Local atmospheric response to the Kuroshio large meander path in summer and its remote influence on the climate of Japan

The Kanto district, Japan, including Tokyo, has 40 million inhabitants and its summer climate is characterized by high temperature and humidity. The Kuroshio that flows off the southern coast of Kanto district has taken a large meander (LM) path since the summer of 2017 for the first time since the 2004–2005 event. Recently-developed satellite observations detected marked coastal warming off the Kanto-Tokai district during the LM path period. By conducting regional atmospheric model experiments, it is found that summertime coastal warming increases water vapor in the low-level atmosphere through enhanced evaporation from the ocean and influences near-surface winds via the vertical mixing effect over the warming area. These two changes induce an increase in water vapor in Kanto district, leading to an increase in downward longwave radiation at the surface and then surface warming through a local greenhouse effect. Resultantly the summer in Kanto district becomes increasingly hot and humid in LM years, with double the number of discomfort days compared with non-LM years. Our simulations and supplementary observational studies reveal the significant impacts of the LM-induced coastal warming on the summertime climate in Japan, which can exceed previously identified atmospheric teleconnections and climate patterns. Our results could improve weather and seasonal climate forecasts in this region.

Ancient and Medieval Events and Recurrence Interval of Great Kanto Earthquakes along the Sagami Trough, Central Japan, as Inferred from Historiographical Seismology

To know the Ancient and Medieval events and the recurrence interval of the great Kanto earthquakes such as the 1923 and 1703 ones generated by the subduction of the Philippine Sea plate along the Sagami trough, central Japan, I examined the latest dataset of historical records. I used only rank-A (contemporary) materials in the Online Database of Historical Documents on Japanese Earthquakes and Eruptions in the Ancient and Medieval Ages. Among destructive earthquakes in the Kanto district before the sixteenth century, the 878 Gangyo earthquake, which has been suspected an inland event generated from the Isehara fault, is considered the oldest candidate of the Kanto earthquake based on the large-scale disaster and intense aftershock activity, though tsunami is not mentioned. The 1293 Showo (Einin) Kamakura earthquake can be regarded as a great Kanto earthquake, because of severe damage in the Kanto district and remarkable aftershock activity. During the fifteenth century, both of the 1433 Eikyo and the 1495 Meio earthquakes are regarded as candidates of the Kanto earthquake; the former caused severe damage around Kamakura on the northern coast of Sagami Bay with high-aftershock activity and presumably caused tsunami, and the latter seems to have caused large tsunamis at Kamakura and the west coast of Sagami Bay. Although further investigation is necessary to clarify which one is the Kanto earthquake, we can say that an interplate earthquake probably occurred in the fifteenth century. Intervals between successive events, in 878, 1293, 1433 or 1495, 1703, and 1923, range from 140 to 270 yr except for the first interval of 415 yr. It is very difficult to confirm whether a great interplate earthquake occurred or not during this interval by means of historiographical seismology, because the Kanto district was in the worst situation of poor historical records in those days.

Postseismic Uplift Along the Pacific Coast of Tohoku and Kanto Districts Associated with the 2011 off the Pacific Coast of Tohoku Earthquake

The 2011 off the Pacific coast of Tohoku earthquake (M9.0) produced up to 1.2 m subsidence along the Pacific coast in northeastern Japan. Based on Global Positioning System (GPS) observations, continuing postseismic coastal uplift has been detected in the past six years after the main shock. By applying a 3D spherical Earth viscoelastic finite element model using the postseismic seafloor and terrestrial GPS observations as constraints, I demonstrate that this uplift is mainly caused by the postseismic viscoelastic relaxation of the asthenosphere. Although the model was constrained only based on horizontal crustal deformation, the vertical displacements predicted for six years after the 2011 Tohoku earthquake agree reasonably well with the time series of the observed uplift at sites along the Pacific coast including the southern Sanriku coast and Kanto district. I estimated the time at which the cumulative postseismic uplift will fully compensate the coseismic subsidence. The results show that large coseismic coastal subsidence on the southern Sanriku coast will be fully offset by the postseismic uplift within several decades. To the immediate north, the model underpredicts the postseismic uplift and possibly indicates unaccounted postseismic fault creep. Farther south, the postseismic uplift of the coast of the Kanto district has already exceeded the small amount of coseismic subsidence over the past six years, as predicted by the model. To prepare for future earthquakes, especially with respect to the coastal construction at fishery ports, it is important to construct a comprehensive rheological structure model based on geophysical observations including GNSS positioning.