Subtropical East Asia Research Articles

Regional Characteristics of Cloud Radiative Effects before and after the South China Sea Summer Monsoon Onset

The South China Sea summer monsoon (SCSSM) onset is characterized by rapid thermodynamical changes in the atmosphere that are critical to regional weather and climate processes. So far, few studies have focused on the changes in the associated cloud and radiative features. This study investigates spatiotemporal characteristics of top-of-atmosphere (TOA) cloud radiative effects (CREs) before and after the SCSSM onset over the South China Sea (SCS) and South China (SC), based on the 2001–2016 Clouds and the Earth’s Radiant Energy System (CERES) Energy Balanced and Filled (EBAF) satellite data and ERA-Interim reanalysis data. Before the SCSSM onset, strong net CRE (NCRE) dominated by its cooling shortwave component occurs over SC, while descending motion and weak NCRE prevail over the SCS. In the SCSSM onset pentad, convection, high clouds, and longwave and shortwave CREs (LWCRE and SWCRE) abruptly increase over the southern and central SCS, and their high-value centers subsequently move northeastward and are strongly affected by the western Pacific subtropical high. The strong offset between LWCRE and SWCRE enables the NCRE intensity (TOA radiation budget) to be quite small (large) between the SCS and the western North Pacific after the SCSSM onset. In contrast, low-middle-level clouds and strong cooling SWCRE remain over SC after the SCSSM onset, but the increasing high clouds and LWCRE weaken (intensify) the regional NCRE (TOA radiation budget) intensity. These marked latitudinal differences in CREs between the SCS and SC primarily arise from their respective dominant cloud types and circulation conditions, which manifest the differences between the tropical SCSSM and subtropical East Asian monsoon processes. The results indicate that regional cloud fractions and CREs before and after the SCSSM onset are strongly modulated by quickly changed large-scale circulation over the East Asian monsoon regions, and the spatiotemporal variation of CREs is a response to the monsoonal circulation adjustment to a large extent.

Development of microsatellite markers for Diaphanosoma dubium (Crustacea, Cladocera) and application to seasonal population dynamics

Diaphanosoma, the “tropical Daphnia”, is common and ubiquitous in South China. Like other ctenopods, Diaphanosoma has a reproductive mode similar to Daphnia’s, but its resting eggs are rarely observed and lack an ephippium. With limited dispersal and reduced buffer effect from resting egg banks, Diaphanosoma is expected to have a population genetic structure different from that of temperate Daphnia. To facilitate genetic comparison, we developed microsatellite markers using next-generation sequencing for the most common species in tropical and subtropical East Asia, Diaphanosoma dubium. Thirty-one polymorphic microsatellite markers were obtained, and 29 of them were efficient for the congeneric species D. excisum, D. orghidani, D. mongolianum and D. chankensis. The markers allowed intra- and interspecific genetic analysis, including population structure, hybridization and introgression. We used 11 selected microsatellite markers to analyze spatial and temporal heterogeneity of genetic diversity in four (sub)tropical D. dubium populations from two large reservoirs and two temporary ponds. In contrast to temperate Cladocera, higher genetic diversity in summer rather than in spring suggested weak contribution from resting eggs in spring. Clustering of DAPC and STRUCTURE analyses indicated a clear-cut genetic structure in the four populations. Variation partitioning revealed that water storage and depth were key factors in genetic differentiation. Within large reservoirs, we detected backward (reversing time) gene flow from resting egg banks. We conclude that resting eggs have an effective contribution to the genetic diversity in large water bodies during growing seasons and that large water bodies can host higher genetic diversity in summer due to environmental heterogeneity and high carrying capacity. Spatial and temporal heterogeneity of genetic diversity detected by our microsatellite markers showed the newly developed markers can be applied for further study of populations of D. dubium and other species of Diaphanosoma at a contemporary scale.

