Subtropical East Asia Research Articles

Diurnal Cycle of Summer Precipitation over Subtropical East Asia in CAM5

Abstract The simulations of summertime diurnal cycle of precipitation and low-level winds by the Community Atmosphere Model, version 5, are evaluated over subtropical East Asia. The evaluation reveals the physical cause of the observed diurnal rainfall variation in East Asia and points to the source of model strengths and weaknesses. Two model versions with horizontal resolutions of 2.8° and 0.5° are used. The models can reproduce the diurnal phase of large-scale winds over East Asia, with an enhanced low-level southwesterly in early morning. Correspondingly, models successfully simulated the diurnal variation of stratiform rainfall with a maximum in early morning. However, the simulated convective rainfall occurs at local noontime, earlier than observations and with larger amplitude (normalized by the daily mean). As a result, models simulated a weaker diurnal cycle in total rainfall over the western plain of China due to an out-of-phase cancellation between convective and stratiform rainfalls and a noontime maximum of total rainfall over the eastern plain of China. Over the East China Sea, models simulated the early-morning maximum of convective precipitation and, together with the correct phase of the stratiform rainfall, they captured the diurnal cycle of total precipitation. The superposition of the stratiform and convective rainfalls also explains the observed diurnal cycle in total rainfall in East Asia. Relative to the coarse-resolution model, the high-resolution model simulated slight improvement in diurnal rainfall amplitudes, due to the larger amplitude of stratiform rainfall. The two models, however, suffer from the same major biases in rainfall diurnal cycles due to the convection parameterization.

FGOALS-s2 simulation of upper-level jet streams over East Asia: Mean state bias and synoptic-scale transient eddy activity

Upper-level jet streams over East Asia simulated by the LASG/IAP coupled climate system model FGOALS-s2 were assessed, and the mean state bias explained in terms of synoptic-scale transient eddy activity (STEA). The results showed that the spatial distribution of the seasonal mean jet stream was reproduced well by the model, except that following a weaker meridional temperature gradient (MTG), the intensity of the jet stream was weaker than in National Centers for Environment Prediction (NCEP)/Department of Energy Atmospheric Model Inter-comparison Project II reanalysis data (NCEP2). Based on daily mean data, the jet core number was counted to identify the geographical border between the East Asian Subtropical Jet (EASJ) and the East Asian Polar-front Jet (EAPJ). The border is located over the Tibetan Plateau according to NCEP2 data, but was not evident in FGOALS-s2 simulations. The seasonal cycles of the jet streams were found to be reasonably reproduced, except that they shifted northward relative to reanalysis data in boreal summer owing to the northward shift of negative MTGs. To identify the reasons for mean state bias, the dynamical and thermal forcings of STEA on mean flow were examined with a focus on boreal winter. The dynamical and thermal forcings were estimated by extended Eliassen-Palm flux (E) and transient heat flux, respectively. The results showed that the failure to reproduce the tripolar-pattern of the divergence of E over the jet regions led to an unsuccessful separation of the EASJ and EAPJ, while dynamical forcing contributed less to the weaker EASJ. In contrast, the weaker transient heat flux partly explained the weaker EASJ over the ocean.

Distinct quasi-biweekly features of the subtropical East Asian monsoon during early and late summers

Using Global Precipitation Climatology Project daily rainfall and ERA interim reanalysis data, we investigate the distinct characteristic of quasi-biweekly variation (QBV: 12–20 days) over East Asia (EA) during early (June 10–July 20) and late (July 21–August 31) summer. The QBV maximum variance is found over the core region of EA (30°–40°N, 110°–130°E), which includes eastern China (lower reaches of the Yellow, Huaihe, and Yangtze rivers) and the Korean Peninsula. At both its peak wet and dry phases, QBV over the core region has a baroclinic structure, but with different spatial distributions, different lower-level prevalent wind anomalies, and different upper-level major circulation anomalies in the two subseasons. Meanwhile, the two subseasons have different propagating tracks prior to reaching the peak phase, and different precursors associated with the local genesis of QBV. Furthermore, during the transition from the peak dry to peak wet phase of QBV, the major monsoon circulations have different behaviors that tropical monsoon trough extends eastward in early summer but retreats westward in late summer and the South Asia high (SAH) and western North Pacific (WNP) subtropical high move toward (away from) each other in early (late) summer. The abrupt change of mean state in mid to late July, which includes the northward migration of westerly jet, SAH and WNP, and the weakening and broken of westerly jet, is considered the root cause of the change in behavior of QBV. Finally, we indicate that the tropical monsoon trough and midlatitude westerly jet are possible sources of QBV over subtropical EA in both subseasons and provide useful guidance for 2–3 week predictions over EA.

