train) propagates towards SC and induces anomalous cyclonic circulation and strong upward motion over SC, resulting in enhanced precipitation over SC. During La Niña and positive SIOD, the low-level cyclone over the northwest Pacific results from both La Niña and positive SIOD. The positive SIOD enhances La Niña-related cyclonic circulation response through cross-equator Hadley circulation, weakening the water vapor transport towards SC. The positive SIOD-related wave train (PC3-like wave train) plays the dominant role in inducing anomalous anticyclonic circulation and weak descending motion, leading to reduced precipitation over SC.

The ENSO Fingerprint on Bangladesh Summer Monsoon Rainfall

In South Asia (SA), the boreal summer monsoon (June to September; JJAS) and the El Niño-Southern Oscillation (ENSO) are connected, though different areas in SA respond differently to ENSO. In this paper, a new 41-year (1981 to 2021) high-resolution gridded rainfall dataset (ENACTS-BMD; Enhancing National Climate Services for Bangladesh Meteorological Department) is used to investigate the linkage between the Bangladesh Summer Monsoon Rainfall (BSMR) and ENSO. Observed BSMR shows a weak positive correlation (r = + 0.21, not statistically significant at the 5% level) with sea surface temperatures (SST) in the central-eastern (Niño3.4) Pacific region. Among the eight El Niño events, seven of them corresponded to above-normal BSMR. However, during the 11 La Niña events, the relationship was more varied, with above-normal BSMR occurring in seven instances. These findings highlight an asymmetric relationship between BSMR and ENSO. Furthermore, BSMR is negatively correlated (r = − 0.47 statistically significant at the 5% level) with Indian Summer Monsoon Rainfall (ISMR: 75°–85oE, 18°–30oN). The potential physical mechanism can be outlined as follows: during El Niño, the Walker circulation tends to be weakened, resulting in a weakening of the summer monsoon circulation, which in turn reduces the intensity of easterly winds along the Bangladesh Himalayan foothills. Subsequently, a lower-level anomalous cyclonic circulation is established, facilitating the convergence of moisture within the boundary layer. This, in turn, leads to intensified rainfall over Bangladesh and the surrounding regions during El Niño. Seasonal forecast models do not adequately capture BSMR and ENSO, BSMR and circulation, and BSMR and ISMR inverse correlations. While the observed BSMR-ENSO relationship is complex and teleconnections are weak, awareness of the inverse relationship with ISMR and the incorrect model behavior could be useful in the context of seasonal BSMR predictions.

The Philippine springtime (February–April) sub-seasonal rainfall extremes and extended-range forecast skill assessment using the S2S database

During the first half month of April 2022, the Philippines experienced severe disasters associated with the weak but deadly tropical storm Megi that caused 214 deaths and two sunken ships. This prompted us to investigate the extended-range prediction skill of the springtime Philippine sub-seasonal scale rainfall extremes in the subseasonal-to-seasonal (S2S) prediction database. The results suggest that the S2S models can well predict the extremity of the 2022 springtime sub-seasonal peak rainfall event (SPRE) ten days ahead. In addition to the La Niña sea surface temperature anomalies, this prolonged rainfall event, from March 26 - April 14, 2022, was associated with an anomalous cyclonic circulation straddled over the South China Sea (SCS) and the Philippine Sea and persisted for two weeks. The strong relationship between the El Niño and Southern Oscillation (ENSO) and the springtime (February–April) rainfall variability in the Philippines is clearly revealed in the analysis of 25 years of observational and hindcast data. The extremely wet SPREs tend to occur during the La Niña springs, while the extremely dry SPREs tend to occur during the El Niño springs. The Madden-Julian oscillation (MJO) and equatorial Rossby (ER) waves that are capable of modulating the sub-seasonal rainfall extremes were weak when the deadly SPRE occurred in April 2022. Thus, the extended-range forecast skill of this example can be interpreted as the baseline skill of the current S2S prediction revealed in the multi-model database. The findings suggest that the SPRE is a useful item to be included in the operational forecast as potential opportunities to harness the benefits of S2S prediction and applications.

Interdecadal Variation in Rossby Wave Source over the Tibetan Plateau and Its Impact on the East Asia Circulation Pattern during Boreal Summer

The wave activity flux representing the energy propagation direction of planetary Rossby wave generally originates from a large wave source area. This study investigates the interdecadal variability and formation mechanism of Rossby wave source over the Tibetan Plateau (TP-RWS) and its impact on the atmospheric circulation and precipitation pattern in East Asia based on the ERA-20C reanalysis dataset in summer (June–July–August) during 1900 to 2010. Results show that the region with the maximum variabilities of Rossby wave source (RWS) in the past 110 years appears over the Tibetan Plateau (TP) during boreal summer, and the TP-RWS shows prominent characteristics of interdecadal oscillation. Secondly, the TP-RWS is mainly composed of the vortex stretching term (RWS-S1) and the absolute vorticity advection term (RWS-S2). The interdecadal TP-RWS is a synergistic result of the snow cover over northwestern TP associated with the RWS-S1, and the deep convection over southeastern TP associated with the RWS-S2. Furthermore, the interdecadal TP-RWS can lead to an alternatively positive and negative pattern of geopotential height anomalies from the northwestern TP to the North Pacific, which has a great climate effect on the precipitation in Huang-huai River Basin, South Korea and Japan Island. Under the guidance of the anomalous cyclonic circulation in East Asia, the prevailing southerly and easterly winds occur over the West Pacific and the Huang-huai River Basin, which lead to the water vapor convergence and upward movement at middle and lower troposphere.