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: Two Pathways for Tropical Deep Convection Evolution in the Top-Heaviness Plane: Perspective of A New Diagnostic
• Time: 10:30 - 12:00 on Feb. 14, 2025.
• Place: General Research Bldg. 2F room 270
• Abstract: TBD

• Title: 地球システムモデルを用いた全球炭素循環及び海洋環境の再構築と予測
• Time: 14:30 - 16:00 on Jan. 28, 2025.
• Place: General Research Bldg. 2F room 270
• Abstract: TBD

• Title: Integrating Simulation, Satellite Observations, High-Performance Computing, and AI to Construct Climate Digital Twin
• Time: 16:00 - 17:30 on Jan. 28, 2025.
• Place: General Research Bldg. 2F room 270
• Abstract: TBD

• Title: Scale- and flow-aware subgrid closure for mesoscale eddies in ocean climate models
• Time: 13:30 - 15:00 on Jan. 20, 2025.
• Place: General Research Bldg. 2F room 270
• Abstract: Ocean mesoscale eddies play a pivotal role in the mixing of momentum, heat, carbon, and other properties, significantly impacting climate dynamics. State-of-the-art ocean climate models, with horizontal resolutions of approximately 0.25 degrees, can resolve the largest mesoscale eddies but often lack an energetic eddy field due to missing feedback from smaller eddies and dissipation by the lateral momentum viscosity. Subgrid closures are necessary to preserve the energy of resolved eddies and represent unresolved eddy fluxes. In this study, we implement and evaluate two eddy closures―the Gent-McWilliams (GM) parameterization for eddy buoyancy flux and the backscatter parameterization for eddy momentum flux― within the GFDL ocean climate model at 0.25-degree resolution, aiming to achieve a scale- and flow-aware representation of ocean mesoscales. The GM parameterization improves the ocean stratification and reduces the over-ventilation bias in the Southern Ocean, crucial for preventing super-polynya events. The backscatter parameterization enhances the eddy energy and strengthens large-scale currents, reducing sea surface temperature and salinity biases in the North Atlantic. However, applying GM and backscatter together reveals a challenge: GM strongly dissipates eddies, fronts, and deep boundary currents energized by the backscatter. To address this issue, we introduce a vertical structure and topography slope function to reduce the GM diffusivity in the deep ocean and steep topography regions, mitigating its damping effects on deep boundary current. In addition, we improve GM with a spatially filtered buoyancy gradient and a flow-aware mixing length, making GM mainly effective on large-scale buoyancy fields and less dissipative to mesoscale eddies. To further reduce energy dissipation, we are implementing a high-order momentum advection scheme with a weak, flow-aware numerical diffusion to damp grid-scale noises. These new approaches show promising results in simulating an energetic eddy field and reducing global-ocean temperature and salinity biases, advancing toward scale- and flow-aware mesoscale large eddy simulations.

• 場所: 総合研究棟270室

• Title: A Geostationary Satellite‐Based Approach to Estimate Convective Mass Flux and Revisit the Hot Tower Hypothesis
• Time: 13:30 - 15:00 on Dec. 13, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: This study aims to revisit the classic “hot tower” hypothesis proposed by Riehl and Simpson (Malkus) in 1958 and revisited in 1979. Our investigation centers on the convective mass flux of hot towers within the tropical trough zone, using geostationary (GEO) satellite data and an innovative analysis technique, known as ML16, which integrates various data sources, including hot tower heights, ambient profiles, and a plume model, to determine convective mass flux. The GEO-based ML16 approach is evaluated against collocated ground-based radar wind profiler observations, showing broad agreement. Our GEO-based estimate of hot tower convective mass flux, 2.8×10¹¹ to 3.4×10¹¹ kg s^-1, is similar to the revisited estimate in Riehl and Simpson (1979), 2.6-3.0×10¹¹ kg s^-1. Additionally, our analysis gives a median count of around 550 hot towers with a median size of about 11 km, in contrast to the previous estimates of 1600-2400 hot towers, each characterized by a fixed size of 5 km. We discuss the causes of these discrepancies, emphasizing the fundamental differences between the two approaches in characterizing tropical hot towers. Finally, we identify future opportunities in Earth Observations that will provide more accurate measurements, enabling further evaluation of the role played by tropical hot towers in mass transport.