East Asian summer monsoon circulation structure controlled by feedback of condensational heating

The East Asian summer monsoon (EASM) features strong humid low-level southerly flows and abundant rainfall over the subtropical East Asia. This study identified how condensational heating generated by the EASM rainfall can affect the EASM circulation by contrasting two 10-member ensembles of atmospheric General Circulation Model experiments with Community Climate Model version 3/National Center for Atmospheric Research respectively with and without feedback of condensational heating over the East Asian domain. Major results inferred from the experiments are as follows. Condensational heating is found to absolutely dominate diabatic heating over East Asia. Exclusion of the feedback of condensational heating leads to a significant weakening of summertime tropospheric warming over land and thus a large reduction of the land-sea thermal contrast between entire Asian continent and surrounding oceans. Associated with this, the lower-level EASM flows are weakened, South Asian High at 200 hPa migrates southward with reduced intensity and breaks over East Asia with southerly flows prevailing in the upper troposphere, in contrast to northerly flows in reality. Consequently, local EASM meridional cell disappears and the baroclinic structure featured by the EASM circulation that is dynamically determined by convective condensational heating over East Asia is altered to a barotropic structure. Therefore, it is concluded that the feedback of condensational heating acts to largely enhance lower-level flows of the EASM and essentially determine its baroclinic structure and meridional cell, once the solar radiation and inhomogeneity of the Earth’s surface form low-level monsoon flows in East Asia by enhancing land-sea thermal contrast.

Distribution of methane in the tropical upper troposphere measured by CARIBIC and CONTRAIL aircraft

We investigate the upper tropospheric distribution of methane (CH4) at low latitudes based on the analysis of air samples collected from aboard passenger aircraft. The distribution of CH4 exhibits spatial and seasonal differences, such as the pronounced seasonal cycles over tropical Asia and elevated mixing ratios over central Africa. Over Africa, the correlations of methane, ethane, and acetylene with carbon monoxide indicate that these high mixing ratios originate from biomass burning as well as from biogenic sources. Upper tropospheric mixing ratios of CH4were modeled using a chemistry transport model. The simulation captures the large‐scale features of the distributions along different flight routes, but discrepancies occur in some regions. Over Africa, where emissions are not well constrained, the model predicts a too steep interhemispheric gradient. During summer, efficient convective vertical transport and enhanced emissions give rise to a large‐scale CH4 maximum in the upper troposphere over subtropical Asia. This seasonal (monsoonal) cycle is analyzed with a tagged tracer simulation. The model confirms that in this region convection links upper tropospheric mixing ratios to regional sources on the Indian subcontinent, subtropical East Asia, and Southeast Asia. This type of aircraft data can therefore provide information about surface fluxes.

Biogeographical Divergence of the Flora of Yunnan, Southwestern China Initiated by the Uplift of Himalaya and Extrusion of Indochina Block

The floral composition of Yunnan is conspicuously linked to the biogeographical history of this extremely species-rich province in southwestern China. The floristic compositions of three representative regions in Yunnan were compared to reveal their variation with geography. From southern Yunnan, 4150 native species (including subspecies and varieties) from 1240 genera and 183 families of seed plants were recognized. From central Yunnan 3389 native species from 1095 genera and 167 families of seed plants were recognized. From northwestern Yunnan 6807 native species from 1296 genera and 166 families of seed plants were recognized. Although these three floras across Yunnan are similar in familial composition, similarities between the floras of southern and northwestern Yunnan are low at the generic and specific levels. The flora of northwestern Yunnan is dominated by families and genera with cosmopolitan and north temperate distributions, while the flora of southern Yunnan is dominated by tropical families and genera. Northwestern Yunnan is composed largely of temperate genera, of which the highest proportion has a north temperate distribution. In contrast, southern Yunnan has mainly tropical genera, of which most have a tropical Asian distribution. The flora of central Yunnan is a combination of southern and northwestern Yunnan. These three floras might be derived from a common Tertiary tropical or subtropical East Asian flora, but the geological history of each region has influenced its flora, and they have remained divergent since the late Tertiary. The flora of northwestern Yunnan has evolved with the uplift of the Himalayas and by gradual proliferation of mainly cosmopolitan and north temperate floristic elements, while the flora of southern Yunnan has evolved with extrusion of the Indochina block and the influence of mainly tropical Asian elements.

