Convergence Zone Research Articles

ENSO weakens the co-variability between the spring persistent rains and Asian summer monsoon: Evidences from tree-ring data in southeastern China

The Spring Persistent Rains (SPR) and the Asian Summer Monsoon (ASM) are the two dominant rainfall systems in East Asia, providing together a majority of annual rainfall in southeastern China (SEC). Since observational data in SEC were mostly unavailable until the 1950s, proxy records that are capable of capturing the SPR and ASM variations are required to examine the long-term co-variability patterns between them. Tree-ring earlywood and latewood δ18O records in SEC were found to respond to relative humidity (RH) during the SPR and ASM seasons, respectively, allowing us, for the first time, to reconstruct the RH changes of SPR and ASM back to 1801. The two reconstructions can explain 44.9 % and 42.3 % of the instrumental variance. We observed a long-lasting wet epoch in the 1920s–60s for both the SPR and ASM, caused by a peak in the land–ocean thermal contrast. The El Niño-Southern Oscillation (ENSO) and the Intertropical Convergence Zone (ITCZ) were found to be the two leading tropical systems that modulated the SPR and ASM co-variability. During a period with weakened ENSO variance, the RH of SPR and ASM showed in-phase changes driven by the ITCZ. However, when the ENSO variance became strengthened, the co-variability collapsed since the ENSO can offset the influence of the ITCZ via teleconnections.

Dynamical Constraint on Precipitation Biases Over the Indo‐Pacific Region During Boreal Summer in AMIP6 Models

AbstractClimate models suffer from longstanding precipitation biases, much of which has been attributed to their atmospheric component owing to unrealistic parameterizations. Here we investigate precipitation biases in 37 Atmospheric Model Intercomparison Project Phase 6 (AMIP6) models, focusing on the Indo‐Pacific region during boreal summer. These models remain plagued by considerable precipitation biases, especially over regions of strong precipitation. In particular, 22 models overestimate the Asian‐Pacific monsoon precipitation, while 28 models underestimate the southern Indian Ocean Intertropical Convergence Zone precipitation. The inter‐model spread in summer precipitation is decomposed into Empirical Orthogonal Functions (EOFs). The leading EOF mode features an anomalous anticyclone circulation spanning the Indo‐northwest Pacific oceans, which we show is energized by barotropic conversion from the confluence of the background monsoonal westerlies and trade‐wind easterlies. Our results suggest precipitation biases in atmospheric models, though caused by unrealistic parameterizations, are organized by dynamical feedbacks of the mean flow.

CloudbandPy 1.0: an automated algorithm for the detection of tropical–extratropical cloud bands

Abstract. Persistent and organized convective cloud systems that arise in convergence zones can lead to the formation of synoptic cloud bands extending from the tropics to the extratropics. These cloud bands are responsible for heavy precipitation and are often a combination of tropical intrusions of extratropical Rossby waves and processes originating from the tropics. Detecting these cloud bands presents a valuable opportunity to enhance our understanding of the variability of these systems and the underlying processes that govern their behavior and that connect the tropics and the extratropics. This paper presents a new atmospheric cloud band detection method based on outgoing longwave radiation using computer vision techniques, which offers enhanced capabilities to identify long cloud bands across diverse gridded datasets and variables. The method is specifically designed to detect extended tropical–extratropical convective cloud bands, ensuring accurate identification and analysis of these dynamic atmospheric features in convergence zones. The code allows for easy configuration and adaptation of the algorithm to meet specific research needs. The method handles cloud band merging and splitting, which allows for an understanding of the life cycle of cloud bands and their climatology. This algorithm lays the groundwork for improving our understanding of the large-scale processes that are involved in the formation and life cycle of cloud bands and the connections between tropical and extratropical regions as well as evaluating the differences in cloud band types between different ocean basins.

The Influential Factors and Prediction of Kuroshio Extension Front on Acoustic Propagation-Tracked

The Kuroshio Extension front (KEF) considerably influences the underwater acoustic environment; however, a knowledge gap persists regarding the acoustic predictions under the ocean front environment. This study utilized the high-resolution ocean reanalysis data (JCOPE2M, 1993–2022) to assess the impact of the KEF on the underwater acoustic environment. Oceanographic factors were extracted from the database using the Douglas-Peucker algorithm, and acoustic propagation characteristics were obtained using the Bellhop raytracing model. This study employed a backpropagation neural network to predict the acoustic propagation affected by the KEF. The depth of the acoustic channel axis and the vertical gradient of the transition layer of sound speed were identified as the fundamental factors influencing the first area of convergence, with correlations between the former and the distance of the first convergence zone ranging from 0.52 to 0.82, and that for the latter ranging from −0.42 to −0.7. The proposed method demonstrated efficacy in forecasting first convergence zone distances, predicting distances with less than 3 km error in >90% of cases and less than 1 km error in 68.61% of cases. Thus, this study provides a valuable predictive tool for studying underwater acoustic propagation in ocean front environments and informs further research.