• Title: 熱収支解析に基づく熱帯季節以下スケール大気海洋相互作用の評価
• Time: 10:00 - 11:30 on Dec. 6, 2024.
• Place: General Research Bldg. 2F room 270

• Title: Surface Impacts of the QBO on East Asia: Its Mechanisms and Prediction
• Time: 13:30 - 15:00 on Nov. 22, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: The quasi-biennial oscillation (QBO) is a dominant interannual variability of stratospheric circulation, characterized by the equatorial zonal wind switching its direction approximately every 14 months. Although the QBO is an equatorial stratospheric phenomenon, it significantly affects the Northern Hemisphere surface climate. In this study, it is found that the QBO, defined as 70-hPa zonal wind in the deep tropics, is negatively correlated with the surface air temperature over the East Asia in February-March. Cold temperature anomaly appears during the QBO westerly phase. Such relationship is likely mediated by the subtropical jet. During the QBO westerly phase, a horseshoe-shaped zonal wind anomaly forms in the upper troposphere and lower stratosphere and is connected to the equatorward shift of the Asia-Pacific jet. This equatorward jet shift is accompanied by a cyclonic circulation anomaly in the subtropical North Pacific and an anticyclonic circulation anomaly over northern Eurasia in the troposphere. The resultant temperature advection brings cold air to East Asia and the western North Pacific. This regional downward coupling in February and March, which is not sensitive to El Nio-Southern Oscillation, has become statistically significant in recent decades. This QBO downward coupling is well predicted by operational forecasting systems, but climate models that resolve QBOs show large inter-model spread.

• Title: Radiation in the climate system
• Time: 10:15 - 12:00 on Nov. 21, 2024.
• Place: General Research Bldg. 3F room 320

• Title: Unveiling abrupt transitions of Antarctic ice sheet response during the mid-Pleistocene transition
• Time: 10:15 - 12:00 on Nov. 21, 2024.
• Place: General Research Bldg. 3F room 320

• Title: The impact of rain on the global ocean carbon uptake
• Time: 10:00 - 11:30 on Oct. 31, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: The ocean plays an important role in the global carbon cycle by absorbing about a quarter of the carbon emitted by human activities every year. Exchanges between the ocean and the atmosphere are governed by various chemicophysical and biological processes. Even though it has been overlooked in past studies due to its intermittence, rain is one of these processes as it (i) increases interfacial turbulence, (ii) dilutes sea surface and alters biogeochemical equilibria and (iii) directly injects CO2 absorbed during its fall into the ocean (wet deposition). We propose the first global estimation of the resultant of these three effects on the global ocean uptake over the 2008-2018 period. This estimate is based on observational products (IMERG, SOCAT) and reanalysis. The study shows that rain increases the oceanic carbon sink by 140 to 190 million tons of carbon per year (MtC/y or 0.14-0.19 PgC/y). This represents an increase of 5 to 7% in the 2.66 billion tons absorbed annually by the oceans. Turbulence and dilution primarily increase the CO2 sink in tropical regions characterized by heavy rainfall events associated with weak winds, which induces noticeable salinity and CO2 dilution. In contrast, the deposition by raindrops is significant in all regions with heavy precipitation: the tropics, of course, but also the storm tracks and the Southern Ocean. Large uncertainties remain on the rain rates themselves and on the associated ocean's surface response. In particular, no observational constraint exists on the wet deposition. It is however possible to evaluate the impact of the rain rate and the associated dilution using various estimates of rainfall (ERA5 and IMERG) and different parameterization of the interfacial ocean. We use radar measurements of rain collected during pre-YMC (2015) and YMC (2017) field studies to compare rain rates from ERA5 and IMERG. ERA5 is shown to produce too frequent and too weak rain rates as compared to radar observation and IMERG. In addition, ERA5 and IMERG do not represent rain events at the same time and positions. We thus introduce a quantile mapping method to correct the hourly rain rates from ERA5 reanalysis that preserves the spatio-temporal distribution of ERA5 precipitations. This ensures that we preserve the coherence between precipitations and surface wind that is particularly important for the CO2 flux. We then apply this method globally on ERA5 using IMERG as a reference and show the sensitivity of our carbon sink increase estimate. Finally, this first global estimate suggests that the impact of rain should be included in ocean and climate models to compute global carbon budgets. Rain effects on CO2 flux have thus been incorporated in a global ocean biogeochemistry model (NEMO-PISCES) forced by an atmospheric reanalysis (JRA55). In this framework, when the effects of rain on CO2 flux are computed as simple diagnostics, we reproduce the impact of rain on the global carbon sink deduced from global products and reanalysis. Next step is to investigate the role of rain in a fully coupled framework where rain acts in the global carbon cycle. Preliminary results will be presented.

