2016年8月後半におけるアジアジェットの蛇行及びロスビー波の砕波に伴うモンスーントラフ強化の予測可能性

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 2016.2016年8月下旬に日本の南海上の対流圏下層で発達した、大規模な低気圧を伴うモンスーントラフの予測可能性を調べた。このモンスーントラフはアジアジェットの蛇行、及びそれに伴う日本の東海上でのロスビー波の砕波と関連する上層での高渦位大気の南西方向への侵入によって強化されたことが分かった。気象庁現業1か月アンサンブル予報は、予測初期1週間においてロスビー波の砕波の強度を過小に予測し、モンスーントラフの強化を予測できなかった。アンサンブル特異ベクトル法に基づく簡易予報感度解析の結果、ベーリング海やアジアジェット入口付近にあった初期摂動は、効率的に成長しながら日本の南海上に向かって伝播し、モンスーントラフを強化する摂動の最大化に寄与しうることが分かった。日本の南海上に向かって伝播する摂動の時間発展は、予測期間におけるアンサンブルスプレッドの時間発展と対応していた。簡易予報感度解析より得られた初期摂動を与えた再予報実験を行った結果、摂動を与えた実験では、摂動を与えなかった実験と比べて日本の南海上でのモンスーントラフの強化がより明瞭となり、簡易予報感度解析の結果と整合していた。これらの結果は、ロスビー波の砕波及びアジアジェット入口付近における初期摂動が、2016年8月後半に強化したモンスーントラフの予測可能性に大きく寄与したことを示している

Mechanism for the Abnormal Extension of North Pacific Subtropical High toward Japan in Late June 2022

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

Tropical Cyclogenesis Triggered by Rossby Wave Breaking over the Western North Pacific

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

Interdecadal Variability of Rossby Wave Breaking Frequency near Japan in August

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

シルクロードパターン事例の何割がロスビー波の砕波を通して太平洋・日本パターンを引き起こすのか？

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 % of the SRP case with RWB is accompanied by the PJ patterns. Conversely, in the case of the SRP without RWB, the composite represents a wave train structure over Eurasia but indicates neither enhanced convection south of the RWB nor PJ patterns. Approximately 40–50 % of the SRP case without RWBs is accompanied by the PJ patterns. Hence, the presence of RWBs increases the percentage of the formation of positive PJ patterns by a factor of 1.2–1.7, indicating that the RWB plays an important role in the excitation of PJ patterns.本研究では、夏季アジアジェット出口付近でロスビー波の砕波を伴うシルクロードパターンが、太平洋・日本（PJ）パターンを引き起こす割合を調査した。ここで、シルクロードパターン事例は、ユーラシア大陸上での対流圏上層の南北風の主成分分析に基づき、黄海及び日本付近が高気圧性偏差となるパターンで特徴づけられる第1、2主成分を用いて抽出した。さらに、抽出した事例を、砕波を伴う事例と伴わない事例に分類した。砕波を伴うシルクロードパターン事例では、アジアジェット出口付近の上層での高気圧性偏差は、砕波を伴わない事例と比べて東西により広がった形状を持ち、振幅も大きい。この事例の合成図では、シルクロードパターンに伴う波列パターンがユーラシア大陸上に存在し、アジアジェット出口付近で砕波を伴っていた。砕波の発生は、砕波域でのアジアジェットの強い減速及び分流と関連する。また砕波は、上層の高渦位気塊の進入を通して、砕波域の南側で活発な対流活動を促し、PJパターンを形成する。合成図において出現する明瞭なPJパターンは、南側で低気圧性偏差、北側で高気圧性偏差を持つ双極子構造を示す。そして、砕波を伴うシルクロードパターン事例の約60～70%が、PJパターンを伴っていた。一方、砕波を伴わないシルクロードパターン事例の合成図では、ユーラシア大陸上で波列パターンは存在するが、砕波域の南側で活発化した対流活動及びPJパターンは存在しない。そして、砕波を伴わないシルクロードパターン事例の約40～50%がPJパターンを伴っていた。したがって、砕波によって正のPJパターンの出現頻度は1.2～1.7倍に増加し、砕波はPJパターンの励起に重要な役割を果たしていることが明らかになった

