Climate Base State Influences on South Asian Monsoon Processes Derived From Analyses of E3SMv2 and CESM2
The E3SMv2 and CESM2 are analyzed to investigate how differences in simulated base state tropical SSTs and ENSO amplitude affect the processes associated with South Asian monsoon precipitation, with a focus on connections between the monsoon and El Niño/Southern Oscillation (ENSO).
Through the analysis of the simulation characteristics of E3SMv2 and CESM2, we quantify differences in monsoon-ENSO connections and identify contributions from ENSO amplitude and tropical SSTs. For the first time, we attribute the differences in the strength of monsoon-ENSO connections to two specific processes, with about half due to mean tropical SSTs and half due to ENSO amplitude. Thus, this two-model analysis provides unique and crucial insights into the processes that affect the predictability of the economically impactful South Asian monsoon.
Historical simulations from 1850-2014 for E3SMv2 and CESM2, 20 ensemble members each, are analyzed for the South Asian monsoon season June-July-August-September (JJAS). To quantify relative contributions to monsoon-ENSO connections from base state SSTs and ENSO amplitude, pacemaker experiments with CESM2 (ten ensemble members) are compared to both models and use time-evolving SST anomalies in the tropical Pacific nudged to observations (ERSSTv5, 1880-2019) whereby the observed evolution of ENSO is maintained along with the model’s base state climate, while the rest of the model’s coupled climate system is free to evolve. Though tropical SSTs are over 1°C cooler in E3SMv2 and there is overall reduced Indian sector precipitation, the regional pattern of South Asian monsoon precipitation is similar in the two models. More significantly, monsoon-ENSO teleconnections are reduced in E3SMv2 compared to CESM2, with about half due to cooler tropical SSTs and half due to reduced ENSO amplitude in E3SMv2.