Interannual Variability of the Baiu Season near Japan Evaluated from the Equivalent Potential Temperature

Abstract

Baiu is a Japanese name for rain and the rainy season that occurs in boreal early summer in the western North Pacific (WNP). This work proposes a methodology to determine the Baiu onset and closing dates using only an atmospheric parameter, the equivalent potential temperature (θ e ) in the lower troposphere, to reveal the interannual variability of Baiu season. First, the location of the Baiu front is determined by the maximum of the negative meridional gradient of θ e , which identifies the zonal boundary of air masses. Then, the Baiu onset and closing dates are fixed on the basis of the northward movement of the Baiu front and development of the southern air mass in the WNP.The mean onset date was estimated to be June 2 near Japan, which is comparable to the date published by the Japan Meteorological Agency (JMA) for South Kyushu, although the correlation was not significant between the two interannual variations. The meridional shift of the monsoon southwesterlies was concurrent with early or late onset dates. The mean closing date was July 21, about 1 week earlier than the JMA date for North Tohoku, but the correlation was significant between the two interannual variations. The closing date varied with an anomalous meridional dipole in atmospheric circulation, largely extending from the eastern Eurasian continent to the central North Pacific.The correlation was insignificant between the interannual variations of the onset and closing dates, although both had dominant 2-4-year periodicity. There was also no significant correlation with typical large-scale interannual variations (i.e., the El Niño/Southern Oscillation, the tropospheric biennial oscillation of the Asian monsoon, and the North Atlantic Oscillation). Hence, these three interannual variations do not appear to be direct causes of the modulation of the onset and closing dates of the Baiu season near Japan.

Isotopic Variations Associated with North-South Displacement of the Baiu Front

“…In early autumn, the Baiu front, which moved northward in early summer, retreats southward as the Akisame (Tomita et al 2011). Similar to the BFZ, the Akisame frontal zone (AFZ) is formed at the transition zone between subtropical warm and subpolar cold air masses and is characterized by a sharp áθ e ñ gradient.…”

“…Previous studies have used the location of the largest north-south gradient of θ e in the lower atmosphere to define the BFZ (Tomita et al 2011;Kanada et al 2012). We therefore find the location of the BFZ using the vertically averaged (925−850 hPa) equivalent potential temperature áθ e ñ in central Japan (132°E−137°E).…”

“…Thus, interannual variations in the position of the Baiu frontal zone (BFZ) can be examined using a global meteorological dataset (e.g., Tomita et al 2011). However, the length of the available dataset is limited to the latter half of the 20th century, a duration which is insufficient to examine the influence of decadal and multi-decadal scale climate variability on the BFZ.…”

Possible Impact of a Tropical Cyclone on the Northward Migration of the Baiu Frontal Zone

“…Because the BFZ is recognized as a large horizontal gradient of the equivalent potential temperature (EPT; Tomita et al 2011), we define the BFZ as the area in which horizontal gradient of the EPT is large. The location of the tropical cyclone MAWAR in the observations is derived from the JMA best track data.…”

Decadal-Scale Modulation of Atmospheric Circulation Change at the Onset of the Western North Pacific Summer Monsoon

“…1a, but for Terms 1, 2, and 3, having the lags of −1, 0, and +1 pentads (not shown). The end date of the Baiu season is also centered on July 21 in Japan with the standard deviation of 9 days for 1979−2008 (Tomita et al 2011). The DSM for the onset date of the WNPSM is relatively small, but not zero.…”