Fast response of global monsoon area and precipitation to regional carbonaceous aerosols

Monsoon exerts a significant social and economic impact on billions of people around the world. Changes in global monsoon (GM) rainfall under global warming have been well studied; however, the understanding of GM connection with regional aerosol emissions is limited. Using an atmospheric general circulation model (AGCM), we attempt to quantify the response of global monsoon area and precipitation to the removal of carbonaceous aerosols over North America (AM), North Africa (AF), Europe (EU), and Asia (AS). Response to the emission removal, the global average aerosol optical depths (AOD) are reduced by 2.21%, 1.8%, 1.16%, and 3.8% respectively. A southward shift in the GM domain is observed, responding to changes in magnitude and position of the Inter-Tropical Convergence Zone, when carbonaceous aerosols are removed over these regions in the Northern Hemisphere. The percentage departure in the area-averaged GM precipitation is found to be 4-fold in magnitude for the removal of emissions over EU when compared to that of AM and AF. A shrinkage in the GM area is also observed, which is maximum for EU (∼−30%) compared to AM and AF (∼−25%). The change in GM precipitation and area are found to be the least and opposite in nature for AS emissions. This study provides insight into the regional nature of carbonaceous aerosol effects on global precipitation.

Investigation into the Performance Characteristics of the Organic Dry Farming Transition and the Corresponding Impact on Carbon Emissions Reduction

Global warming affects food security and ecological security, and it threatens economic stability and sustainable agricultural development. The transformation and development of agriculture have significant implications for the achievement of the “dual-carbon” goals and the promotion of sustainable agricultural development. Based on panel data on organic dry farming in China from 2005 to 2020, this study aimed to comprehensively assess the transformation performance of organic dry farming (TRODF) in 15 provinces. It explored the impact of the transformation of organic dry farming on carbon emissions by utilizing a spatial Markov chain and spatial measurement models. Our findings are as follows: (1) The performance of the organic dryland agriculture transformation has gradually improved and is accompanied by a corresponding trend of fluctuating regional disparities, which are on the rise. Moreover, the disparities between the five major regions mainly stem from intra-regional differences. (2) TRODF agriculture presents the possibility of state transfer during different periods, featuring four convergent zones: a lagging zone, a starting zone, a crossing zone, and an advanced zone. The spatial Markov chain indicates that state transitions typically occur between adjacent levels, with fewer instances of “jump”-type transitions. Moreover, there is a clear trend of differentiation in the state transitions between non-adjacent areas. (3) The organic dry farming transformation exhibits a significant carbon reduction effect, which is characterized by heterogeneity across different stages of agricultural development, provinces, and time periods. This study emphasizes that economic and industrial transformation, along with the transformation of the ecological environment, represents a crucial direction for conserving resources and achieving a further reduction in carbon emissions.

Burial and thermal history modeling of basins in convergent oblique-slip mobile zones: A case study of the Ardmore Basin, southern Oklahoma

