岡崎 淳史

Abstract Stable water isotopes in inland Antarctic ice cores are powerful paleoclimate proxies; however, their relationship with dynamical atmospheric circulations remains controversial. Using a water isotope climate model (MIROC5‐iso), we assessed the influence of the Last Glacial Maximum (LGM; ∼21,000 years ago) sea surface temperatures (SST) and sea ice (SIC) on Antarctic precipitation isotopes (δ¹⁸O_p) through atmospheric circulation. The results revealed that the synoptic circulation mostly maintained southward moisture transport, reaching inland Antarctica. The steepened meridional SST gradient in the mid‐latitudes increased δ¹⁸O_p in inland Antarctica with the enhanced baroclinic instability and synoptic moisture transport. In contrast, expanded SIC distribution decreased δ¹⁸O_p over Antarctica by enhanced preferential removal of heavy isotopes during vapor transport due to the increased transport distance and enhanced surface cooling. These findings propose to use Antarctic ice cores to describe the southern hemisphere atmospheric circulation, represented by the westerly jets, during the LGM and other past climates.

Abstract Modeling tritium content in water presents a meaningful way to evaluate the representation of the water cycle in climate models as it traces fluxes within and between the reservoirs involved in the water cycle (stratosphere, troposphere, and ocean). In this study, we present the implementation of natural tritium in water in the atmospheric general circulation model (AGCM) MIROC5‐iso and its simulation for the period 1979–2018. Owing to recently published tritium production calculations, we were able to investigate, for the first time, the influence of natural tritium production related to the 11‐yr solar cycle on tritium in precipitation. MIROC5‐iso correctly simulates continental, latitudinal, and altitude effects on tritium in precipitation. The seasonal tritium content peaks, linked to stratosphere‐troposphere exchanges, are accurately simulated in terms of timing, even though MIROC5‐iso underestimates the amplitude of the changes. Decadal tritium concentration variations in precipitation owing to the 11‐yr solar cycle are well simulated in MIROC5‐iso, in agreement with the observations at Vostok in Antarctica for example, Finally, our simulations revealed that the internal climate variability plays an important role in tritium in polar precipitation. Owing to its influence on the south polar vortex, the Southern Annular Mode enhances the effect of the production component on tritium in East Antarctic precipitation. In Greenland, we found an east‐west contrast in the detection of the 11‐yr solar cycle in tritium in precipitation owing to the influence of the North Atlantic Oscillation on humidity conditions.

Abstract The products from the Stable Water Isotope Intercomparison Group, Phase 2, are currently used for numerous studies, allowing water isotope model‐data comparisons with various isotope‐enabled atmospheric general circulation model (AGCMs) outputs. However, the simulations under this framework were performed using different parameterizations and forcings. Therefore, a uniform experimental design with state‐of‐the‐art AGCMs is required to interpret isotope observations rigorously. Here, we evaluate the outputs from three isotope‐enabled numerical models nudged by three different reanalysis products and investigate the ability of the isotope‐enabled AGCMs to reproduce the spatial and temporal patterns of water isotopic composition observed at the surface and in the atmospheric airborne water. Through correlation analyses at various spatial and temporal scales, we found that the model's performance depends on the model or reanalysis we use, the observations we compare, and the vertical levels we select. Moreover, we employed the stable isotope mass balance method to conduct decomposition analyses on the ratio of isotopic changes in the atmosphere. Our goal was to elucidate the spread in simulated atmospheric column δ¹⁸O, which is influenced by factors such as evaporation, precipitation, and horizontal moisture flux. Satisfying the law of conservation of water isotopes, this budget method is expected to explain various fractionation phenomena in atmospheric meteorological and climatic events. It also aims to highlight the spreads in modeled isotope results among different experiments using multiple models and reanalyses, which are primarily dominated by uncertainties in moisture flux and precipitation, respectively.

