[A-HW26] 同位体水文学2022

[AHW26-04] Influence of the stable isotope ratios of precipitation in the Nordics by the North Atlantic Oscillation

*高倉 健仁¹、一柳 錦平²、田上 雅浩³ (1.熊本大学理学部、2.熊本大学先端科学研究部、3.国立環境研究所地球システム領域)

One of the main atmospheric oscillations in the Northern Hemisphere is the North Atlantic Oscillation (NAO), which effects the climate and weather in the mid- and high-latitudes. The stable isotope ratios in precipitation are strongly correlated with temperature in the high-latitudes. There are a lot of studies which have been related with correlation between δ¹⁸O in precipitation and NAO, however, and the studies how the NAO is affected to the variation in δ¹⁸O are not so much. The purpose of this study is to explain the effect of NAO on the δ¹⁸O in precipitation by using the isotope-incorporated atmospheric circulation model (IsoGSM). From the Global Network of Isotopes in Precipitation (GNIP) database, two stations (Espoo in Finland and Reykjavik in Iceland) were selected because of their strong correlation with the NAO index. Since the NAO is mainly observed in the winter period (December - February), winter average of δ¹⁸O in precipitation were calculated weighted by the precipitation amount from 1981 to 2020. The correlation coefficients between δ¹⁸O and air temperature are 0.85 for the observation and 0.98 for the model at Espoo, and those are 0.47 for the observation and 0.97 for the model at Reykjavik, these values are above statistical significance. These results indicate that the IsoGSM can reproduce the seasonal variation of δ¹⁸O in precipitation well. Hereafter, influence of the δ¹⁸O in precipitation by the NAO are investigated in the IsoGSM. The correlation coefficients between winter average of δ¹⁸O or temperature and the NAO index are 0.57 or 0.73 at Espoo and -0.65 or -0.50 at Reykjavik, respectively, these values are above statistical significance.
To compare the δ¹⁸O in precipitation and air temperature at two stations, the NAO-high and NAO-low phases were defined as the highest 5 years and the lowest 5 years in the NAO index from 1981 to 2020. Difference in mean δ¹⁸O between the NAO-high and NAO-low phases are about 2.4‰ and 1.4‰ for Espoo and Reykjavik, respectively. These differences in δ¹⁸O could not be explained only by the temperature effect, which is the empirical relationship between δ¹⁸O in precipitation with temperature (1.9‰ for Espoo and 0.2‰ for Reykjavik). In addition, temperature and winds at 850hPa were averaged for the NAO-high and NAO-low phases to compare atmospheric circulations in the North Atlantic between these phases. During the NAO-high phase, high temperature anomaly around the Scandinavia corresponding to the warm air transport by the southwesterly winds from the low latitudes in the Atlantic Ocean, while low temperature anomaly is found around the Iceland corresponding to the cold air transport by the northwesterly winds from the Arctic Ocean. On the other hand, during the NAO-low phases, low temperature anomaly is found around the Scandinavia because warm air from the low latitudes in the Atlantic Ocean is not transported, while high temperature anomaly is found around the Iceland corresponding to the southerly winds from the mid-latitudes in the Atlantic Ocean due to the cyclonic circulation of the Icelandic Low. These atmospheric circulations affect to the moisture origin and distillation ratio of moisture flux, so the NAO influence to δ¹⁸O in precipitation might be explained quantitatively in addition to the temperature effect.