Inter-annual Controls on Oxygen Isotopes of Precipitaion in the Asian Monsoon Region

Abstract:

The complex nature of speleothem δ¹⁸O from the Asian monsoon region is a result of the varying influences of monsoon strength, moisture source region, transport history, local cave hydrology and other effects on cave dripwater δ¹⁸O. In order to provide a more robust interpretation of speleothem δ¹⁸O data from the broader Asian monsoon region, we utilize existing simulations from the isotope-enabled GCM, IsoGSM (Yoshimura el al. 2008), to investigate the climatic controls on precipitation δ¹⁸O (^δ18O_p) at four cave locations: Dongge Cave, China (25°17′ N, 108°5′ E); Tham Mai Cave, Laos (20.75 N, 102.65 E); Mawmluh Cave, India (25°15′44′′N, 91°52′54′′E); and Qunf Cave, Oman (17°10′ N, 54°18′ E). Our composite speleothem records from Laos—a key site at the interface between the Indian and East Asian monsoon systems—will be used as a case study for interpreting speleothem δ¹⁸O in the South-East Asian Monsoon (SEAM) region.

Our results show that δ¹⁸O_p extracted from the grid point closest to four cave sites from IsoGSM shows very low correlation between δ¹⁸O_p and local precipitation. δ¹⁸O_p at Dongge cave reveals a negative correlation (0.4 to 0.5) with precipitation in the Bay of Bengal, suggesting that δ¹⁸O_p from the East Asian monsoon area reflects upstream distillation over the Indian monsoon region. δ¹⁸O_p in Laos exhibits a negative correlation with precipitation over the broad Indo-Pacific warm pool region, indicating increased convection over this area leads to more negative δ¹⁸O_p over SE Asia. Given the low correlation between local precipitation and δ¹⁸O_p at all four cave sites, we interpret the δ¹⁸O_p at these locales as reflective of regional changes in hydroclimate, rather than local precipitation amount. In addition, δ¹⁸O_p from IsoGSM at all fours sites, especially Qunf, Mawnluh, and Tham Mai cave, show a positive correlation with Pacific SSTs over the NINO3.4 region and in the western and northern Indian Ocean, suggesting that the δ¹⁸O_p of annual rainfall may be influenced by ENSO and IOD.As a case study, after detrending, Laos δ¹⁸O_p from IsoGSM since 1900 AD is moderately correlated with the NINO3.4 index (r=0.53) and with the IOD index at a lag of 1 year (r = -0.35). This indicates that interannual variability of δ¹⁸O_p at our site is likely controlled by ENSO.