<jats:p id="p1">Stable water isotope signals in inland Antarctic ice cores have provided wealth of information about past climates. This study investigated atmospheric circulation processes that influence precipitation isotopes in inland Antarctica associated with atmospheric circulations in the southern mid-latitudes during the Last Glacial Maximum (LGM, ~21 000 year ago). We simulated this climate period using circulation model (MIROC5-iso) forced with different sea surface boundary conditions. Our results showed a steepened meridional sea surface temperature gradient in the southern mid-latitudes associated with a strengthening of the southern westerlies. This change in the atmospheric circulation enhanced the intrusion of warm and humid air from low latitudes that contributes to precipitation events, inducing heavy isotope precipitation inland East Antarctica. Our results suggest that the representation of past southern westerlies can be constrained using water isotopic signals in Antarctic ice cores.</jats:p>

© 2019. American Geophysical Union. All Rights Reserved. Proxy system models (PSMs) are an important bridge between climate simulations and climate records prior to the period where instrumental observations are available. PSMs help to interpret what proxies show and how they record climate. Although previous studies have evaluated PSMs for individual sites, their systematic evaluation on a global scale has not yet been conducted. This study evaluated the performance of PSMs for stable water isotopes in ice cores, corals, and tree-ring cellulose for the period 1950–2007. Spatial distributions of the mean state were well simulated for all proxy types, albeit with a bias for tree-ring cellulose. Interannual variability was well simulated for corals and tree-ring cellulose. These results indicate that the models represent key mechanisms for the proxies. In contrast, the reproducibility of interannual variability in ice cores was markedly lower than that for the other proxies. Although the reproducibility was limited by the atmospheric forcing used to drive the model, the results suggest that the PSM may be missing postdepositional processes, such as sublimation for ice cores on the interannual time scale.

Abstract. Spaceborne precipitation radars, such as the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM) Core Observatory, have been important platforms to provide a direct measurement of three-dimensional precipitation structure globally. Building upon the success of TRMM and GPM Core Observatory, the Japan Aerospace Exploration Agency (JAXA) is currently surveying the feasibility of a potential satellite mission equipped with a precipitation radar on a geostationary orbit. The quasi-continuous observation realized by the geostationary satellite radar would offer a new insight into meteorology and would advance numerical weather prediction (NWP) through their effective use by data assimilation. Although the radar would be beneficial, the radar on the geostationary orbit measures precipitation obliquely at off-nadir points. In addition, the observing resolution will be several times larger than those on board TRMM and GPM Core Observatory due to the limited antenna size that we could deliver. The tilted sampling volume and the coarse resolution would result in more contamination from surface clutter. To investigate the impact of these limitations and to explore the potential usefulness of the geostationary satellite radar, this study simulates the observation data for a typhoon case using an NWP model and a radar simulator. The results demonstrate that it would be possible to obtain three-dimensional precipitation data. However, the quality of the observation depends on the beam width, the beam sampling span, and the position of precipitation systems. With a wide beam width and a coarse beam span, the radar cannot observe weak precipitation at low altitudes due to surface clutter. The limitation can be mitigated by oversampling (i.e., a wide beam width and a fine sampling span). With a narrow beam width and a fine beam sampling span, the surface clutter interference is confined to the surface level. When the precipitation system is located far from the nadir, the precipitation signal is obtained only for strong precipitation.

Water availability is fundamental to societies and ecosystems, but our understanding of variations in hydroclimate (including extreme events, flooding, and decadal periods of drought) is limited because of a paucity of modern instrumental observations that are distributed unevenly across the globe and only span parts of the 20th and 21st centuries. Such data coverage is insufficient for characterizing hydroclimate and its associated dynamics because of its multidecadal to centennial variability and highly regionalized spatial signature. High-resolution (seasonal to decadal) hydroclimatic proxies that span all or parts of the Common Era (CE) and paleoclimate simulations from climate models are therefore important tools for augmenting our understanding of hydroclimate variability. In particular, the comparison of the two sources of information is critical for addressing the uncertainties and limitations of both while enriching each of their interpretations. We review the principal proxy data available for hydroclimatic reconstructions over the CE and highlight the contemporary understanding of how these proxies are interpreted as hydroclimate indicators. We also review the available last-millennium simulations from fully coupled climate models and discuss several outstanding challenges associated with simulating hydroclimate variability and change over the CE. A specific review of simulated hydroclimatic changes forced by volcanic events is provided, as is a discussion of expected improvements in estimated radiative forcings, models, and their implementation in the future. Our review of hydroclimatic proxies and last-millennium model simulations is used as the basis for articulating a variety of considerations and best practices for how to perform proxy-model comparisons of CE hydroclimate. This discussion provides a framework for how best to evaluate hydroclimate variability and its associated dynamics using these comparisons and how they can better inform interpretations of both proxy data and model simulations. We subsequently explore means of using proxy-model comparisons to better constrain and characterize future hydroclimate risks. This is explored specifically in the context of several examples that demonstrate how proxy-model comparisons can be used to quantitatively constrain future hydroclimatic risks as estimated from climate model projections.