The burial and thermal history of sedimentary basins within oblique-slip mobile zones are unique and multifaceted, with irregular periods of subsidence that are closely related to compressional and shear stress. Modern basin modeling techniques can constrain the timing of tectonic events and thermal history as well as determine rates and magnitudes of basin subsidence, which in turn, helps guide exploration for hydrocarbons. The work presented here is the first modern basin modeling effort in the Ardmore Basin in southern Oklahoma. The study uses 12 one-dimensional burial history models to evaluate the thermal maturity of the Late Devonian (Famennian)–Early Mississippian (Tournaisian) Woodford Shale and the Early–Late Mississippian (Tournaisian–Serpukhovian) Caney Shale hydrocarbon source rocks. All models display a similar tectonic evolution with subsidence during and following Cambrian Iapetan rifting, tectonic stability during a Silurian–Late Mississippian passive margin phase, Pennsylvanian synorogenic subsidence, Permian post-orogenic subsidence, Late Permian–Early Cretaceous regional uplift and unroofing, and Early Cretaceous–Paleogene subsidence of the Gulf of Mexico Basin. Episodic Pennsylvanian subsidence appears to have been synchronous with sequential uplift of the Wichita Uplift and the Arbuckle Uplift in response to major left-lateral transpression. Rapid and high magnitude Late Mississippian–Permian subsidence (>250 m/m.y.; 820 ft/m.y. in basin synclines) suggests the Ardmore Basin functioned episodically as an elevator basin, which is typical of sedimentary basins in oblique-slip mobile zones. The Devonian–Mississippian shale section has a broad range of thermal maturity (vitrinite reflectance, 0.40–2.00% Ro), which is strongly dependent on structural position. Isoreflectance lines are subhorizontal and cross-cut structure, indicating post-kinematic thermal maturation in which strata are thermally immature in uplifts and thermally overmature in the deepest synclines. The post-kinematic pattern is a product of a rapid, early phase of synkinematic thermal maturation that has been obscured by a prolonged period of post-kinematic thermal maturation. Variations in basal heat flow, structural history, and general variability of hydrocarbon source rock organic composition, particularly at low thermal maturity levels, have resulted in a modest degree of scatter in the vitrinite reflectance-depth data.

The North Equatorial Current and rapid intensification of super typhoons

Super Typhoon Mangkhut, which traversed the North Equatorial Current (NEC; 8–17 °N) in the western North Pacific in 2018, was the most intense Category-5 tropical cyclone (TC) with the longest duration in history—3.5 days. Here we show that the combination of two factors—high ocean heat content (OHC) and increased stratification — makes the NEC region the most favored area for a rapid intensification (RI) of super typhoons, instead of the Eddy Rich Zone (17–25 °N), which was considered the most relevant for RI occurrence. The high OHC results from a northward deepening thermocline in geostrophic balance with the westward-flowing NEC. The stratification is derived from precipitation associated with the Inter-Tropical Convergence Zone in the summer peak typhoon season. These factors, which are increasingly significant over the past four decades, impede the TC-induced sea surface cooling, thus enhancing RI of TCs and simultaneously maintaining super typhoons over the NEC region.

Northward Shift and Narrowing of the ITCZ in 20 Years of AIRS Data

AbstractA time series analysis of Deep Convective Clouds (DCC) in Atmospheric InfraRed Sounder data between 2002 and 2023 shows that the mean latitude of the Inter Tropical Convergence Zone (ITCZ) has shifted northward for land at a rate of about 30 km/decade. The shift for ocean is north as well at about half of the rate for land. Comparisons of our results with the existing literature are complicated by the use of different methodologies and data sources: We used the daily observations of DCCs, which by their association with extreme rain rates are indicators of locally extreme vertical velocity on 15 km horizontal scales, while most of the literature used model or reanalysis gridded data with typically 100 km bins. The southern boundary of the ITCZ shifts north faster than the northern boundary, as qualitatively stated in the literature. The resulting contraction of the ITCZ width with global warming was qualitatively predicted based on theoretical consideration. Our observation of the rate of decrease in the width for ocean is statistically consistent with the contraction deduced from European Centre for Medium Range Forecast Reanalysis and Modern‐Era Retrospective analysis for Research and Applications data. The observed northward shift of the ITCZ was not predicted and is not seen in climate models in the 21st century.

Analysis of seamount propagation through the first convergence zone

During the recent Office of Naval Research (ONR) New England Seamounts Acoustics (NESMA) Pilot experiment, continuous measurements were made of underwater acoustic propagation and scattering from close range through the first deep-water acoustic convergence zone (CZ) using a stationary mid-frequency source and the ONR Three Octave Research Array (THORA) at Penn State. The source was positioned at a shallow depth over the plateau of one of the Atlantis II seamounts; the THORA was towed from 1 km range to a maximum of 85 km from the source. High-level goals of this work are to: 1) assess the influence of seamount scattering on mid-frequency transmission loss (TL) within the shadow zone, and 2) characterize how seamount interaction and the CZ is influenced by the shallow sound speed profile (SSP) in this region. The data was beamformed and analyzed using narrowband and broadband techniques, while ray-based and parabolic equation (PE) based models were used to develop an understanding of the influence of the seamount bathymetry, roughness, and the SSP. This talk will discuss a frequency dependent decrease in TL (acoustic enhancement) within the shadow zone due to seamount interaction and the limited influence of the seamount within the first CZ.

