Water vapor anomaly over the tropical western Pacific in El Niño winters from radiosonde and satellite observations

Abstract. Using radiosonde observations at five stations in the tropical western Pacific and reanalysis data for 15 years from 2005 to 2019, we report an extremely negative anomaly in atmospheric water vapor during the super El Nino winter of 2015/16, and compare the anomaly with that in the other three El Nino winters. Strong specific humidity anomaly is concentrated below 8 km of the troposphere with a peak at 2.5–3.5 km, and column integrated water vapor mass anomaly over the five radiosonde sites has a large negative correlation coefficient of −0.63 with oceanic Nino3.4 index, but with a lag of about 2–3 months. In general, the tropical circulation anomaly in the El Nino winter is characterized by divergence (convergence) in the lower troposphere over the tropical western (eastern) Pacific, thus the water vapor decreases over the tropical western Pacific as upward motion is suppressed. The variability of the Hadley circulation is quite small and has little influence on the observed water vapor anomaly. The anomaly of the Walker circulation makes a considerable contribution to the total anomaly in all the four El Nino winters, especially in the 2006/07 and 2015/16 eastern-Pacific (EP) El Nino events. The monsoon circulation shows a remarkable change from one to the other event, and its anomaly is large in the 2009/10 and 2018/19 central-Pacific (CP) El Nino winters and small in the two EP El Nino winters. The observed water vapor anomaly is caused mainly by the Walker circulation anomaly in the supper EP event of 2015/16 but by the monsoon circulation anomaly in the strong CP event of 2009/10. Owing to the anomalous decrease in upward transport of water vapor during the El Nino winter, less cloud amount and more outgoing longwave radiation over the five stations are clearly presented in satellite observation.