• Title: Bridging between weather and climate from high-resolution to long timescale phenomena
• Time: 10:00 - 11:30 on Oct. 30, 2024.
• Place: General Research Bldg. 2F room 270（＊気候コロキウムと合同開催）
• Abstract: There is deep uncertainty, how short-term weather extremes, and comparably longer-lived bio-geo-climatological extremes such as persistent high ocean temperatures, decadal/centennial droughts, or abrupt ice-shelf retreat interact on decadal to centennial timescales, for which warming levels this matters, and whether this adds global and/or regional uncertainty to future projections. In this talk I will outline potential avenues to bridge the gap between the meteorological world of high-resolution and process understanding, to the geological evidence from ice cores, sediments and caves for longterm climate variability.

• Title: 全球超高解像度大気シミュレーションの紹介
• Time: 15:00 - 16:30 on Oct. 29, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: 富岳を最大限に活用し全球雲解像モデルNICAMを用いて水平メッシュ間隔220mまでの全球大気シミュレーションを行った。本発表では解像度間の大規模場の違い、対流表現の違いなどについて発表する。数kmから数百m解像度は大気シミュレーションで用いる乱流パラメタリゼーションにおいて、レイノルズ平均モデル(RANS)、Large Eddy Simulation(LES)モデルの双方の境界に位置する、グレーゾーンに該当する。本研究では、RANSスキームであるMYNNとLESスキームであるSmagorinskyスキームをそれぞれ用いた実験を行い比較を行った。

• Title: Coupled climate-ice sheet simulations of the long-term past and future
• Time: 10:30 - 11:30 on Oct. 28, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: In my presentation, I will show an ensemble of transient simulations from the last glacial maximum (LGM; 20.000 years before present) to pre-industrial times (PI). The fully-interactive simulation of this transition represents a hitherto unsolved challenges for state-of-the-art climate models. Therefore, we use a novel coupled comprehensive atmosphere-ocean-vegetation-ice sheet-solid earth model to simulate the transient climate. An ensemble of transient model simulations successfully captures the main features of the last deglaciation, as depicted by proxy estimates. In addition, our model simulates a series of abrupt climate changes, which can be attributed to different drivers and will be discussed throughout the presentation. I will furthermore show, how the model can be applied for simulations of the long-term future. The future simulations show, that parts of the Antarctic ice sheet become unstable even under low-emission scenarios, with significant implications for the modelled climate response. Sensitivity experiments additionally show that, the Greenland ice sheet may exhibit multiple steady-states under pre-industrial climate conditions. This has significant implications for a potential regrowth, once disintegrated entirely.

• Title: Efficient spin-up of Earth System Models using sequence acceleration
• Time: 13:30 - 15:00 on Oct. 22, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: The ocean and land carbon cycles plays a critical role in the climate system and are key components of the Earth System Models (ESMs) used to project future changes in the environment. However, their slow adjustment time also hinders effective use of ESMs because of the enormous computational resources required to integrate them to a pre-industrial quasi-equilibrium, a prerequisite for performing any simulations with these models. Here, a novel solution to this ``spin-up'' problem is shown to accelerate the equilibration of state-of-the-art marine and land biogeochemical models typical of those embedded in ESMs by over an order of magnitude. Based on a ``sequence acceleration'' method originally developed in the context of electronic structure problems, the new technique can be applied in a ``black box'' fashion to any existing model. Preliminary results suggest that the approach can also be applied to ocean physical model. The ability to efficiently spin-up ESMs would enable for the first time a quantification of major parametric uncertainties in these models, lead to more accurate estimates of metrics such as climate sensitivity, and allow increased model resolution beyond what is currently feasible.

• Title: Effects of present-day SST biases on the projections of the tropical Pacific SST warming pattern
• Time: 13:00 - 14:30 on Oct. 21, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: Reliable projections of the tropical Pacific SST warming (TPSW) pattern are key to understanding how the climate will change in response to global warming. Most climate models that participate the phase five or six of the CMIP project an El Nino-like warming pattern with a weakened zonal SST gradient, yet such results are always questioned given that there are various robust common biases when simulating the present-day climate. In this talk, we will focus on how the two robust present-day SST biases―the excessive cold tongue bias within the tropical Pacific and the cold SST bias in the nearby tropical north Atlantic―affect the projections of the future TPSW patterns in climate models. The emergent constraints on the projected TPSW patterns by these two biases both yield a more El Nino-like warming pattern with more weakened zonal SST gradient than the original projections.

