気候システムセミナー

日時：金曜日 13:30-15:00
場所：東京大学柏キャンパス 総合研究棟270室
（＊変更の場合もありますので、詳細は下記の予定をご確認ください）

今後の予定

• Title: 台風渦が全球の気候場の表現に与える影響
• Time: 14:00 - 15:30 on June 24, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: 台風渦が全球気候場の表現にどのように寄与しているかを調査した。観測された大気には常に台風渦が含まれているため、気候場に埋め込まれたその寄与を直接評価することは困難であるため、本研究では全球再解析データから台風成分を診断的に分離する枠組みを開発した。具体的には、従来の渦位逆変換法（PV inversion）に台風の表現に重要な力学的効果を組み込み、改良したものである。
  　本手法をJRA-3Q再解析データに適用し、1979年から2023年までの6時間間隔の台風渦除去データセットを構築した。事例解析の結果、本手法により、下層の低気圧性渦度や上部対流圏の暖気核などの台風に起因する循環構造を、力学的な不整合を生じさせることなく適切に分離できることを確認した。さらに、台風渦は全球平均循環に顕著な寄与を持ち、モンスーントラフの強化、亜熱帯高気圧の弱化、および上部対流圏の昇温に寄与していることが明らかとなった。また、台風渦はその通過域の広範な領域において、下層相対渦度の季節内変動（20–80日周期）の50%以上を説明し、一部の格子点では90%以上を占めることが示された。
  　本データセットにより、元の解析場と台風渦除去場（背景場）を同一時刻で直接比較することが可能となる。台風渦除去場は、各解析時刻において台風渦のみを診断的に取り除いた大気場を表すものであり、「台風が存在しない仮想的な大気場」を再現するものではない。しかし、本データセットは、さまざまな時空間スケールにおける気候場への台風渦の寄与を評価するうえで有用である。

• Title: Responses of extreme fire weather to CO2 emission reductions and underlying mechanisms
• Time: 15:30 - 16:30 on May 14, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: While fire weather risk is projected to increase under global warming, its response under mitigation scenarios remains unclear. Using idealized emission-driven simulations, we compare extreme fire weather under net-zero (ZeroE) and net-negative (NegE) pathways. NegE leads to substantial reductions in fire danger, especially in high-risk regions, due to cooler temperatures and higher humidity. In contrast, fire danger remains elevated under ZeroE, particularly in low-latitude Northern Hemisphere regions. These differences are linked to changes in atmospheric moisture supply associated with the Atlantic Meridional Overturning Circulation and shifts in the Intertropical Convergence Zone. Overall, our results highlight that aggressive CO₂ removal is essential, as net-zero emissions alone may not sufficiently reduce future fire weather risk.

• Title: Climate histories from noble gases in ice
• Time: 10:30 - 12:00 on April 23, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: Because of their chemical and biological inertness, noble gases are powerful tracers of physical processes. Their elemental and isotopic ratios in ice provide constraints on local and global conditions. Here we discuss the use of ice core noble gas ratios to reconstruct past mean ocean temperature, and briefly highlight noble gas isotope dating of old ice to probe ocean temperature evolution on million-year timescales.

• Title: 能動型衛星観測に基づく雲の全球的な力学特性および微物理特性に関する研究
• Time: 13:00 - 15:00 on April 21, 2026.
• Place: General Research Bldg. 2F room 270

• Title: Long-term sea-level consequences of greenhouse gas emissions imply urgency of net negative emissions
• Time: 13:30 - 15:00 on April 13, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: Sea-level rise is one of the most impactful consequences of anthropogenic climate change. It results principally from thermal expansion of sea water and the transfer of mass from glaciers and ice sheets to the ocean. Net-zero emissions of carbon dioxide, the dominant greenhouse gas, would likely stabilise surface temperatures at a level proportional to total emissions. However, the immense inertia of the ocean and cryosphere mean global mean sea level (GMSL) will continue to rise for centuries to millennia. Here, we demonstrate how past and future GMSL rise are related to a simple policy relevant metric: the product of how much carbon has been emitted and how long ago those emissions occurred, termed Carbon Air Time (CAT). Observations and GMSL projections collapse onto the same GMSL versus CAT curve with each Tt of Carbon causing an ongoing 30 to 70 cm of rise per century. GMSL rises as CAT increases, even after net emissions are reduced to zero. Halting continued multi-century GMSL rise will require halting increases in CAT, only possible with negative emissions that reduce warming to near preindustrial levels. The Carbon Air Time model is a potentially valuable policy tool for estimating how future emissions drive sea level change, and for guiding mitigation goals, notwithstanding the possibility for abrupt ice-sheet collapse. This is joint work with Prof. John Church.

• Title: Perspectives on Climate Sensitivity
• Time: 15:00 - 16:30 on April 13, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: The notion of climate sensitivity has become synonymous with Equilibrium Climate Sensitivity (ECS), or the equilibrium response of the Earth system to a doubling of CO2. But, there is actually a hierarchy of measures of climate sensitivity, some of which probe important physics beyond that of ECS and which are more relevant to 21st century warming. This talk will focus on two of them, the well known Transient Climate Response (TCR) and the lesser known Transient Climate Response to Cumulative Emissions (TCRE). Using the two-box model for ocean heat uptake as a common theoretical framework, we derive a TCR scaling which applies quite generally across multi-decadal scenarios, but also find discrepancies with ocean heat uptake on longer timescales. Generalizing to emissions-driven simulations, we find that the TCRE depends critically on the airborne fraction of cumulative emissions, which tends towards 0.5-0.6 but for which a theoretical understanding is lacking.

• Title: QBO–MJO connection: observation and mechanisms
• Time: 10:30 - 12:00 on April 9, 2026.
• Place: General Research Bldg. 2F room 270
• Abstract: The Quasi-Biennial Oscillation (QBO) and the Madden–Julian Oscillation (MJO) are strongly coupled during the boreal winter. Observational evidence shows that the MJO is approximately 40% stronger and persists about 10 days longer during the QBO easterly phase (EQBO) than during its westerly phase (WQBO). Despite the robustness of these observations, numerical models fail to reproduce this QBO–MJO connection, and the underlying physical mechanisms remain a subject of active debate. In this presentation, I explore two leading hypotheses explaining the QBO–MJO connection. The first focuses on the Kelvin-wave-like cold anomalies in the upper troposphere and lower stratosphere (UTLS) that weaken local static stability. Idealized model experiments indicate that these MJO-induced cold anomalies are intensified during EQBO, as QBO zonal winds modulate the vertical propagation of Kelvin waves in the UTLS. The second hypothesis concerns longwave cloud–radiative feedback within the MJO envelope. Analysis of satellite datasets reveals that cloud-top pressure and brightness temperature of deep convection are systematically lower during EQBO. These deeper clouds are more effective at trapping outgoing longwave radiation, thereby enhancing the cloud–radiative feedback that sustains and strengthens the MJO. Finally, the current challenges and limitations in modeling the QBO–MJO connection are briefly discussed.
  
  https://www.nature.com/articles/s41467-023-39465-7
  https://www.nature.com/articles/s43017-021-00173-9
  https://journals.ametsoc.org/view/journals/clim/30/6/jcli-d-16-0620.1.xml
  

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