Abstract
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.
Highlights
As a dominant greenhouse gas in the atmosphere, water vapor has a profound impact on global energy budgets through latent heat release upon phase transitions (Held and Soden, 2000) and cloud formation that reflects longwave radiation from below and shortwave radiation from above (Stevens et al, 2017); water vapor plays a substantial role in the formation and evolution of the climate system
Sea surface temperature (SST) anomalies in the tropical Pacific have an important influence on water vapor transport, cloud cover, and precipitation distribution due to the tropical circulation changes caused by El Niño–Southern Oscillation (ENSO)
Some discrepancies can be noticed over small tropical islands characterized by steep orography (Lees et al, 2020), and it is reported that precipitable water vapor (PWV) from the ERA5 reanalysis is in good agreement with the retrievals from the Global Navigation Satellite System over 268 stations, there is a bias of 4 mm PWV in the southwest of South America and western China due to the terrain limitations and fewer observations (Wang et al, 2020)
Summary
As a dominant greenhouse gas in the atmosphere, water vapor has a profound impact on global energy budgets through latent heat release upon phase transitions (Held and Soden, 2000) and cloud formation that reflects longwave radiation from below and shortwave radiation from above (Stevens et al, 2017); water vapor plays a substantial role in the formation and evolution of the climate system. 7. In present study, we investigate the atmospheric water vapor by using radiosonde observations at five tropical stations for 15 years from January 2005 to December 2019, which are provided by the National Oceanic And Atmosphere Administration (NOAA) at the following website: https://www.ncei.noaa.gov/pub/data/igra/derived/ (last access: 6 September 2021). P − 0.378e where e is the vapor pressure, RH is the relative humidity, and p is the pressure in units of hPa. In addition, we use the monthly specific humidity and horizontal winds from the surface to 300 hPa during the period of 2005–2019, obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, to investigate the water vapor anomaly and tropical atmospheric circulation in the region of the radiosonde stations. We use the CALIPSO version 1.00 lidar level 3 cloud occurrence monthly data in a latitudinal and longitudinal grid of 2◦ × 2.5◦ with an altitude resolution of 60 m above the mean sea level, and the available data are from June 2006 to December 2016, downloaded from the website of the NASA at https://asdc.larc.nasa.gov/project/CALIPSO/CAL_LID_L3_ Cloud_Occurrence-Standard-V1-00_V1-00 (last access: 6 September 2021)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
