Tropical Intraseasonal Oscillation Research Articles

Coherent amplification of the Okhotsk high, Korean trough, and northwestern Pacific subtropical high during heavy rainfall over Japan in August 2021

In August 2021, rain front stagnation in Japan resulted in prolonged and disastrous rainfall across the entire country. During the heavy rainfall period, the large-scale atmospheric field over the East Asian–western North Pacific region was characterized by meridional tripolar circulation anomalies: the Okhotsk high (OH), the trough over the Korean Peninsula (Korean trough), and the northwestern Pacific subtropical high (NWPSH). Simultaneously, tropical convective activity was enhanced over the eastern Indian Ocean and suppressed over the tropical western–central Pacific. This study investigates the dynamic mechanism of linkage of the extratropical tripolar anomalies and the effects of tropical convective modulation using a reanalysis dataset, a cutoff low detection scheme, the potential vorticity inversion method, and numerical experiments. Upper-tropospheric blocking over eastern Siberia connected to the surface OH is conducive to the stagnation of synoptic depressions, including cutoff lows and troughs, over the Korean Peninsula, contributing to the development and maintenance of the quasi-stationary Korean trough. Rossby waves emanating from the Korean trough excite an anticyclonic anomaly over the northwestern Pacific. This upper-level anomalous anticyclone acts to enhance the surface NWPSH through zonal heat transport, accompanied by a northward tilting structure with height. Simultaneously, the tropical intraseasonal oscillation is amplified over the Indo–western Pacific Ocean sector under the negative-phase Indian Ocean dipole and multi-year La Niña conditions. The combination of enhanced convection over the eastern Indian Ocean and suppressed convection across the tropical western–central Pacific reinforces the NWPSH. The anomalous circulation associated with the extratropical tripolar pattern and concurrent tropical heat forcing causes more moisture transport, convergence, and anomalous ascent, which contribute to heavy rainfall in Japan. These results suggest that the dynamically correlated amplification of tropical and extratropical circulation anomalies plays a crucial role in precipitation variability in East Asia.

Influences of intraseasonal oscillations on a compound cold and wet event in southern China during the Spring Festival travel rush

AbstractIn January 2023 during the Spring Festival travel rush, a compound cold and wet event (CCWE) hit southern China, causing heavy precipitation followed by two cold wave processes. Through an analysis from the intraseasonal oscillation (ISO) perspective, it was found that the cold event is caused by two consecutive cold events, in which upper level midlatitude ISO wave train propagating across the Eurasia favours the formation of blocking high, leading to the expansion of Siberian High and intensification of East Asian winter monsoon. As a result, strong cold advection appeared in the northern boundary of southern China and caused the cooling process. As for the wet event, it was found that a cyclonic ISO anomaly from South China Sea conveyed abundant moisture through the western and southern boundaries and caused the precipitation. It was revealed in the partial lateral forcing (PLF) experiment that the midlatitude ISO signal from the northern boundary caused minimum 2 m temperature to drop by 1.84°C, mainly affecting the northern part of the research area. The tropical ISO from the western and southern boundaries resulted in heavy precipitation for over 40 mm in a belt‐shaped area in 3 days. The combination of the ISOs is the key factor behind this CCWE and our study provides intuitive evidences for this, which is helpful in understanding the mechanisms and providing empirical precursory signals for the forecast of CCWEs.

Minimal Mechanisms Responsible for the Dispersive Behavior of the Madden–Julian Oscillation

An attempt has been made to explore the relative contributions of moisture feedback processes on tropical intraseasonal oscillation or Madden–Julian Oscillation (MJO). We focused on moisture feedback processes, including evaporation wind feedback (EWF) and moisture convergence feedback (MCF), which integrate the mechanisms of convective interactions into the tropical atmosphere. The dynamical framework considered here is a moisture-coupled, single-layer linear shallow-water model on an equatorial beta-plane with zonal momentum damping. With this approach, we aimed to recognize the minimal physical mechanisms responsible for the existence of the essential dispersive characteristics of the MJO, including its eastward propagation (k>0), the planetary-scale (small zonal wavenumbers) instability, and the slow phase speed of about ≈5 m/s. Furthermore, we extended our study to determine each feedback mechanism’s influence on the simulated eastward dispersive mode. Our model emphasized that the MJO-like eastward mode is a possible outcome of the combined effect of moisture feedback processes without requiring additional complex mechanisms such as cloud radiative feedback and boundary layer dynamics. The results substantiate the importance of EWF as a primary energy source for developing an eastward moisture mode with a planter-scale instability. The eastward moisture mode exhibits the highest growth rate at the largest wavelengths and is also sensitive to the strength of the EWF, showing a significant increase in the growth rate with the increasing strength of the EWF; however, the eastward moisture mode remains unstable at planetary-scale wavelengths. Moreover, our model endorses that the MCF alone could not produce instability without surface fluxes, although it has a significant role in developing deep convection. It was found that the MCF exhibits a damping mechanism by regulating the frequency and growth rate of the eastward moisture mode at shorter wavelengths.

