To elucidate dynamics of significant downward propagation (SDP) events of zonal wavenumber 1 (WN1) planetary waves from the stratosphere after sudden stratospheric warming (SSW) events and their influence on the extratropical troposphere, a case study of an SDP event in March 2023 and composite analyses for SDP events with and without SSW using JRA-3Q reanalysis are conducted. The March 2023 event is characterized by the equatorward propagation of enhanced WN1 components in the troposphere, which follows the WN1 downward propagation after the SSW. Exceptionally warm anomalies in East Asia including Japan in early March are associated with the equatorward propagation. A statistical investigation on the timing of all SDP and SSW events reveals that an SDP event is significantly more likely to occur after an SSW. The composite of the SDP event with SSW is characterized by stratospheric easterlies, in contrast to that without SSW, in which westerlies prevail in the stratosphere. Downward propagated WN1 components in the troposphere propagate equatorward and produce temperature anomalies in the extratropics, depending on the WN1 phase at high latitudes for both SDP events. When the WN1 ridge is positioned around the date line in SDP events with SSW, cold anomalies tend to cover East Asia including Japan. The WN1 component during the March 2023 event has the largest amplitude among SDP events with SSW and a ridge location around 90°W, which is far apart from the rest. These peculiar characteristics of the WN1 component would contribute to the extraordinary warm anomalies near Japan.

In 2025 Japan underwent record-high summer-mean temperature exceeding the preceding records set in 2023 and 2024. The prolonged heat in 2025 was attributable to a markedly persistent poleward shift of the subtropical jet (STJ) over the Asian continent and around Japan, associated with the early onset and persistent enhancement of Asian monsoonal convection. The early termination of the Baiu period caused extremely dry conditions nationwide in July. The remarkably-enhanced convection around the Philippines, where several typhoons sequentially formed, contributed to the marked northwestward extension of the North Pacific Subtropical High (NPSH) toward Japan through the Pacific-Japan (PJ) teleconnection in July. Sub-monthly events of Rossby-wave propagation along the STJ occasionally enhanced the extension of the upper-level Tibetan High toward Japan, causing the accentuated extreme heat. Persistent anomalous descent and increased insolation under the NPSH contributed to the record-high surface air temperatures. Global warming as well as above-normal sea-surface temperatures around Japan and over the midlatitude North Pacific may also have contributed to the record heatwave.In early August, intra-seasonally suppressed convection around the Philippines and Rossby-wave propagation over the Asian continent caused the southward-meandered STJ, with ascent along enhanced low-level moist airflow to cause locally heavy rainfall over Japan.

In summer 2024, record-high temperatures were observed over Japan, where the area-averaged summer-mean temperature was tied highest with 2023. The extreme heatwave in July was attributable primarily to the poleward-deflected upper-tropospheric subtropical jet (STJ). The low-level North Pacific Subtropical High (NPSH) intensified to the immediate south of Japan under the remote influence of enhanced cumulus convection over the northern Indian Ocean. Persistent anomalous descent and increased solar radiation associated with the NPSH contributed to the record-high temperatures in the southern portion of Japan. In August, the persistent heatwave, particularly over western Japan, was attributable to the poleward-deflected STJ and enhanced convection associated with a lower-tropospheric cyclonic gyre to the southeast of Japan where several typhoons sequentially formed. Meanwhile, heavy rainfall over northern Japan in late July was attributable primarily to developed convective systems organized just to the south the Baiu front under the intensified moist westerly airflow to the north of the markedly-extended NPSH over western Japan. Other factors that could contribute to the extreme heatwave and heavy rainfall in 2024 summer include extreme warmth of the surrounding ocean, global warming, and extremely high zonal-mean temperatures in the midlatitude Northern Hemisphere troposphere during a post El Niño summer.

This study examines the seasonal characteristics of the interannual stratospheric variability that impact the polar tropospheric climate in the northern hemisphere winter, herein referred to as the Arctic Stratospheric Oscillation (ASO). The westerly wind anomalies associated with the ASO begin in the middle stratosphere around 60°N in early winter, gradually strengthen to reach their maximum in the upper stratosphere in January, and then move downwards with a decrease in intensity to the lower stratosphere in March. The seasonal progression of the ASO is found to be associated with increasing negative sea level pressure anomalies at the polar cap, reaching their maximum in March. It has been determined that the main driving force for ASO is planetary waves, with a major contribution from non-stationary waves. It is suggested that the ASO could have an impact on the occurrence of Sudden Stratospheric Warmings and Vortex Intensifications until midwinter. November signal of the ASO is found to be a promising candidate for predicting the polar climate for the subsequent winter season in both the troposphere and the stratosphere.