Regimes of Diurnal Variation of Summer Rainfall over Subtropical East Asia

Using hourly rain gauge records and Tropical Rainfall Measuring Mission 3B42 from 1998 to 2006, the authors present an analysis of the diurnal characteristics of summer rainfall over subtropical East Asia. The study shows that there are four different regimes of distinct diurnal variation of rainfall in both the rain gauge and the satellite data. They are located over the Tibetan Plateau with late-afternoon and midnight peaks, in the western China plain with midnight to early-morning peaks, in the eastern China plain with double peaks in late afternoon and early morning, and over the East China Sea with an early-morning peak. No propagation of diurnal phases is found from the land to the ocean across the coastlines. The different diurnal regimes are highly correlated with the inhomogeneous underlying surface, such as the plateau, plain, and ocean, with physical mechanisms consistent with the large-scale “mountain–valley” and “land–sea” breezes and convective instability. These diurnal characteristics over subtropical East Asia can be used as diagnostic metrics to evaluate the physical parameterization and hydrological cycle of climate models over East Asia.

Morphometric and genetic differentiation of two sibling gossamer-wing damselflies, Euphaea formosa and E. yayeyamana, and adaptive trait divergence in subtropical East Asian islands.

Insular species frequently demonstrate different tendencies to become smaller or larger than their continental relatives. Two sibling gossamer—wing damselflies, Euphaea formosa (Odonata: Euphaeidae) from Taiwan and E. yayeyamana from the Yaeyama Islands of Japan, have no clear structural differentiation, and can only be recognized by their geographical distribution, sizes, and subtle differences in wing shape and coloration. This study combined morphometric and genetic techniques to investigate the adaptive significance of trait divergence and species status in these two Euphaea damselflies. Phylogenetic analyses of the mitochondrial cox2 sequences demonstrated that the two damselflies are monophyletic lineages and constitute valid phylogenetic species. The landmark—based geometric morphometrics indicated that the two damselflies are different morphological species characterized by distinctive wing shapes. The larger E. formosa exhibited broader hind wings, whereas E. yayeyamana had narrower and elongated forewings. The body size and wing shape variations among populations of the two species do not follow the expected pattern of neutral evolution, suggesting that the evolutionary divergence of these two traits is likely to be subjected to natural or sexual selection. The decreased body size, elongated forewings, and narrower hind wings of E. yayeyamana may represent insular adaptation to limited resources and reduced territorial competition on smaller islands.

Quasi-4-Yr Coupling between El Niño–Southern Oscillation and Water Vapor Transport over East Asia–WNP

Abstract The summer moisture circulation anomaly over East Asia and the western North Pacific (WNP) couples well with the El Niño–Southern Oscillation (ENSO) in a quasi-4-yr period. The moisture circulation is dominated by two well-separated modes. The first mode exhibits an anticyclonic (cyclonic) moisture circulation over tropical–subtropical East Asia–WNP with an easterly (westerly) transport over the tropical WNP–Indian Ocean; the second mode displays an alternating pattern with an anticyclonic (cyclonic) moisture circulation over the subtropical WNP layered between two cyclonic (anticyclonic) circulations. Both modes couple well with the ENSO signal during its quasi-4-yr cycle. Within the cycle, in the summer of a developing warm episode, the positive phase of the second mode plays a key role, while in the transitional summer between a decaying warm episode and a developing cool episode, the positive phase of the first mode tends to take effect. In the summer of a developing cool episode, the negative phase of the second mode plays an important role, while the negative phase of the first mode tends to take effect in the transitional summer between a decaying cool episode and a developing warm episode. The anticyclone (cyclone) over the Philippine Sea region serves as a bridge in the quasi-four-year coupling. Its establishment and eastward extension modify moisture circulation over East Asia–WNP. Conversely, the easterly (westerly) wind to the south of the anticyclone (cyclone) is beneficial for the formation and eastward propagation of the Kelvin wave and, hence, to the development of the quasi-4-yr periodic ENSO episode.

