Southerly Monsoon Research Articles

Summertime secondary convection and interaction with sea‐breeze circulations

AbstractSummertime convection in the arid United Arab Emirates (UAE) is mostly confined to the eastern and northeastern sides of the country, known as primary convection. Convection can also occur in the drier central and western parts of the country; this is called secondary convection. However, the mechanisms behind the latter are still not fully understood, even though it represents a crucial water source in the hotter months. To this end, a series of real‐case and semi‐idealized numerical simulations are performed to explore the role of sea breezes emanating from the nearby water bodies in triggering convection. We found the primary convection is mostly driven by convergence of the sea breezes from the Sea of Oman and Arabian Gulf with the topography‐driven circulations at the Al Hajar mountains. When the Arabian Gulf sea breeze is removed, which is achieved by replacing the water body with flat desert land, the convection is weaker. Secondary convection is triggered by the convergence of the sea breeze from the Arabian Gulf with the circulation associated with the Arabian heat low (AHL). In the absence of the Arabian Gulf sea breeze, it develops in response to low‐level convergence within the southerly monsoon flow reinforced by the AHL's circulation. It is fed by moisture from the Arabian Sea advected by the southerly flow into the UAE enhanced by surface evaporation. The sea breeze from the Sea of Oman plays a minor role in the secondary convection. Sensitivity experiments confirmed these findings. The results presented here stress that any variation in the characteristics of the sea breezes, such as through changes in surface properties (e.g., land‐use land cover), will impact the nature and intensity of the primary and secondary convection in the UAE. However, summertime convection will continue to occur, owing to favourable dynamic and thermodynamic conditions at regional scale.

A comparison of intense rainfall characteristics and mechanisms between monsoon onset and retreat over the Yangtze River Basin

ABSTRACT The thermodynamic and convective characteristics of the seasonal progression of the monsoon over eastern China are examined, with the aim of understanding why regional heavy rainfall events (RHREs) over the Yangtze River Basin (YRB) are more frequent and intense during the monsoon progression from mid-June to mid-July than during the retreat from mid-August to mid-September. During the monsoon progression, the southerly monsoon flow at low and mid-levels intensifies and moves northward, while the Western Pacific subtropical high shifts northeastward. These changes result in enhanced moisture advection over eastern China. The stronger southerly flow brings warmer and moist air into the YRB, leading to a higher equivalent potential temperature (θE) and favouring convection. Conversely, during the monsoon retreat, the northerly flow becomes stronger and drier, causing lower θE air over the YRB. Additionally, as the Western Pacific subtropical anticyclone retreats, easterly prevailing winds prevail over eastern China, causing reduced specific humidity. The monsoon progression period exhibits higher convective available potential energy but also higher convective inhibition, which is overcome by the presence of the monsoon front providing sufficient dynamical uplift. Understanding the severity of RHREs, particularly over the YRB, is crucial for improving forecasting capabilities and reducing societal vulnerability.

Synoptic Control on the Initiation and Rainfall Characteristics of Warm‐Season MCSs Over the South China Coast

AbstractThe South China coast (SCC) experiences frequent heavy rainfall during the warm season (May–September). Objective classification analysis on 925‐hPa geopotential height shows that the majority of warm‐season precipitation (>80%) occurs under three typical synoptic patterns: the southerly monsoon pattern (P1), the southwesterly monsoon pattern (P2), and the low‐level vortex pattern (P3). Using 20 years of satellite observations and cloud tracking, this study highlights that mesoscale convective systems (MCSs) play a pivotal role in generating precipitation under all three synoptic patterns, while the initiation and rainfall characteristics of MCSs are strongly modulated by the background synoptic circulations. The diurnal MCS precipitation under P1 and P2 is predominantly influenced by land‐sea breeze circulation, which is characterized by a morning offshore propagation and an afternoon onshore propagation. The rainfall propagation speeds, however, are strongly modulated by the prevailing low‐level monsoonal flows. Under P3, MCS precipitation initiates near the coast around midnight and then propagates offshore, merging with the widespread offshore precipitation. The analysis also shows that the majority of MCSs contributing to SCC warm‐season precipitation were locally initiated. This underscores the critical role of locally initiated MCSs in driving the SCC precipitation characteristics, as opposed to propagating MCSs. Statistical correlation analysis further indicates that the precipitation area and intensity of locally initiated MCSs are closely related to the lower‐tropospheric moisture transport and the convective available potential energy over the upstream South China Sea, and the deep‐layer wind shear (from surface to 400‐hPa) over the SCC.

