Reference: Yin Q. Z. and Berger A., 2010. Insolation and CO2 concentration contribution to the interglacials before and after Mid-Brunhes Event. Nature Geoscience, in press.
1. Seasonally evolving dominant modes of the East Asian Monsoon
Two dominant modes of the East Asian monsoon correspond to post-ENSO year and ENSO turnabout, respectively. In the first mode, during the El Ni��o decaying summer, negative precipitation anomalies and associated anticyclone anomalies appears in the western North Pacific (WNP). The anticyclone anomalies cause the positive precipitation anomalies extending from the middle and lower reaches of the Yangtze River valley and Huaihe River valley to the southern Japan. During the subsequent fall, cyclone anomalies replace the anticyclone anomalies and cover the WNP. The cyclone anomalies cause the negative precipitation anomalies over southeastern China. The anomalous precipitation pattern persists during the following winter and spring. In the second mode, during El Ni��o developing summer, twin cyclonic couplet stimulated by the positive heating over the equatorial central Pacific enhance the WNP monsoon. Meanwhile, an anomalous anticyclone appears in the tropical northern Indian Ocean. During the following fall and spring, the anomalous anticyclone gradually moves to the South China Sea and WNP. The anticyclone intensifies the precipitation over southeastern China through increasing moisture transport. The anomalous anticyclone and associated precipitation anomalies maintain during the following spring through local positive air-sea feedback.
2. Relative roles of the Indian Ocean and local SSTA in forcing circulation anomalies over the WNP during El Ni��o decaying summers
Numerical experiments indicate that the WNPAC is maintained by the combined effects of the local forcing of the negative SSTA in the WNP and the remote forcing from the Indian Ocean basin-wide warming. The local SSTA forcing is crucial for the WNPAC in early summer. With the attenuation of the negative SSTA, the local forcing gradually weakens as the summer progresses. With the development of the WNP monsoon trough, the remote forcing from the tropical Indian Ocean, on the other hand, strengthens from June to August. From the prediction point of view, the negative SSTA in the WNP may be a better predictor for the East Asian summer monsoon. This is because the WNPAC exerts a dominant impact on the EASM in June, when the climatological Meiyu-Baiu rain band is strongest.
3. Asymmetry of circulation anomalies in the WNP between El Ni��o and La Ni��a
The asymmetry of the WNP low-level atmospheric circulation anomalies between El Ni��o and La Ni��a mature winter is examined. An anomalous cyclone (WNPC) center during La Ni��a tends to shift westward relative to an anomalous anticyclone (WNPAC) center during El Ni��o. Two factors may contribute to this asymmetric response. The first factor is the longitudinal shifting of El Ni��o and La Ni��a anomalous heating. The second factor is attributed to the amplitude asymmetry of SST anomalies (SSTA) in the WNP viz. the amplitude of local cold SSTA during El Ni��o is greater than that of warm SSTA during La Ni��a. The asymmetry of SSTA is originated from the asymmetric SSTA tendencies during the ENSO developing summer. Although both the precipitation and surface wind anomalies are approximately symmetric, the surface latent heat flux anomalies are highly asymmetric over the key WNP region, where the mean wind speed is small. Both the anomalous westerly during El Ni��o and the anomalous easterly during La Ni��a in the region lead to an enhanced surface evaporation, strengthening (weakening) the cold (warm) SSTA in situ during El Ni��o (La Ni��a). The asymmetric circulation in the WNP leads to significantly different precipitation anomalies over southeastern China, viz. the precipitation anomaly during the El Ni��o mature winter is greater than its La Ni��a counterpart.