Increase in optimal configuration of 25–60-day atmospheric circulations for Yangtze heavy rainfall under global warming background

In summer, the middle and lower reaches of the Yangtze River (MLYR) often suffers from severe floods as a result of a dominant intraseasonal oscillation (ISO) of rainfall with the quasi-period of 25–60 days. 75% of flooding events during summers of 1961–2022 are found to occur following wet phases of the 25−60-day ISO in rainfall. This study objectively extracts the optimal configuration (OC) of 25−60-day atmospheric ISOs for the wet phase of MLYR rainfall. The OC is characterized by the quadruple pattern of tropical convection configuring with the 25−60-day Silk Road Pattern (SRP) in the mid−high latitudes. The 25−60-day SRP in cooperation with tropical ISO over the western Pacific induces dipolar vertical cells over eastern China, with ascending branch between the vertical cells providing the vitally dynamical condition for MLYR rainfall. With topographic blocking of the Tibetan Plateau, the northward-propagating tropical ISO over the Indian Ocean enhances maintenance of the 25−60-day SRP via upper-tropospheric divergent flows, thus anchoring anomalous ascents to the MLYR. Numerical experiments further verify the remarkable tropical-extratropical interaction around the Tibetan Plateau in this OC. Under the global warming background, the tropical 25−60-day ISO tends to strengthen, leading to an increase in the occurrence frequency of strong OC through tropical-extratropical interaction, and thus the severe flood events doubled in summers of 1991–2020.

Mitigating Model Error via a Multimodel Method and Application to Tropical Intraseasonal Oscillations

Mitigating Model Error via a Multimodel Method and Application to Tropical Intraseasonal Oscillations

Interplay between Boreal Summer Intraseasonal Oscillation and Southern Hemisphere Stratospheric Polar Vortex Warming

Abstract The boreal summer intraseasonal oscillation (BSISO) features more distinctive and complex propagation characteristics than its wintertime counterpart, the Madden–Julian oscillation (MJO). While the relationship between the MJO and the Arctic stratosphere during boreal winter has been widely documented, the linkage between the BSISO and the Antarctic stratosphere during austral winter has not been extensively discussed. Here, after identifying the Southern Hemisphere (SH) stratospheric polar vortex warming (SPVW) events, we reveal the bidirectional connection between BSISO and SPVW. Before onset of the SPVW events, the occurrence frequency and amplitude of BSISO phase 5 (P5) shows a significant increase. The most significant responses of the SH polar stratospheric temperature to the BSISO are found about 10 days after BSISO P5. Thus, to some extent, BSISO P5 can be regarded as a precursor to the SH SPVW event, which is attributed to the enhanced upward propagation and dissipation of planetary waves in the SH stratosphere induced by the BSISO P5. After onset of the SPVW events, a significant increase in the occurrence and amplitude of BSISO P6 is observed, corresponding to the enhanced convection over the South China Sea and southern Philippine Sea. Tropical upwelling associated with the strengthened Brewer–Dobson circulation (BDC) induced by the SPVW tends to result in unstable circumstances in the tropical upper troposphere. Then the high correlation between static stability at 150 hPa and outgoing longwave radiation (OLR) anomalies over the South China Sea and southern Philippine Sea provides robust evidence that the intensity of convective activity in the tropics can indeed be modulated by the variability in SH stratospheric polar vortex. Significance Statement The relationship between the boreal summer intraseasonal oscillation (BSISO) and the Southern Hemisphere (SH) polar stratosphere remains unclear. After selecting the stratospheric polar vortex warming (SPVW) events, we reveal the two-way connection between BSISO and SPVW. Before the SPVW, the significant increase in occurrence and amplitude of BSISO phase 5 (P5) suggests that BSISO P5 can be regarded as a precursor to the weakened polar vortex. After the SPVW, the significant increase in occurrence and amplitude of BSISO phase 6 (P6) establishes that environmental alteration in tropical upper troposphere induced by the SPVW provides a favorable condition for the growth of ensuing convective activity. The results here help us better understand the potential link between stratospheric polar vortex and tropical intraseasonal oscillation.

