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

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 Nino-Southern Oscillation (ENSO) cycles. Results reveal greater upper-tropospheric stability during El Nino, resulting in a lower level of neutral buoyancy compared to La Nina. However, outflow levels remain relatively consistent across ENSO cycles. This suggests that, despite less favorable conditions for deep convection during El Nino, stronger convective intensity is required to achieve outflow levels comparable to those in La Nina. Indeed, our results suggest that convection observed during El Nino tends to have broader cores and lower entrainment rates, translating to greater intensity compared to La Nina. 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. 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 Nino to La Nina, 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 Nino. However, the heights where convective outflow occurs do not change much during El Nino and La Nina events. This happens because during El Nino, 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. LNB is lower during El Nino due to a greater UT stability, but convective outflow levels remain consistent across ENSO cycles The intensity of convective cores during El Nino has to be stronger than that during La Nina Our finding necessitates a modification to FAT and the proportionately higher anvil temperature (PHAT)