近年の研究では、海洋中のTIWに対する応答が大気境界層内で発見され注目されて来ている。しかし、それらの偏差が海洋の不安定波に与えるフィードバックついてはあまり知られていない。また、熱帯不安定波が全球の気候に及ぼす影響についても明らかになっていない。
本研究では、TIWの大気海洋相互作用を明らかにすることを目的に、次の3点に注目して大気海洋結合モデルMIROCを用いた解析を行った。
1. TIWに対する大気の応答は、TIWに対して何らかのフィードバックを及ぼすのか?
2. TIWに対する大気の応答が大気大循環場に与える影響
3. 熱帯不安定波とENSOの相互作用
TIWに対する大気の役割を調べるために、TIWを解像可能な高解像度海洋モデルに、T42(TIW解像不可)、T106(TIW解像可)、T213(TIW解像可)の大気モデルを結合したCGCM MIROCの実験結果を比較した。その結果、解像度の違いによるITCZ及びそれに伴う基本場の風の再現の違いが海洋の東西流シアの形成に大きく影響し、TIWの強度に影響を及ぼすことが明らかとなった。
次に、熱帯東太平洋においてTIWを含む短周期成分を除去したSSTを境界条件としてT106 AGCMに与える感度実験を行い、同じAGCMの標準実験と比較することで、TIWに対する大気の応答が存在することにより大循環場がどのような影響を受けるかを調べた。この実験から、TIWがITCZを南下させる働きを持つという結果が得られた。前述の解像度別実験の比較からITCZはTIWに対して重要な影響を及ぼすことが分かっているため、AGCM実験から得られたITCZの南下に伴う風応力場の変化が海洋表層の流速場にどの程度影響を与えるかを、中解像度OGCMを用いて検証した。その結果、TIWに伴うITCZの変化は、TIWに対して小さいながらも負のフィードバックの傾向を持つことが明らかとなった。
最後に、中解像度MIROCにTIWのパラメタリゼーションを導入して、TIWがENSOに与える影響を調べた。先行研究では、観測等からTIWがENSOのskewnessを増加させる(エルニーニョがラニーニャよりも大きくなる)働きをすることが示唆されてきたが、本パラメタリゼーション実験からその説を支持する結果が得られた。また、TIWによる赤道域の加熱の効果が表現されるようになったことで、冷舌のcoldバイアスが緩和する結果となり、本パラメタリゼーションがCGCMの再現性の向上や季節予報の精度向上に貢献することが期待される。
このような現状のもと、我々の研究グループでは空間解像度を高め、詳細な雲微物理過程を計算する事で直接的に雲の成長過程を表現しようと試み、非静力大気大循環モデルNICAMの開発を進めてきた(Satoh etal., 2008)。しかし、現状ではNICAMに導入されている雲微物理モデルでは雲の質量のみを予報する1momentバルク法を採用しており(Tomita,2008)、粒径に対する情報に任意性が残っている為に雲の光学的性質が十分ではない(Masunaga etal.,2008)。NICAMを用いてCRFを定量的に評価する為には現状の雲微物理モデルに含まれている粒径に対する任意性の除去が不可欠であり、この情報を予報する必要がある。そこで、本研究では雲の質量に加えて個数を予報変数として扱う2momentバルク法を開発し(NDW6)、粒径の任意性を排し雲微物理過程に基づいて雲の光学特性を評価出来るようにした。
始めに熱帯のスコールラインを想定した理想化実験でNDW6のパフォーマンスを調べ、次にMISMO(Yoneyama etal.,2008)を対象とした領域実験を行いNDW6のバリデーションを行った。
NDW6のスキームを変更した感度実験を行った所、特に予報するモーメントの数、氷粒子の形状は短波、長波放射に対するCRFに大きな影響を持つ事が確認された。また、光学的物理量の衛星との比較により適切な雲微物理スキームの検討を行った。
*Large scale aspects
The large-scale aspects that affected Isobel’s lifecycle will be presented in the first half. The westerly wind burst (WWB) accompanying the onset of a Madden-Julian Oscillation (MJO) event over the Java Sea enhanced the cyclonic shear and convergence in the lower troposphere, providing the pre-conditioned large-scale environment for the genesis of Isobel. In the subsequent evolution, five stages are identified for the simulated Isobel, namely, the initial eddy, intensifying, temporary weakening, re-intensifying, and decaying stages. At the initial eddy stage, small-/meso-scale cyclonic vortices (eddies) developed in the zonally-elongated rainband organized in a convergent shear-line in the lower troposphere. As the MJO propagated eastward, the cyclonic eddies moved southward with intensifying convective activities, showing the signal of cyclogenesis over the Timor Sea. In an environment with weak vertical shear and low-level strong cyclonic vorticity enhanced by the trade easterlies and WWB associated with the MJO, a typical tropical cyclone structure appeared, leading to the development of tropical storm Isobel (intensifying stage). An approaching subtropical high from the southwest exposed Isobel to strong vertical shear and