Changes in ENSO Frequency and Character as Recorded in NE Indian Speleothem Oxygen Isotopes

Abstract:

Oxygen isotopes (δ¹⁸O) from speleothems in monsoon regions such as NE India have provided important records of monsoon strength on decadal to multi-millennial timescales. However, recent precipitation and cave drip water studies suggest that regional rain and drip water δ¹⁸O variability may be driven by changes in moisture sources and transport pathways rather than mainly reflecting changes in rainfall amount. Hence, we compare a modern δ¹⁸O speleothem time-series with historical meteorological records and global climate indices to identify dominant influences on speleothem δ¹⁸O.We have constructed a high resolution (7-8 samples per year) δ¹⁸O time-series using a stalagmite (MAW 02-01) from Mawmluh Cave in NE India that grew during the ~50 years preceding collection in 2013. U-series dates (±1-2 years) provide an age model and confirm that visible stalagmite laminations are approximately annual. We compared the stalagmite time-series with local meteorologic records and indices of the El Niño/Southern Oscillation (Niño3, Niño4) and the Indian Ocean Dipole (IOD). Our record shows no significant correlation with rainfall amount and reveals the same seasonal signal observed in local rain and drip water δ¹⁸O. We observe significant positive correlations, with a 6 month lag, between our dataset and the Niño4 Sea Surface Temperature (SST) index (r = 0.49, p < 0.001) and the IOD (r = 0.41, p < 0.001). A cross-wavelet analysis between MAW 02-01 δ¹⁸O and Niño4 highlights a shared 4-6 year frequency throughout and a strong in-phase 8-12 year frequency beginning in 1990 and persisting for the duration of the record. The latter may reflect an increase in strength and frequency of central Pacific (CP) El Niño events since 1990. Such events, with stronger SST anomalies in the central Pacific, are more likely to restrict Indian Summer Monsoon (ISM) strength through enhanced westward migration of the descending arm of the Walker Cell. HYSPLIT back-trajectory analysis suggests that moisture transport distance is reduced during years with CP-El Niño events compared to traditional El Niño and non-El Niño years. Our proposed interpretation of the Mawmluh stalagmite record aids interpretation of longer-term speleothem records, including their relationship to ISM variability, and supports refined predictions of future ISM behavior.