Causes of 2022 Pakistan flooding and its linkage with China and Europe heatwaves

In boreal summer of 2022, Pakistan experienced extremely high rainfall, resulting in severe flooding and displacing over 30 million people. At the same time, heatwaves persisted over central China and Europe. The coexistence of these extreme events suggests a possible linkage. Our analysis indicated that the record rainfall was mainly induced by compounding factors. These included (1) La Niña-induced strong anomalous easterlies over the northern Indian subcontinent, (2) intense southerlies from the Arabian Sea with an upward trend in recent decades, (3) an interaction between extratropical and tropical systems, specifically the northerly flow downstream of the Europe blocking and the southerly monsoon flow from the Arabian Sea. Wave activity flux and regression analyses unveiled a distinct stationary Rossby wave-like pattern connecting the flooding in Pakistan and heatwaves in Europe and China. This pattern, an emerging teleconnection pattern in recent decade, exhibited substantial differences from the reported teleconnection patterns. We also noted the positive feedback of the excessive Pakistan rainfall could further enhance the large-scale background flow and the heavy rainfall itself. The 2022 Pakistan flood event was an intensified manifestation of the 2010 Pakistan flood event, which was also caused by compounding factors, but occurred in a more pronounced upward trend in the both tropics and extratropics.

Impact of the Eurasian Wave Train on the Interannual Variability of Autumn Precipitation in the Central Region of China

AbstractAutumn precipitation in the central region of China (APCC) can considerably influence on agricultural production and human livelihood. With reanalysis data from 1979 to 2020, it revealed that APCC is significantly affected by the second mode of the meridional wind field at 200 hPa over the Eurasian continent on the interannual scale, which featured a quasi‐barotropic “+‐+” wave‐like pattern from Europe to China (EC pattern) in extratropical zone. When the EC pattern is in a positive phase, central China lags behind the downstream positive geopotential height (anticyclone) anomaly. In this context, the transport of moisture from the south and east is enhanced and the advancement of the southerly monsoon is blocked, providing sufficient water vapor to support the occurrence of APCC. Further diagnosis revealed that local ascending motion is strengthened owing to the warm advection induced by anomalous southerly and positive vorticity advection associated with the anomalous anticyclone over East Asia. Additionally, wave flux analysis and model simulation suggest that the EC pattern results from the propagation of stationary Rossby wave, which could be excited by dipole sea surface temperature anomalies in the North Atlantic. These discoveries will contribute to improving the predictability of APCC.

Diurnal Characteristics in Summer Water Vapor Budget and Transport over the Tibetan Plateau