• Title: 衛星搭載降水レーダ観測による梅雨期の降水特性およびその将来変化予測
• Time: 10:00 - 11:30 on Oct. 9, 2024.
• Place: General Research Bldg. 2F room 270（＊気候コロキウムと合同開催）
• Abstract: 雲降水システムに伴う非断熱加熱は、地球のエネルギー収支の重要な要素であると共に、様々な気象現象において重要な役割を果たしている。そのため、雲降水システムの実態解明は気候・気象システムの理解向上に不可欠である。同時に、雨がいかに降るかは我々の生活に直結する重要な情報でもある。発表者はこれまでに、衛星観測データを主に用いた解析から全球の降水特性や大規模場との関係に関する研究を行ってきた。本発表ではその中から、衛星搭載降水レーダ観測が捉えた梅雨期の雨の降り方に関する研究、及び一連の研究から得られた知見に基づいてマルチ気候モデル群の大規模場予測から推定された梅雨期降水特性の将来変化予測について紹介する。特に、その予測不確実性をもたらす大規模場について議論する。

• 場所: 本郷キャンパス理学部１号館１０５号室（＊本郷の大気海洋グループに合流して開催）

• Title: Subsurface Adjustment and Heat Transport Response of the Tropical Pacific to Hemispheric Energy Forcing
• Time: 15:30 - 16:30 on Aug. 28, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: Changes in cross-equatorial ocean heat transport (OHT) damp hemispheric asymmetries in anomalous extratropical radiative forcing. Prior studies have suggested that these OHT changes occur due to wind-driven changes in the Indo-Pacific’s shallow subtropical cells (STCs) and buoyancy-driven changes in the deep Atlantic meridional overturning circulation. Here, I introduce a method of overriding surface ocean wind stress in a coupled global climate model (CGCM) to linearly partition the ocean’s response to anomalous extratropical forcing into surface buoyancy-driven and surface momentum-driven responses. In contrast with prior expectations, buoyancy-driven changes in the STCs are the primary driver of cross-equatorial heat transport in the Indo-Pacific’s response to Northern Hemisphere aerosol-like cooling. This buoyancy forced STC response arises from extratropical density perturbations that are amplified by the low cloud feedback in the Northeast Pacific marine stratocumulus regime. While prior studies have explored the important role of surface ocean-atmosphere pathways in connecting the extratropics and tropics, I mechanistically explore the subsurface teleconnection (the so-called “oceanic tunnel”) further by using an ocean-only general circulation model forced by subtropical SST anomalies. Cooling in the Northeast Pacific low cloud deck dynamically adjusts the subtropical thermocline through baroclinic wave activity; within ten years the equatorial Pacific features a shoaled thermocline and La Nia-like cooling. This dynamically driven hemispheric temperature asymmetry drives an equatorially asymmetric subtropical cell adjustment, which transports heat to the cooled hemisphere and qualitatively matches our CGCM results. This overturning is a result of a basin-scale thermal wind response. I find similar equatorial responses when forcing is applied in the Northwest Pacific or the Southeast Pacific, highlighting the importance of wave activity for understanding this adjustment. These results provide a clearer understanding of how the tropical ocean adjusts to hemispheric energy forcing, which has important implications for climate sensitivity, tropical basin interactions, and historical observations of the tropical Pacific.

