A 225-year pine (Pinus latteri) tree-ring record of pre-monsoon relative humidity variation in Nan province of northern Thailand and the linkage with large-scale ocean-atmospheric circulations

Thailand is bounded by the Gulf of Thailand in the Pacific Ocean and the Andaman Sea in the Indian Ocean. Variations in the Thailand monsoon (TM) are strongly influenced by the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) phenomenon. The climatology of Thailand is of great importance for un-derstanding seasonal responses to large-scale climatic variations and associated ocean-atmospheric forcing. This study presents the first longest reconstruction (1792-2016 CE) of March-May relative humidity (RH) based on the tree-ring width chronology of Pinus latteri trees from Nan province, northern Thailand. The growth of P. latteri was mainly limited by pre-monsoon RH variability (r = 0.773, p < 0.001, n = 66). The field spatial correlations with the gridded climate datasets demonstrated that the climatic signals retained in our reconstruction repre-sented regional climate variations. Extremely wet years were observed in 1879, 1887, 1908, 1929, 1942, 1974, and 2013 CE, whereas extremely dry years were found in 1795-1796, 1905, 1960, 1992, and 2010 CE. Spectral analysis and spatial correlation analysis revealed pre-monsoon RH variability in the northern Thailand is linked to large-scale ocean-atmospheric circulations, mainly ENSO and IOD activities due to sea surface temperatures variation in the Pacific and Indian Oceans. ENSO-related climate anomalies during the past 225 years were strongly associated in our reconstruction, of which a total of 63% extremely and abnormally wet years occurred during the La Nina events, whereas 59% of extremely and abnormally dry years corresponded to El Nino events. Our reconstruction indicated increasing extreme wet and dry events since 1900 CE attributed to ENSO activities, and thus provide useful insights to understand occurrence of extreme weather events in Thailand climatology.