太平洋・日本パターンに関する新たな視点：ロスビー波の砕波によって引き起こされる事例の推定割合

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 % of the whole cases of the positive PJ pattern. The number of positive PJ cases with the RWB further accounts for approximately 80 % of those in the cases associated with the RWB. Results of a lag composite analysis and the related Q-vector diagnosis for the cases of the positive PJ pattern with the presence of RWB show that the RWB east of Japan promotes the formation of the PJ pattern through the southwestward intrusion of high potential vorticity air mass toward the subtropical western North Pacific (WNP) and the consequent dynamically induced enhanced convection over the region, consistent with results of previous studies. By contrast, the composite for the cases the negative PJ pattern accompanied by the RWB indicates that the RWB-related upper-tropospheric zonally elongated anomalous circulation and basin-wide sea surface temperature (SST) warming over the Indian Ocean can contribute suppressed convective activities over the subtropical WNP and the consequent formation of the negative PJ pattern although the RWB occurs. The composite for the cases of positive and negative PJ patterns with the absence of RWB further indicates that the convective activities over the subtropical WNP move northwestward with time, causing the formation of the PJ pattern. The formation of the PJ pattern with the absence of RWB is also closely associated with tropical SST and phase of the boreal summer intraseasonal oscillation, illuminating “pure” tropical impacts on the formation of a PJ pattern.太平洋・日本（Pacific–Japan; PJ）パターンの形成に及ぼす日本の東海上で生ずるロスビー波砕波の寄与の重要性を、熱帯大気海洋変動の寄与と比較しながら定量的に調べた。まず、正及び負の位相を持つPJパターンのそれぞれの事例を、砕波が発生した事例と発生しなかった事例に分類した。その結果、砕波によって引き起こされた正位相のPJパターン事例数は、正位相全事例の約20％を占めることが分かった。また、砕波を伴う正位相のPJパターン事例数は、砕波を伴う正負両位相事例の約80％を占めていた。次に、砕波を伴う正位相のPJパターン事例について、ラグ合成図を用いたQベクトルに基づく診断を行った。その結果、日本の東海上での砕波が、北西太平洋亜熱帯域へ向かう南西方向への高渦位大気の進入と、それによって北西太平洋亜熱帯域で力学的に惹起される強い対流活動を通して、PJパターンの形成を促進することが示された。このPJパターンの形成メカニズムは先行研究で示されたものと一致する。一方、砕波を伴う負位相のPJパターン事例を対象とする合成図解析によって、砕波と関連する対流圏上層での東西方向に延伸した大気循環場偏差、及びインド洋全体での正の海面水温偏差が北西太平洋亜熱帯域での対流活動を弱化させたために、砕波の発生にも拘わらず、負位相のPJパターンが形成されたことが示された。最後に、砕波を伴わない正負両位相のPJパターン事例では、北西太平洋亜熱帯域での対流活動域が、時間とともに北西進してPJパターンの形成を促すことが合成図解析の結果より示された。砕波を伴わないPJパターンは、熱帯での海面水温偏差や北半球夏季季節内振動の位相と密接に関連し、「純粋な」熱帯の影響によって形成されることが明らかとなった

大規模アンサンブル気候予測実験において見出された地球温暖化に伴う夏季北太平洋中央部でのロスビー波の砕波頻度の減少

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.現在気候及び将来気候を対象とする大気大循環モデルによる大規模アンサンブルシミュレーションの結果を用いて、８月の北太平洋中央部におけるロスビー波の砕波頻度の将来変化、及びそれに関連する大気循環場の特徴を調べた。現在気候実験における北太平洋中央部での砕波頻度は、再解析データと同様に、エルニーニョ・南方振動と関連することが相関解析より示された。将来気候実験における北太平洋中央部での砕波頻度は、現在気候実験と比べて顕著に減少することが分かった。将来気候実験では、アジアモンスーン循環が顕著に弱化し、その結果としてアジアジェット気流が南偏する傾向が見られた。このアジアジェット気流の将来変化に伴って、北太平洋中央部ではジェット気流の分流・減速が弱化し、それは砕波頻度の減少と関連していた。また将来気候実験では、ユーラシア大陸及び北太平洋の中緯度でロスビー波の波束伝播が弱化する傾向が明瞭であり、このことは砕波頻度の減少と整合的である。相関解析及び頻度分布の解析より、将来気候実験における砕波頻度の減少は、フィリピンの東海上での積雲対流活動の弱化と関連することが示された。さらに、ω方程式を用いた診断より、砕波頻度の減少は、中部太平洋トラフの弱化及びそれに伴う力学的上昇流の弱化を通して、フィリピンの東海上での積雲対流活動の弱化に影響を及ぼすことが示された