Masahiro Tanoue

Global-scale river flood vulnerability in the last 50 years

The impacts of flooding are expected to rise due to population increases, economic growth and climate change. Hence, understanding the physical and spatiotemporal characteristics of risk drivers (hazard, exposure and vulnerability) is required to develop effective flood mitigation measures. Here, the long-term trend in flood vulnerability was analysed globally, calculated from the ratio of the reported flood loss or damage to the modelled flood exposure using a global river and inundation model. A previous study showed decreasing global flood vulnerability over a shorter period using different disaster data. The long-term analysis demonstrated for the first time that flood vulnerability to economic losses in upper-middle, lower-middle and low-income countries shows an inverted U-shape, as a result of the balance between economic growth and various historical socioeconomic efforts to reduce damage, leading to non-significant upward or downward trends. We also show that the flood-exposed population is affected by historical changes in population distribution, with changes in flood vulnerability of up to 48.9%. Both increasing and decreasing trends in flood vulnerability were observed in different countries, implying that population growth scenarios considering spatial distribution changes could affect flood risk projections.

Spatial analysis of annual mean stable isotopes in precipitation across Japan based on an intensive observation period throughout 2013

ABSTRACT Spatial distribution of annual mean stable isotopes in precipitation (δ18O, δ2H) was observed at 56 sites across Japan throughout 2013. Annual mean δ18O values showed a strong latitude effect, from −12.4 ‰ in the north to −5.1 ‰ in the south. Annual mean d-excess values ranged from 8 to 21 ‰, and values on the Sea of Japan side in Northern and Eastern Japan were relatively higher than those on the Pacific Ocean side. The local meteoric water line (LMWL) and isotope effects were based on the annual mean values from all sites across Japan as divided into distinct regions: the Sea of Japan side to the Pacific Ocean side and Northeastern to Southwestern Japan. Slopes and intercepts of LMWL ranged from 7.4 to 7.8 and 9.8 to 13.0, respectively. Slopes for latitude, altitude, and temperature effects ranged from −0.27 to −0.48 ‰/°N, −0.0034 to −0.0053 ‰/m, and 0.36 to 0.46 ‰/°C, respectively, with statistically significance at the 99 % level. However, there was no precipitation amount effect. From the result of a multiple regression analysis, the empirical formula of annual mean δ18O in precipitation from latitude and altitude for all sites across Japan was determined to be δ18 O = −0.348 (LAT) − 0.00307 (ALT) + 4.29 (R2 = 0.59). Slopes for latitude and altitude ranged from − 0.28 to − 0.51, and − 0.0019 to − 0.0045, respectively. Even though site distribution was uneven, these equations are the first trial estimation for annual mean stable isotopes in precipitation across Japan. Further research performed on the monthly basis is required to elucidate factors controlling the spatiotemporal variability of stable isotopes in precipitation across Japan.

Large-scale vapor transport of remotely evaporated seawater by a Rossby wave response to typhoon forcing during the Baiu/Meiyu season as revealed by the JRA-55 reanalysis

The modulation of large-scale moisture transport from the tropics into East Asia in response to typhoon-induced heating during the mature stage of the Baiu/Meiyu season is investigated using the Japanese 55-year reanalysis (JRA-55), aided by a Rayleigh-type global isotope circulation model (ICM). We highlighted the typhoons that migrate northward along the western periphery of the North Pacific subtropical high and approach the vicinity of Japan. Anomalous anticyclonic circulations to the northeast and southeast of typhoons and cyclonic circulation to their west become evident as they migrate toward Japan, which could be interpreted as a Rossby wave response to typhoon heating. These resultant anomalous circulation patterns form moisture conveyor belt (MCB) stretching from the South Asian monsoon region to East Asia via the confluence region between the monsoon westerlies and central Pacific easterlies. The ICM results confirm that the well-defined nature of the MCB leads to penetration of the Indian Ocean, South China Sea, Philippine Sea, and Pacific Ocean water vapors into western Japan. The typhoons have the potential to accumulate large amounts of moisture from distant tropical oceans through the interaction of their Rossby wave response with the background flow. In the case of a typical typhoon, the total precipitable water around the typhoon center as it approaches Japan is maintained by the moisture supply from distant oceans rather than from the underlying ocean, which indirectly leads to the occurrence of heavy rainfall over western Japan.

