[Japanese]
修士論文要旨(松島 史弥)
気候変動に伴う台風特性の変化
松島 史弥
台風は東南アジア・太平洋地域に大きな被害をもたらすため，温暖化に伴い台風がどのように変化するかは重要な問題である．先行研究には，強い台風の数や割合が多くなっているという研究もある．しかし，高解像度で多数の台風についてシミュレーションが行われた研究はなく，台風強度の将来変化について，完全には解明されていない．そこで，本研究では，水平解像度約2 ㎞で「現在気候・近未来気候・将来気候」という3つの気候実験ごとに約60 個の台風について解析を行うことで，温暖化に伴う台風強度や台風内部構造の変化を明らかにすることを目的とする.
解析の結果から，台風が生涯で最大強度となった平均緯度は，現在気候，近未来気候，将来気候でそれぞれ20.38°，21.61°，21.95°であることが示された．将来気候では現在気候に比べて約1.5°平均的な最大強度緯度が北上していた．
気候実験間で強い台風にどのような変化があるのかを示すため，最大強度上位30％の台風について暖気核構造と風速場を調べた．高度2㎞の最大風速半径で規格化した最大強度時のコンポジット解析より，最大の温位偏差が現在気候に比べ将来気候で大きくなっており，暖気核が強くなっていた．また，上向きの鉛直風について，現在気候に比べて将来気候で強まっており，二次循環が強まっていることが確認できた．
本研究で得られた新知見として，詳細な物理に基づいたシミュレーションによって台風強度の将来変化を示したということが挙げられる．台風中心気圧の平均は気候実験間で傾向が見られなかった．これは，将来気候において強い台風と弱い台風の両方が増えることに起因すると考えられる．より強い台風に注目して，最大強度上位30％の台風にについて統計をとると，平均最低中心気圧は将来気候ほど低くなる．
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[English]
Change of Tropical Cyclone Characteristics under the Changing Climate
Fumiya MATSUSHIMA
Since typhoons cause great damage to Southeast Asia and north western Pacific regions, it is an important problem how the typhoons change with global warming. Previous studies have shown that the number and ratio of strong typhoons are increasing. However, there are no simulations that have been performed for many typhoons with high resolution, and the future changes in typhoon intensity have not been thoroughly examined. Therefore, the purpose of this study is to clarify the changes in typhoon intensity and internal structure of typhoons with global warming by examining about 60 typhoons in each climate experiment of "current climate, near-future climate, and future climate" with about 2 km horizontal resolution.
Latitudes of typhoons lifetime maximum intensity are 20.38 ° , 21.61 ° and 21.95 ° N for the present climate , the near future climate and the future climate, respectively. In the future climate, the maximum intensity latitude was about 1.5 ° north of the present climate.
In order to show how strong typhoons change between climate experiments, we investigated the warm core structure and wind field for the strongest 30% typhoons in each climate. From composite analyses at the maximum intensity normalized by the radius of maximum wind speed at an altitude of 2 km, the maximum potential temperature anomaly was larger in the future climate than in the present climate, and the warm core was stronger. In addition, we found that the upward vertical velocity is stronger in the future climate than the present climate. This means that secondary circulation of typhoon is stronger in the future climate.
A new finding in this study is the future change of typhoon intensity shown by detailed physics-based simulations. The average minimum central pressure showed no trend with global warming. This is attributed to increases in both strong and weak typhoons in the future climate. On the other hand, focusing on typhoons with the 30％ of the highest maximum intensity as strong typhoons, the average minimum central pressure in the future climate was significantly smaller than that of the present climate.
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