resulted in strong low-level large-scale stretching deformation field over Isobel as the WWB of MJO propagated eastward. This change led to the development of asymmetric structure in the inner core of Isobel and interrupted its intensification, causing a temporary weakening (temporary weakening stage). Therefore, the MJO had both positive and negative effects on the simulated Isobel’s lifecycle. As the vertical shear weakened and changed the direction due to the development of upper-level anticyclonic circulation, Isobel re-intensified in response to the eyewall reformation process as a result of the inward spiraling outer rainband that was formed downshear of the vertical shear vector (re-intensifying stage). Finally Isobel decayed due to the land effect as it approached the land and made landfall in northwest Australia (decaying stage).
*Mesoscale process on genesis
In the second part, both the mesoscale and storm-scale processes in the lifecycle of the simulated Isobel will be presented. In the preconditioned favorable environment over the Java Sea, mesoscale convective vortices (MCVs) developed in the mesoscale convective systems (MCSs) enhanced convection and triggered the genesis of vortical hot towers (VHTs). The merging of multiple VHTs was found to play an important role in the formation of MCV with concentric PV monolith structure, leading to the genesis of Isobel. The genesis of Isobel was accomplished by the multiscale vortex interaction and showed a bottom-up development.
*Storm-scale process on intensification
After its genesis, Isobel developed into a tropical storm over the Timor Sea under the favorable environment. The system scale intensification (SSI) process was found to result primarily from the response of the mid-upper-level secondary circulation to the axisymmetric condensational heating in the eyewall. A potential vorticity (PV) budget analysis indicated that the cyclonic PV sources including condensing PV due to the vertical divergence and inward PV flux were enhanced by the mid-upper-level secondary circulation, leading to the intensification of Isobel. Under the unfavorable environment during the temporal weakening stage, the development of asymmetric structure almost terminated the SSI process due to the loss of axisymmetric diabatic heating and thus interrupted the intensification of Isobel. Later on, Isobel experienced axisymmetrization and the eyewall reformation with the recovery of eyewall convection, leading to the SSI process to operate effectively and thus the re-intensification of Isobel.
そこで本研究では、SPRINTARSの硫酸過程を改変した。特に硫酸塩の生成反応の解法を変えることで硫酸塩の分布を改良することができた。硫酸塩の直接効果放射強制力の見積もりの絶対値も大きくなり、IPCCの見積もり値は-0.4W/m2に近い値が計算された。さらに、新しく硝酸エアロゾルをシミュレーションした。地表付近での質量は観測と比べて概ね合っており、季節変動も再現できた。硫酸塩/アンモニウム/硝酸塩の直接効果放射強制力は-0.6W/m2と見積もられた。つまり、新しく導入したアンモニウムと硝酸塩によって約0.2W/m2さらに小さくなった。そして、人為起源エアロゾルによる間接効果放射強制力を見積もり、IPCC第四次報告書(2007)の見積もり値に近いものが得られたが、この値は、人為起源エアロゾルの見積もりだけではなくてバックグラウンドエアロゾルの見積もりにも大きく依存していることがわかった。