An Objective Frontal Data Set to Represent the Seasonal and Interannual Variations in the Frontal Zone around Japan

摘要

 A long-term (1948-2009) frontal data set was created with an objective method by using NCEP-NCAR (National Centers for Environmental Prediction-National Center for Atmospheric Research) reanalysis data. This method utilizes a gradient and a thermal front parameter (TFP) of a potential temperature and an equivalent potential temperature at 850 hPa (dθ, TFP(θ), and TFP(θe)). The TFP defined as the directional derivative of a gradient of a thermodynamic variable along its gradient is one of the measures of frontal intensity and is often used for objectively analyzing frontal positions on surface weather maps. On the basis of the frontal data set, the average seasonal behavior of the frontal zone around Japan, its influence on the weather in mid-summer, and the seasonal march of the frontal zone during El Niño/La Niña events were examined. The main results are summarized in the following points: 1) The frontal data set generated under the conditions of dθ 0.04 K (100 km)−1, TFP(θ) 0.05 K (100 km)−2, and TFP(θe) 0.69 K (100 km)−2 showed the strongest correlation to that compiled by counting the number of fronts on the surface weather maps around Japan. 2) Although the long-term frontal data set created in this study retained some differences in frequency, the seasonal march of the frontal zone was consistent with that created from fronts on the weather maps. 3) The relationship of interannual mid-summer variations (July 20-August 16) between the clear-sky ratio of Japan and the frontal zone and various mean characteristics of the Japanese climate during El Niño/La Niña events, most of which have been discussed in previous reports, were verified from the perspective of the variability in the frontal zone, which has not been clarified so far. These results of this work show that the frontal data set created herein has the advantage of being simple and objective and is useful for research on the detailed relationship in interannual variations between the regional climate around Japan and large-scale atmospheric conditions.