Investigating Convective Processes Underlying ENSO: New Insights Into the Fixed Anvil Temperature Hypothesis

• Published in: Geophysical research letters Vol. 51; no. 6
• Main Authors: Takahashi, Hanii, Luo, Zhengzhao Johnny, Masunaga, Hirohiko, Storer, Rachel, Noda, Akira T.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-03-2024; Wiley
• Subjects: Air flow; Annual variations; Anvils; Climate; Climate change; Cloud properties; Convection; convective cores; convective outflow; Cores; Dynamic control; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; ENSO; Entrainment; Environmental conditions; Environmental factors; FAT; Global warming; Hypotheses; Interannual variations; La Nina; La Nina events; Outflow; PHAT; Southern Oscillation; Stabilizing; Storms; Temperature; Translating; Troposphere; Upper troposphere; Variation
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL107113

Interannual variations provide insight into the sensitivity of convective processes. Thus, CloudSat and ERA5 are used to explore the relationship among convective cores, outflows and environmental conditions during El Niño‐Southern Oscillation (ENSO) cycles. Results reveal greater upper‐tropospheric stability during El Niño, resulting in a lower level of neutral buoyancy compared to La Niña. However, outflow levels remain relatively consistent across ENSO cycles. This suggests that, despite less favorable conditions for deep convection during El Niño, stronger convective intensity is required to achieve outflow levels comparable to those in La Niña. Indeed, our results suggest that convection observed during El Niño tends to have broader cores and lower entrainment rates, translating to greater intensity compared to La Niña. These findings emphasize the importance of considering both large‐scale and convective‐scale processes, providing an update to the fixed anvil temperature (FAT) and the proportionately higher anvil temperature (PHAT) hypotheses as originally proposed.
Plain Language Summary
Examining year‐to‐year variations provides unique insights into understanding how storms may change in a warmer climate. We use CloudSat and ERA5 reanalysis to examine variations in convective outflow (i.e., an airflow pushed out of storms), environmental factors, and cloud properties during ENSO cycles. Comparing El Niño to La Niña, we find that the atmosphere higher up in the troposphere tends to be more stable, which usually slows down the development of storms, during El Niño. However, the heights where convective outflow occurs do not change much during El Niño and La Niña events. This happens because during El Niño, the upward movement of air in storms is more powerful. This stronger upward movement offsets the stabilizing effect of the upper troposphere, so the overall outflow from the storms stays about the same. Our study aligns with the main idea of the FAT hypothesis, which postulates that in a warming climate, the temperature of the anvils (i.e., wide, flat clouds crawling out of the storm top) stays relatively constant primarily due to a thermodynamic constraint. However, our results show that convective‐scale processes with dynamic control play a key role as well, providing an update to the FAT as originally proposed.
Key Points
LNB is lower during El Niño due to a greater UT stability, but convective outflow levels remain consistent across ENSO cycles
The intensity of convective cores during El Niño has to be stronger than that during La Niña
Our finding necessitates a modification to FAT and the proportionately higher anvil temperature (PHAT)