Tropical water vapor variations during the 2006-2007 and 2009-2010 El Ninos: Satellite observation and GFDL AM2.1 simulation

Water vapor measurements from Aura Microwave Limb Sounder (MLS, above 300 hPa) and Aqua Atmospheric Infrared Sounder (AIRS, below 300 hPa) are analyzed to study the variations of moisture during the 2006-2007 and 2009-2010 El Ninos. The 2006-2007 El Nino is an East Pacific (EP) El Nino, while the 2009-2010 El Nino is a Central Pacific (CP) El Nino or El Nino Modoki. Results show that these two types of El Nino events produce different patterns of water vapor anomalies over the tropical ocean, approximately resembling the cloud anomalies shown in Su and Jiang (2013). Regression of water vapor anomalies onto the Nino-3.4 SST for the A-Train period shows a clear upper tropospheric amplification of the fractional water vapor change, i.e., the ratio of the change in specific humidity to the layer-averaged specific humidity. Furthermore, tropical water vapor anomalies in different circulation regimes are examined. It is shown that the variations of water vapor during the 2006-2007 El Nino are mainly controlled by the thermodynamic component, whereas both dynamic and thermodynamic components control the water vapor anomalies during the 2009-2010 El Nino. GFDL AM2.1 model simulations of water vapor and cloud anomalies for the two El Ninos are compared with the satellite observations. In general, the model approximately reproduces the water vapor anomalies on both zonal and meridional planes but it produces too strong a cloud response in the mid- and lower troposphere. The model fails to capture the dynamic component of water vapor anomalies, particularly over the Indian Ocean.