293 Cenozoic vegetation and climate changes in the subtropical East Asia : a case study from Guangxi, south China

293 Cenozoic vegetation and climate changes in the subtropical East Asia : a case study from Guangxi, south China

Onset of East Asian subtropical summer monsoon and rainy season in China

Here we use harmonic analyses to examine seasonal variations of China land rainfall, low-level winds, and atmospheric heating over East Asia during spring to summer and the associated subtropical summer monsoon activities. Our results indicate that the South China spring rainfall (SCSR) in March is the prophase of East Asian sub-tropical summer monsoon (EASSM), and the onset of EASSM and China summer rainy season starts in early April, characterized by the enhanced rainfall in South China and the seasonal reverse of zonal land-sea thermal contrast in sub-tropical East Asia. The EASSM onset is earlier than that of South China Sea summer monsoon, and it is active in east of 100°E and north of 20°N. Our analyses suggest that the subsequent heating appears over India-China Peninsula in March and South China in April and causes the low-level atmospheric warming and the zonal land-sea thermal contrast seasonal reverse in East Asian subtropics. The atmospheric heating over South China is the main force to drive the southwesterly winds, updrafts and strengthen the summer precipitation in South China.

Large-Scale Control of Summer Precipitation in Taiwan

Taiwan is located at the western stretch of the North Pacific high pressure (NP high) ridge in boreal summer, and its climate is highly sensitive to the NP high. By grouping years of anomalously high and low summer precipitation in Taiwan, this study investigated the large-scale atmospheric circulation and the land–sea temperature contrast during these two groups of years and identified the control of summer precipitation in Taiwan. It is found that in years when summer precipitation in Taiwan is anomalously high, the western stretch of the NP high weakens. Weakening of the western stretch of the NP high induces strengthened southerly wind and enhanced vertical motion in East Asia and the western NP (EA–WNP) region, which is essentially an invigorated summer monsoon circulation. Corresponding to the invigorated circulation, precipitation increases in the southern section of the EA–WNP but decreases in the midlatitude section of the EA–WNP. It is further found that in those wet years, the land–sea temperature contrast between Asia and the surrounding seas is anomalously large and that the westerly wind in the tropical Indian Ocean and the southerly wind in the South China Sea and the subtropical East Asia are strengthened, which is an accelerated cyclonic circulation surrounding South and Southeast Asia. Coincident with the invigorated monsoon circulation in the EA–WNP region is a weakened Asian high pressure (Asian high). This is in variance with the expectation that the invigorated monsoon circulation in the EA–WNP region is related to a strengthened Asian high. The weakened Asian high is related to the weakened monsoon circulation in South and Southeast Asia. It is suggested that these unexpected results might be due to the interannual time scale of this study as opposed to either climatological or decadal scales of previous studies.

The tropospheric biennial oscillation in the East Asian monsoon region and its influence on the precipitation in China and large‐scale atmospheric circulation in East Asia