The Potential of Using Tree-Ring Chronology from the Southern Coast of Korea to Reconstruct the Climate of Subtropical Western North Pacific: A Pilot Study

Tree-ring width (TRW) chronologies have successfully been used as climate proxies to infer climate variabilities over the past hundreds to thousands of years worldwide beyond observational records. However, these data are scarce over parts of subtropical East Asia, and especially over the Korean Peninsula. In this pilot study, Korean red pine (Pinus densiflora Siebold and Zucc.) TRW chronologies from Mt. Mudeung and Mt. Wolchul, South Korea, were developed, and their local- to large-scale climatic responses were investigated. Mt. Mudeung TRW had a positive association with local temperature in the preceding December and April. Mt. Wolchul TRW had a positive association with local temperature in the preceding December and most of the early summer to autumn months, and with local precipitation in February and October. On a large scale, both TRWs retained meaningful temperature and monsoon precipitation signals over East Asia and sea surface temperature signals over the Western North Pacific. The results suggest that the subtropical trees from South Korea can be used to infer past long-term climate variability at both local and large scales over East Asia and the Western North Pacific, such as the East Asian summer monsoon, the Kuroshio Current, the Western North Pacific Subtropical High, and El Niño–Southern Oscillation.

Journey from the West: first record of the Indian Ocean pilumnid crab Pilumnopeus convexus (Maccagno, 1936) (Decapoda: Brachyura: Pilumnidae) in Hong Kong

Abstract The pilumnid crab Pilumnopeus convexus (Maccagno, 1936), native to the Arabian Sea and western Indian Ocean, has been discovered among intertidal fouling communities along shores of Hong Kong. The species was first discovered in 2011 and is believed to have become established by around 2016. Material from local sites was compared with specimens from Iran and Pakistan, and the identification was supported by molecular analyses using mtDNA COI gene sequences. This species is not yet known from East and Southeast Asia, and was probably transported to Hong Kong by international maritime shipping, likely in the ballast water or on the hulls of ships, and subsequently survived as a non-indigenous species (NIS). This represents the first case of west-to-east invasion of brachyuran crabs from the Indian Ocean reaching sub-tropical East Asia. Past studies on the local intertidal fauna, and documentations of NIS, are briefly discussed.

A Bridging Role of Winter Snow over Northern China and Southern Mongolia in Linking the East Asian Winter and Summer Monsoons

AbstractEurasian snow, one of the most important factors that influence the Asian monsoons, has long been viewed as a useful predictor for seasonal monsoon prediction. In this study, observations and model simulations are used to demonstrate a bridging role of the winter snow anomaly over northern China and southern Mongolia (NCSM) in the relationship between the East Asian winter monsoon (EAWM) and the East Asian summer monsoon (EASM). Enhanced snow in NCSM results in local surface and tropospheric cooling, strengthening the EAWM through cold-air intrusion induced by northerly wind anomalies. In turn, the stronger EAWM provides a favorable condition for enhanced snowfall over East Asia to the south, indicating an active snow–EAWM interaction. The continental cooling could be maintained until summer due to the memory effect of snowmelt and moistening as well as the snow–monsoon interaction in the spring, causing changes in the meridional temperature gradient and associated upper-level westerlies in the summer. The interaction between the strengthened westerlies over the northern Tibetan Plateau and the topography of the plateau could lead to anomalous downstream convergence and compensating divergence to the south. Therefore, anomalous cyclonic circulation and increased rainfall occur over northeastern China and the Korean Peninsula, but anticyclonic circulation and decreased rainfall appear over the subtropical East Asia–Pacific region. Moreover, limited analysis shows that, compared to sea surface temperature feedback, the direct impact of snow anomaly on the EAWM–EASM connection seems more important.

The East Asian Subtropical Jet Stream and Atlantic Tropical Cyclones

AbstractAtlantic tropical cyclones (TCs) can cause significant societal and economic impacts, as 2019's Dorian serves to remind us of these storms' destructiveness. Decades of effort to understand and predict Atlantic TC activity have improved seasonal forecast skill, but large uncertainties still remain, in part due to an incomplete understanding of the drivers of TC variability. Here we identify an association between the East Asian Subtropical Jet Stream (EASJ) during July–October and the frequency of Atlantic TCs (wind speed ≥34 knot) and hurricanes (wind speed ≥64 knot) during August–November based on observations for 1980–2018. This strong association is tied to the impacts of EASJ on a stationary Rossby wave train emanating from East Asia and the tropical Pacific to the North Atlantic, leading to changes in vertical wind shear in the Atlantic Main Development Region (80–20°W, 10–20°N).