Using the ERA5 reanalysis dataset during the period 1979–2019, the diurnal variation in summer water vapor budget (Bt) over the Tibetan Plateau (TP) is investigated in this study. It is found that the TP Bt shows a distinct diurnal cycle. It tends to increase in the morning, reaches a peak in the afternoon, and falls to a minimum in the early morning. The diurnal variations in four boundary water vapor budgets of the TP contribute to the growth in the TP Bt from the early morning to the afternoon, of which the western and eastern boundaries are more important. To understand the reasons for the diurnal variations in boundary water vapor budgets, the temporal evolutions of water vapor transports and relevant circulations at the four boundaries are examined. The results show that the temporal evolutions of water vapor transports and budgets at the four boundaries are essentially regulated by the changes in the orographic thermodynamic effect. Specifically, rapid and strong warming (cooling) on the TP slopes generates anomalous water vapor inputs (outputs) by anomalous upslope (downslope) flows during the daytime (nighttime). At the southern and western boundaries, apart from the terrain effects, the diurnal variation in the Indian southerly monsoon also has an effect on the changes in water vapor budgets by modulating the water vapor input towards the TP below 700 hPa. At the northern and eastern boundaries, under the orographic thermodynamic effects, low-level water vapor transports towards the TP accompanying by plateau-scale vertical circulations, exist significant diurnal variations and thereby adjust the boundary water vapor budgets. In this study, it is also found that the deviated water vapor flux vectors over the TP present a daily clockwise rotation, which mainly results from the diurnal variation in wind below 450 hPa. In addition, the largest amount of precipitation over the TP occurs 2–3 h after the Bt peak.

Threnody, 立秋, and: Calques for the Passing of the High Season: from: archipelago

Threnody, 立秋, and: Calques for the Passing of the High Seasonfrom: archipelago Heather H. Yeung (bio) A flash of insight suspended in a liquid blue Today was a yat hauh 女女, your father saysit is also 鬼門開; we had fun with the radicals; it is worth waiting to start anything new;the cabbages are hearting (would you believe it, after 3 years trying and in such a dry year) (my mam messaging to let me know it is officially autumn), soI start menstruating; start this poem; the moon will be newtomorrow which means the cycle is as anticipated; fires ravagethe grain or the pine trees and the white horses of the world oceanrun wild, gulls wheel foundationless. I know these things meanthat on this day strange liminalities will exert themselves. I do not usually dream. The telephone rings in another countryor another room and I cannot move to help you stopthe ringing I know you are there because the ringtone is familiar I know we are before ’97 because the international ringtone is familiar, familiar too the smell of the southerly monsoon season in 香港仔 against which all other strong smells cower and recede, I know2I always wish to stay in this season whose richnesses overpower my own where we can take umbrellas to the streets, riot in the puddles, this somewhere impossible The apartment is not big enough for such movements and has shrunkin time. I am standing and my toe catches a snag in the bedsiderug, so I know I am not real here or know because of the weathertoday we stay inside where there is the smell of hot rice and frying and on the countertop sits a macaque eating a peanut butter sandwich. I know the macaque is not me as here I have no tongue and know it is a peanut butter sandwich because it catches in my throat [End Page 57] you pad into the kitchen, speak my name in the versionrarely heard nowadays (all tones right) tell me to get off the damnedcountertop, finish the sandwich, stop fooling around Your brother visited with your 侄仔 who is a cheeky monkey 甩繩馬騮! your father and I are compiling a list of ‘spoken only’ phrases At what moment have I been monkey, monster, untethered horse, wereyou alive? When did red stain this conversation and the firesstart and how can we be sure today is a day of thresholds? Parched, I veer toward the bathroom, catch toein inevitable rug, cannot distinguish 媽媽 from 嫲嫲with the mirror all steamed up with the pressure built insidemy tongueless dream-ears, tone-deaf to this momentin which your eye cannot sound the differencebetween characters a part of me asks how is it then that we have passed dissident sensibilities through generations if you could not read what my 爺爺 had printed (or perhaps that is exactly how such things are passed) and the ghostsof inescapable soap operas on the cathode-ray TVwail messages, deafen with the cliché patterning of truthin the sound of a backalley printshop, the smellof soon-to-be-burned inkpressed pamphlets, the secrecyand flight, pragmatic exile, impossibility of return. on our 10 o’clock news I was shocked by the beauty of the images of the wildfires in the Mediterranean and by my reaction that it was beauty I saw or was it terror [End Page 58] I know I will never choke more than a tourist’s nicetiesin knotted mandarin, will fall back on things too ancientto know and flinch at the sense stripped from each ‘simplified’ ‘character’ I meetand flinch when the friendly academician at a collegial supper where we eat light spiced tagine only with our right hands Englishes to me ‘Chinese’ as a single nation and language tells me to go home and look up my origins to understand but cannot hear the difference between whitemaa and maa and maa andalthough I know I cannot sound the names of the poets rightI hear in the lilt of tones that are not mine the condescension of something torn from its roots...