AbstractThe precipitation in China manifests a remarkable quasi‐biennial signal. For interannual variability, about 70% stations over China indicate the dominance of a quasi‐2‐year period. The maximum precipitation variability associated with the quasi‐biennial oscillation is located over the Yangtze River valley (YRV) and Huaihe River valley (HRV) as well as South China. This paper attempts to reveal the spatial–temporal evolution of the precipitation in China and related large‐scale atmospheric circulation associated with the tropospheric biennial oscillation (TBO) in East Asia by using a season‐dependent empirical orthogonal function (S‐EOF) analysis approach.The leading two modes of the TBO of summer precipitation in China and associated large‐scale circulations are examined by the S‐EOF analysis and regression analyses based on the S‐EOF time coefficients. The first TBO mode is characterized by an elongated band of positive precipitation anomalies along the YRV and negative precipitation anomalies over both North and South China. The second TBO mode is characterized by an elongated band of positive precipitation anomalies along the HRV and negative precipitation anomalies to the north of the Yellow River and to the south of the Yangtze River, respectively. Meanwhile, the leading modes of the TBO in East Asia may be determined by the meridional teleconnection Rossby wave pattern extending from the WNP to the midlatitudes of East Asia that is forced by the heating source fluctuation over the WNP during boreal summer. And the cold air activity, which is associated with the East Asian winter monsoon, may further influence the leading modes of the TBO through a modulation of the large‐scale atmosphere circulation over East Asia during the following boreal summer.Also found is that the TBO in East Aisa depends on both the large‐scale air–sea coupling over the tropical Indo‐Pacific Ocean regions and the tropical–midlatitude interaction in the western North Pacific (WNP)‐East Asia region. A fundamental element for the TBO in East Asia is the WNP monsoon. It is not only an important component of the TBO cycle in the tropics but also serves as a major source of the TBO signal for the subtropical East Asia. The meridional teleconnection Rossby wave train over the WNP‐East Asia region acts as a conveyer belt that transports the tropical TBO signal to the midlatitudes of East Asia, and produces the TBO footprints in the large‐scale circulation and precipitation in East Asia. Furthermore, the cold air activity over the East Asia during boreal summer also services as an important link in the chain events of the tropical–midlatitude interaction, with enhancing the role of the TBO modes in East Asia. Copyright © 2011 Royal Meteorological Society

Typhoon Track Changes Associated with Global Warming

Abstract Increasing tropical cyclone (TC) influence in the subtropical East Asia and decreasing TC activity in the South China Sea over the past few decades have been researched in previous studies. The singular value decomposition (SVD) of observational data and the Intergovernmental Panel on Climate Change (IPCC) climate change simulations in the Fourth Assessment Report (AR4) shows that the observed TC track changes are linked to the leading SVD mode of global sea surface temperature (SST) warming and the associated changes in large-scale steering flows. The selected five IPCC models can generally simulate the leading mode in their ensemble control run and prediction, suggesting the possible persistence of the reported track changes by 2040.

Seasonal and rainfall-type variations in inorganic ions and dicarboxylic acids and acidity of wet deposition samples collected from subtropical East Asia

Rainfall samples were collected over a period of 3 years and 8 months in subtropical East Asia. They are categorized into different rainfall types and analyzed to assess the ionic composition and its effect on the acidity of wet deposition in southern Taiwan. Only 4% of samples had a pH of <5.0, indicating that the study area is not impacted significantly by acid rain. The volume-weighted mean (VWM) pH by rainfall type was Spring Rain 5.74, Typhoon Rain 5.56, Summer Rain 5.46, Typhoon Outer Circulation (TOC) Rain 5.45, Plum Rain 5.32 and Autumn–Winter Rain 5.29. Dilution effects were important to the equivalent ionic concentration of different rainfall types. HCO3−, SO42− and Cl− were detected as major anions whereas NH4+, Na+ and Ca2+ were major cations. CO2-derived HCO3− was the major ionic species in all but Typhoon Rain and Spring Rain, in which the major species were Na+ and Cl− and Ca2+, respectively. Excluding HCO3−, the major species were NH4+, Na+ and Ca2+ in Plum Rain, the secondary photochemical products SO42−, NO3− and NH4+ in TOC Rain and Summer Rain, and Na+ and Ca2+ in Autumn–Winter Rain. Calculation of arithmetic means showed that dicarboxylic acids contributed between 0.25% and 0.53% of the total ionic concentration and of these, oxalic acid contributed the least (81.3% of the dicarboxylic acid) to TOC Rain and the most (96.1% of the dicarboxylic acid) to Spring Rain, suggestive of long-range transport in the latter. Differences in wet deposition composition were shown to be a result of differences in local emissions and long-range transport (hence of prevailing wind direction) during the period of rainfall and of the frequency and volume of rain that typifies each rainfall type. Principal component analysis (PCA) further revealed that traffic-related and industrial organic and inorganic pollutants, their secondary photochemical products, sea salts, and dust are important contributors to wet deposition. Moreover, the ratio of malonic acid to succinic acid (M:S ratio) indicated that both traffic and secondary photochemical reactions are major contributors to all but TOC Rain, for which the M:S ratio of 4.54 indicates a relative abundance of pollutants from secondary photochemical reactions. An ion balance (IB) ratio analysis demonstrated the validity of the results in this research.