Boreal Summer Extratropical Intraseasonal Waves over the Eurasian Continent and Real-Time Monitoring Metrics

Abstract Boreal summer extratropical intraseasonal oscillation (EISO) is crucial in modulating regional subseasonal variation and particularly causing extreme meteorological events, but it has yet to be well clarified and operationally monitored. This study first objectively sorts out three dominant EISOs trapped along two extratropical westerly jet streams over Eurasia, and then proposes the corresponding real-time metrics. The three dominant EISOs are (i) an 8–25-day eastward-propagating wave along the subtropical westerly jet (EISO-SJE) initiating at the exit of the North America–North Atlantic jet and strengthening over the Black Sea–Caspian Sea–arid central Asia region; (ii) a 10–30-day eastward-traveling wave along the polar front jet (EISO-PJE), starting near Scandinavia and enhancing from the East European Plain to the West Siberian Plain and then decaying over the Okhotsk region; (iii) a 10–40-day westward-migrating wave along the polar front jet (EISO-PJW), which enhances near the Ural Mountains and weakens over Scandinavia. The real-time metrics then, following the three EISOs, have been constructed, and they are able to capture the spatiotemporal features of three EISOs in application. Moreover, the close linkages between these EISOs and the regional extremes/the blocking occurrence have been clearly demonstrated, confirming the importance of real-time EISO metrics. Together with tropical intraseasonal oscillation, this study provides the subseasonal-to-seasonal (S2S) community with a well-portrayed unified picture of extratropical intraseasonal waves and the real-time metrics for monitoring boreal summer intraseasonal signals over Eurasia and facilitate subseasonal predictions. Significance Statement Boreal summer extratropical intraseasonal oscillation (EISO) has drawn increasing attention owing to its importance in triggering extreme weather events and affecting regional subseasonal prediction. However, despite the urgent need of the subseasonal-to-seasonal (S2S) community, a comprehensive delineation of EISO diversity and real-time EISO monitoring remain the gap of knowledge. This study objectively sorts out and comprehensively clarifies three dominant EISOs trapped along two extratropical westerly jet streams over Eurasia. More importantly, the well-portrayed real-time EISO metrics are constructed based on three EISOs, which are applicable for operational real-time monitoring, subseasonal prediction, and model evaluation. This study stimulates an extratropical focus in the S2S community as a complementary component in addition to monitoring the MJO’s teleconnection to the mid- to high latitudes.

Equatorial Rossby waves and their impacts on monsoon region deep convection

Convectively coupled equatorial Rossby waves are the dominant mode of westward-moving subseasonal convection in the tropics. A portion of the variance in these waves has been shown to associate with the tropical intraseasonal oscillation, along with a process often mediated by the extratropical Rossby wave response to tropical convection that yields Rossby waves breaking back into the tropical atmosphere. The potential vorticity anomalies driven by Rossby wave breaking become the equatorial Rossby waves. This work creates an index of planetary scale equatorial Rossby waves and applies the method of seasonally varying regression slope coefficients to diagnose their preferred associations with tropical and extratropical circulation features. Results confirm the already known association between these waves and the extratropical atmosphere and they reveal a pattern of westward-and northward-moving anomalies of tropical convection over the Indian Ocean and Southern Asia during the Northern Hemisphere summer. These patterns are associated with a cycle of suppression and enhancement of convection in which negative anomalies of outgoing longwave radiation are found to be 3-times as likely during the wet than the dry phases of the waves.

