Western North Pacific Monsoon Research Articles

Western North Pacific monsoon vorticity distribution as a potential driver of interannual meridional migration of the boreal summer synoptic scale wave train

Abstract On inter-annual time scales, there is significant meridional migration of the boreal summer (May–October) synoptic-scale wave train (SSW) relative to the summer monsoon trough line over the western North Pacific (WNP) during 1979–2021. The associated plausible physical reasons for the SSW meridional migration are investigated by comparing analyses between two distinct groups: atypical SSW years where SSWs tend to prevail northward of the summer monsoon trough line and typical SSW years where SSWs largely occur along the summer monsoon trough line. During typical SSW years, SSWs originate primarily from equatorial mixed Rossby-gravity (MRG) waves and then develop into off-equatorial tropical depression (TD) waves in the lower troposphere of the monsoon region. During atypical SSW years, SSWs appear to be sourced from upper-level easterlies, propagating downward to the lower troposphere in the monsoon region, with a prevailing TD wave structure. A budget analysis of barotropic eddy kinetic energy suggests that interannual meridional SSW migration is closely related to changes in the vorticity distribution along the summer monsoon trough over the WNP, especially the western part of the summer monsoon trough. These changes cause low-frequency zonal convergence and shear differences, changing barotropic conversion around the monsoon trough and modulating interannual SSW meridional movement. In response to these changes, there are corresponding differences in SSW sources: a predominate MRG-TD wave pattern in typical SSW years and a predominate TD wave pattern in atypical SSW years. These results improve our understanding of the interannual variability of the large-scale circulation and tropical cyclones.

Impact of Extratropical Circulation in East Asia on Western North Pacific Tropical Cyclone Frequency and Intensity

Abstract This study examines the East Asia and western North Pacific (WNP) monsoon circulation patterns for strong and weak WNP tropical cyclone (TC) numbers in summer. It suggested that years with more intense TCs are coupled with the tropical monsoon circulations, including the northward cross-equatorial airflow and the extending tropical monsoon trough toward the central-eastern WNP. However, a higher frequency of weak TCs can be largely attributed to the mutual interactions among the tropical monsoon trough west of 140°E, the westward South Asia high, and the high pressure anomaly in Northeast Asia (NEA). Then the potential influence of the NEA extratropical system is focused on. The resultant local negative potential vorticity (PV) anomaly is carried southeastward by the prevailing flow. It stimulates a descending flow around 30°N, which favors the westward retreat of the South Asian high and the decreased zonal vertical wind shear around 20°N. The associated lower-level outflow converges in the tropical WNP and reinforces the ascending motion around 10°–20°N. Meanwhile, the warm air column in NEA also contributes to anomalous easterlies in a band around 30°N, intensifying the lower-level cyclonic vorticity in the northwestern WNP. Consequently, the ascending motion, cyclonic vorticity, and the weakened zonal vertical wind shear in northwestern WNP promote the WTC formation. A set of physically based empirical models is developed using various physically based predictors to reconstruct the number of intense and weak TCs. Cross-validated hindcasts suggest that the NEA extratropical circulation can serve as an additional source of predictability for the weak TC variability. Significance Statement Tropical cyclones (TCs) are a highly destructive type of natural disaster that have garnered widespread attention. By comparison with intense TCs (ITCs), weak TCs (WTCs) are much more numerous and often form closer to the coastal regions of East Asia, whose mechanism has not been fully understood. In this study, we suggest that more ITCs are controlled by tropical monsoon circulations, while the WTC variability is closely coupled with both tropical and extratropical monsoon systems. In addition to the tropical monsoon trough west of 140°E and the westward South Asian high, the Northeast Asian circulation can regulate the WTC number by changing the lower-level vorticity, vertical motion, and vertical wind shear in the WTC genesis-prone region, which can be applied to improve the seasonal prediction skill of WTCs.

