Land–sea thermal contrast associated with summer monsoon onset over the Chao Phraya River basin

Theoretical and Applied Climatology https://doi.org/10.1007/s00704-022-04131-3 ORIGINAL PAPER Land–sea thermal contrast associated with summer monsoon onset over the Chao Phraya River basin Tomohito J. Yamada1 · Sourabh Shrivastava2 · Ryosuke Kato2,3 Received: 4 January 2022 / Accepted: 26 June 2022 © The Author(s) 2022 Abstract Earlier onset of the Southeast Asian summer monsoon (SAM) was observed over the Chao Phraya River basin in Thailand using Thai Meteorological Department-derived high-resolution merged rainfall data from 1981 to 2016. SAM variability depends on numerous local and global factors, including thermal conditions over the Bay of Bengal (BoB) and Tibetan Plateau (TbT). Despite tremendous past research efforts, the effect of thermal heat contrast on the SAM remains unclear. Using observational and reanalysis datasets, we found that the absolute value of total heat over the BoB was increasing. However, the interannual variability of total heat was greater over the TbT. Changes in surface temperature (± 1.5 °C), air thickness (± 20 m), and geopotential height over the TbT were associated with the timing of SAM onset. The results also suggested that significant changes in air thickness are driven by surface temperature differences over the TbT, while changes in the integrated apparent heat source and integrated apparent moisture sink of ± 100 W m−2 resulted in anomalous convective activities over the BoB and mainland of the Indochina Peninsula in years of early and late SAM onset. At the intraseasonal timescale, Madden–Julian oscillation (MJO) was observed over the Indian Ocean and Western Hemisphere for 4–10 days in years of early SAM onset. The opposite situation was found for years of late SAM onset, with MJO located over the Western Pacific Ocean and Maritime Continent. 1 Introduction Thailand receives approximately 80% or more of its annual total rainfall between May and October according to a Thai Meteorological Department (TMD, 2021) during the South Asian summer monsoon (SAM). However, the variability of SAM onset has been raised as a topic of concern (Zhou et al. 2019) due to its effects on agriculture, human population, and industry. For example, rice production strongly depends on the parameters of SAM as well as its onset. In addition, the SAM affects more than 69.6 million people in Thailand (The World Bank, 2019), particularly in the Chao Phraya River basin (CPB), which covers approximately 30% (157,925 km2) of the mainland area of Thailand. Moreover, * Tomohito J. Yamada 1 Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, Japan 2 Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido, Japan 3 Ministry of Land, Infrastructure, Transport and Tourism, Tokyo, Japan SAM onset has major effects on the gross domestic product of Thailand. The characteristics of SAM onset have been extensively studied (Zhang et al. 2002; Takahashi and Yasunari, 2006; Ding, 2007; Kuraji and Arthorn, 2011; Kajikawa et al. 2012; Nguyen-Le, 2015; Shrivastava et al., 2017; Tanaka et al. 2019). Using a rainfall index for 1951–1996, Zhang et al. (2002) defined the mean onset date over the central Indochina Peninsula (ICP) as approximately the 26th pentad of the year. Kajikawa et al. (2012) found that the SAM has arrived earlier over the ICP in the three decades from 1979 to 2008 based on Climate Prediction Center merged analysis of precipitation (CMAP) at 2.5 × 2.5° spatial resolution. Minoura et al. (2003) discussed the mechanism of SAM onset and characterized the ICP region as having relatively slow onset based on European Centre for Medium-Range Weather Forecasts reanalysis data with a spatial resolution of 2.5° × 2.5° from 1982 to 1993. Using ground observations, a trend of earlier SAM onset was also identified by Kuraji and Arthorn (2011) and Tanaka et al. (2019) in the Mae Chaem watershed of the ICP. In recent decades, long-term high-resolution gauge rainfall data for the ICP have become available for research purposes. Therefore, our first question 13 Vol.:(0123456789) T. J. Yamada et al. is whether SAM onset has continued to shift earlier in recent decades or whether this trend has changed. Extensive efforts have been made to determine the relationships between SAM onset and various contributing factors (e.g., Minoura et al. 2003; Kiguchi et al. 2016). Takahashi and Yasunari (2006) determined climatological mean values of rainfall over 50 years and showed a distinct climatological monsoon break occurring at approximately the 33rd pentad over the ICP, with a dramatic change in largescale monsoon circulation in the seasonal progression. Kiguchi et al. (2016) analyzed premonsoon rainfall to identify its relationship with SAM onset, but found no clear relationship. Minoura et al. (2003) discussed the mechanism of SAM onset, determined that onset of the Indian summer monsoon (ISM) was slower than SAM onset, and suggested that the onset dates of ISM and SAM should be distinguished. The question of whether global teleconnections affect SAM onset over the ICP remains unanswered. Takahashi and Yasunari (2006) identified a trough over the Bay of Bengal (BoB) during early SAM onset; at the same time, weak southwesterlies appeared over the eastern Indian Ocean and high precipitation was subsequently recorded over the ICP. Many efforts have focused on SAM onset measured based on the El Niño–Southern Oscillation (ENSO) and Indian Ocean dipole, and the relationship between SAM onset timing and large-scale variability modes has been documented (Wu and Wang, 2000; Wang et al. 2001; Ding, 2007). Wu and Wang (2000) identified a link between ENSO and SAM onset. Their research indicated that physical processes occurring on land and in the ocean are critically responsible for reducing land–ocean thermal contrast. The Madden–Julian oscillation (MJO) may also affect SAM onset at the intraseasonal scale (Chi et al. 2015). The MJO is one of the dominant modes of intraseasonal variability in the tropics (Madden and Julian, 1972). The eastward-moving MJO is associated with deep convection and circulation from eastern Africa, the Indian Ocean, and the Maritime Continent to the Western Pacific (Madden and Julian, 1971, 1972). When the MJO moves to the eastern and western portions of the Indian Ocean, it impacts SAM onset (Chi et al. 2015). The MJO, which is influenced by thermal conditions over the Indian Ocean, is not associated with either late or early SAM. The relationships of SAM onset with local factors also remain unclear. Therefore, we explored whether the intensity and position of the MJO can influence SAM onset. Heat contrast between the Asian landmass and the tropical Indian Ocean is essential to changes in SAM onset (Kajikawa et al. 2012). The relationship between sea surface temperature (SST) over the BoB and rainfall over the ICP has not been fully clarified (Kanae et al. 2002). Therefore, some studies have focused on the effect of l (...truncated)