Causes of the Record-breaking Drought in Taiwan in 2020-2021

<p>2020年梅雨季提早結束，夏季沒有侵臺颱風，秋冬季雨量亦明顯偏少，2021年春雨也是寥寥可數，就在水庫快要見底之際，望眼欲穿的梅雨終於在5月底到來，臺灣旱象才得以逐漸緩解。此次旱災除在經濟上有許多損失外，亦對民眾生活造成諸多不便，值得深入了解形成旱象的物理機制。</p> <p>統計2020年6月至2021年5月的臺灣雨量，達到1910年有紀錄以來的最少，堪稱為百年大旱。本報告分析此段期間的大尺度環流，推測造成臺灣百年大旱的可能原因，並歸納主要發現如下：①2020年6月至9月的少雨，主要是極端偏強的副熱帶高壓造成，當時的副高不僅在強度上達到最強，持續時間亦為1949年之後的最持久；副高偏強可能與熱帶各洋面同時出現海溫異常有關，包含反聖嬰在中東太平洋的偏冷、印度洋及大西洋的偏暖。②臺灣2020年秋冬季雨量偏少，可能與反聖嬰及菲律賓海偏暖有關。③2021年臺灣春雨偏少，主因應該是菲律賓海溫偏暖，其次為逐漸減弱的反聖嬰，負相位的太平洋年代際震盪亦有少許的貢獻；當菲律賓海偏暖時，會在其西側產生氣旋式環流，為臺灣及東亞沿岸帶來水氣較少的東北風距平，進一步造成臺灣春雨較少；反聖嬰有利上述偶合系統的維持。④2021年3月中旬至5月期間的熱帶季內振盪較為活躍，季內振盪可能使臺灣4月中下旬的西南水氣源減少，5月季內振盪乾區逐漸接近臺灣，進而造成臺灣少雨。簡而言之，2020-2021年的臺灣百年大旱不是單一因素造成的極端事件，而是許多因素同時出現的複合現象。</p> <p> </p><p>During 2020, an earlier-ended Mei-Yu season combined with a typhoon-absent summer, followed by notably low rainfall from following autumn to spring in 2021, Taiwan’s accumulated rainfall from June 2020 to May 2021 broke the lowest record since 1910, we report the unique characteristics and physical mechanism of this extreme event.</p> <p>The possible causes of the drought event are presented as follows. ①The duration and strength of subtropical high over the Western North Pacific from Jun to September were the longest and strongest since 1949. The abnormal subtropical high maybe caused by the compounding effects from cool sea surface temperature (SST) over the central to eastern equatorial Pacific associated with La Niña, and warm SST in the northern Indian Ocean and the tropical Atlantic. ②The warmest SST appeared in the Philippine Sea through October 2020 to Marth 2021, concurrent with La Niña, generated the dry condition near Taiwan. ③ The less spring rainfall in Taiwan in 2021 could be directly attributed to the warmest SST in the Philippine Sea and indirectly to the weakening La Niña , whereas negative phase of the Pacific Decadal Oscillation seemed to have little effect. An anomalous cyclone usually appeared in the western side of the warm Philippine Sea and brought dry northeasterly anomaly to the East Asian coast, resulting in less spring rainfall in Taiwan, and the La Niña would favor and maintain this atmosphere-ocean coupled system. ④ In April-May 2021, active tropical intra-seasonal oscillation caused weaker moist southeasterly in middle to late April, and also resulted in dry condition before the end of May.</p> <p>In conclusion, the most serious drought event in 2020-2021 since 1910 was caused by the compounding effect from various influencing factors that occurred concurrently and sequentially. How much of these abnormal conditions were caused by the warming trend in recent decades, which is not investigated in this study, warrants further studies.</p> <p> </p>

Moisture Modes of Tropical Intraseasonal Oscillations—High Order and Anti‐Symmetric Solutions

AbstractThe Madden Julian Oscillation (MJO) and the Boreal Summer Intraseasonal Oscillation are fundamental climate modes in the tropical atmosphere on the intraseasonal time scales. A recent study developed a linear moisture mode theory for unified treatment of these intraseasonal oscillations under different meridional moisture gradients. Using scaled Hermite polynomials as the basis for the meridional structure, it is shown that the system has infinite number of normal modes under the same parameter values, as n → ∞. The v = 0 and n = 1 analyzed in the previous study are two special cases. These moisture modes are non‐orthogonal. The idealized MJO solutions derived using the parameters in the boreal winter conditions bear similar meridional structure for planetary scale waves in the Earth‐like atmosphere, unlike the higher order solutions (which are often considered unrealistic) in the classical tropical waves theory. The solutions include both symmetric and anti‐symmetric modes. The symmetric modes are analogous to the Kelvin‐Rossby wave paradigm of the MJO. The anti‐symmetric modes features cross‐equatorial flow near the equator and subtropical gyres. Both growth rates and frequencies of the scaled higher order n modes display small increments compared to lower order modes. In the asymptotic limit n → ∞, a growth rate higher than other modes with finite n is achieved.

