An Increasing Trend in the Early Winter Precipitation during Recent Decades along the Coastal Areas of the Sea of Japan

Long-term changes in the monthly precipitation along the coastal areas of the Sea of Japan are examined using monthly operational observation data from the Japan Meteorological Agency. The monthly precipitation in December significantly increased from the mid-1980s to 2015, even though no remarkable changes were found in January, February, or November. Significant positive trends in the December precipitation extend widely over the coastal areas of the Sea of Japan, and the amounts increase to approximately 50% of the climatological precipitation in December at most observational stations. The interannual variations in other variables, including the monthly accumulated actual sunshine duration, days with lightning detection, and satellite-retrieved outgoing longwave radiation, also show significant trends that are consistent with the precipitation increase in December. The effect of the sea surface temperature (SST) on precipitation change is discussed based on correlation and regression analyses. The interannual variations in the December precipitation averaged over the observational stations near the Sea of Japan are significantly correlated with the SSTs in the prior month (November). However, the SST increase in November is insufficient to account for the increase in precipitation. In addition, it was found that the satellite-retrieved surface wind speed in December has grown stronger in recent years over the Sea of Japan. It is suggested that the stronger wind corresponds to the enhanced monsoonal flow and is the primary cause of the precipitation increase in December.