Data assimilation (DA) has been successfully applied in the field of paleoclimatology to reconstruct past climate. However, data reconstructed from proxies have been assimilated, as opposed to the actual proxy values. This prevented full utilization of the information recorded in the proxies. This study examined the feasibility of proxy DA for paleoclimate reconstruction. Isotopic proxies (delta O-18 in ice cores, corals, and tree-ring cellulose) were assimilated into models: an isotope-enabled general circulation model (GCM) and forward proxy models, using offline data assimilation. First, we examined the feasibility using an observation system simulation experiment (OSSE). The analysis showed a significant improvement compared with the first guess in the reproducibility of isotope ratios in the proxies, as well as the temperature and precipitation fields, when only the isotopic information was assimilated. The reconstruction skill for temperature and precipitation was especially high at low latitudes. This is due to the fact that isotopic proxies are strongly influenced by temperature and/or precipitation at low latitudes, which, in turn, are modulated by the El Nino-Southern Oscillation (ENSO) on interannual timescales. Subsequently, the proxy DA was conducted with real proxy data. The reconstruction skill was decreased compared to the OSSE. In particular, the decrease was significant over the Indian Ocean, eastern Pacific, and the Atlantic Ocean where the reproducibility of the proxy model was lower. By changing the experimental design in a stepwise manner, the decreased skill was suggested to be attributable to the misrepresentation of the atmospheric and proxy models and/or the quality of the observations. Although there remains a lot to improve proxy DA, the result adequately showed that proxy DA is feasible enough to reconstruct past climate.

We observed stable isotopes in precipitation and atmospheric water vapor over a humid subtropical rice paddy field in Tsukuba, Japan, from June 2013 to May 2014. We used observed isotope ratios, in combination with an isotope-enabled general circulation model (GCM; Isotopes-incorporated Global Spectral Model: IsoGSM) to improve our understanding of the impacts of moisture sources and transport on the variability of water vapor isotopes. The isotopic measurements of water vapor and precipitation suggested that vapor isotopes in the study area were controlled by not only air-rain isotopic exchange, but also other kinetic effects associated with land evapotranspiration and large scale atmospheric circulation at the seasonal time scale. The contribution of land evapotranspiration to local water vapor content (F-ET) was approximately 16.0 +/- 12.3% as an annual average, with a summer maximum of 20.5 +/- 12.9%. Our results show that large-scale atmospheric circulation is the primary control on the variability of near surface water vapor delta D. An IsoGSM tagging simulation experiment demonstrated that the large temporal variation of surface water vapor isotopes can primarily be attributed to advection and mixing of moisture from different oceanic source regions. (C) 2015 Elsevier B.V. All rights reserved.

Partitioning ecosystem evapotranspiration (ET) into soil evaporation (E) and transpiration (T) is crucial for understanding hydrological processes. In this study, by using high-frequency isotope measurements and continuous surface water measurements, we investigated the isotope ratios in soil-vegetation-atmosphere transfer and the physical mechanisms involved over a paddy field for a full growing season. The isotopic signals of delta(ET), delta(T), and delta(E) were determined by the Keeling plot method, surface water isotopic measurements, and the Craig-Gordon model, respectively. The fraction of transpiration in evapotranspiration (FT) ranged from 0.2 to 1, with an almost continuous increase in the early growing season and a relatively constant value close to 1 later in the year. The result was supported by FT derived from simulated T and eddy correlation measured ET. The seasonal change in the transpiration fraction could be described quite well as a function of the LAI (FT50.67LAI(0.25), R-2 = 0.80), implying that transpiration plays a dominant role in the soil-vegetation-atmosphere continuum during the growing season. The two end-member uncertainty analysis suggested that further improvement in the estimation of delta(T) and delta(ET) is necessary for partitioning evapotranspiration using the isotopic method. In the estimation of delta(ET), the assumptions underlying Keeling plot method were rarely met and the uncertainty was quite large. A high frequency of precise isotopic measurements in surface water was also necessary for delta(T) estimation. Furthermore, special care must be taken concerning the kinetic fractionation parameter in the Craig and Gordon Equation for delta(E) estimation under low-LAI conditions. The results demonstrated the robustness of using isotope measurements for partitioning evapotranspiration.