Relationship between the western Pacific subtropical high and the subtropical East Asian diabatic heating during south China heavy rains in June 2005

Based on the daily NCEP/NCAR reanalysis data, the position variation of the western Pacific subtropical high (WPSH) in June 2005 and its relation to the diabatic heating in the subtropical East Asia are analyzed using the complete vertical vorticity equation. The results show that the position variation of the WPSH is indeed associated with the diabatic heating in the subtropical East Asian areas. In comparison with June climatology, stronger heating on the north side of the WPSH and relatively weak ITCZ (intertropical convergence zone) convection on the south side of the WPSH occurred in June 2005. Along with the northward movement of the WPSH, the convective latent heating extended northward from the south side of the WPSH. The heating to the west of the WPSH was generally greater than that inside the WPSH, and each significant enhancement of the heating field corresponded to a subsequent westward extension of the WPSH. In the mid troposphere, the vertical variation of heating on the north of the WPSH was greater than the climatology, which is unfavorable for the northward movement of the WPSH. On the other hand, the vertical variation of heating south of the WPSH was largely smaller than the climatology, which is favorable for the anomalous increase of anticyclonic vorticity, leading to the southward retreat of the WPSH. Before the westward extension of the WPSH in late June 2005, the vertical variation of heating rates to (in) the west (east) of the WPSH was largely higher (lower) than the climatology, which is in favor of the increase of anticyclonic (cyclonic) vorticity to (in) the west (east) of the WPSH, inducing the subsequent westward extension of the WPSH. Similar features appeared in the lower troposphere. In a word, the heating on the north-south, east-west of the WPSH worked together, resulting in the WPSH extending more southward and westward in June 2005, which is favorable to the maintenance of the rainbelt in South China.

TRMM 위성의 강수레이더에서 관측된 동아시아 여름 강수의 특성

The characteristics and vertical structure of the rainfall are examined in terms of rain types using TRMM (Tropical Rainfall Measuring Mission) PR (Precipitation Radar) data during the JJA period of 2002-2006 over three different regions; midlatitude region around the Korean Peninsula (EA1), subtropical East Asia (EA2), and tropical East Asia (EA3). The convective rain fraction in the EA1 region is 12.2%, which is smaller by 6% than those in the EA2 and EA3 regions. EA1 shows less frequent convective rain events, which are about 0.5 times as many as those in EA3. EA1 produces the mean convective rain rate of 10.4 mm/h that is about 40% larger than EA2 and EA3 while all regions have similar mean stratiform rain rate. The relationships between storm height and rain rate indicate that the rain rate is proportional to the storm height. Based on the vertical structure of radar reflectivity, EA1 produces deeper and stronger convective clouds with higher rain rate compared to the other regions. In EA3, radar reflectivity increases distinctly toward the land surface at altitude below 5 km, indicating more dominant coalescence-collision processes than the other regions. Furthermore, the bright band of stratiform rain clouds in EA3 is very distinct. In convective rain clouds, the first EOFs of radar reflectivity profiles are similar among the three regions, while the second EOFs are slightly different. The larger variability exists at upper layers for EA1 while it exits at lower levels for EA3.

Simulation of the East Asian Subtropical Westerly Jet Stream with GFDL AGCM (AM2.1)

The present study validated the capability of the AM2.1, a model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), in reproducing the fundamental features of the East Asian Subtropical West- erly Jet Stream (EASWJ). The main behaviors of the EASWJ are also investigated through the reanalysis of observational NCEP/NCAR data. The mean state of the EASWJ, including its intensity, location, structure, and seasonal evolution is generally well-portrayed in the model. Compared with the observation, the model tends to reproduce a weaker jet center. And, during summer, the simulated jet center is northward-situated. Results also demonstrate the model captures the variability of EASWJ during summer well. The results of the empirical or- thogonal function (EOF) applied on the zonal wind at 200 hPa (U200) over East Asia for both the observation and simulation indicate an inter-decadal shift around the late 1970s. The correlation coefficient between the corre- sponding principle components is as great as 0.42 with significance at the 99% confidence level.