The Characteristics of Thunderstorms and Their Lightning Activity on the Qinghai-Tibetan Plateau

This paper discusses the temporal and spatial distribution characteristics of cloud-to-ground (CG) lightning activity over the Qinghai-Tibetan Plateau (QTP) from 2009 to 2018 and their dependence on meteorological factors. It is found that (1) the number of CG flashes fluctuates, reaches a maximum in 2014, and then gradually decreases. The main active period of CG lightning is from June to September each year, after which it decreases rapidly. CG lightning is mainly distributed in the valley areas at around 4800 m above sea level at Lhasa, Nagqu, and Chamdo, and there are differences in the characteristics of CG activity in these three areas. The peak of daily CG lightning occurs at 1000 UTC, and the lowest value is at 0400 UTC. The distribution of CG lightning in all seasons has obvious differences in peak time and the proportion of positive CG (+CG) lightning, with the ratio of +CG lightning to total CG lightning flashes in spring and autumn exceeding 50%. (2) The ratio of +CG lightning to total CG lightning flashes over the QTP is influenced by a combination of thermodynamic and microphysical factors. Over the QTP, greater vertical wind shear leads to the movement of upper positive charges and promotes the occurrence of +CG lightning. Also, the higher total column liquid water content implies higher cloud water content in the warm-cloud region, and the higher cloud-base height implies a thicker warm-cloud region, which is not conducive to the occurrence of +CG lightning. (3) During high-value years (in this study, 2010, 2012, 2014, and 2016), the midlatitude (30°N–60°N) high pressure is strong and the plateau is situated at the intersection of the East Asian and South Asian monsoons and the cold air from the northwest, which strengthens the water vapor convergence and increases the frequency of thunderstorms. When the plateau is under the control of the southerly monsoon from June to September every year, its atmosphere is full of water vapor and lightning activity is accordingly high, with the proportion of +CG lightning being about 10%. Meanwhile, in the remaining months, when controlled by the westerly wind belt, the plateau’s water vapor condition is poor, the level of lightning activity weakens, and the proportion of +CG lightning gradually increases to more than 50%.

Formation and Breakdown of an Offshore Summer Cold-Water Zone and Its Effect on Phytoplankton

Estuarine plume frontal zones typically form a vertical two-layer structure with low-salinity and a high-temperature plume during the summertime. However, two field surveys in the Changjiang River Estuary and its adjacent shelf waters identified a significant surface cold-water zone (CWZ) formation in the summers of 2014 and 2015. The sea surface temperature of the CWZ was 4°C lower than the multi-year summer average. Satellite images showed that the CWZ mainly appeared in the Yangtze Shoal during the periods of July 1–17, 2014, and July 3–19, 2015. A three-dimensional physical-biogeochemical coupled model was used to explore the formation mechanism of the CWZ. Our investigation revealed that an uncharacteristic northerly wind during the southerly monsoon resulted in a significant onshore retreat of the plume front. Vertical tidal mixing is stronger than the decreased stratification in the former plume-covered region, which resulted in the formation of the CWZ. This process was accompanied by relatively lower net heat flux, which also promoted CWZ formation. The formation of CWZ had a strong ecological impact; enhanced vertical mixing transported nutrients from the lower layer to the surface column, relaxing the CWZ’s phosphate limitation. CWZ formation also increased the depth of the mixed layer and turbidity level in the water column, forming a temporary light limitation in the center. At the margin of the CWZ, it formed a patch with a high concentration of chlorophyll a. The underwater light was sufficient once the plume was restored and the CWZ was stratified again, and the phytoplankton grew rapidly in the center of the CWZ.