In summer 2023, record-high temperatures were observed in many parts of the Northern Hemisphere, including Japan, where summer-mean temperature was the highest over the last 126 years. Under an unprecedented heatwave in late July through September, record-high temperatures were successively observed particularly over northern and eastern Japan. The late-July heatwave is attributable primarily to the markedly-intensified North Pacific Subtropical High over Japan, accompanied by the poleward-deflected subtropical jet (STJ). This situation occurred under the influence of the Pacific–Japan pattern driven by northwestward-moving enhanced tropical convection over the western North Pacific and the Silk-Road pattern. The enhanced convection was influenced by upper-level cyclonic vortices detached from the intensified mid-Pacific trough. Seemingly, it was also under the remote influence from positive sea-surface temperature (SST) anomalies in the western equatorial Pacific as well as negative ones in the central–eastern equatorial Indian Ocean, considered as remnant and delayed impacts of long-lasted La Niña until the preceding winter. The August heatwave occurred under the persistent poleward-shift of STJ as well as warm, moist low-level southerlies and their downslope-wind effects. Both extremely high SST around northern Japan and a long-term warming trend in air temperature could also contribute to the record-setting air temperature.

In this study, we investigate the rate at which the Silk Road pattern (SRP) with Rossby wave breaking (RWB) near the Asian jet exit causes the Pacific–Japan (PJ) pattern in boreal summer. In this study, the SRP case is detected using the two principal components of upper-tropospheric meridional winds over Eurasia and characterized by the presence of an upper-level anticyclonic anomaly over the Yellow Sea or near Japan. They are further classified into cases with and without RWBs. In the SRP case with RWB, the upper-level anticyclonic anomaly near the Asian jet exit has a more extended shape in the zonal direction and larger amplitude than in the case without RWB. In the composite, a wave train associated with the SRP appears over Eurasia, which is accompanied by the RWB near the Asian jet exit. The occurrence of RWB is associated with strong deceleration and diffluence in the basic state there. The RWB promotes enhanced convection on its southern side due to the intrusion of upper-level high potential vorticity toward the southwest, resulting in the formation of the PJ pattern. The excited PJ pattern in the composite has a dipole structure with cyclonic anomalies to the south and anticyclonic anomalies to the north. Approximately 60–70

This study shows a possible mechanism of abnormal extension of North Pacific subtropical high (NPSH) toward western and eastern Japan observed in late June 2022, when Japan experienced an unprecedented heat wave lasting more than a week. During the period, an upper-tropospheric anticyclonic circulation anomaly amplifies to the east of northern Japan associated with the Silk-Road pattern. A diagnosis using quasi-geostrophic potential vorticity (QGPV) inversion shows that the lower-level anticyclonic circulation anomalies induced by the upper-level anticyclone are the primary factor in the generation of lower-level negative QGPV anomalies from mainland Japan to its south associated with the anomalous extension of the NPSH. The induced circulation causes the lower-level negative QGPV anomalies by upgradient horizontal advection of the climatological QGPV, which has a strong gradient from mainland Japan to its south associated with the Baiu frontal zone. The proposed mechanism well explains a peculiar vertical structure of the observed anticyclone near Japan. A sensitivity diagnosis considering the sub-seasonal variation of the climatological lower-tropospheric QGPV distribution during summer indicates that the proposed NPSH extension mechanism toward Japan becomes most efficient from late June to early July, when the Baiu frontal zone is most enhanced near Japan.

The seasonal predictability of the Rossby wave breaking (RWB) frequency near Japan in July-August (JA) is examined using daily JMA/MRI-CPS3 (CPS3) hindcast data, which is an operational seasonal prediction system of the Japan Meteorological Agency. Although the RWB frequency near Japan during JA in CPS3 is underestimated in com-parison with the reanalysis, interannual variabilities of the frequency are generally predicted with moderate or high skill for hindcasts, initiating from February to June. The RWB frequency forecast skill in CPS3 is much higher than that in the previous version of the seasonal prediction system due to the improvement in the model bias of the Asian jet stream merid-ional position. A regression analysis for the RWB frequency near Japan utilizing all ensemble members is conducted to evaluate the reproducibility of the increased (decreased) RWB frequency associated with La Nina (El Nino) conditions, as indicated by previous studies. The regressed anomalies demonstrate an anomalous sea surface temperature (SST) pattern similar to that of La Nina and a negative phase of the Indian Ocean dipole mode with the associated anomalous convection in the tropics. For the La Nina condition, the regressed geopotential height in the upper troposphere demonstrates nega-tive anomalies over the tropical Pacific and positive anomalies in the extratropical Northern Hemisphere, corresponding to the enhanced mid-Pacific trough and northward-shifted subtropical jet. The regressed meridional wind anomalies demon-strate a wavy pattern along the Asian jet over Eurasia, consistent with the relationship between the Silk Road pattern and the RWB near the Asian jet exit region.