An estimation of water origins in the vicinity of a tropical cyclone’s center and associated dynamic processes

To clarify the time evolution of water origins in the vicinity of a tropical cyclone (TC)’s center, we have simulated Typhoon Man-yi (July 2007) in our case study, using an isotopic regional spectral model. The model results confirm that the replacement of water origins occurs successively as the TC develops and migrates northward over the western North Pacific. It is confirmed that, in this case, a significant proportion of total precipitable water around the cyclone center comes from external regions rather than the underlying ocean during the mature stage of a TC. Similar features can also be seen in the proportion of each oceanic origin to total condensation. Indian Ocean, South China Sea, and Maritime Continent water vapors begin to increase gradually at the developing stage and reach their peak at the decay stage when the TC approaches southwestern Japan. These remote ocean vapors are transported to the east of the cyclone via the moisture conveyor belt, a zone characterized by distinct low-level moisture flux that stretches from the Indian Ocean to the TC, and are further supplied into the inner region of the TC by inflow within the boundary layer associated with its secondary circulation. Since it takes time to undergo these two dynamic processes, the delayed influence of remote ocean vapors on the TC appears to become evident during the mature stage.

Verification of the isotopic composition of precipitation simulated by a regional isotope circulation model over Japan.

ABSTRACT The isotopic composition (δ18O and δ2H) of precipitation simulated by a regional isotope circulation model with a horizontal resolution of 10, 30 and 50 km was compared with observations at 56 sites over Japan in 2013. All simulations produced reasonable spatio-temporal variations in δ18O in precipitation over Japan, except in January. In January, simulated δ18O values in precipitation were higher than observed values on the Pacific side of Japan, especially during an explosively developing extratropical cyclone event. This caused a parameterisation of precipitation formulation about the large fraction of precipitated water to liquid detrained water in the lower troposphere. As a result, most water vapour that transported from the Sea of Japan precipitated on the Sea of Japan side. The isotopic composition of precipitation was a useful verification tool for the parameterisation of precipitation formulation as well as large-scale moisture transport processes in the regional isotope circulation model.

Estimation of the Isotopic Composition and Origins of Winter Precipitation Over Japan Using a Regional Isotope Circulation Model

The deuterium excess (d-excess) of precipitation aids in identifying vapor source regions because it reflects humidity conditions in those regions. For Japan, studies have assumed that the Sea of Japan was the dominant source of winter precipitation when the d-excess value in winter is >20‰ or higher than the average value in summer. Because this assumption is based on an interpretation that the high d-excess value is due to an interaction between the continental winter monsoon (WM) and warm Sea of Japan, it may not be appropriate for winter precipitation due to extratropical cyclones (ECs). Here, we clarify local patterns of water isotopic composition and the origins of precipitation in WM and EC types over Japan using a regional isotope circulation model. The results indicate that the Sea of Japan made the highest contribution to precipitation on the Sea of Japan side in the WM type, whereas the Pacific Ocean was the dominant source of precipitation over Japan in the EC type. Because d-excess values were higher in the WM type than the EC type, we can assume that the Sea of Japan is the dominant source of precipitation on the Sea of Japan side when the d-excess value is high. In comparison, we cannot identify the source regions from d-excess values alone for Honshu Island bordering the Pacific Ocean, because the difference in the d-excess value between the WM and EC types is unclear. WM variability can be estimated from observed d-excess values due to their clear positive correlation.

Contributions of natural and anthropogenic radiative forcing to mass loss of Northern Hemisphere mountain glaciers and quantifying their uncertainties

Observational evidence indicates that a number of glaciers have lost mass in the past. Given that glaciers are highly impacted by the surrounding climate, human-influenced global warming may be partly responsible for mass loss. However, previous research studies have been limited to analyzing the past several decades, and it remains unclear whether past glacier mass losses are within the range of natural internal climate variability. Here, we apply an optimal fingerprinting technique to observed and reconstructed mass losses as well as multi-model general circulation model (GCM) simulations of mountain glacier mass to detect and attribute past glacier mass changes. An 8,800-year control simulation of glaciers enabled us to evaluate detectability. The results indicate that human-induced increases in greenhouse gases have contributed to the decreased area-weighted average masses of 85 analyzed glaciers. The effect was larger than the mass increase caused by natural forcing, although the contributions of natural and anthropogenic forcing to decreases in mass varied at the local scale. We also showed that the detection of anthropogenic or natural influences could not be fully attributed when natural internal climate variability was taken into account.