Impacts of monsoon break events in the western North Pacific on the cross-equatorial flows over the Maritime Continent

This study examines the possible impacts of western North Pacific (WNP) monsoon break events on the cross-equatorial flows (CEFs) over the Maritime Continent using daily data from ERA5 for 1979–2020. It reveals that WNP monsoon break events can cause significant weakening of CEF in the both upper and lower troposphere, i.e., weaker southerlies in the lower level and northerlies in the upper level. These CEF anomalies are associated with a baroclinic structure of circulation anomalies over the WNP, consisting of upper-level cyclonic and lower-level anticyclonic anomalies. Furthermore, statistical analysis indicates that WNP monsoon break events correspond more closely to weakening of upper-level CEF, in comparison with lower-level CEF: 92% of WNP monsoon break events correspond to upper-level CEF weakening, while this decreases to 70% for lower-level CEF. This relatively lower proportion for lower-level CEF is largely attributable to the impact of the warm phase of ENSO.摘要本文利用ERA5逐日再分析资料, 探讨了1979–2020年间西北太平洋季风中断事件对海洋性大陆越赤道气流的影响. 合成结果表明, 西北太平洋季风中断事件会造成高, 低空越赤道气流减弱, 即高层南风异常, 低层北风异常, 与此相关的环流异常表现为西北太平洋高层气旋, 低层反气旋的斜压结构. 特别的是, 西北太平洋季风中断对高空越赤道气流的影响更为显著, 92%的季风中断事件都导致高空越赤道气流减弱, 而只有70%的事件造成低空越赤道气流减弱, 这是由于低空越赤道气流同时还受到赤道中东太平洋海温异常的调控.

Impact of centennial-scale solar activity reduction on the weakened Asian monsoon event at 9.2 ka BP

The weakened Asian monsoon event at 9.2 ka has been documented in many proxy-based reconstructions, but its underlying causes are uncertain. In this study, we investigated this event from a regional monsoon perspective by analysing simulations of Holocene transient solar activity forcing using the Community Earth System Model and high-resolution proxies. Our results revealed two separate periods of decreasing precipitation in the western North Pacific (WNP) and South Asian (SA) monsoons from 9.5 to 9.2 ka, each lagging behind two remarkable weakening events (0.38 and 0.21 W m−2) in solar forcing. However, there was no significant change in East Asian summer monsoon precipitation at 9.2 ka based on our experiment. Moisture budget analysis indicated that the dynamic effects term (i.e., anomalous descending motion) figured prominently in decreasing WNP and SA monsoon precipitation. Such a change was affected by an anomalous WNP anticyclone, which was maintained by the ‘wind-evaporation-sea surface temperature’ feedback during the decay phase of the centennial-scale El Niño-like event. The development of centennial El Niño-like conditions was caused by a reduction in solar irradiance. Reduced solar irradiance induced cooling over northern Australia, generating anomalous zonal sea level pressure gradients and westerlies over the tropical Pacific, ultimately resulting in the development of an El Niño-like pattern.

Intermodel Biases of the Western North Pacific Monsoon Trough in CMIP6 Models

Abstract The impact of climate change on tropical cyclone (TC) activity is often assessed by various downscaling approaches, statistical–dynamical frameworks, and high-resolution global climate models using the projected changes of environmental factors. Uncertainty in simulating and projecting TC-relevant, large-scale circulation is closely linked to the projection of TC activity in a warming climate. Based on the model output in phase 6 of the Coupled Model Intercomparison Project (CMIP6), this study examines the intermodel biases in simulating the western North Pacific monsoon trough (MT), which is one of the most important large-scale circulation systems for TC activity, especially TC formation. It is found that most CMIP6 models can successfully simulate the climatological mean structure of the MT, although considerable biases remain in its exact location and its simulated historical changes. The mean latitude of the simulated MT spreads between 10° and 20°N, with noticeable differences in its orientation. The multimodel ensemble mean indicates that the MT exhibits no significant long-term zonal and poleward shifts in the future scenarios, consistent with the projection in the selected models in which the simulated MT resembles the observed spatiotemporal characteristics of the counterpart. Further analysis suggests that the intermodel bias in the simulated MT location is closely related to the east–west contrast of sea surface temperature (SST) anomalies in the tropical Pacific. More attention is required on improving the simulation of the basinwide SST distribution and its associated MT to reduce the uncertainty in predicting the future location of TC formation.

