The Iris Illusion in the Tropical Sky Seen Through Two Decades of Aura MLS Ice Water Contents

TL;DR

This study examines the relationship between ice water content, infrared leakage, and sea surface temperature in the tropics over two decades, challenging the Iris hypothesis by showing that cirrus clouds strengthen with warming.

Contribution

It provides a two-decade analysis of tropical cloud behavior, revealing a negative correlation between IR leakage derivatives and SST, contradicting the Iris hypothesis.

Findings

IR leakage decreases as SST increases

Tropical cirrus clouds strengthen with warming

Contradicts the Iris hypothesis

Abstract

I analyzed ice water content (IWC) data from the Aura Microwave Limb Sounder (MLS) and sea surface temperature (SST) data from NOAA's Optimum Interpolation SST (OISST) product from 2004 to 2024. Using these data, I derived monthly infrared (IR) leakage over the tropics and computed derivatives of both the IR leakage and tropical SST time series from 2005 to 2023. These two derivatives produced a Pearson correlation of -0.49, indicating that IR leakage decreases when SST increases. This behavior contradicts the trend predicted by the Iris hypothesis, suggesting that tropical cirrus clouds strengthen, rather than weaken, as the ocean warms.