Using an isotope enabled model to interpret precession-driven δ18Ο variability in the South Asian monsoon region

Figure: Change in annual δ18Ο of precipitation in the SASM region
Figure: Change in annual δ18Ο of precipitation in the SASM region. Simulated change in annual δ18Ο of precipitation in the SASM region between two high eccentricity periods with perihelion in NH summer vs. winter solstice. Negative values indicate precipitation is more depleted in the heavier oxygen isotope for the case with higher NH summer insolation. Stars mark Bittoo and Tianmen cave record locations.

Isotopic signals captured in speleothem records (calcium carbonate deposits in caves) located in the monsoon regions in southern Asia reveal long periodic cycles associated with changes in the precession of the Earth’s orbit modulated by the orbital eccentricity. These signals are usually interpreted as changes in the strength of the South Asian Summer Monsoon (SASM) due to what is known as the “amount effect” where precipitation during strong events is typically more depleted in the heavier water isotopes than weaker events.

Figure: Image of a collected speleothem ready for analysis
Figure: Image of a collected speleothem ready for analysis. Speleothems are cave formations, such as stalactites and stalagmites, of layers of minerals deposited from groundwater that flows into the cave over tens of thousands of years. Dated and analyzed, records of the thickness and chemical and isotopic composition of the layers are used to reconstruct past climate change over these long time periods.
Image credit: paleoclimate archives

Indeed, climate models have long simulated a strong response in the intensity of the SASM due to precession-eccentricity driven changes in Northern Hemisphere (NH) summer insolation. However, there are multiple possible confounding mechanisms responsible for the variability seen in the speleothem records which need to be disentangled before the records can be accurately interpreted as a hydroclimate change and used to reconstruct past climate change.

Using a version of the Community Earth System Model (CESM), enabled with isotopic water tracers and water-tagging capability, developed in part by NCAR scientists with university partners, Clay Tabor, ASP post-doctoral scholar, in collaboration with NCAR scientists Bette Otto-Bliesner and Esther Brady, other researchers from the Paleo and Polar Climate section of NCAR’s Climate and Global Dynamics Lab, and university collaborators, investigated the mechanisms contributing to these proxy climate signals captured in the records. Tabor was able to show that the simulated precession-eccentricity driven response in the local isotopic content of precipitation and soil moisture reflected changes seen in the speleothem records. The response however, was found to be mostly due to changes in the relative proportions of water vapor transported into the region from different source regions with a secondary but important role for changes in local evaporation that amplified the isotopic signal of soil water relative to the local precipitation signal. This study demonstrates the novel application and utility of the isotope-enabled CESM in understanding the mechanisms underlying the proxy climate signals in the paleoclimate archives that are used to reconstruct our knowledge of past climate change.

Tabor, C. R., Otto-Bliesner, B. L., Brady, E. C., Nusbaumer, J., Zhu, J., Erb, M. P., Wong, T. E., Liu, Z., Noone, D. (2018). Interpreting precession-driven δ18O variability in the South Asian monsoon region. Journal of Geophysical Research: Atmospheres, 123, 5927–5946. Doi: 10.1029/2018JD028424