Role of precession on the transition seasons of the Asian monsoon

While there have been many paleoclimate studies on the precessional control of climate, typically only the orbital phase where perihelion occurs close to the solstices has received attention. Here, we explore how precession affects the seasonal evolution of the Asian summer monsoon in the transitional seasons of boreal spring and autumn. With perihelion occurring in boreal spring, the Hadley circulation weakens over the northern Indian Ocean, linked to precession-enhanced sensible heating over the Tibetan Plateau. There is an early northward migration of the midlatitude westerly jet stream, and the advancement of the pre-monsoon along the Asian–Australian land bridge. The pre-monsoon response to precession may have had a major role in the early part of the last deglaciation, when perihelion last occurred during boreal spring. A weak continental summer monsoon and autumn aphelion during the early part of the last deglaciation led to a weak Pacific high over the east of coastal East Asia, allowing for a vigorous oceanic western North Pacific monsoon in the late summer. Additionally, the seasonal expansion of oceanic monsoon trough could shed light on the quasi-stationarity of the oceanic monsoon during a precessional cycle.

The Study of Wind Field ERA-20C in Monsoon Domains for Rainfall Predictor in Indonesia (Java, Sumatra, and Borneo)

In recent years, various research institutions have developed diverse global data reanalysis projects. This provides an opportunity to gain long-term of meteorological data for local scale. This study aims to select the potential predictor of wind fields u and v of the ERA-20C dataset, a reanalysis dataset, at 850 mb from seven domains or windows of Asian, Maritime Continent, Australian, and Western North Pacific monsoon related physically to rainfall anomaly patterns in Indonesia. The vector wind velocity scalar was obtained by using a Helmholtz decomposition to separate the total circulation v = (u,v) into the divergent component/velocity potential (χ) or Phi and rotational component/stream function (ψ) or Psi for obtaining the scalar variable of vector wind velocity. The method applied Singular value decomposition (SVD) to identify pairs of spatial patterns (expansion coefficients) between the predictors of Phi and Psi in seven domains, with rainfall data from 48 stations in Java, Sumatra, and Borneo Islands from 1981 to 2010. The results revealed that spatial patterns correlations of Java Islands were the highest in the Maritime Continent monsoon domain (80o−150o E and 15oS−15o N), while Sumatra and Borneo Island were in the Western North Pacific monsoon domain (100o–130o E and 5o–15o N) with predictor Psi. The lowest correlation for Java, Sumatra, and Borneo was the Australian monsoon domain (110o E–130o E and 5o S–15o S) with predictor Phi. In general, spatial pattern correl-ations of Java Island were higher than others, agreeing with monsoonal rainfall type dominantly in the region.

Quasi-biennial modulation of rapidly intensifying tropical cyclones by the western North Pacific monsoon

Previous publications have highlighted the relationship between several climate modes such as El Niño–Southern Oscillation and tropical cyclones (TCs) experiencing rapid intensification (RI) over the western North Pacific (WNP), particularly on a 3–7-yr timescale. This study investigates the modulation of TCs experiencing RI by the WNP monsoon on biennial timescales. There is a significant positive relationship between rapidly intensifying TC (RITC) frequency over the WNP during July–November from 1980 to 2021 and the simultaneous WNP monsoon index. After classifying different WNP monsoon years on biennial timescales, we find significantly more TCs forming over the key region from 5°–25°N, 140°–160°E during strong WNP monsoon years. Some of these TCs then move westward into the portion of the WNP that climatologically has the most RI-favorable environmental conditions. Alternatively, other TCs forming in the key region move northward and undergo RI quickly after genesis, subsequently leading to an increase in rapidly intensifying WNP TC frequency. The WNP monsoon influences rapidly intensifying TC activity predominantly through modulation of large-scale atmospheric conditions. During strong WNP monsoon years, increased low-to-mid-level humidity, low-level vorticity and upper-level divergence and decreased vertical wind shear all favor TC genesis and RI development over the key region. A strong WNP monsoon is also associated with an anomalous 850-hPa cyclone, an anomalous 200-hPa anticyclone, increased 600-hPa moisture convergence and a decrease in the magnitude of 200-hPa winds over the key region. Our study highlights that the WNP monsoon significantly modulates TC and RITC activity at distinct timescales.