Interdecadal variation in the frequency of extreme hot events in Northeast China and the possible mechanism

Based on the daily maximum temperature data from Chinese metrological stations, the frequency of extreme hot event (EHE) in Northeast China (NEC) over the period from 1960 to 2018 is investigated. The results show that the EHE frequency in summer (June and July) experienced a significant interdecadal increase around the mid-1990s. The analysis of the possible mechanism indicates that the interdecadal strength of the summer geopotential height in the mid-to-upper troposphere over the Mongolian region could be directly responsible for the interdecadal increase in the NEC EHE frequency. The enhanced geopotential height can weaken the subtropical East Asian westerly jet and lead to anomalous convergence and subsidence over NEC. Consequently, there were less cloud cover, favoring more incoming short-wave solar radiation to warm the land surface of NEC. Under such a background, there are more outgoing sensible heat, latent heat, and long-wave radiation from the land surface to warm the overlying atmosphere, favoring more EHE frequency over NEC. Further analysis indicates that the interdecadal variation in the summer geopotential height over the Mongolian region is related to a barotropic stationary Rossby wave train at the midlatitude from the North Atlantic to East Asia. Dynamic diagnosis and numerical simulation both suggest that the Atlantic Multidecadal Oscillation (AMO) can excite the pronounced Rossby wave train and further influence the interdecadal change in the geopotential height over the Mongolian region, finally contributing to the interdecadal increase in the NEC EHE frequency around the mid-1990s.

Impact of the September Silk Road Pattern on the South China Sea summer monsoon withdrawal

AbstractThe Silk Road Pattern (SRP) is the dominant teleconnection in the upper troposphere and is closely linked to the Asian summer monsoon in both India and subtropical East Asia. This study reveals a close interannual relationship between the SRP in September and the South China Sea (SCS) summer monsoon (SCSSM) withdrawal. During positive phases of the September SRP (characterized by negative geopotential height anomalies over eastern Europe and central China and positive geopotential height anomalies over western Europe, western and central Asia, and Northeast Asia), withdrawal of the SCSSM tends to be later than normal. The September SRP could impact the SCSSM withdrawal via modulating atmospheric circulation anomalies over the SCS. In particular, the notable low‐level cyclonic anomaly over central China related to the positive phase of the September SRP induces significant southwesterly wind anomalies over the northern SCS, which favour a late withdrawal of the SCSSM. This study suggests that besides the tropical factors that have been identified by previous studies, the climate systems over mid‐latitudes may also play an important role in contributing to interannual variability of the SCSSM withdrawal.

Juxtaposition of Western Pacific Subtropical High on Asian Summer Monsoon Shapes Subtropical East Asian Precipitation

AbstractIncreasing lines of evidence question the homogenous response of Asian Summer Monsoon (ASM) precipitation patterns, requiring rethinking of the forcing mechanisms. Here we show a ~15,000‐year quantitative precipitation history based on well‐dated lake levels at Lake Chenghai, subtropical China. Lake levels and the inferred precipitation were high during the Bølling‐Allerød, early and late Holocene, but low during the middle Holocene. The orbital scale precipitation trend is out of phase with boreal summer insolation, the later has been widely suggested as the driver of ASM precipitation. Lake Chenghai long‐term lake levels are synchronous with trends in tropical Pacific sea surface temperatures, the related zonal sea surface temperature gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high, which is juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia.

Wetland expansion on the continental shelf of the northern South China Sea during deglacial sea level rise