An interpretation experiment on eddies’ role in sound propagation in the ocean using ray model

The relationship between mesoscale eddies and ambient acoustic propagation under different conditions were studied using high resolution ocean reanalysis products in South China Sea, which was summarized by a linear-equation representing the convergence zone change with respect to eddy signatures and source positions. Beyond the experiments scales, warm eddies “dragged” the sound propagation and cold eddies “squeezed” that in contrast, and the acoustic amplifying mechanism can be summarized as stronger(eddies), nearer(eddies) and deeper(source).

FRACTUS – Looking for evidence of Saharan dust

There is a presence of aerosols that come from the Sahel (Africa) in the Amazonian atmosphere and are maximum in the atmosphere in the months of February to April, coinciding with the humid season in the central Amazon region (MARTIN et al., 2010). The transport of this particulate matter from Africa to the Central Amazon only occurs when the ITCZ (Intertropical Convergence Zone) is south of Manaus (ANSMANN et al., 2009; BEN-AMI et al., 2010; RIZZOLO et al., 2017 ).

Mid-frequency convergence zone propagation

Convergence zone propagation has a cycle distance of order 50 km with most of the path spent below the thermocline. Calculations using Garrett-Munk statistics and rays computed with a smoothed background sound speed profile over this single cycle predict propagation in the partially saturated regime and an integration time of 500 seconds (time spent in a single output bin of a discrete Fourier transform). Parabolic equation simulations with synthetic fine structure and ocean dynamics validate the ray-based calculations. Data of narrowband transmissions from 1.5 to 7.5 kHz from a shallow towed source (150 m) to a shallow drifting array (150 m) depth at a range of 59 km demonstrate smoothly varying phase in a beam permitting coherent integrations of 3.5 minutes. The shorter integration time in data is consistent with non-uniform motion estimated from GPS.

Tidal Intrusion Fronts, Surface Convergence, and Mixing in an Estuary with Complex Topography

Abstract Observations from a tidal estuary show that tidal intrusion fronts occur regularly during flood tides near topographic features including constrictions and bends. A realistic model is used to study the generation of these fronts and their influence on stratification and mixing in the estuary. At the constriction, flow separation occurs on both sides of the jet flow downstream of the narrow opening, leading to sharp lateral salinity gradients and baroclinic secondary circulation. A tidal intrusion front, with a V-shaped convergence zone on the surface, is generated by the interaction between secondary circulation and the jet flow. Stratification is created at the front due to the straining of lateral salinity gradients by secondary circulation. Though stratification is expected to suppress turbulence, strong turbulent mixing is found near the surface front. The intense mixing is attributed to enhanced vertical shear due to both frontal baroclinicity and the twisting of lateral shear by secondary circulation. In the bend, flow separation occurs along the inner bank, resulting in lateral salinity gradients, secondary circulation, frontogenesis, and enhanced mixing near the front. In contrast to the V-shaped front at the constriction, an oblique linear surface convergence front occurs in the bend, which resembles a one-sided tidal intrusion front. Moreover, in addition to baroclinicity, channel curvature also affects secondary circulation, frontogenesis, and mixing in the bend. Overall in the estuary, the near-surface mixing associated with tidal intrusion fronts during flood tides is similar in magnitude to bottom boundary layer mixing that occurs primarily during ebbs.

Astronomical Time Scale of the Late Pleistocene in the Northern South China Sea Based on Carbonate Deposition Record