This study was performed to examine the relationship between isotopic composition in near-surface vapor (delta O-18(v)) over western Africa during the monsoon season and El Nino-Southern Oscillation (ENSO) activity using the Isotope-incorporated Global Spectral Model. The model was evaluated using a satellite and in situ observations at daily to interannual timescales. The model provided an accurate simulation of the spatial pattern and seasonal and interannual variations of isotopic composition in column and surface vapor and precipitation over western Africa. Encouraged by this result, we conducted a simulation stretching 34 years (1979-2012) to investigate the relationship between atmospheric environment and isotopic signature on an interannual timescale. The simulation indicated that the depletion in the monsoon season does not appear every year at Niamey. The major difference between the composite fields with and without depletion was in the amount of precipitation in the upstream area of Niamey. As the interannual variation of the precipitation amount is influenced by the ENSO, we regressed the monsoon season averaged delta O-18(v) from the model and annually averaged NINO3 index and found a statistically significant correlation (R = 0.56, P &lt; 0.01) at Niamey. This relationship suggests that there is a possibility of reconstructing past western African monsoon activity and ENSO using climate proxies.

本研究では，同位体の境界条件の違いがGCMの同位体再現結果に与える変化を定量的に把握するため，陸面における同位体分別と海洋における同位体比の時間発展を考慮する同位体GCMを開発し，これを調べた． 使用したモデルはMIROC5で，この大気陸面部に同位体比を導入した．行った実験は以下の4つである：陸面の同位体分別を考慮しない実験をコントロール実験（CTL），陸面の同位体分別を考慮した実験，海水同位体比の空間分布を考慮するが時間方向に一定とした実験（OCN1），時空間変化も考慮した実験（OCN2）である．海水同位体比の計算にはボックスモデルを使用した．このモデルは降水・蒸発・深層水との混合による同位体比の変化のみを計算し，海流などを考慮するものではない． コントロール実験の降水同位体比の気候値をGNIPと比較したところ，よく知られる同位体の基本的な特徴を確認することができた．SWINGに参加したモデルと比較すると相関は最も低く，バイアスも最も大きかった．水蒸気同位体比を比較したところ，モデルは空間的な分布の特徴をよく再現した．相関係数を他の同位体GCMと比較すると低いが，バイアスについては9&permil;で他の同位体GCMと比較しても好成績であった．以上の比較から，同位体MIROCは空間的な特徴はよく再現できているものの，他の同位体GCMと比較すると成績はやや劣ることが分かった．現段階では同位体に関するパラメタをチューニングしていないためこのような結果になったと考えられるが，さらなる改良が望まれる． 結果については水蒸気の最大のソースである海水の同位体比変化を考慮した場合の変化について記す．注目する変数は，海洋から大気への直接の影響を見るため，海洋起源である蒸発同位体比である．一様0&permil;としたCTLに比べると，空間分布を与えたOCN1，時間変動も考慮したOCN2では結果が大きく変化することがわかった．なかでも空間分布を与えた影響が顕著であった（CTLとの平均二乗誤差：RMS=2.4&permil;）．一方で時間変動を考慮した影響はこれに比べると小さかった（OCN2とOCN1のRMS=0.6&permil;）．この結果は同位体GCMにとって同位体境界条件の空間分布が重要であることを示唆する．これらの結果が降水同位体比の再現性にどう影響を与えるのか，陸面の影響はどうか等の詳細な比較については発表で述べる．

We evaluated change in flood risk under global warming using the output from the latest version of the Model for Interdisciplinary Research on Climate (MIROC5), an atmosphere-ocean general circulation model. River discharge for the 21st century were simulated for the two Representative Concentration Pathway (RCP4.5 and RCP8.5) scenarios and converted to the Discharge Probability Index (DPI) to evaluate future flood risk. The occurrence of flood events corresponding to various DPI categories was calculated for each continental region. The results show a significant increase in the risk of massive flood incidents during the 21st century in Asia, Africa, Oceania, and South America, with relatively large differences between the two scenarios. In contrast, both scenarios showed only slight increases in massive flood risk in North America and almost no change in Europe. For the RCP8.5 scenario in particular, the risk of massive flood occurrence will increase approximately ten times in Africa, seven times in Asia, and five times in South America by the end of the current century. Further analyses indicated that these projected flood increases will occur mainly due to the increases in the number of rainy days and the annual maximum daily precipitation, and the decrease in snowmelt in high latitudinal regions will play an important role on the unchanged risk in Europe in spite of the projected increase in precipitation.