The Roles of Atmospheric and Air–Sea Interaction Processes in Causing the Eastward Extension of the Western North Pacific Monsoon Trough in Boreal Summer

What causes the eastward extension of the climatological monsoon trough over the western North Pacific in the boreal summer was investigated through both observational analyses and numerical simulations. It was found that the highest SST is always located to the east of maximum precipitation, and this asymmetric SST pattern favors the eastward extension of the monsoon trough through SST induced boundary layer convergence. A mixed-layer heat budget analysis further indicates that the SST asymmetry arises primarily from the asymmetric pattern of cloud-modified downward shortwave radiation. In addition, two internal atmospheric mechanisms are identified. Firstly, there is a zonal asymmetry in the lower-tropospheric moisture advection. Southeasterlies to the east of the convection, in association with the subtropical high advect high mean moisture from south, leads to low-level moistening to the east of the convective center. Secondly, the heating-induced Kelvin wave response leads to a boundary layer convergence ahead of the convection. Both the processes lead to the setup of a convectively unstable stratification to the east, favoring the eastward extension of the monsoon trough. Two sets of the WRF model experiments that specify a fixed and a time-dependent SST field confirm the roles of the aforementioned atmospheric internal processes as well as the air–sea interaction process in causing the eastward progression of the climatological monsoon trough over the western North Pacific.

A Regional Air‐Sea Coupled Model Developed for the East Asia and Western North Pacific Monsoon Region

AbstractIn this study, we developed a regional coupled model for the western North Pacific (WNP) and East Asian region. The atmospheric/oceanic component of the coupled model was the Weather Research and Forecasting and the Regional Ocean Modeling System, coupled through the National Center for Atmospheric Research Coupler version 7 (CPL7). An evaluation is provided through the simulation of the climatology and interannual variability of the summer monsoon climate from 1982 to 2015. The coupled model is also compared with a solely atmospheric run, which is driven by the observed sea surface temperature (SST) as the surface boundary. The results show that the coupled model has good capability in modeling the climate in the WNP and East Asian domain, including the climatology of summer precipitation and circulation, the subseasonal (month‐to‐month) variation in the western Pacific subtropical high and associated precipitation, and the seasonal cycle of the East Asian rain belt. One merit of this model is its ability to simulate the northward and eastward shift from early to late summer. With respect to the interannual variability, the model captures the spatial pattern of the leading mode of summer precipitation, with a north–south dipole mode in the studied region. The model also reproduces the larger SST variability in the northern part of this region. The temporal correlation coefficient between the coupled simulation and observation was 0.70 for the precipitation and 0.53 for the SST leading mode. The coupled model also captures the precipitation–SST relationship, which results from reasonably considering the air–sea coupling processes in the coupled run.