Biweekly and 21–30-Day Variations of the Subtropical Summer Monsoon Rainfall over the Lower Reach of the Yangtze River Basin

Abstract The lower reach of the Yangtze River basin (LYRB) is located at the central region of the mei-yu and baiu front, which represents the subtropical East Asian (EA) summer monsoon. Based on the newly released daily rainfall data, two dominant intraseasonal variation (ISV) modes are identified over the LYRB during boreal summer (May–August), with spectral peaks occurring on day 15 (the biweekly mode) and day 24 (the 21–30-day mode). These two modes have comparable intensities, and together they account for above about 57% of the total intraseasonal variance. Both ISV modes exhibit baroclinic structures over the LYRB at their extreme phases. However, the genesis and evolutions associated with the two modes are different. Considering the genesis of their extreme wet phases over the LYRB, the biweekly mode is initiated by a midlatitude jet stream vorticity anomaly moving southeastward, while the 21–30-day mode is primarily associated with a low-level westward propagation of an anticyclonic anomaly from 145° to 120°E, which reflects the westward extension of the western North Pacific subtropical high (WNPSH). The development of the biweekly mode at LYRB is enhanced by the northwestward movement of a low-level anticyclonic anomaly from the Philippine Sea to the south of Taiwan, which is a result of the enhancement of the WNPSH resulting from its merger with a transient midlatitude high. In contrast, the development of the 21–30-day mode is enhanced by an upper-level trough anomaly moving from Lake Baikal to far east Russia. These two ISV periodicities are also found to be embedded in their corresponding source regions. The new knowledge on the sources and evolutions of the two major LYRB ISV modes provides empirical predictors for the intraseasonal variation in the subtropical EA summer monsoon.

How Much Do Tropical Cyclones Affect Seasonal and Interannual Rainfall Variability over the Western North Pacific?

Abstract The authors investigate the effects of tropical cyclones (TCs) on seasonal and interannual rainfall variability over the western North Pacific (WNP) by using rainfall data at 22 stations. The TC-induced rainfall at each station is estimated by using station data when a TC is located within the influential radius (1000 km) from the station. The spatial–temporal variability of the proportion of TC rainfall is examined primarily along the east–west island chain near 10°N (between 7° and 13°N) and the north–south island chain near 125°E (between 120° and 130°E). Along 10°N the seasonality of total rainfall is mainly determined by non-TC rainfall that is influenced by the WNP monsoon trough. The proportion of the TC rain is relatively low. During the high TC season from July to December, TC rainfall accounts for 30% of the total rainfall in Guam, 15%–23% in Koror and Yap, and less than 10% at other stations. In contrast, along 125°E where the WNP subtropical high is located, the TC rainfall accounts for 50%–60% of the total rainfall between 18° and 26°N during the peak TC season from July to October. In Hualien of Taiwan, TC rainfall exceeds 60% of the total rainfall. The interannual variability of the TC rainfall and total rainfall is primarily modulated by El Niño–Southern Oscillation (ENSO). Along 10°N, the ratio of TC rainfall versus total rainfall is higher than the climatology during developing and mature phases of El Niño (from March to the following January), whereas the ratio is below the climatology during the decaying phase of El Niño. The opposite is true for La Niña, except that the impact of La Niña is shorter in duration. Furthermore, in summer of El Niño developing years, the total seasonal rainfall increases primarily because of the increase of TC rainfall. In the ensuing autumn, an anticyclonic anomaly develops over the Philippine Sea and TC rainfall shifts eastward; as a result, the total rainfall over the Philippines and Taiwan decreases. The total rainfall to the east of 140°E, however, changes little, because the westward passage of TCs enhances TC rainfall, which offsets the decrease of non-TC rainfall. Along the meridional island chain between 120° and 130°E, the total rainfall anomaly is affected by ENSO starting from the autumn to the following spring, and the variation in TC rainfall dominates the total rainfall variation only in autumn (August–November) of ENSO years. The results from this study suggest that in the tropical WNP and subtropical East Asian monsoon regions (east of 120°E), the seasonal and interannual variations of rainfall are controlled by changes in nonlocal circulations. These changes outside the monsoon domain may substantially affect summer monsoon rainfall by changing TC genesis and tracks.