An underlying factor of increasing early winter precipitation in the Hokuriku region of Japan in recent decades

Using a reanalysis dataset and large-ensemble simulation results, this study examines a possible factor of increasing trend in early winter precipitation in recent decades in the Hokuriku region of Japan. Monthly precipitation in December has a significant increasing trend after the early 1990s, which is different from those in January and February. The increasing precipitation in December is related to that in the sea surface upward latent heat flux due to intensified winter monsoon circulation and warming sea surface temperatures (SSTs) over the Sea of Japan. December averaged SSTs show a trend pattern in recent decades that is similar to the negative phase of the interdecadal Pacific oscillation (IPO), accompanied by positive trends from the eastern Indian Ocean to the western tropical Pacific. The enhanced trend of convection over the Bay of Bengal is seen; suggesting a combined effect of climatologically high SSTs and IPO-related warmed SSTs over the region. Trends in recent decades of an upper-level wavy pattern from South Asia to near Japan along the subtropical jet associated with enhanced convection near the Bay of Bengal and the related pressure drop from Japan to the north are seen, which contribute to intensified winter monsoon circulation.