Summer relative humidity in northern Japan inferred from δ 18 O values of the tree ring in (1776–2002 A.D.): Influence of the paleoclimate indices of atmospheric circulation

The summer relative humidity (RH) changes in Hokkaido, northern Japan, since 1776 were reconstructed using the oxygen isotope ratios of the tree ring cellulose of two living oak trees. We investigated the direct relationships between the decadal‐centennial variations in the summer RH in northern Japan and the climate indices of atmospheric circulation to understand the factors affecting the changes in the hydrological climate in northern Japan. The variations in the summer RH are negatively correlated with those in the annual PDO indices since 1781. This is probably because the humid southerly wind from the western Pacific Ocean blows toward northern Japan with the intensified Pacific high when the PDO index is lower. Further, the fluctuations in the summer RH are positively correlated with those in the summer AO index during 1781–1930, but they are negatively correlated with those in the summer AO during 1940–1997. During the 1930s, the AO index changed from the negative to positive on the average. The drastic shift in its correlation is explained by the difference between atmospheric circulations in the low‐AO period (1899–1930) and the high‐AO period (1970–2000). The summer RH in northern Japan was regulated by the summer AO during 1781–1930 (the cold period) and the annual PDO during 1940–1997 (the warm period). As a consequence of global warming, the midlatitude forcing such as PDO might become stronger than the high‐latitude forcing such as AO on the hydrological climate in northern Japan.