Diurnal Variations of Southerly Monsoon Surge and Their Impacts on East Asian Summer Rainfall

AbstractMonsoon southerlies can be particularly active for days and produce substantial rainfall over East Asia. These multiday episodes of southerly monsoon surge may exhibit distinct diurnal variations due to regional forcings under given large-scale conditions. This study categorizes the southerly surges into two types with different wind diurnal variations to clarify their influence on rainfall over East Asia. In the summers of 1998–2019, there are 63 episodes of southerly surges with large wind diurnal cycles and 55 episodes with small diurnal cycles. The first type of southerly surges usually occurs with anomalous low-level warming over southeastern China related to the westward extension of the western Pacific subtropical high. The second type of southerly surges instead occurs with anomalous cooling due to the deepened midlatitude trough. They thus represent the different mechanisms downscaling from large-scale conditions to regional diurnal forcings. After the onset of the first type, the intensified monsoon southerlies at night lead to the northward displacement of large-scale ascent and northward water vapor transport with warm moist energy. The monsoon rainband tends to move to the north of 35°N with a robust response in precipitation systems, especially in the meso-α-scale rain events from midnight to morning. As a comparison, the rainband stays at 30°–35°N after the onset of the second type when the strengthened large-scale ascent and water vapor convergence are located relatively south. These differences between the two types of southerly monsoon surges highlight that the multiday large-scale conditions interact with subdaily regional forcings and greatly regulate the detailed evolution of summer rainband over East Asia.

Spatially differentiated changes in regional climate and underlying drivers in southwestern China

The climate in Southwest China are predominantly under the influences of three contrasting climate systems, namely the East Asian monsoon, the South Asian monsoon, and the westerlies. However, it is unclear if the diversified climate systems, in combination with the complex terrain and varying vegetation types, would result in contrasting patterns of changes in climate across the region. Based on the CRU TS data for the period 1901−2017, we examined the spatiotemporal characteristics of the regional climate, and identified types of climate change patterns and drivers. Overall, the region experienced significant increases in annual mean temperature during 1901−2017, with occurrence of a significant turning point in 1954 for a more pronounced warming (0.16 °C/10 a). The annual precipitation fluctuated greatly over the study period without apparent trend, albeit the occurrence of a significant turning point in 1928 for a slight increase in the later period (1.19 mm/10 a). Spatially the multi-year averages of selective climate variables during 1901–2017 displayed a trend of decreases from southeast to northwest, but with increasing variability. We identified five major climate change types across the study region, including warmer (T+), drier (P−), warmer-drier (T+P−), warmer-wetter (T+P+), and no significant changes (NSC). The type T+P+ mainly occurred in the western parts over the plateau sub-frigid semiarid ecozone (77.0%) and the plateau sub-frigid semihumid ecozone (19.9%). The central parts of the region are characterized by the type T+, corresponding to six ecozones, including the mid-subtropical humid ecozone (33.1%), the plateau temperate humid-semihumid ecozone (28.8%), the plateau sub-rigid semihumid ecozone (9.5%), the southern subtropical humid ecozone (8.1%), the plateau sub-frigid arid ecozone (7.3%), and the plateau temperate semiarid ecozone (6.6%). No significant change in climate was detected for the eastern parts over the mid-subtropical humid ecozone (67.3%), the plateau temperate humid and semihumid ecozone (19.5%) and the plateau sub-frigid semihumid ecozone (8.8%). The types P− and T+P− together accounted for less than 5% of the entire study region, which predominantly occurred in central Yunnan-Guizhou Plateau and south of the southeastern Xizang, corresponding predominantly to the mid-subtropical humid ecozone. Across the region and within the zonal climate change types, vegetation and topography both played a significant role in determining the climate variability and magnitude of changes. Our results suggest that the southwestern China experienced intensified influences of the southeasterly monsoon and the southerly monsoon in the regional climate, while the westerly alpine influences subsided; topography and vegetation affected the magnitudes of the directional changes in climate at a local scale.

