Tropical Clouds and Circulation Changes during the 2006/07 and 2009/10 El Niños

Abstract

hanges in tropical cloud vertical structure, cloud radiative forcing (CRF), and circulation exhibit distinctly different characteristics during the 2006/07 and 2009/10 El Niños, revealed by CloudSat and Cloud?Aerosol Lidar and Infrared Pathfinder Satellite (CALIPSO) observations and reanalysis data. On the tropical average, the 2009/10 has a decrease of clouds from 2 to 14 km, an increase of clouds in the boundary layer, and an increase of cirrus clouds above 14 km. The tropical-mean cloud anomalies in the middle to upper troposphere (6?14 km) for the 2006/07 El Niño are nearly opposite to those in 2009/10 El Niño. The tropical averaged net CRF anomaly at the top of the atmosphere (TOA) is 0.6?0.7 W m?2 cooling (0.02?0.5 W m?2 warming) for the 2009/10 (2006/07) El Niño. The 2009/10 El Niño is associated with a strengthening of tropical circulation, increased high (low) clouds in extremely strong ascending (descending) regimes, and decreased clouds in the middle and high altitudes in a broad range of moderate circulation regimes. The strengthening of tropical circulation is primarily caused by the enhancement of the Hadley circulation. The 2006/07 El Niño is associated with a weakening of the tropical circulation, primarily caused by the reduction of the Walker circulation. The cloud anomalies in each circulation regime are approximately opposite for these two El Niños. The analysis herein suggests that both the magnitude and pattern of sea surface temperature anomalies in the two events contribute to the differences in clouds and circulation anomalies, with magnitude playing a dominant role. The contrasting behaviors of the two El Niños highlight the nonlinear response of tropical clouds and circulation to El Niño SST forcing.