The El Niño–Southern Oscillation effect on tropical outgoing longwave radiation: A daytime versus nighttime perspective

Trends of tropical (30°N–30°S) mean daytime and nighttime outgoing longwave radiations (OLRs) from the Clouds and Earth's Radiant Energy System (CERES) and the Atmospheric Infrared Sounder (AIRS) are analyzed using data from 2003 to 2013. Both the daytime and nighttime OLRs from these instruments show decreasing trends because of El Niño conditions early in the period and La Niña conditions at the end. However, the daytime and nighttime OLRs decrease at different rates with the OLR decreasing faster during daytime than nighttime. The daytime-nighttime OLR trend is consistent across CERES Terra, Aqua observations, and computed OLR based upon AIRS and Moderate-Resolution Imaging Spectroradiometer (MODIS) retrievals. To understand the cause of the differing decreasing rates of daytime and nighttime OLRs, high cloud fraction and effective temperature are examined using cloud retrievals from MODIS and AIRS. Unlike the very consistent OLR trends between CERES and AIRS, the trends in cloud properties are not as consistent, which are likely due to the different cloud retrieval methods used. When MODIS and AIRS cloud properties are used to compute OLR, the daytime and nighttime OLR trends based upon MODIS cloud properties are approximately half as large as the trends from AIRS cloud properties, but their daytime-nighttime OLR trends are in agreement. This demonstrates that though the current cloud retrieval algorithms lack the accuracy to pinpoint the changes of daytime and nighttime clouds in the tropics, they do provide a radiatively consistent view for daytime and nighttime OLR changes. The causes for the larger decreasing daytime OLR trend than that for nighttime OLR are not clear and further studies are needed.