Cloud sensitivity to tropical warming in observations

The inter-annual variability of the ocean- atmosphere system in the tropics is dominated by El Niño Southern Oscillation (ENSO) in the Pacific. Sea surface temperature (SST) changes in the tropics during ENSO cause large-scale cloud variability through atmospheric teleconnections. How the clouds will respond to future warming is still uncertain and needs further research. Here, the cloud response on inter-annual timescale over the tropics from MODIS for a period of 18 years is used as a proxy to study the expected large-scale cloud responses in a future warmer climate. We explore the cloud response to both the tropical mean SSTs and Niño 3.4 index. Different cloud classes respond to this inter-annual warming differently and we concentrate on the cirrostratus and stratocumulus clouds, which show the largest and most consistent variability: a reduction in cloud fraction with warming. The radiative effects associated with these clouds are determined using top-of-atmosphere fluxes measured by Clouds and Earth Radiant Energy System (CERES). The possible mechanisms associated with the control of cirrostratus cloud fraction over the tropics are explored by using mean profiles of vertical static stability, radiative cooling, and radiatively driven mass divergence derived from the ERA 5 dataset. We have strong evidence showing reduction in clouds which are not as optically thin as what was previously known. We support the previous findings that the reduction in cloud fraction is associated with a significant decrease in upper-level divergence, by demonstrating detailed balance in height categories using a longer record and quantify the radiative importance. In addition to the known increase in stability with height, we find that there is also a significant local decrease in stability at anvil height with warming. We confirm that, apart from the tropical mean SSTs, the Niño 3.4 index is also an important parameter that is strongly correlated to the high cloud responses in terms of height of cloud tops, cover, and the associated meteorological variables in the tropics