Variations in solar insolation caused by changes in the Earth’s orbit—specifically its eccentricity, obliquity, and precession—can leave discernible marks on the geologic record. Astrochronology leverages these markers to establish a direct connection between chronological measurements and different facets of climate change as recorded in marine sediments. This approach offers a unique window into the Earth’s climate system and the construction of high-resolution, continuous time scales. Our study involves comprehensive bulk carbonate analyses of 390 discrete samples from core SCS1, which was retrieved from the deep-sea floor of the northern South China Sea. By utilizing carbonate stratigraphic data, we have developed a carbonate stratigraphic age model. This was achieved by aligning the carbonate sequence from core SCS1 with the established carbonate standard stratigraphic time scale of the South China Sea. Subsequently, we construct an astronomically tuned time scale based on this age model. Our findings indicate that sediment records in this core have been predominantly influenced by a 20,000-year cycle (precession cycle) throughout the Late Pleistocene. We have developed an astronomical time scale extending back approximately 110,000 years from the present, with a resolution of 280 years, by tuning the carbonate record to the precession curve. Time-domain spectral analysis of the tuned carbonate time series, alongside the consistent comparability of the early Holocene low-carbonate event (11–8 kyr), underscores the reliability of our astronomical time scale. Our age model exposes intricate variations in carbonate deposition, epitomizing a typical “Pacific-type” carbonate cycle. Previous research has illustrated that precession forcing predominantly influences productivity changes in the South China Sea. The pronounced precession-related cycle observed in our record suggests that changes in productivity significantly impact carbonate content in the area under study. Furthermore, the clear precession period identified in the carbonate record of core SCS1 reflects the response of low-latitude processes to orbital parameters, implying that carbonate deposition and preservation in core SCS1 are chiefly influenced by the interplay between the Intertropical Convergence Zone (ITCZ) and the monsoon system within the precession band. Our astronomical time scale is poised to enhance paleoceanographic, paleoclimatic, and correlation studies further. Additionally, the independent evidence we provide for using proxy records for astronomical age calibration of marine sediments lends additional support to similar methods of astronomical tuning.

Understanding the Spatiotemporal Variability of Tropical Orographic Rainfall Using Convective Plume Buoyancy

Abstract Mechanical forcing by orography affects precipitating convection across many tropical regions, but controls on the intensity and horizontal extent of the orographic precipitation peak and rain shadow remain poorly understood. A recent theory explains this control of precipitation as arising from modulation of lower-tropospheric temperature and moisture by orographic mechanical forcing, setting the distribution of convective rainfall by controlling parcel buoyancy. Using satellite and reanalysis data, we evaluate this theory by investigating spatiotemporal precipitation variations in six mountainous tropical regions spanning South and Southeast Asia, and the Maritime Continent. We show that a strong relationship holds in these regions between daily precipitation and a measure of convective plume buoyancy. This measure depends on boundary layer thermodynamic properties and lower-free-tropospheric moisture and temperature. Consistent with the theory, temporal variations in lower-free-tropospheric temperature are primarily modulated by orographic mechanical lifting through changes in cross-slope wind speed. However, winds directed along background horizontal moisture gradients also influence lower-tropospheric moisture variations in some regions. The buoyancy measure is also shown to explain many aspects of the spatial patterns of precipitation. Finally, we present a linear model with two horizontal dimensions that combines mountain wave dynamics with a linearized closure exploiting the relationship between precipitation and plume buoyancy. In some regions, this model skillfully captures the spatial structure and intensity of rainfall; it underestimates rainfall in regions where time-mean ascent in large-scale convergence zones shapes lower-tropospheric humidity. Overall, these results provide new understanding of fundamental processes controlling subseasonal and spatial variations in tropical orographic precipitation.

Equatorward shift of the boreal summer intertropical convergence zone in Maritime Continent and the impacts on surface black carbon concentration and public health

In Maritime Continent, the shift of intertropical convergence zone (ITCZ) location directly regulates the distribution of black carbon and hence affects public health in the region, but the mechanism and human health impacts have not yet been comprehensively revealed. Here we used multiple reanalysis datasets to investigate the long-term shift of seasonal-mean zonal-mean ITCZ location in this region from 1980 to 2014, and to assess the influences on black carbon distribution and the resultant health impact in terms of premature mortality. Results show that recent human-related equatorial warming contributed to an equatorward shift (∼2.1°) of ITCZ location in Maritime Continent. Spatially, the equatorward shift of ITCZ reduced surface black carbon concentration over the maritime area by enhancing updrafts and wet deposition, but raised the concentration in the continental area by inhibiting updrafts. Meanwhile, anomalous low-level northeasterlies weakened summer circulation and prevented black carbon from being transported to the Philippines. Our results also suggest that the equatorward shift decreased ∼13% of black carbon-associated monthly premature mortality in maritime countries, but increased ∼6% of that in continental countries based on the population and mortality rate in 2010. We therefore recommend considering climate change impacts in the design of adaptation strategies against regional air pollution.

