El Nino/Southern Oscillation (ENSO) is a typical natural oscillation in interannual timescale. Under global warming, rainfall variability will become stronger over the tropical central-eastern Pacific modulated by ENSO. Surface warming pattern of mean state is the dominant dynamic contributor to enhanced precipitation variability there. But the detailed physical mechanisms remain unclear.

SUN Ning, a PhD student in research group of Prof. ZHOU Tianjun with the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, investigated the impact of background sea surface temperature (SST) warming on the ENSO-driven rainfall variability.

They cooperated with the Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA) and used a high-resolution Atmosphere General Circulation Model (AGCM) developed by MRI/JMA to isolate the influence of mean-state warming.

"We found under global warming, warmer SST over the tropical central-eastern Pacific would enhance local upward motion through increased the moist static energy (MSE) in lower troposphere and positive feedback loop between cloud longwave warming and enhanced upward motion," said SUN. "The enhancement of upward motion is a dominant contributor to the enhanced ENSO-driven rainfall variability, while increasing mean-state moisture plays a secondary role."

A flowchart of how SST warming pattern amplify rainfall variability in the tropical Pacific. (Image by SUN NIng) 

Under a warming climate, projected changes in interannual variability of precipitation over tropical Pacific plays an important role in modulating Earth's climate through atmospheric bridge. The proposed physical mechanisms of enhanced ENSO-driven rainfall variability can help us understand the variability changes in other faraway locations, such as land monsoon and polar regions.