Abstract. Using radiosonde observations at five stations in the tropical
western Pacific and reanalysis data for the 15 years from 2005 to 2019, we
report an extremely negative anomaly in atmospheric water vapor during the
super El Niño winter of 2015/16 and compare the anomaly with that in
the other three El Niño winters of the period. A strong specific humidity anomaly is
concentrated below 8 km of the troposphere with a peak at 2.5–3.5 km, and a
column-integrated water vapor mass anomaly over the five radiosonde sites
has a large negative correlation coefficient of −0.63 with the oceanic
Niño3.4 index but with a lag of about 2–3 months. In general, the
tropical circulation anomaly in the El Niño 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 four El Niño winters,
especially in the 2006/07 and 2015/16 eastern Pacific (EP) El Niño
events. The monsoon circulation shows a remarkable change from one event to
another, and its anomaly is large in the 2009/10 and 2018/19 central Pacific
(CP) El Niño winters and small in the two EP El Niño winters. The
observed water vapor anomaly is caused mainly by the Walker circulation
anomaly in the super EP event of 2015/16 but is caused by the monsoon circulation
anomaly in the strong CP event of 2009/10. The roles of the Hadley, Walker,
and monsoon circulations in the EP and CP events are confirmed by the
composite EP and CP El Niños based on the reanalysis data for 41 years.
Owing to the anomalous decrease in upward transport of water vapor during
the El Niño winter, lower cloud amounts and more outgoing longwave
radiation over the five stations are clearly presented in satellite
observation. In addition, a detailed comparison of water vapor in the
reanalysis, radiosonde, and satellite data shows a fine confidence level for
the datasets; nevertheless, the reanalysis seems to slightly underestimate
the water vapor over the five stations in the 2009/10 winter.