This study quantitatively examined the relative importance of Rossby wave breaking (RWB) east of Japan to a formation of the Pacific–Japan (PJ) pattern compared with that of tropical atmospheric and oceanographic variabilities. First, cases of the positive and negative PJ patterns are classified into those with and without the RWB occurrence. The result of the classification indicates that the cases of the positive PJ pattern triggered by the RWB account for approximately 20

To compare the predictability of two stratospheric sudden warming (SSW) events occurring in 2009 and 2010, ensemble forecast experiments are conducted using an Atmospheric General Circulation Model. It is found that the predictable period of the vortex-splitting SSW in 2009 is approximately 7 days that is much shorter than that of the vortex-displacement SSW in 2010. The latter event is predictable more than 13 days in advance. The ensemble spread in the upper stratosphere for medium-range forecasts is found to be enlarged just prior to the onset of the 2009 SSW event, whereas no such enlargement is seen for the 2010 SSW event. Stability analysis of the zonally asymmetric basic states specified by the ensemble mean forecast using a non -divergent barotropic vorticity equation reveals that the extremely distorted polar vortex in the upper stratosphere just before the onset of the 2009 SSW event is highly unstable to infinitesimal perturbations, whereas there is no such unstable mode with an extremely large growth rate during the 2010 SSW event. In addition, the most unstable mode during the onset of the 2009 SSW event has a similar horizontal structure to the 1st EOF of the ensemble spread. Thus, it is suggested that a predictability barrier inherent in the upper-stratospheric circulation, characterized by the presence of dynamically unstable modes with large growth rates, limits the predictable period of the 2009 SSW event.

Seasonal predictability of summertime Asian jet deceleration near Japan is examined using monthly mean data of hindcasts based on an operational seasonal prediction system of the Japan Meteorological Agency. Interannual variabilities of the Asian jet deceleration averaged during July–August are generally well predicted with moderate to high forecast skill starting from initial months from January to June. The seasonal predictability of the Asian jet deceleration in specific years is, by contrast, limited with large forecast errors. An inter-member regression analysis for the forecast errors of the Asian jet deceleration using ensembles shows that the forecast errors of the Asian jet are associated with those of the Asian jet deceleration near Japan. Furthermore, the forecast errors of El Niño Southern Oscillation (ENSO)-related excessive upper-tropospheric divergence near Southeast Asia can account for the errors of the northward shifted Asian jet. The above-mentioned results indicate that more accurate seasonal prediction of ENSO can further improve the seasonal prediction skill of the Asian jet deceleration and summer climate near Japan.

Using data from recent 64 years, we found that the zonal wind signal associated with the winter‐mean North Atlantic Oscillation (NAO) tends to extend into the upper stratosphere when solar activity is high, but the vertical extension is limited when solar activity is low. To examine the physical source of this phenomenon, the momentum and wave‐energy budget are analyzed using the 63‐winter record of the Japanese Reanalysis data set from 1958 to 2021. These analyses show that (a) the zonal‐mean momentum acceleration is driven by eddies throughout the entire depth of the atmosphere around 60°N, (b) the barotropic energy conversion from eddies to zonal‐mean flow is centered in the middle stratosphere around 55°N, and (c) the baroclinic energy conversion from zonal‐mean flow to eddies from the surface to the middle stratosphere around 60°N tends to be enhanced from December to February associated with the positive winter‐mean NAO index in High Solar (HS) activity years. These energy transfers are closely related to enhanced zonal wind. These analyses suggest that the structural modulation of the NAO due to the solar cycle comes from the enhanced sensitivity of the wave‐mean flow interaction in the stratosphere in HS years, which is broadly shared with the solar cycle modulation of the late‐winter Southern Annular Mode (SAM).