• Title: Dissecting climate and climate models with the aid of radiative kernels
• Time: 13:30 - 15:00 on Jul. 25, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: Radiation is a crucial shaping factor of the Earth climate. Based on the global reanalysis data (ERA5), we have produced a new set of radiative kernels that can be used for diagnosing the radiation budgets at the Top-of-Atmosphere (TOA), surface and inside the atmosphere. In this talk, I will exemplify the use of the radiative kernels in different applications. The first case is an analysis of the "radiator fin" effect of the Arctic in the warming climate. This effect refers to a distinct positive trend of the Earth thermal radiation in the Arctic, which has been observed by the satellites in the past two decades. This effect acts to radiate excess heating accumulating in the climate system to the space during global warming. Using the aid of the band-by-band radiative kernels, we find that compared with other regions such as the tropics, the prominent thermal radiation trend in the Arctic results from a stronger surface and atmospheric warming and a less offsetting greenhouse effect of water vapor. The second case is a critical examination of the Global Climate Models (GCMs). Given the importance of the radiation budget, GCMs are often validated with regard to their simulated TOA radiation fluxes. Here we use the radiative kernels to diagnose the radiation biases in the CMIP6 GCMs. We find that many of them have a cold air temperature bias and a moist tropospheric humidity bias, which lead to considerable TOA radiation biases but are compensated by cloud-induced biases. These findings disclose that seemingly good radiation simulations can be due to compensating errors. This possibility can and should be checked with kernels during GCM development.

• Title: イベントアトリビューション迅速化のための新手法
• Time: 10:00 - 11:30 on Jul. 24, 2024.
• Place: General Research Bldg. 2F room 270（＊気候コロキウムと合同開催）
• Abstract: この１０年ほど地球温暖化の加速とともに、全球的に熱波や豪雨が頻発しており、極端気象に対する人為的気候変動の影響を見積もるイベントアトリビューション(EA)の迅速化が求められている。従来のEAは、全球モデルや領域気候モデルによるダウンスケーリングによる大規模アンサンブル実験を実施した後で行うため、公表までに時間を要した。そこで、既存のデータのみを用いて、統計的にかつ迅速に気候変化影響を推定する新EA手法を開発した。日本の気象現象は、熱帯-中緯度域の海面水温偏差とそれに伴う大気循環場に影響を受ける事が知られているが、新手法は極端イベント発生時の海洋背景場の影響が考慮されている事が特徴である。発表では、新手法と日本の極端高温イベントへの適用例について紹介する。

• Title: Tropical Deep Convection, Cloud Feedbacks and Climate Sensitivity
• Time: 10:00 - 11:30 on Jul. 16, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: The lecture is concerned with how the diabatically-forced overturning circulations of the atmosphere, established by the deep convection within the tropical trough zone (TTZ), first introduced by Riehl and Simpson (1979), fundamentally shape the distributions of tropical and sub-tropical cloudiness and the changes to cloudiness as Earth warms. The talk first draws on analysis of a range of observations to understand the connections between the energetics of the TTZ, convection and clouds. These observations reveal a tight coupling of the two main components of the diabatic heating, the cloud component of radiative heating, shaped mostly by high clouds formed by deep convection, and the latent heating associated with the precipitation. Interannual variability of the TTZ reveal a marked variation that connects the depth of the tropical troposphere, the depth of convection, the thickness of high clouds and the TOA radiative imbalance. The study the examines connections between this convective zone and cloud changes further afield in the context of CMIP6 model experiments of climate warming. The warming realized in the CMIP6 SSP5-8.5 scenario multi-model experiments, for example, produces an enhanced Hadley circulation with increased heating in the zone of tropical deep convection and increased radiative colling and subsidence in the sub-tropical regions that then impacts low cloud changes and in turn the model warming response through low cloud feedbacks. The pattern of warming produced by models, also influenced by convection in the tropical region, also has a profound influence on the projected global warming.

• Title: Evaluating fine-scale ESMs for Southeast Asian Climate and Extreme
• Time: 15:00 - 16:30 on May 29, 2024.
• Place: General Research Bldg. 2F room 270
• Abstract: Even state-of-the-art CMIP-class Earth system models (ESMs) still exhibit biases in simulating realistic climate state and these biases are source of uncertainties in climate prediction and projection. One of possible solutions could be a refinement of model’s resolution like High-ResMIP. Towards this direction, NextGEMS project (EU Horizon 2020) is developing European “storm-resolving” ESMs whose resolution is a range of 10km to a few km (atmosphere and ocean) globally. 　This very fine resolution allows us to investigate climate, air-sea interaction, and extreme weather in marginal oceanic areas where common CMIP6 might have difficulty to resolve properly. This study assesses how 10km-resolution ESMs from NextGEMS project and other fine-resolution ESM can reproduce the climate and some extreme weather (wet and dry) in Southeast Asia that is related strongly to monsoon system, comparing to observation and some of CMIP6 ESMs. 　In the seminar, I will focus more on winter monsoon season (Dec-Jan-Feb) and show some preliminary results of this verification. (*セミナーは日本語で開催)

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