Distinct Influences of Two Boreal Summer Intraseasonal Oscillation Modes on Synoptic-Scale Eddies over the Western North Pacific

Abstract The boreal summer intraseasonal oscillation (BSISO), which consists of a 10–30-day component and a 30–90-day component, is vigorous over the East Asian and the western North Pacific (WNP) monsoon regions where synoptic-scale eddies are also active. In this study, we systematically compare and diagnose the modulating effects of the 10–30- and 30–90-day BSISO modes on the development and maintenance of the WNP synoptic-scale eddies in terms of eddy energetics. During the developing phase of the synoptic eddies, the eddies tend to grow faster under the background conditions associated with the 10–30-day BSISO mode, compared with that associated with the 30–90-day mode, indicating the importance of the 10–30-day mode in eddy development. In contrast, the 30–90-day BSISO mode shows a larger contribution to the maintenance of the synoptic eddy intensity after the eddies reach their maximum amplitude. On the basis of the diagnoses of the eddy kinetic energy (EKE) budget equation, we find that the positive EKE advection induced by the 10–30-day flow is the leading process resulting in the increased EKE for eddy development. However, the 30–90-day circulation anomalies provide long-lasting favorable conditions (relative to those of the 10–30-day anomalies) to maintain the eddy intensity by generating a positive barotropic energy conversion from 30- to 90-day kinetic energy to EKE. These results not only advance our understanding of multiscale interactions over the WNP but also help toward better monitoring and prediction of synoptic-scale eddies, including tropical cyclones, using the background BSISO information. Significance Statement The western North Pacific is a region with active but complex multiscale interactions among synoptic-scale disturbances, intraseasonal oscillation, and low-frequency background monsoonal activity. The relative effects of 10–30- and 30–90-day intraseasonal variability on the synoptic-scale disturbances were quantitatively examined based on the eddy kinetic energy budget analysis. The results show that the 10–30-day (30–90-day) mode of intraseasonal oscillations plays a key role in the growth (maintenance) of synoptic-scale disturbances through the advection (barotropic energy conversion) process. The results suggest the implication of monitoring and predicting the activity of synoptic-scale disturbances using the information of two different intraseasonal modes.

Impact of the interannual variability of the South China Sea monsoon trough on tropical cyclone activity over the western North Pacific

AbstractThe present study reveals the impact of the interannual variability of the South China Sea monsoon trough (SCSMT) on tropical cyclone (TC) activity over the South China Sea (SCS) and the western North Pacific (WNP) from June to August during the period of 1958–2019. The SCSMT can influence TC genesis and occurrence through affecting the large‐scale environmental conditions and barotropic energy conversion. The intensification of the SCSMT can lead to enhanced low‐level vorticity and upper‐level divergence, decreased vertical wind shear, and increased midlevel humidity over the northern SCS and subtropical WNP. The strengthening of the SCSMT is accompanied by an intensification and eastward extension of the WNP monsoon trough and eastward retreat of the WNP subtropical high. All these changes associated with the SCSMT are favourable for TC genesis and occurrence over the north SCS and WNP. More TCs form over the WNP around 20° N, while higher TC occurrence frequency appears over the north SCS and WNP north of 20° N. Diagnostic results of eddy kinetic energy distribution and barotropic energy conversion provide a possible explanation for the impact of the SCSMT on TC activity. The synoptic‐scale disturbances are more active in the WNP region north of 20° N which might lead to an increase in TC occurrence frequency. While the barotropic energy conversion is intensified in the northern SCS and east of Philippines, more eddy energy converted from the mean flow is favourable for TC genesis. The meridional shear of mean zonal wind and the meridional convergence of mean meridional wind, which are related with the interannual variability of the SCSMT, primarily contribute to the barotropic energy conversion.

Influence of the pace of El Niño decay on tropical cyclone frequency over the western north pacific during decaying El Niño summers