This study assesses the predictability of an enhanced monsoon trough south of Japan in late August 2016, which is accompanied by Rossby wave propagation over Eurasia and a consequent anticyclonic Rossby wave breaking east of Japan, with a relaxation technique using an atmospheric general circulation model. Three types of the relaxation experiments are conducted, with nudging the model forecast in the upper troposphere toward reanalysis, for regions of the Rossby wave breaking east of Japan, the Rossby wave propagation over Eurasia, and both the regions from Eurasia to the east of Japan. All types of the relaxation experiments show improved reproducibility of the enhanced monsoon trough, which the operational one-month ensemble prediction in Japan Meteorological Agency failed to predict. Compared with a result of a control experiment, the relaxation experiments show the more amplified Rossby wave propagation over Eurasia and Rossby wave breaking east of Japan, as seen in the reanalysis. The upper-level wave amplification contributes to the improved reproducibility of the enhanced monsoon trough, through that of southwestward intrusion of upper-level high potential vorticity airmass toward the southeast of Japan. The results of relaxation experiments indicate primary and secondary contributions from corrected forecast errors of the Rossby wave breaking east of Japan and the Rossby wave propagation over Eurasia to the predictability of the monsoon trough, respectively. Their relative contributions to the enhanced monsoon trough are consistent with a result of ensemble-based simple sensitivity analysis shown in a related previous study.

This study examines the predictability of an enhanced monsoon trough, which is accompanied by a largescale cyclone in the lower troposphere, south of Japan seen in late August 2016. The monsoon trough is found to be enhanced by a meandering of the Asian jet and a consequent southwestward intrusion of upper-level high potential vorticity associated with a Rossby wave breaking east of Japan. Japan Meteorological Agency's operational one-month ensemble prediction during the forecast period of a week underestimates the intensity of the Rossby wave breaking and fails to predict the enhanced monsoon trough. A simple sensitivity analysis based on ensemble singular vectors indicates that initial perturbations over the Bering Sea and near the Asian jet entrance region can efficiently grow and propagate toward the region to the south of Japan, contributing to maximize the perturbations of the enhanced monsoon trough. The time evolution of the perturbations propagating toward the region to the south of Japan is consistent with that of the ensemble spread during the forecast period. Perturbed hindcast experiments were conducted with the initial perturbations obtained from the simple sensitivity analysis. The monsoon trough to the south of Japan in the perturbed experiment is significantly more enhanced than the unperturbed experiment, supporting the simple sensitivity analysis. These results indicate a crucial contribution of the initial perturbations associated with the Rossby wave breaking and near the Asian jet entrance region to the limited predictability of the enhanced monsoon trough in late August 2016.

<p>We investigated the spatial structure of the intraseasonal variation (15-30 day) in cloud cover in the mid-latitudes during winter. We attempted to interpret the spatial pattern of clouds in　the context of Rossby waves.</p><p> </p><p>We used the total cloud cover in H-series dataset (1984-2016) by the International Satellite Cloud Climatology Project (ISCCP) based on the satellite observations, and ERA-Interim re-analysis data (1980-2016) including high, medium, and low cloud covers defined by σ coordinate.</p><p> </p><p>We calculated correlation coefficients between the geopotential height at 300hPa (Z300) at a certain position and the cloud covers, meridional wind, and vertical velocity in the surrounding area. The positions of the maximum of high (0.45≧σ) and medium cloud cover (0.8≧σ＞0.45) relative to Z300 are longitudinally constant for all longitudes except the region from east Asia to western part of the Pacific. The position of the maximum of the high cloud cover is located just west of the ridge and just east of the maximum positions of the upward motions of re-analysis vertical velocity and its adiabatic component. These results suggest that the adiabatic upward motion in the southerly wind region west of the ridge contributes to the generation of high cloud cover. In contrast, the position of the maximum of medium cloud cover is located just east of the trough. The position of the maximum of diabatic upward motion, which is consider to be due to condensation process is located near the maximum of medium cloud cover. These results suggest that Rossby waves modulate activity of short-period disturbances with precipitation. Apart from high and medium cloud covers, the position of the maximum of low cloud cover (σ＞0.8) has large longitudinal dependency. While the position of the maximum is located at almost the same as that of medium cloud cover mainly over the continent, the position of the maximum is located just east of the ridge mainly over the ocean.</p><p> </p><p>The correlation coefficients between ISCCP total cloud cover and Z300 are statistically significant only over the continent, where the positions of the maximum of high, medium, and low cloud covers are all located east of the trough and west of the ridge.</p>