To identify environmental causes for past changes in vegetation in subtropical East Asia, we present carbon isotope compositions of plant-wax n-alkanes and provide estimates of the C4-plant contribution across the past four glacial terminations and interglacials, based on cores recovered from the northern South China Sea. Our results show a comparable C4-plant contribution between the Last Glacial Maximum (LGM) and the Holocene. An increase of the C4-plant contribution by 15–20% is found for Terminations IV, II and I relative to subsequent interglacial peaks, coeval with an expansion of Cyperaceae and Poaceae. In contrast, Termination V reveals a lower C4-plant contribution than Marine Isotope Stage (MIS) 11c. The data exhibit a long-term trend, with a stepwise increase of the C4-plant contribution across interglacials MIS 11c, 9e, 7e and 1. We suggest that no substantial changes in humidity levels over glacial-interglacial cycles occurred facilitating a similar C3/C4-plant ratio for the LGM and the Holocene. Instead, deglacial sea-level rises caused an extensive development of floodplains and wetlands on the exposed continental shelf, providing habitats for the spread of C4 sedges and grasses. The progressive subsidence of Chinese coastal areas and the broadening of the continental shelf over the late Quaternary explains the nearly absence of C4 plant occurrence during Termination V and a gradual increase of the C4-plant contribution across interglacial peaks. Taken together, changes in coastal environments should be considered when interpreting marine-based vegetation reconstructions from subtropical Asia.

Climate‐Dependent Sediment Composition and Transport of Mountainous Rivers in Tectonically Stable, Subtropical East Asia

AbstractWhile mountainous rivers in subtropical East Asia deliver a disproportionately large amount of sediments to the global ocean, the controlling mechanisms for sediment supply and transport remain understudied. Here we target a mesoscale tectonically stable mountainous river, the Minjiang River, in southeast China. We present petrography, heavy minerals, and Sr‐Nd isotopic data from suspended particulate matter (SPM) and riverbed sediments to characterize sediment‐climate feedback processes. Results show Sr‐Nd isotopic compositions of the SPM vary seasonally, corresponding well with the spatiotemporal variations of precipitation. River sands display low compositional and textural maturity and represent first cycle‐dominated and fast‐transported detritus. Provenance analysis suggests prominent contributions of upstream Proterozoic‐Paleozoic rocks to downstream SPM and riverbed sediments. We propose that climate‐induced hydrological variations exert a major control on sediment supply, transport, and compositions. Our study highlights the crucial role of climate in driving physical erosion of mountains in tectonically stable, subtropical East Asia.

Seasonal geochemical heterogeneity of sediments from a subtropical mountainous river in SE China

The Zhe-Min mountainous rivers in SE China are characterized by stable tectonics, small–mesoscale drainage basins, relatively homogenous bedrocks, dense vegetation cover and high rainfall conditions. These rivers deliver loads of sediments and nutrients to the oceans and always indicate considerably high variability of sediment flux on short-term timescales. However, short-term sediment compositional variations and transport mechanism by these mountainous rivers remain poorly understood. Here we target the Minjiang River, use mineralogical and trace element geochemical data of suspended particulate matters (SPMs) within a one-year hydrological cycle (from October 2016 to October 2017), riverbed sediments (<0.063 mm fractions) and SPMs from the mainstream and major tributaries, to investigate their provenance and transport process. The results indicate that the SPMs in different seasons have strong seasonal geochemical heterogeneity, with highly positive correlations between several trace element ratios (e.g. La(CN)/Yb(CN) and Th/Sc) and runoff of the river. Riverbed sediments and SPMs collected in pairs have distinct geochemical compositions. The riverbed sediments have relatively low La(CN)/Yb(CN) (ca. 10–15) and Th/Sc (ca. 2–5) ratios, whereas the SPMs have La(CN)/Yb(CN) and Th/Sc ratio ranges of ca. 14–17 and ca. 4–7, respectively. These findings suggest a predominant hydrologic control on the Minjiang River sediment transport and geochemical compositions on a seasonal timescale. We conclude that exposed riverbed sediments are probably selectively recycled in dry-season sediment transport, whereas the flood-season SPMs are less mixed with riverbed sediments and are likely dominated by particulates directly from those tributaries and the upper mainstream. We favor that the Zhe-Min mountainous rivers are featured by rapid sediment delivery in response to hydroclimatic changes and have similar sediment transport behaviors with those small mountainous rivers in tectonically active regions of the subtropical East Asia (e.g. Taiwan). This case study also highlights the importance of multi-spatio-temporal scale, heterogeneous river input of chemical elements to the oceans.