Physical Controls of Summer Variations in Bottom Layer Oxygen Concentrations in the Coastal Hypoxic Region off the Northeastern Shandong Peninsula in the Yellow Sea

AbstractHypoxia has been observed worldwide and causes great damage to marine ecosystems. In the northeastern mariculture areas of the Shandong Peninsula, summer oxygen depletion and even hypoxia have been observed in the bottom layer, but the underlying dynamics are still poorly understood. In the current study, the physical controls of variations in bottom layer oxygen concentrations in summer and their interannual variations in the northeastern mariculture areas of the Shandong Peninsula were first investigated using in situ observations collected during 2016–2020. Over the past 5 years, the dissolved oxygen (DO) concentrations of the bottom layer have ranged from 0.8 to 13.0 mg L−1 and exhibited a robust annual cycle, with the lowest value occurring in August. Hypoxia events covering several days were observed intermittently in the summers of 2016 and 2017. Summer oxygen depletion was caused by the combined effects of ocean stratification and enhanced biogeochemical oxygen consumption. At the interannual time scale, stratification and bottom layer DO concentrations showed synchronous variations and both peaked in 2018. The underlying mechanism was related to bottom layer currents, which were created by southerly monsoon winds. Strong/weak southerly surface winds induced a strong/weak bottom shoreward advection of cold, oxygen‐rich water and resulted in strong/weak stratification and high/low DO concentrations. The results provide a good reference for coastal mariculture management in the Shandong Province and enrich our understanding of the global distribution of coastal hypoxia.

A Case Study on the Variability of Summer Water Properties in the Southeastern Yellow Sea Based on the Hydrological Data from 1995 to 2019

The long-term variability of the summer water properties in the southeastern Yellow Sea is described using the hydrological data. The results indicate warming trend in the upper layer and cooling trend in the deeper layer resulting in a strengthen thermocline. While, the mean and the core temperatures in the southeastern Yellow Sea Cold Water Mass both tend to rise slowly, which are previously thought to be fallen. At the deep layer (below 30 m depth), the temperature cooling trend in summer is even stronger than that in winter, which contradict to the stronger surface heat flux in summer. This study proposes that the strengthen thermocline during warming seasons (spring and summer) prevents the heat transferring from surface to the deep layer across isopycnal lines. Furthermore, combined with the metrological data, the strengthen thermocline induces stronger southward tidal residual current at the deep layer, which facilitates the enhanced complementary wind-driven current induced by the southerly monsoon. Thus, the southward tidal induced residual current effectively contributes to the southward motion of the colder water from the northern area. The declined trending of salinity due to the increase of the Changjiang River discharge can be conducive to the strengthen summer thermocline.

Stable isotopes of precipitation in Nepal Himalaya highlight the topographic influence on moisture transport

The Nepal Himalaya is a transition zone for pollutants transport from the South Asia onto the Tibetan Plateau, with the moist convection identified as dominating the chemical composition that would be transported onto the Plateau. Yet little is known regarding the atmospheric water vapor transport over this region, and how the summer monsoon flows would interact with local topography, with traces detectable from ground sample and observations. The stable oxygen and hydrogen isotopes in precipitation (δ18Op and δDp) are important tracers for understanding various hydrological processes. This study reports, for the first time, stable isotope data in daily precipitation at five stations along a north-south transect on the southern slope of Himalaya, with altitudes ranging from 102 m to 5050 m above sea level (a.s.l). The altitude effect of δ18Op across the region of interest is apparent with an altitudinal lapse rate as 0.17‰/100 m. Seasonal trends in δ18Op and δDp show low values during June–September and higher values during October–May at all of the study sites, with the noticeable decrease of δ18Op during summer indicative of the presence of the Indian monsoon over the eastern Nepal Himalaya. The average d-excess values in the summer precipitation increase with altitudes til 3800 m a.s.l before decreasing afterward to 5050 m a.s.l. This corroborates with previous meteorological discovery in the region, and probably confirms the replacement of prevailing westerly by the southerly monsoon flows during summer monsoon season.