This study investigated the distinct responses of western North Pacific (WNP) tropical cyclone (TC) activity during different decaying El Niño summers. The El Niño events were classified into two types according to the periodicity of the ENSO cycle, with positive SST anomalies in the equatorial central-eastern Pacific maintaining positive values into the following summer as the slow decaying (SD) cases, but transforming to negative values in the following summer as the rapid decaying (RD) cases. Compared with that in SD El Niño summers, the TC occurrence frequency over the WNP is significantly lower in RD El Niño summers, led by a much weaker WNP monsoon trough with more unfavorable environmental factors for TC genesis and development. Further examination showed that the apparent warming over the tropical Indian Ocean basin and cooling over the equatorial central-eastern Pacific contribute together to an enhanced lower-tropospheric anticyclone through modulation of the descending branch of the large-scale Walker circulation over the WNP, which may play a crucial role in suppressing the TC activity during the decaying summer of RD El Niño cases. In contrast, the warming equatorial central-eastern Pacific and remote western Indian Ocean induce a weakening WNP anticyclone and less suppressed deep convection during the decaying summer of SD El Niño cases. Thus, the different evolution of SST anomalies associated with different paces of El Niño decay results in the linkage between the preceding winter El Niño and the decreased WNP TC frequency in summer being more (less) robust for RD (SD) El Niño cases.摘要本文分析了El Niño事件衰减速度的差异对衰退年夏季西北太平洋热带气旋 (tropical cyclone, TC) 频数的不同影响. 按照El Niño事件衰减速度不同, 将其划分为迅速衰减 (rapid decaying, RD) 和缓慢衰减 (slow decaying, SD) 的El Niño事件. SD (RD) El Niño事件的衰退年夏季, 赤道中东太平洋海温仍维持正异常 (衰减为负异常) . 与SD El Niño事件相比, RD El Niño事件衰退年夏季西北太平洋TC频数显著减少. 进一步的分析揭示了导致TC频数差异的大尺度环境要素, 指出热带印度洋-太平洋海温异常密切相关的西北太平洋低层反气旋异常在其中起到了关键作用.

Climatological spatial scales of meteorological droughts in China and associated climate variability

This study investigates the climatological spatial scales (CSSs) of meteorological droughts in China and the linkages to climate variability. The Global Precipitation Climatology Centre monthly gridded precipitation with a spatial resolution of 25 km × 25 km for 1961–2010 is used. The standardized precipitation index at different timescales (1-, 3-, 6-, 9-, 12-, 24-, 36-, and 48-month) is applied to characterize meteorological droughts. The CSSs of meteorological droughts are calculated using a method considering spatial correlation and anisotropy. The relationships between the CSSs and main monsoons and climate teleconnections in five selected regions are quantified using dynamic spatial panel models. The five regions are South China (SC), the Yangtze River Valley (YRV), North China (NC), Northeast China (NE), and the Tibetan Plateau (TIB). The following results are obtained: (1) The CSSs of multi-timescale meteorological droughts in China are determined. The means of the CSSs at various timescales in China are 68.9, 73.8, 71.8, 69.7, 68.8, 65.9, 62.9, and 60.8 × 104 km2, respectively. The CSSs in eastern China generally show a significant decrease with increasing timescale. (2) Significant quantification relationships (R2 greater than 0.85) suggest that there is not a simple linear relationship between climate anomalies and the CSSs, but rather a complex spatiotemporal interaction including exogenous and endogenous interaction effects. The CSSs not only strongly depend on their neighbors on the same timescale but also significantly depend on the CSSs of neighbors on adjacent timescales. Three monsoon indices (Indian monsoon, East Asian monsoon, and western North Pacific monsoon) have significant impacts on the multi-timescale CSS variations in the five regions, especially for NE, YRV, SC, and TIB, while the El Niño–Southern Oscillation and Pacific Decadal Oscillation affect mainly SC and YRV. These findings could broaden our understanding of the spatiotemporal relationships of droughts and be useful for drought risk management.

Correction: Shi et al. Interactive Contribution of Indian Summer Monsoon and Western North Pacific Monsoon to Water Level and Terrestrial Water Storage in the Mekong Basin. Remote Sens. 2021, 13, 3399

In the original article [...]

