Kenjiro Sho

Synchronized Northern Hemisphere climate change and solar magnetic cycles during the Maunder Minimum

The Maunder Minimum (A.D. 1645–1715) is a useful period to investigate possible sun–climate linkages as sunspots became exceedingly rare and the characteristics of solar cycles were different from those of today. Here, we report annual variations in the oxygen isotopic composition (δ18O) of tree-ring cellulose in central Japan during the Maunder Minimum. We were able to explore possible sun–climate connections through high-temporal resolution solar activity (radiocarbon contents; Δ14C) and climate (δ18O) isotope records derived from annual tree rings. The tree-ring δ18O record in Japan shows distinct negative δ18O spikes (wetter rainy seasons) coinciding with rapid cooling in Greenland and with decreases in Northern Hemisphere mean temperature at around minima of decadal solar cycles. We have determined that the climate signals in all three records strongly correlate with changes in the polarity of solar dipole magnetic field, suggesting a causal link to galactic cosmic rays (GCRs). These findings are further supported by a comparison between the interannual patterns of tree-ring δ18O record and the GCR flux reconstructed by an ice-core 10Be record. Therefore, the variation of GCR flux associated with the multidecadal cycles of solar magnetic field seem to be causally related to the significant and widespread climate changes at least during the Maunder Minimum.

TREE-RING WIDTH AND STABLE CARBON ISOTOPE COMPOSITION OF JAPANESE CYPRESS IN THE LAKE BIWA AREA, CENTRAL JAPAN, AND THEIR HYDROLOGIC AND CLIMATIC IMPLICATIONS

Chronologies of tree-ring width and stable carbon isotope composition of Japanese cypress were developed to help reconstruct a 300-year record of past hydrologic and climatic environments in the Lake Biwa area, central Japan. Site chronologies were built with 37 trees for ring width and four trees for carbon isotope composition, respectively. Correlation analysis with monthly climatic data revealed that radial growth of the trees is related to temperature in early spring, precipitation (or number of precipitation days) in early summer and precipitation in previous-year summer to autumn. Tree-ring cellulose carbon isotopic composition is correlated most significantly with the number of precipitation days in early summer months. Consequently, a chronology of the number of precipitation days in May was reconstructed by multiple regression analysis with ring-width and carbon-isotope predictors and was validated by comparison with the recent observed record.

FIELD OBSERVATIONS ON CHARACTERISTICS OF SALINITY INTRUSION AND VERTICAL MIXING IN AN URBAN TIDAL RIVER

Field observations on characteristics of salinity intrusion and vertical mixing were conducted in an urban tidal river, the Horikawa River in Nagoya City, in order to understand the effects of stratified flow and reciprocating flow on water quality. We measured time series of vertical distribution of water depth, velocity, salinity, temperature, DO etc. during one tide by using an electromagnetic velocimeter and a multiple water quality meter. The vertical mixing type in the measured region was basically classified as a mild mixing type, but a considerable change of mixing type was observed with time and location. An increase of installed water slightly affects a stratified flow structure at the measuring site. Salt water has lower DO value than that in fresh water and it is moving up and down through a bottom layer during tidal motion.

EVALUATION OF H-Q RELATION OF THE SETA RIVER IN 18-19 CENTURIES USING HISTORICALDOCUMENTS

The Seta River is the only one natural outlet river of Lake Biwa. This paper examines the relationship between water level of Lake Biwa and discharge of the Seta River in historical years. Since there are no gauged discharge data in historical years, water level data are transformed into differences of outflow from inflow (Qout-Qin) using the relationship between water level and storage volume (H-V relation) of Lake Biwa. The results indicate that flow capacity of the Seta River was expanded after the dregging in Tempo years (1831-1833), and that there were no notable changes in flow capacity during Meiji Period (1874-1899).

Transformation Process of Flood Waves Passing Compound Channels with Locally Vegetated Flood Plains

Transformation process of flood waves and hydraulic characteristics of unsteady flows were investigated experimentally in compound channels with locally vegetated flood plains. Some noticeable differences between unsteady flow structure in compound channels with and without vegetation were revealed. The shortly vegetated zone in the flood plains causes the sharp decrease of the flow depth in front of that and increases the upstream stage extremely. 2-D depth-averaged numerical calculation technique for unsteady flow with vegetation in flood plains was developed. The calculated results reproduce well the characteristic tendency of the experiments. This calculation method is useful for the flood routing passing such a complex flow field.