Monitoring early-flood season intraseasonal oscillations and persistent heavy rainfall in South China

Abstract

Guangxi and Fujian provinces, has the most abundant rainfall in China (Lu 1990). The rainy season in this area is usually divided into the early-flood season (April-June) and the late-flood season (July-September). During the early-flood season, heavy rainfall events and the associated floods frequently cause severe property damages in South China. The late-flood season rainfall is mainly caused by tropical systems moving into South China, whereas the rainfall during the early-flood season is largely associated with the quasistationary subtropical front, called the Mei-Yu front (Chen et al. 2006). The early-flood season persistent heavy rainfall events (PHR events hereafter) are responsible for most severe flood disasters associated with significant economic losses in South China. Thus, they have received much attention (Tao and Chen 1987;Lau et al. 1988;Tang et al. 2006;Bao 2007Bao , 2008.The intraseasonal oscillation (ISO) has been found responsible for most rainfall variations in the Asian summer monsoon region (Yang 1992; Mao and Chan 2005; Li and Wang 2005; Zhou and Chan 2005; Miura et al. 2007; Lin et al. 2008; Mao et al. 2010; Yang et al. 2010; Ren et al. 2013, 2015; Pan et al. 2013; Chen and Zhai 2014). Two different periodicities of the boreal summer ISO (BSISO) have been identified (Li and Wang 2005): 30-60 days (e.g., Wang et al. 2005) and 10-20 days (e.g., Kikuchi and Wang 2010). The wet and dry spells of the BSISO strongly influence extreme hydro-meteorological events, major driving forces of natural disasters, and thus the socio-economic activities in South Asia (Lau and Waliser 2005). Huang et al. (2008) investigated the climatological characteristics of the ISO related precipitation variability in China and found that the ISO begins to amplify in April and weakens in November. Ju and Zhao (2005) showed that the 30-60 days ISO is the most remarkable in strong summer Abstract Rainfall variability during the early-flood season (April-June) in South China is largely controlled by both the 10-20 and 20-70-day intraseasonal oscillations (ISO). In this study, a method is described to monitor the ISO and persistent heavy rainfall in South China. Three existing daily real-time 20-70-day ISO indices are compared. It is found that the regional East Asia-western North Pacific (EAWNP) ISO index best represents the earlyflood season 20-70-day ISO in South China. A new bivariate boreal summer ISO index is designed to describe the 10-20-day ISO in the EAWNP region. Composite analysis shows that the rainfall anomaly in South China is well captured by the northward propagation of both the 10-20 and 20-70-day ISO. With different phase combinations of the 10-20 and 20-70-day EAWNP ISO, nine conditions are defined ranging from those favorable to those unfavorable to heavy rainfall in South China that can be used to effectively monitor the early-flood season ISO and persistent heavy rainfall in South China.

Simultaneous modulations of precipitation and temperature extremes in Southern parts of China by the boreal summer intraseasonal oscillation

“…The BSISO influences on precipitation in the Yangtze River Valley are fulfilled via modulating the position of the western Pacific subtropical high (Ren et al 2013;Lu et al 2014). Precipitation extremes also show close relationship with two BSISO components (Hsu et al 2016;Gao et al 2016). For instance, both Lu et al (2014) and Zhu et al (2003) revealed that floods in the Yangtze River Basin during 1998 were in tandem with the active phase of 30-60-day BSISO.…”

Statistical analysis of the impacts of intra‐seasonal oscillations on the South China Sea summer monsoon withdrawal

“…There are two commonly applied techniques in studying ISOs, namely, the ISO indices (e.g., Wheeler and Hendon, 2004;Lee et al, 2013;Lin, 2013;Gao et al, 2016;He et al, 2019;Qian et al, 2019) and the bandpass filter technique (e.g., Tong et al, 2009;Hsu, 2012;Qiao et al, 2015;Hu et al, 2018c). Using the first technique, spatial patterns of ISO modes are extracted by multivariable empirical orthogonal functions (MV-EOFs).…”

Impact of the intra‐seasonal oscillation on tropical cyclone genesis over the western North Pacific

“…The data used in this study are listed as follows: (a) the National Centers for Environmental Prediction (NCEP)-National Center for Atmospheric Research (NCAR) daily reanalysis data set with a horizontal resolution of 2.5 (Kalnay et al, 1996;Kistler et al, 2001Kistler et al, ) during 1982Kistler et al, -2016(b) the NOAA high resolution SST daily data with a horizontal resolution of 0.25 during the same time which are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.esrl. noaa.gov/psd/; (c) the TC best-track data sets from the Joint Typhoon Warning Center (JTWC; western North Pacific; data available online at http://metocph.nmci.navy.mil/jtwc/best_ tracks/), where the tropical cyclone genesis location is defined as the position of a storm whose maximum sustained 1-m winds first reaches 17 m/s along its trajectory, a definition identical to McDonald et al (2005); (d) the daily outgoing longwave radiation (OLR) obtained from the NOAA polar orbiting satellites (Liebmann, 1996); (e) three sets of daily real-time ISO indices for the period of 1982-2016, where the RMM indices are downloaded from the Australian Bureau of Meteorology website (http://www.bom.gov.au/climate/mjo/), the BSISO-1 and BSISO-2 indices are from the International Pacific Research Center website (http://iprc.soest.hawaii.edu/users/jylee/bsiso), and the EAWNP ISO-1 and EAWNP ISO-2 indices are calculated according to Lin (2013) and Gao et al (2016).…”

“…Studies indicate that there is an increase of TC genesis in the convective ISO phases while a decrease in the non-convective phases over the WNP basin (Liu and Lin, 1990;Kim et al, 2008;Wang and Zhou, 2008;Gao and Li, 2011;Zhu et al, 2013;Li and Zhou, 2013a;2013b). In addition to the ISO of 30-60-day period, a second dominant mode of ISO with 10-20-day prevailing period, referred to as the quasi-biweekly oscillation (QBWO), has also been identified over the Asian summer monsoon region (Chen and Chen, 1993;Chen and Sui, 2010;Kikuchi and Wang, 2010;Lee et al, 2013;Gao et al, 2016). Previous studies suggested that the kinetic energy of the QBWO is much stronger than that of the MJO over the WNP basin (Li and Zhou, 1995), so the QBWO can strongly impact on the local TC activity (Wang and Chen, 2009;Li and Zhou, 2013a;2013b).…”

“…One is the daily Real-time Multivariate MJO indices (RMMs; Wheeler and Hendon, 2004) which represent the eastwards propagation feature of the tropical intraseasonal variability. The second set of ISO index is the daily real-time boreal summer intra-seasonal oscillation indices (BSISO-1 for the 30-60-day oscillation and BSISO-2 for the 10-30-day oscillation, Lee et al, 2013) which capture the main feature of the northwards/northwestwards propagation in the region of 10 S-40 N, 40 -160 E. The third set of ISO index is the daily real-time ISO indices in the East Asian-western North Pacific (EAWNP ISO-1 for the 20-70-day oscillation, Lin, 2013 and EAWNP ISO-2 for the 10-20-day oscillation, Gao et al, 2016) that were designed specifically to represent the prominent northwards propagation of the ISO in the EAWNP summer monsoon region (10 S-40 N, 90 -150 E). Both the BSISO and EAWNP ISO indices were designed to describe the Asian summer monsoon ISO and its propagation, but due to their different analysis domains, the BSISO indices are for a larger area including the Indian ocean, SCS and WNP regions while the EAWNP ISO indices mainly focus on the East Asian-western North Pacific.…”