Resolution of Summertime East Asian Pressure Pattern and Southerly Monsoon Wind in CMIP5 Multi-model Future Projections

Resolution of Summertime East Asian Pressure Pattern and Southerly Monsoon Wind in CMIP5 Multi-model Future Projections

Contrasting frontal and warm-sector heavy rainfalls over South China during the early-summer rainy season

Heavy rainfalls occur frequently during early summer (April–June) over South China, causing severe floods. Using 12 years of hourly rain-gauge data, we identify a large number of regional heavy rainfall events and categorize them into two major types based on the strength of synoptic forcing. It is shown that frontal heavy rainfalls associated with fronts or shear lines mainly occurs over inland regions, whereas warm-sector heavy rainfall under weakly forced synoptic environment is observed in coastal areas. The rainfall maxima of both types tend to form over the low-lying plains or sea surfaces adjacent to windward mountains. Frontal heavy rainfall usually propagates southwards with a cold front, while warm-sector events move relatively slowly and produce coherent patterns of rainfall. The occurrence of warm-sector rainfall increases markedly from April to June in a close association with the onset of summer monsoon, in contrast to frontal rainfall with less monthly variation. Warm-sector rainfall also exhibits pronounced diurnal variation, with a peak in the early morning, as a result of intensified convergence between nocturnal low-level jets in southerly monsoon and land breezes. In contrast, frontal rainfall has an afternoon peak, due to the arrival of eastward-propagating rain systems and daytime heating on land. As frontal (warm-sector) heavy rainfall has relatively high (low) predictability in numerical models, understanding their occurrence and formation is of benefit to operational forecasting and decision-making for disaster prevention.

Decadal Variations of the East Asian Summer Monsoon Forced by the 11-Year Insolation Cycle

AbstractStatistical evidence suggests that solar activity may affect the atmospheric circulation over East Asia (EA), but the way in which the 11-yr solar radiation cycle affects the East Asian summer monsoon (EASM) remains unexplained. Based on one control experiment and four solar-only forcing experiments performed during the Community Earth System Model–Last Millennium Ensemble (CESM-LME) model project, we explore the potential impacts of the 11-yr solar cycle on EASM variability and the physical processes through which solar forcing influences EASM decadal variability. The model results show that the warm season [May–September (MJJAS)] mean precipitation over EA exhibits significant decadal variation with a “northern wet–southern dry” pattern during peak years in the strong 11-yr solar cycle epoch (AD 900–1285), which is in contrast to the absence of decadal signals during the weak 11-yr solar cycle epoch (AD 1400–1535). For the four-member ensemble averaged solar-only forcing experiment, the summer mean precipitation over northern EA is significantly correlated with the solar forcing (r = 0.414, n = 68, p < 0.05) on a decadal time scale during the strong cycle epoch, whereas there is no statistical link between the EASM and solar activity during the weak cycle epoch (r = 0.002, n = 24). A strong, 11-yr solar cycle is also shown to excite an anomalous sea surface temperature (SST) pattern that resembles a cool Pacific decadal oscillation (PDO) phase, which has a significant 11-yr periodicity. The associated anomalous North Pacific anticyclone dominates the entire extratropical North Pacific and enhances the southerly monsoon over EA, which results in abundant rainfall over northern EA. We argue that the 11-yr solar cycle affects the EASM decadal variation through excitation of a coupled decadal mode in the Asia–North Pacific region.