The Relationship Between Abnormal Meiyu and Medium-Term Scale Wave Perturbation Energy Propagation Along the East Asian Subtropical Westerly Jet

The East Asian subtropical westerly jet (EASWJ) is one of the most important factors modulating the Meiyu rainfall in the Yangtze-Huaihe River Basin, China. This article analyzed periods of the medium-term EASWJ variation, wave packet distribution and energy propagation of Rossby waves along the EASWJ during Meiyu season, and investigated their possible influence on abnormal Meiyu rain. The results showed that during the medium-term scale atmospheric dynamic process, the evolution of the EASWJ in Meiyu season was mainly characterized by the changes of 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves. The strong perturbation wave packet and energy propagation of the 3-8 d synoptic-scale and 10-15 d low-frequency Rossby waves are mostly concentrated in the East Asian region of 90°-150°E, where the two wave trains of perturbation wave packets and wave-activity flux divergence coexist in zonal and meridional directions, and converge on the EASWJ. Besides, the wave trains of perturbation wave packet and wave-activity flux divergence in wet Meiyu years are more systematically westward than those in dry Meiyu years, and they are shown in the inverse phases between each other. In wet (dry) Meiyu year, the perturbation wave packet high-value area of the 10-15 d low-frequency variability is located between the Aral Sea and the Lake Balkhash (in the northeastern part of China), while over eastern China the wave-activity flux is convergent and strong (divergent and weak), and the high-level jets are strong and southward (weak and northward). Because of the coupling of high and low level atmosphere and high-level strong (weak) divergence on the south side of the jet over the Yangtze-Huaihe River Basin, the low-level southwest wind and vertically ascending motion are strengthened (weakened), which is (is not) conducive to precipitation increase in the Yangtze-Huaihe River Basin. These findings would help to better understand the impact mechanisms of the EASWJ activities on abnormal Meiyu from the perspective of medium-term scale Rossby wave energy propagation.

Soil carbon stocks and their primary origin at mature mangrove ecosystems in the estuary of Fukido River, Ishigaki Island, southwestern Japan

ABSTRACT Mangrove ecosystems play an important role in carbon (C) accumulation in tropical and subtropical regions. Below-ground deep anoxic soil is especially important for C accumulation. However, quantitative data on below-ground soil C stocks in mangrove ecosystems are lacking compared with data on above-ground biomass. In addition, soil C accumulation processes in mangrove ecosystems have not been sufficiently clarified. In this study, we quantified soil C stocks and focused on the mass of fallen litter and below-ground roots, which are produced by tree and that may directly influence soil C stocks in a mature subtropical mangrove in the estuary of Fukido River, Ishigaki Island, southwestern Japan. The principal species in this study site were Bruguiera gymnorhiza and Rhizophora stylosa, and total above-ground biomass at the site was 80.7 ± 1.3 (mean ± SD) Mg C ha−1 over the period from 2014 to 2016. Litter was collected in six litter traps from May 2013 to November 2016, it ranged from 7.8 to 11.5 Mg C ha−1, with the major proportion of litter being from foliage (leaves and stipules). The root C density at 90-cm depth was 27.1 ± 11.3 Mg C ha−1. The soil C stock in the mangrove forest at a depth of 90 cm at the study site was 251.0 ± 34.8 Mg C ha−1, and it seems to be lower value in the tropical region but it to be higher in subtropical East Asian mangrove sites. Dead roots, especially dead fine roots, but not fallen litter, were significantly positively correlated with soil C stocks. The δ13C values obtained from soils ranged from −29.3‰ to −27.0‰; these values are consistent with those for below-ground fine roots. These results strongly suggest that dead fine roots could be a main factor controlling soil C stocks at this study site.

Remote effect of a tropical cyclone in the Bay of Bengal on a heavy-rainfall event in subtropical East Asia

Torrential frontal rainfall occurred over Northern Taiwan on June 2, 2017. Prior to this rainfall, tropical cyclone (TC) Mora formed in the Bay of Bengal. This study investigated the triggering effect of TC Mora on the heavy-rainfall system through numerical experiments. The numerical experiments, in which TC Mora was included or excluded in the initial conditions, revealed that TC Mora strengthened the southwesterly winds, which effectively transported tropical warm moist air toward Taiwan, and the cool dry northerly winds to the north of Taiwan. The strengthened southeasterly and northerly winds created a strong frontal system near Taiwan. TC Mora contributed to the heavy frontal rainfall by enhancing tropical–extratropical interaction and vertical coupling in East Asia.

