Estimated cloud-top entrainment index explains positive low-cloud-cover feedback

Area of Science:

• Climate Science
• Atmospheric Physics
• Global Warming Research

Background:

• Subtropical marine low cloud cover (LCC) response to global warming remains a key climate change uncertainty.
• Estimated Inversion Strength (EIS) predicts LCC but fails in warming scenarios, showing LCC decreases despite increasing EIS in global climate models (GCMs).

Purpose of the Study:

• To introduce and validate the Estimated Cloud-Top Entrainment Index (ECTEI) as a reliable predictor of subtropical marine LCC changes under global warming.
• To resolve the discrepancy between EIS and LCC changes in GCMs during warming.

Main Methods:

• Utilized state-of-the-art Global Climate Models (GCMs) to simulate climate responses.
• Applied the newly proposed Estimated Cloud-Top Entrainment Index (ECTEI) to analyze LCC feedback mechanisms.
• Investigated patterned sea surface temperature (SST) warming scenarios.

Main Results:

• The ECTEI consistently decreases with LCC in warmer SST climates, unlike EIS.
• ECTEI constrains subtropical marine LCC feedback to -0.41 ± 0.28% K⁻¹ (90% CI), indicating a virtually certain positive feedback.
• ECTEI provides a physical explanation for heuristic LCC models based on EIS and SST changes, linking them to cloud-top entrainment.

Conclusions:

• The ECTEI is a robust index for predicting subtropical marine LCC changes in response to global warming.
• The study confirms a virtually certain positive feedback loop between warming and LCC, improving climate model reliability.
• Understanding cloud-top entrainment processes is crucial for accurately projecting future climate change.