Tectonic stress around the South Caspian basin deduced from earthquake focal mechanisms

ABSTRACT In this paper, we study the tectonic stress around the South Caspian Basin (SCB), which includes the Kopeh Dagh, Alborz, Talesh, eastern Greater Caucasus mountain belts, the Apsheron sill, and the Balkhan-Ashkabad fault zone. We apply the stress inversion to focal mechanisms of 410 mainshocks that have occurred over the last 69 years. These mechanisms indicate that the surrounding fault zones of the SCB exhibit diverse types of faulting, ranging from thrust to strike-slip, normal, and their combinations. The results of the stress inversion align with the kinematics of the major fault zones bounding the SCB and emphasize the spatial heterogeneity of the stress field in this region. The region is predominantly under compression, but transpressive and strike-slip regimes are also present. This highlights the role of obliquely oriented basement faults with respect to the maximum horizontal compressive stress (S Hmax ) in accommodating deformation through convergent zones. The orientation of the S Hmax is in the NE to NNE direction in the Kopeh Dagh, Alborz, Talesh, and Ashkabad-Balkhan fault zones, being rotated to NNE-N in the Greater Caucasus. The orientation of the S Hmax relative to the convergence direction of the Arabian and Eurasian plates indicates that the Arabian-Eurasian oblique convergence-derived tectonic stress is the primary contributor to the total stress and deformation in this region.

MERIDIONAL MIGRATIONS OF THE INTERTROPICAL CONVERGENCE ZONE DURING THE LAST DEGLACIATION IN THE TIMOR SEA DETECTED BY EXTENSIVE RADIOCARBON DATING

ABSTRACT At the Intertropical Convergence Zone (ITCZ), the northern and southern Tradewinds converge, and this region is characterized by low atmospheric pressure and high precipitation. The climate in the Timor Sea is characterized by seasonal precipitation changes driven by meridional migrations of the ITCZ and the monsoonal front. The ITCZ shifts in response to changes in the thermal balance between the northern and southern hemispheres. Thus, reconstruction of paleo-precipitation in the Timor Sea is expected to reveal past changes in both regional and global climate, the latter through inference of the ITCZ position. To reconstruct paleo-precipitation in the Timor Sea, we performed extensive radiocarbon analysis on both planktonic foraminifera and total organic carbon (TOC), which is derived from terrestrial and marine sources. Increased precipitation enhances the fraction of relatively old, terrestrial carbon to the core site, which in turn increases the difference between the ages of TOC and planktonic foraminifera. Variations in radiocarbon ages reveal that during northern hemisphere cooling intervals such as Heinrich Stadial 1 and the Younger Dryas, the ITCZ was in a southern position, thus increasing precipitation in the Timor Sea. However, the Timor Sea was dryer during the Bølling–Allerød warming as the ITCZ shifted northward.

Analysis and comparison of water vapor transport features and circulation anomalies during the super-strong Meiyu period of 2020 and 1998*

2020 and 1998 are the strongest Meiyu years in recent decades. The characteristics of the super-strong Meiyu precipitation and water vapor sources in 2020 and 1998 were compared, and the atmospheric circulation anomalies and the forcing factor SST were examined. (1) In 2020, the Meiyu duration, accumulated precipitation, and number of rainstorm days were greater than in 1998, and the highest since 1961. The Meiyu period in 2020 experienced 11 rainstorm processes. In 1998, a typical “second Meiyu” phenomenon occurred, and the area of heavy rainfall in 1998 was located further southward than that in 2020 (2) The contribution of the Bay of Bengal-South China Sea (BOB-SCS) to the total supply of water vapor in 2020 and 1998 was 43.0% and 42.0%, respectively, i.e., much higher than that of the climatological mean (25.5%). In 2020, the sources that provide most water vapor were the BOB, SCS, and central Pacific Ocean, while in 1998 were the Arabian Sea, BOB, and the western Pacific Ocean. (3) During the Meiyu period in 2020 and 1998, the position of atmospheric circulation pattern “two ridges and one trough” are different. Analysis of the vertical structure revealed that the specific humidity intensity above the area of heavy rainfall in 1998 was weaker than that in 2020, and the low-level convergence zone was further south and not as strong as in 2020. The positions of the western Pacific subtropical high (WPSH) and the western North Pacific anticyclone (WNPAC) in 1998 were both further south than those in 2020, which resulted in the more southerly locations of the southwesterly jet stream and rain belt. It should be pointed out that, the important contributions of the SST anomalies in the equatorial central eastern Pacific and the tropical Indian Ocean to the anomalous WNPAC in 1998 and 2020, respectively.