Effects of the Tibetan Plateau and its second staircase terrain on rainstorms over North China: From the perspective of water vapour transport

The effects of the Tibetan Plateau (TP) and its second staircase terrain (i.e., the extension of the TP) on summer rainstorms over North China (NC) from the perspective of large‐scale water vapour transport were investigated based on the frequency of summer rainstorms from 70 observational stations in NC and National Centers for Environmental Prediction reanalysis data from 1961 to 2010. Seven rainstorm cases over NC during 1981–2016 acquired from ERA‐Interim reanalysis data and observational hourly precipitation data were selected. Two water vapour transport channels provided favourable backgrounds for rainstorms over NC. One channel flowed along the northern edge of the TP via westerlies. The other channel flowed along the eastern edges of the TP and its second staircase terrain accompanied with southerly monsoon airflow, with water vapour sources from the Bay of Bengal, South China Sea, and western Pacific. The abundant water vapour that was transported along the two channels not only offered a favourable moisture background for rainstorms over NC but also resulted in the development of water vapour flux vortexes that supplied convergence conditions for rainstorms in situ. At low levels, water vapour transport mainly flowed along the eastern edge of the TP second staircase terrain. The results from the Weather and Research Forecasting model simulation of a randomly selected rainstorm case originating from the TP to NC confirmed the vital effect of the TP second staircase terrain on rainstorms over NC accompanied with water vapour transport.

Key Dynamical Factors Driving the Kuroshio Subsurface Water to Reach the Zhejiang Coastal Area

AbstractObservations indicate a seasonal variation of the Kuroshio subsurface water (KSSW) intrusion into the coastal region off the Zhejiang Province of China. The observed temperature and salinity distributions suggest a stronger intrusion in June and weaker intrusion in December. A hydrodynamic model well reproduces the seasonal variation of temperature and salinity characteristics and the current structure over the continental shelf. A passive tracer in the subsurface layer is used to investigate the movement of the KSSW. Tracer experiments confirm the seasonal variation of the KSSW intrusion, showing that the intrusion is stronger in summer and weaker in autumn and winter. Moreover, tracer concentrations suggest that in general it takes at least 1.5 months for the KSSW from east of Taiwan to reach the Zhejiang coastal area. Analyses of the momentum balances indicate that the KSSW's northward movement is regulated by the geostrophic current, while the shoreward intrusion results from the bottom Ekman effect. The seasonal changes in pressure gradients and vertical eddy viscosities contribute to the KSSW intrusion into the Zhejiang coastal area in June rather than in December. Sensitivity experiments show that the seasonal variation of the KSSW intrusion is mainly induced by the different wind stresses in summer and winter. The stronger southerly monsoon, the weaker and warmer Taiwan Warm Current, and the stronger and colder Kuroshio Current are favorable to the KSSW intrusion.

Upwelling characteristics during El Nino 2015 in Maluku Sea

Investigation of upwelling characteristics in Banggai-Maluku Sea (BMS) during super El Nino 2015 event and its interannual variation related to ENSO used validated INDESO model output and satellite imagery datasets between 2008 and 2015. The result shows the upwelling episode occurs during the Southeast Monsoon period from June to October, and its maximum in September. It is mainly forced by fully developed southerly monsoon winds, dragging surface water northward similar to the wind direction since the effect of Coriolis vanishes near the equator. Warm surface water in the center upwelling is replaced by upwelled colder water from about 60 m, which evolves from southern to northern region then curving to northeastern due to the boundary of Sulawesi mainland. Upwelling characteristics in 2015 event is indicated by drastic changes in ocean-atmosphere variables, such as increased northward meridional winds/infrared heat flux/transport volume, decreased sensible heat flux/mixed layer depth, upwelled 26°C isotherm from 60 m depth to surface, and blooming surface chlorophyll-a. Between 2008 and 2015, upwelling intensity in 2015 was the most powerful (index: -5.35), in contrast to upwelling intensity during 2010 strong La Nina event (+0.27). Hence, ENSO influences significantly on fluctuation of upwelling intensity in BMS.