Observational Evidence for a Stability Iris Effect in the Tropics

Marion Saint‐lu (1) , Sandrine Bony (1) , Jean-Louis Dufresne (1)

1 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)

Anvil clouds cover extensive areas of the tropics, and their response to global warming can affect cloud feedbacks and climate sensitivity. A growing number of models and theories suggest that when the tropical atmosphere warms, anvil clouds rise and their coverage decreases, but observational support for this behavior remains limited. Here we use 10 years of measurements from the space-borne CALIPSO lidar to analyze the vertical distribution of clouds and isolate the behavior of anvil clouds. On the interannual time scale, we find a strong evidence for anvil rise and coverage decrease in response to tropical warming. Using meteorological reanalyses, we show that this is associated with an increase in static stability and with a reduction in clear-sky radiatively driven mass convergence at the anvil height. These relationships hold over a large range of spatial scales. This is consistent with the stability Iris mechanism suggested by theory and modeling studies. Plain Language Summary Anvil clouds cover about 40% of the tropics. Their response to global warming, especially changes in their height or in their horizontal extent, has the potential to affect the Earth's surface temperature. By analyzing 10 years of observations of the vertical distribution of clouds from a space-borne lidar, we show that the anvils rise and reduce their coverage during the years that are anomalously warm. By using meteorological reanalyses, we further show that this behavior is consistent with the stability Iris effect suggested by theory and modeling studies. These results improve our physical understanding of the response of tropical clouds to warming and present relationships that may be used to test climate models.

Domaines: Océan, Atmosphère

Marion Saint‐lu, Sandrine Bony, Jean-Louis Dufresne. Observational Evidence for a Stability Iris Effect in the Tropics. Geophysical Research Letters, 2020, 47 (14), ⟨10.1029/2020GL089059⟩. ⟨hal-02909896⟩