Process‐Level Assessment of the Iris Effect Over Tropical Oceans
Masato Ito, Hirohiko Masunaga
Abstract
Abstract The iris hypothesis suggests a cloud feedback mechanism that a reduction in the tropical anvil cloud fraction (CF) in a warmer climate may act to mitigate the warming by enhanced outgoing longwave radiation. Two different physical processes, one involving precipitation efficiency and the other focusing on upper‐tropospheric stability, have been argued in the literature to be responsible for the iris effect. In this study, A‐Train observations and reanalysis data are analyzed to assess these two processes. Major findings are as follows: (a) the anvil CF changes evidently with upper‐tropospheric stability as expected from the stability iris theory, (b) precipitation efficiency is unlikely to have control on the anvil CF but is related to mid‐ and low‐level CFs, and (c) the day and nighttime cloud radiative effects are expected to largely cancel out when integrated over a diurnal cycle, suggesting a neutral cloud feedback.