Warmer SSTs in the Equatorial Eastern Pacific Delay the Seasonal March of the Asian–Pacific Summer Monsoon Onset

Abstract This study investigates the impact of El Niño–Southern Oscillation (ENSO) on the seasonal evolution of the Asian–Pacific summer monsoon. The results show that during the summers when the SSTs in the equatorial eastern Pacific are above normal (i.e., during developing El Niño), the seasonal march of the Asian–Pacific summer monsoon is delayed. Specifically, the stepwise northeastward extension of the western North Pacific monsoon and the northward migration of East Asian and Indian monsoon are all delayed, resulting in rainfall deficiency in the western North Pacific and India, and more rainfall in the Yangtze River valley but less rainfall in North China due to the delayed northward migration and resultant prolonged rainy season in the Yangtze River valley. In addition, the seasonal march of the Asian upper-tropospheric westerly jet, which is mainly characterized by the northward migration during the monsoon development, is also delayed, concurrent with the delayed march of monsoonal rainfall. During La Niña summers, however, the seasonal march of the monsoon and Asian jet does not exhibit clear variations, suggesting asymmetrical impacts between El Niño and La Niña. Furthermore, we explain these variations in the seasonal evolution of rainfall and jet and the asymmetrical impacts between El Niño and La Niña through the variations in tropical tropospheric temperatures and the resultant changes in meridional temperature gradient. Significance Statement The seasonal march of the Asian–Pacific summer monsoon exhibits large spatial and temporal variations, but shows some regular features such as stepwise northward migrations. The impacts of ENSO on the seasonal evolution of the components of Asian–Pacific summer monsoon have been shown in previous studies. However, there have been remarkable uncertainties in the previous results, and more emphasis has been put on the impacts of preceding ENSO. This study starts from El Niño and La Niña summers to reduce uncertainty caused by defining the seasonal march of the monsoon, and includes the variations in circulations that correspond well to monsoonal rainfall, making the conclusions more reliable.

Gravimetry-based terrigenous freshwater extension in the southwestern South China Sea and its response to monsoon under ENSO

Terrigenous discharge represents a mass movement from land to oceanic environment. While previous studies characterized terrigenous freshwater via oceanographic (physical and biochemical) data, the persistent fresh water in the far-field ocean via satellite-gravimetric observation has been rarely explored. This paper aims to characterize the spatiotemporal extension of Mekong freshwater and the interchangeable role of runoff and climatic factors in the southwestern South China Sea. Employing wavelet coherence analysis between the in situ runoff and oceanic freshwater variations inferred from satellite gravimetry, the coherence and transport duration were obtained at annual, intra- and inter-annual time scales during 2003–2015. Despite weak relationship at 6-month and 24-month scales in regions away from the estuary, the two time series remained significantly correlated at the 12-month scale with a highly positive coherence over 0.97. Spatial pattern of the annual transport duration further indicated that freshwater firstly flowed alongshore before turning eastward offshore, qualitatively consistent with the northeastward western boundary current and an anticyclonic eddy during the summertime generated from the ocean circulation model. Using partial wavelet coherence, the time-variable relationship at all these three scales was found closely related to the Indian Monsoon and Western North Pacific Monsoon. A series of alternating ENSO events during 2007–2011 were responsible for the inter-annual variations, contributing <5 % to the seasonal freshwater extension. Compared with the averaged transport duration of the isotope method (i.e., 21.5 days) and the geostrophic current computation (i.e., 38.8 days) in the summer of 2007, our method yielded a comparable transport duration of 23.9 days with smaller uncertainties. The wind-driven Ekman transport, however, was primarily responsible for the anticyclonic movement of freshwater transport in the southwestern South China Sea during late summer.

