Climatological Evolution of the Okinawa Baiu and Differences in Large-Scale Features during May and June

Abstract

The Okinawa baiu (summer rainy season) starts in early May and ends in late June, preceding the baiu in mainland Japan by approximately 1 month. This study investigates the time evolution of the large-scale circulation associated with the Okinawa baiu using 10-yr (1997-2006) climatologies of precipitation and meteorological fields, with particular focus on temperature advection at 500 hPa.The onset of the Okinawa baiu occurs in early May and is followed by an initial peak in precipitation during mid-May. The baiu rainband then moves southeastward, leading to a short break in baiu precipitation during late May. The rainband returns to Okinawa in early June, and a second peak in precipitation occurs during mid-June. The baiu rainband withdraws northward in late June. The mid-May precipitation peak is associated with warm advection at 500 hPa, mainly due to the meridional temperature gradient and the prevailing southerly winds. This warm advection coincides with upward motion near Okinawa; however, the warm advection is insufficient to explain the peak precipitation amount. Enhancement of precipitation by a transient disturbance probably contributes to the peak amount. The break period during late May coincides with the peak of South China Sea monsoon. Warm advection at 500 hPa strengthens again in June because of the strong zonal thermal contrast between the warm Tibetan Plateau and cold Pacific. This warm advection is able to adequately explain both the upward motion and precipitation. These results indicate that the large-scale meteorological characteristics are different during the first and second peaks.

Future Changes in Atmospheric Conditions for the Seasonal Evolution of the Baiu as Revealed from Projected AGCM Experiments

“…In P_OB, precipitation peaks at approximately 12 mm day −1 in mid-June, and this peak is consistent with the GPCP_1DD. Precipitation in the GPCP_1DD shows a temporal peak in mid-May, before the peak in mid-June, which corresponds to the Okinawa Baiu (Okada and Yamazaki 2012). However, the AGCM does not reproduce this peak (Fig.…”

“…With the progress of the Baiu season in East Asia, the zonal contrast of temperature in the mid-troposphere becomes enhanced; this is because the heated surface of the Asian continent, especially the Tibetan Plateau, and latent heating by monsoonal convection in South Asia, warm the troposphere aloft, leading to a larger difference in mid-troposphere temperature between the Tibetan Pla- teau and the Baiu frontal zone (Kato 1989;Kodama 1993;Ose 1998;Sampe and Xie 2010). The westerlies jet emanating from the eastern flank of the Tibetan Plateau transports the warmer air to the southern part of China and toward the Japanese islands (Sampe and Xie 2010;Okada and Yamazaki 2012). Therefore, warm-air advection is examined here.…”

“…Sampe and Xie (2010) also showed a good correlation between the band of warm-air advection at the 500-hPa level and the frontal zone over the Japanese islands during the Baiu season. Okada and Yamazaki (2012) closely examined whether this correlation applies to the Baiu around the Okinawa region (referred to as the Okinawa Baiu; 23 -28°N, 123 -129°E). In the pre-Baiu season (i.e., May), a jet stream is present in the south of the Tibetan Plateau; however, the westeast contrast of temperature is weak over the Okinawa region.…”

Interdecadal change of the active‐phase summer monsoon in East Asia (Meiyu) since 1979

“…Previous studies have indicated that interannual variability of the EASM circulations is linked to the Tibetan Plateau thermal conditions and India Ocean sea surface temperature (SST) anomalies (Li and Yanai, ; Zhao et al , ; Liu and Wang, ; Hu and Duan, ). These processes are complicated by the varying mid‐tropospheric temperature advection within the Meiyu rainband (Kosaka et al , ; Okada and Yamazaki, ). However, few studies have focused on the interdecadal variability of Meiyu.…”

East Asian presummer precipitation in the CMIP5 at high versus low horizontal resolution

“…A global climate simulation conducted at a horizontal resolution that is too low to resolve topographic effects and small-scale processes encounters an inevitable drawback in reproducing the East Asian climate (Lau and Ploshay, 2009;Li et al, 2015). Regarding the East Asian quasi-stationary precipitation during spring-early summer (Ding and Chan, 2005;Chiang et al, 2015), that is, spring persistent rains (Tian and Yasunari, 1998;Wan and Wu, 2007;Linho et al, 2008) through pre-Meiyu and Okinawa Baiu to Meiyu-Baiu (Chen et al, 2004;Srivastava et al, 2004;Okada and Yamazaki, 2012;Wang et al, 2016), in addition to the effects of the massive Tibetan Plateau (Sampe and Xie, 2010;Chen and Bordoni, 2014), mesoscale high mountains have received increasing attention because of their substantial effects on the synoptic-to-large-scale monsoon dynamics (Shi et al, 2008;Qi and Wang, 2012;Wang and Chang, 2012;Wu et al, 2014;Wu and Hsu, 2016).…”