Australia-Asian monsoon in two versions of the UK Met Office Unified Model and their impacts on tropical–extratropical teleconnections

Many climate models have dry biases in tropical monsoon regions, but it is less clear how these errors can affect these model-simulated tropical–extratropical interactions and rainfall teleconnections. In this study, we evaluate the Australia and Asian (A–A) monsoon rainfall in two versions [Global Atmosphere version 6 (GA6), and version 7 (GA7)] of the UK Met Office Unified Model (UM) with uncoupled atmosphere-only simulations using two horizontal resolutions (N96 ~ 135 km and N216 ~ 60 km). Although UM can capture broad features of rainfall seasonal variations in the monsoon region, there are significant model errors in the locations of monsoon rainfall, its magnitudes and time evolutions, especially a climatological rainfall dry bias over the Indian subcontinent. This dry bias is progressively reduced by increased model resolutions from N96 to N216 and improved model physics from GA6 to GA7. Area-averaged peak monsoon rainfall over the Indian subcontinent (70°–90°E, 5°–25°N) increases by ~ 70% from 3 mm day−1 in GA6N216 to 5 mm day−1 in GA7N216, and rainfall dry bias averaged over a large tropical Asian monsoon domain is reduced by 2/3. In the uncoupled atmosphere-only experiment, the model rainfall is excessively sensitive to its underlying SST conditions, with positive local SST-rainfall correlations occurring over whole tropical oceans, while observations show negative correlations in regions dominated by atmospheric influence although such model performance is improved in its fully coupled runs. Most significant improvement of monsoon rainfall and ENSO (El Nino Southern Oscillations) relationship in GA7 occurred in the Australian continent, with GA7N216 better capturing the observed relationship than GA6N216. Although GA7N216 still suffers a dry bias in the Indian monsoon region, the increased monsoon rainfall substantially increases atmospheric diabatic heating over the region in the middle-upper troposphere, leading to more realistic extratropical circulation Gill-type responses to the heating. This results in improved monsoon-desert rainfall teleconnections, more realistic linkage between tropical Asian monsoon with its subtropical East Asian component, and its interhemispheric influence in Southern Hemisphere. Such results are further confirmed by the model nudging experiments in which the atmospheric wind and temperature are nudged towards observations over the Indian monsoon domain. Our study clearly demonstrates that uncertainty associated with monsoon simulations needs to be considered in future climate projections even outside the monsoon domain.

Two new taxa of the Auriscalpium vulgare species complex with substrate preferences

Basidiomes of the cone-inhabiting Auriscalpium species, usually regarded as A. vulgare Gray, are widely distributed across the Northern Hemisphere and easily recognized because of their unique macro-morphology. However, phylogenetic diversity patterns among different geographic populations and different substrates are completely unknown. In this study, samples of A. vulgare s.l. in different areas of China were studied phylogenetically, morphologically, and ecologically. For comparison, European collections of A. vulgare were included. Our phylogenetic analyses, inferred from partial nucleotide sequences including the internal transcribed spacers (ITS) 1 and 2 with the 5.8S nrDNA, the large subunit of nuclear ribosomal RNA (nrLSU), and the region between the conserved domain 6 and 7 of the gene for the second largest subunit of RNA polymerase II (RPB2), indicated that three phylogenetic species can be recognized, which correspond well with morphological and ecological evidence. Therefore, three species, having distinct substrate preferences, are documented here, namely the well-known A. vulgare, and the newly described species A. orientale and A. microsporum. Auriscalpium vulgare is widely distributed throughout the northern temperate Eurasia and North America, while the new species have been known mainly from subtropical East Asia and northeastern India to date.