A Filtered Model for the Tropical Intraseasonal Moisture Mode

AbstractThe authors propose a new analytic approximation to aid understanding of the “moisture modes” that arise in idealized models of tropical intraseasonal oscillations. The approximation makes use of the separation between the time scale of the large scale flow and that of convective adjustment, but does not neglect the moisture tendency. The approximation filters out equatorial Kelvin waves, so that the only remaining mode is the intraseasonal moisture mode. Some physical insights are more transparent under the approximation: (a) vertical advection of moisture or temperature lead to a diffusion‐type operator that damps short waves, and instability is strongest at planetary scale. (b) A positive effective gross moist stability appears in a wave operator, while a negative effective gross moist stability leads to instabilities whose growth rates increase with zonal wavenumber. (c) Zonal moisture advection and surface flux feedbacks are crucial for both planetary instability and eastward propagation.

A Multivariate Index for Tropical Intraseasonal Oscillations Based on the Seasonally‐Varying Modal Structures

AbstractThe spatial structure and propagation characteristics of tropical intraseasonal oscillations vary substantially by season. In this study, these seasonal variations are identified using a multivariate sliding‐window Empirical Orthogonal Function (EOF) analysis. The two modes comprising the leading EOF pair have equal variances and depict the propagation of intraseasonal oscillations in convection and low‐level circulation over the Indian Ocean, the Maritime Continent, and the western Pacific region in the equatorial summer hemisphere. In contrast, the upper tropospheric circulation shows more structure in the winter hemisphere. It is suggested that this variation in seasonality with height is an inherent feature of intraseasonal oscillations. A new multivariate index for tropical intraseasonal oscillations (MII) is developed based on the leading EOFs and represents the three‐dimensional structure of intraseasonal variability in all seasons. The MII is computed by projecting intraseasonal anomalies onto the leading EOFs pair, and it exhibits clearly delineated but smooth seasonal transitions and rich meridional structure. The real‐time version of this new index, rMII, is shown to be similar to MII, with a correlation of 0.9. Compared to the widely used Real‐time Multivariate MJO (RMM) index, the power spectrum of rMII represents substantially greater intraseasonal variance, and the application of rMII in dynamical forecast models indicates rMII is skillfully predicted for an additional week compared to RMM.

Origins of the Intraseasonal Variability of East Asian Summer Precipitation

AbstractAccurate subseasonal prediction of East Asian summer precipitation (EASP) requires a deep understanding of the origins of its intraseasonal variability (ISV). However, the relative contributions to the EASP ISV from local processes, tropical intraseasonal oscillation (ISO), and extratropical influence remain unknown. We conducted a set of numerical experiments with a grid nudging method to confirm the roles of external forcing from mid‐to‐high latitude wave trains and tropical ISO and quantify their relative contributions. The Community Earth System Model can reproduce realistic EASP ISV and associated preceding signals. The tropical and mid‐to‐high latitude forcings account for, respectively, 53% and 40% of the total variance for the two leading ISV modes of EASP. The tropical forcing contributes more significantly to the two leading modes (31% and 40%, respectively) compared to the extratropical forcing (24% and 15%). These external forcings mainly affect EASP regional averages rather than the values of local (grid) scale.