Future change of Rossby wave breaking (RWB) frequency over the middle North Pacific (MNP) in August and the related features of large-scale atmospheric circulation are examined using large-ensemble simulations of current and future climates with a global circulation model. Correlation analysis indicates that the RWB frequency over the MNP in the current climate can show a relationship with El-Niño Southern Oscillation as reanalysis. The RWB frequency in the future climate shows significant decreases over the MNP, compared with that in the current climate. The large-scale atmospheric circulation in the upper troposphere in the future climate indicates a significant weakening of the Asian summer monsoon circulation and the consequent southward shifted Asian jet. The decreased RWB frequency over the MNP is associated with the modulated Asian jet through reduced diffluence and deceleration of the jet in the basic state over the region. Rossby wave propagation over Eurasia and the North Pacific in midlatitudes is also clearly reduced in the future climate, consistent with the decreased RWB frequency over the MNP. The correlation and histogram analyses of the current and future experiments indicate that the significantly decreased RWB frequency over the MNP is associated with significantly suppressed convective activities east of the Philippines in the future climate. The diagnosis using ω -equation further shows the dynamical impact of the decreased RWB frequency on the suppressed convective activities through the weakened extension of the Mid-Pacific trough and the consequent weakening of dynamically induced ascent east of the Philippines.

This study presents a possible large-scale factor of tropical cyclogenesis over the western North Pacific, which is triggered by Rossby wave breaking to the east of Japan. More than half of the wave breaking cases is accompanied by the tropical cyclogenesis. Results from a composite analysis for the wave breaking cases indicate that the genesis and development of tropical cyclones are dominant over the southwest quadrant of the wave breaking center, where an intrusion of the upper-level potential vorticity caused by the wave breaking and the consequent enhanced convection are seen. The number of tropical cyclones in the wave breaking cases exponentially increases in time during the developing stage of the wave breaking. The results of composite analysis further indicate that lower-level strong wind convergence and the associated enhanced convection, which are resulting from the wave breaking, is favorable conditions for the tropical cyclogenesis. An enhanced monsoon trough accompanied by the Pacific–Japan pattern resulting from the enhanced convection can regulate tracks of the tropical cyclones. These results show that the Rossby wave breaking can trigger the tropical cyclogenesis over the western North Pacific, through the southwestward intrusion of the upper-level potential vorticity and the consequent enhanced convection.

Rossby waves propagating along the Asian jet frequently cause the breaking near the jet exit region. This study examines characteristics of oceanographic condition and atmospheric circulation associated with interdecadal variability of Rossby wave breaking frequency near Japan in August. Sea surface temperature during a period of the higher Rossby wave breaking frequency is cooler over the central part of the tropical North Pacific, compared with that during a period of the lower frequency. Convective activities are suppressed over the region consistent with the cooler sea surface temperature, contributing to an enhanced and southwestward extended mid-Pacific trough. Deceleration and diffluence of the Asian jet are stronger during the period of the higher frequency than that during the period of the lower one. The enhanced deceleration and diffluence of the jet are associated with the enhanced and southwestward extended mid-Pacific trough. The abovementioned dynamical influence is also shown by a numerical simulation using an atmospheric linear baroclinic model. These results indicate that the interdecadal variability of sea surface temperature over the central part of the tropical North Pacific has an impact on that of the Rossby wave breaking frequency near Japan, through the modulated convective activities and mid-Pacific trough.

Rossby wave propagation along the Asian jet during boreal summer, such as the Silk Road pattern, frequently causes wave breaking near the Asian jet exit region. This study examines the statistical relationship between interannual variability of the Rossby wave breaking frequency near Japan and large-scale atmospheric circulation during the boreal summer. The Rossby wave breaking frequency in the midlatitudes climatologically shows its maximum near Japan, and significantly increases during La Nina years. The upper-tropospheric circulation regressed onto the Rossby wave breaking frequency near Japan in August shows large-scale anomalous convergence from the tropical central to eastern Pacific and divergence around the Indian Ocean. The consequent northward anomalous divergent wind over Eurasia contributes to enhancement and northward shift of the Asian jet. The Asian jet also shows meridional meandering with a phase of anomalous anticyclonic circulation near Japan accompanied by the frequent Rossby wave breaking, which is associated with the Silk Road pattern. The frequent Rossby wave breaking is related to southwestward intrusion of anomalous low potential temperature air mass toward the subtropical western North Pacific associated with an enhanced mid-Pacific trough. West of the southwestward cold-air intrusion, enhanced cumulus convection is seen around the northern Philippines, and the Pacific-Japan pattern is significantly seen in the lower troposphere. This result is consistent with a previous study that revealed a linkage mechanism between the Rossby wave breaking near Japan and the Pacific-Japan pattern through dynamically induced ascent resulting in an intrusion of high potential vorticity associated with the Rossby wave breaking.