On Relationships between Nonrecurving Western North Pacific Tropical Cyclones, the Madden–Julian Oscillation, and the East Asian Subtropical Jet

Abstract A 200-hPa zonal momentum budget is performed to examine the role that western North Pacific tropical cyclones (TCs) play in helping to organize intraseasonal extratropical circulation anomalies that occur with the Madden–Julian oscillation (MJO). Zonal wind is linearly decomposed into components that occur on MJO time scales (i.e., 20–100-day periods), as well as those that occur with lower and higher frequency. Dates during Northern Hemisphere fall that feature nonrecurving TCs within a search radius centered on a South China Sea grid point when the MJO is convectively active over the Maritime Continent and west Pacific warm pool are used to generate composites of relevant budget terms. These composites are then compared to others that are based on the full list of dates that feature a convectively active MJO in the same location during NH fall without regard for TC presence. Composite results highlight the primary momentum sources that guide the evolution of the NH extratropical zonal wind and associated mass field in each event set. TCs help to accelerate the East Asian subtropical jet that evolves with the MJO by modulating the high-frequency subtropical circulation over Southeast Asia. The phasing of this circulation with its underlying MJO time-scale component enables it to transfer momentum to the emerging subtropical jet. This momentum is integrated into the more slowly evolving flow and carried forward by other processes, which leads to the development of a westerly momentum surge along the subtropical jet that spans the length of the North Pacific Ocean.

Inter‐decadal change of the middle‐upper tropospheric land–sea thermal contrast in the late 1990s and the associated Northern Hemisphere hydroclimate

Although the middle‐upper tropospheric land–sea thermal contrasts play very important roles in modifying global monsoon, the inter‐decadal influences of the middle‐upper tropospheric land–sea thermal contrasts in the Northern Hemisphere on the global monsoon have not investigated specifically. In this study, it is found that the summer middle‐upper tropospheric zonal land–sea thermal contrast in the Eurasia–Pacific region changed from weak during the late 1980s and earlier and middle 1990s to strong after the late 1990s. This feature indicates a strengthened zonal land–sea thermal contrast during summer, which drives a stronger than normal Northern Hemisphere summer monsoon, with a hypothermal and wet climate over most of the subtropical East Asian and tropical African monsoon regions. In addition, the extensive anticyclonic circulation anomalies together with the less precipitation results in a warm and dry climate over the arid and semiarid regions to the west and north of the monsoon region, such as central Asia, Mongolia, and Northeast Asia. The inter‐decadal change of the middle‐upper tropospheric zonal thermal contrast is also closely associated with the warming trends of the extratropical North Pacific and North Atlantic during summer. However, this link possibly reflects a response of the warming oceans to the local atmospheric variability.

Stand dynamics and aboveground net primary productivity of a mature subtropical mangrove forest on Ishigaki Island, south-western Japan

There are insufficient data regarding forest productivity of mature mangroves near the northern limit of mangrove distribution in subtropical East Asia. We conducted the present study to determine the stand dynamics and net primary production (NPP) of a mature mangrove forest on Ishigaki Island in south-western Japan over three successive years, using a large permanent plot. The two mangrove species present were Bruguiera gymnorrhiza and Rhizophora stylosa. The aboveground biomass in the plot was high, despite the high latitude (24 ∘29′N) of the site, gradually increasing from 158.4 Mg ha−1 to 164.6 Mg ha−1 over the three-year study period. This increase was attributable to an increase in the biomass of B. gymnorrhiza. The biomass of R. stylosa, however, decreased during the same period because of a threefold-higher mortality rate and a lower relative growth rate of this species compared with B. gymnorrhiza. The mangrove forest was in the late successional stage, following a pioneer R. stylosa forest that was widely distributed throughout the study plot as standing dead trees and logs. The aboveground NPP of the mangrove forest was 10.66 ± 1.46 Mg ha−1 y−1, partitioned into 3.10 ± 0.51 Mg ha−1 y−1 as woody NPP (net increase in aboveground woody parts, SI), and 7.56 ± 0.99 Mg ha−1 y−1as foliage NPP (litter production, including foliage and reproductive organs, Ln). The mature mangrove forest had a relatively low SI/Ln ratio (0.41 ± 0.03), although litter fall production was within the range previously recorded for mangroves. The lower woody NPP in the mature mangroves was due to the exclusion of R. stylosa once the stand was in a late successional stage dominated by B. gymnorrhiza.