Changes in MJO Characteristics and Impacts in the Past Century

Abstract The changes of the MJO behavior and its impact on global precipitation, precipitation extremes, and high-frequency variability during the past century (1901–2010) were investigated using the NOAA-20CR dataset. It was found that the MJO amplitude was significantly strengthened, while its eastward phase speed hardly changed. The impacts of the MJO on precipitation in South China (SC), northern Australia (AU), and California (CA) were investigated. The anomalous rainfall in the regions was strengthened from the early to late twentieth century, with the percentage increase ranging from 67% in AU to 14% in CA. A moisture budget analysis indicated that the enhanced precipitation was primarily attributed to the effect of anomalous wind while the effect of the mean moisture change was small. The impact of the local meridional wind anomaly was critical in SC, while in AU and CA the zonal wind component dominated. The precipitation extremes had a significant increase from the early to late twentieth century. The cumulative extreme precipitation amount increased by 140%–150% in SC and AU and by 100% in CA, and the number of the extreme days increased by 110% in SC and AU and 70% in CA. Such increases are consistent with the strengthening of the high-frequency (HF; with a period less than 10 days) variability across the globe. The pattern of percentage increase of the HF variability resembles the pattern of percentage increase of MJO-scale precipitation. This implies that the MJO teleconnection pattern could exert a large-scale control on the HF variability and weather extremes. Significance Statement The Madden–Julian oscillation (MJO), a dominant mode of tropical intraseasonal oscillations, plays an important role in affecting global weather and climate. To understand to what extent the MJO and associated teleconnections change with global warming, this study adopts an alternative approach by analyzing a century-long reanalysis dataset rather than the outputs of climate models that consist of large uncertainties due to poor MJO simulations. We found that MJO-induced tropical and midlatitude precipitation and associated extremes and high-frequency variabilities were strengthened during the past century while the global mean surface temperature increases approximately by 1°C. The results derived from the present study provide a basis for assessing the future MJO behavior and associated climate impacts and for improving the extended-range prediction of severe weather and climate extremes.

Synergistic Effect of the 25–60‐day Tropical and Midlatitude Intraseasonal Oscillations on the Persistently Severe Yangtze Floods

AbstractSignificant 25–60‐day intraseasonal oscillations (ISOs) in rainfall over the middle‐lower reaches of the Yangtze River (MLYR) are found for eight summers during 1979–2020. These 25–60‐day ISOs of MLYR rainfall account for 66.7% of persistently severe floods (PSFs) during 1979–2020, indicating their major role in inducing PSFs and the potential for extended‐range forecasting of PSFs. The 25–60‐day ISO in MLYR rainfall results from the synergistic effect of tropical and midlatitude ISOs. During the eight identified summers, more barotropic energy is transferred from summer‐mean to 25–60‐day circulation, thus forming a midlatitude ISO wave train, which cooperates with the tropical ISO in triggering twin vertical cells. Phase locking of ascending branch from the vertical cell induced by the tropical ISO with that from the reversed vertical cell triggered by the midlatitude ISO brings about the wet phase of 25–60‐day ISO of MLYR rainfall.

Role of Microphysics and Convective Autoconversion for the Better Simulation of Tropical Intraseasonal Oscillations (MISO and MJO)

AbstractThe seasonal prediction of the Indian summer monsoon (ISM) and Monsoon Intraseasonal Oscillations (MISO), as well as the Madden Julian Oscillations (MJO) that strongly modulate MISO, is of much importance to the agriculture sectors of the Indian Sub‐continent. This study highlights the importance of selecting proper “autoconversion” coefficients for precipitation formation in Coupled Global Climate Model's convective and microphysical schemes in simulating MISO and MJO. With a series of sensitivity experiments, we systematically examined the convective autoconversion and microphysical autoconversion coefficients in the National Centers for Environmental Prediction Coupled Forecast System version 2 (CFSv2). The mean and intraseasonal features of MISO and MJO are improved with a proper combination of autoconversion coefficients, which not only provide a better partition of cloud water and ice but also better feedback between the large‐scale condensation and convective parameterization. The results demonstrate a road map for the improvement of MISO as well as MJO simulation using a coupled climate model.

East Asian Summer Monsoon Rainfall Anomalies in 2020 and the Role of Northwest Pacific Anticyclone on the Intraseasonal‐to‐Interannual Timescales

AbstractIn summer 2020, extreme rainfall occurred over the middle and lower reaches of the Yangtze River Basin (YRB). The accumulated precipitation amount of this rainfall event broke the record since 1961. On the interannual timescale, seasonal mean rainfall anomalies in the YRB during summer 2020 are related to the tropical North Indian Ocean (NIO) sea surface temperature (SST) warming and a La Niña event in the equatorial eastern Pacific. Both the NIO SST warming and La Niña help to generate the anomalous tropospheric low‐level Northwest Pacific anticyclone (NWP‐AAC) via the Matsuno‐Gill type response. On the subseasonal timescale, rainfall anomalies in the YRB exhibit two significant periods during summer 2020, intraseasonal (40–60 days) and quasi‐biweekly (∼10 days). The former intraseasonal component is the focus of this study. The intraseasonal NWP‐AAC, which is a part of tropical intraseasonal oscillation, plays a dominant role in connecting the East Asian summer monsoon (EASM) rainfall with tropical convection. Using the pattern regression, a real‐time index (RTI) is proposed to represent the intraseasonal‐to‐interannual variability of the NWP‐AAC. The RTI captures the quasi‐biennial tendency of NWP‐AAC as well as its intraseasonal variability. These suggest that the NWP‐AAC is a common mode across timescales. Also, the RTI well depicts the EASM rainfall variability in 2020. Our results provide an alternative view of cross‐scale EASM variability, with an emphasis on the conduit effect of NWP‐AAC from intraseasonal to interannual timescale. Hence, the RTI could be utilized for subseasonal to seasonal climate monitoring in the Asian summer monsoon region.