Flood inundation in the Lancang-Mekong River Basin: Assessing the role of summer monsoon

• Monsoons have positive impacts on inundation frequency, area and depth. • Inundation timing is either advanced or delayed when monsoons strengthen. • A change of monsoon can cause on average up to 14.2% change in inundation. • Different monsoons dominate different inundation characteristics. Although it is well known that precipitation and flood pulses in the Langcang-Mekong River Basin (LMRB) are largely impacted by monsoons, it is unclear to what extent flood inundation characteristics (i.e., inundation frequency, depth, area, and timing) in the basin respond to different monsoon types and monsoon combined effect, i.e., the Indian summer monsoon (ISM), the Western North Pacific Monsoon (WNPM), and their combined effect (ISWN). In this study, flood inundation in the LMRB during 1967–2015 was simulated by a hydrological-hydrodynamic model, from which the inundation characteristics were extracted and calculated. The monsoon impact on inundation characteristics was then quantified using the slope from linear regression model. The results show the monsoons and the ISWN overall have a positive impact on inundation frequency, depth, and area, while the inundation timing is usually advanced when the WNPM or the ISWN strengthens but delayed when the ISM strengthens. On average, a unit change in different monsoons can cause, 7.7%–14.2% change in inundation frequency, 5.3%–8.1% change in inundation depth and 4.3 days–5.8 days change in inundation timing for depth, which can also lead to 1.0%–4.3% change in inundation area and 2.8 days–3.8 days change in inundation timing for area. Also, the relative contributions of different monsoons and spatial distributions of the dominant monsoon were discussed. The results indicate different monsoons regulate different inundation characteristics, and suggest the coexistence of monsoon impacts. If the impact of the ISWN is ignored, the WNPM will play a more important role than the ISM in affecting the inundation.

Holocene Multi‐Centennial Variations of the Asian Summer Monsoon Triggered by Solar Activity

AbstractSolar activity affects Asian summer monsoon (ASM) at various time scales. However, it remains unknown if and how solar activity can influence ASM on the centennial time scale. Using the Community Earth System Model, we conduct a solar activity forced Holocene transient simulation with an acceleration factor of 10 and show that during the middle–late Holocene ASM precipitation exhibits a significant 300–600‐year periodicity under solar forcing. This model‐produced multi‐centennial variation is also suggested by proxy data. The leading mode of the multi‐centennial ASM variation shows a “wet tropics–dry subtropics” pattern, which lags the corresponding solar activity by about a quarter cycle. The western North Pacific (WNP) circulation system is responsible for the multi‐centennial ASM variation, through enhancing the climatological south Asia–WNP monsoon trough. We suggest that solar activity modulates the zonal SST gradients of the tropical Pacific, inducing the anomalous WNP cyclone and enhancing ASM precipitation in a delayed mode.

Possible impacts of spring subtropical Indian Ocean Dipole on the summer tropical cyclone genesis frequency over the western North Pacific

AbstractThe linkage between the spring subtropical Indian Ocean Dipole (SIOD) events and tropical cyclone genesis frequency over the western North Pacific (WNP) in the following summer is investigated in this study. It is found that their correlation is positive and statistically significant with a greater number of TCs genesis over the WNP in summer of a positive SIOD year. After removing the ENSO effects, diagnostic analysis reveals that under positive SIOD conditions the summertime WNP monsoon trough becomes intensified. Correspondingly, there are enhanced lower‐level vorticity, reduced vertical zonal wind shear, increased middle‐level relative humidity and vertical velocity over the WNP, conductive to generation of more TCs. Further examination indicates that the Mascarene high and Australian high are strengthened successively from spring to subsequent summer in a positive SIOD year, contributing to the maintenance of SIOD‐related SST dipole anomalies in southern Indian Ocean due to SST–wind–evaporation feedback mechanism. The resulting cyclonic anomaly is formed at lower troposphere over the WNP induced by the prominently enhanced cross‐equator airflows and the related Ekman upwelling. At upper troposphere, there tend to be remarkably WNP divergence anomalies of the interhemispheric vertical circulation forced by the summertime SST patterns closely associated with positive SIOD cases. By contrast, the monsoon trough is weakened and the dynamic and thermodynamic conditions are unfavourable for the WNP TC genesis in summer of a negative SIOD year, in agreement with a lower‐level anticyclonic anomaly and suppressed deep convection over the WNP forced by the SIOD‐related SST anomalies.