Impact of the Boreal Summer Intraseasonal Oscillation on Typhoon Tracks in the Western North Pacific and the Prediction Skill of the ECMWF Model

AbstractThe tropical intraseasonal oscillation (ISO) strongly influences tropical cyclone (TC) genesis. However, little is known concerning its impact on TC tracks. Here, we examine how TC tracks in the western North Pacific are modulated by the boreal summer ISO (BSISO) during each month of the TC season (June–October) using the best track data and how well the modulation are reproduced in the European Centre for Medium‐Range Weather Forecasts (ECMWF) model forecasts. The results reveal that the impact of the BSISO on the typhoon tracks varies month to month. The ECMWF model successfully reproduces this impact up to a month in advance. A simple advection model shows that the reproduction of the modulations in the TC genesis location leads to accurate TC track forecasts. These results suggest that the BSISO is one of the major sources of TC track predictability at the subseasonal time scale (2 weeks to 2 months).

Effect of tropical sub‐seasonal variability on heatwaves over India

AbstractOne of the important consequences of changing climate is the increase in temperature extremes and heat stress over many regions of the world. Understanding the various factors that modulate these events on different time scales is of utmost importance due to their great societal relevance. For example, understanding and prediction on sub‐seasonal time scales is crucial for reducing losses due to extremes and diligent management of limited resources. In this study, the effect of tropical intraseasonal oscillations like convectively coupled equatorial waves (CCEWs) such as equatorial Rossby wave (ER), Kelvin wave, mixed Rossby gravity waves and the Madden and Julian Oscillations (MJO) in modulating heatwaves occurrence over India is explored. We find that these oscillations significantly modulate extreme heat events. The extent of the modulation is also found to be highly dependent on the location, type and phase of these waves. The maximum effect is observed in case of ER with positive phase (defined as high outgoing longwave radiation phase) enhancing the frequency of the events by almost twice the normal values, and supporting long duration and more intense heatwaves while the opposite phase totally suppresses the heatwave events. The effect of MJO is region dependent with positive (negative) phase increasing (decreasing) the heatwave frequency over South and Eastern (North‐western) parts of the country. Kelvin and MT waves are also found to have lesser but significant effect on heatwaves, and mostly reduces the occurrence of heatwaves. Overall, the study shoes that the ER wave has the most significant effect on the frequency and duration of heatwaves over the study region.

Asymptotic Models for Tropical Intraseasonal Oscillations and Geostrophic Balance

AbstractIn the tropics, rainfall is coupled with waves in the form of, for example, convectively coupled equatorial waves (CCEWs) and the Madden–Julian oscillation (MJO). In perhaps the simplest viewpoint of CCEWs, the effects of moisture and convective adjustment can predict the basic aspects of their propagation and structure: reduced propagation speeds and reduced meridional length scales. Here, a similar simple viewpoint is investigated for the MJO’s propagation and structure. To do this investigation, budget analyses of a model MJO are first presented to illustrate and motivate the asymptotic scaling assumptions. Asymptotic models are then derived for the MJO. In brief, the structure of the asymptotic MJO is described by a tropical geostrophic balance, and the slow propagation arises from the dynamics of moist static energy. To be specific, if the moist static energy has a background vertical gradient that is asymptotically weak (i.e., a moist stability that is nearly neutral), then it supports a slowly propagating wave. Beyond these main aspects, other processes also have an influence, such as eddy diffusion of moisture. In comparing the simple viewpoints of CCEWs and the MJO, one main difference is in the propagation speeds: relative to a dry wave speed of 50 m s−1, the MJO has a speed of 5 m s−1, resulting from a reduction factor of 0.1 related to moist stability, whereas the basic CCEW speed is 15 m s−1, resulting from a reduction factor of the square root of 0.1, related to the square root of the moist stability.