Study on ENSO Modulated Seasonal Variations in Atmospheric Humidity Using Global Positioning System Radio Occultation Data

Abstract

The role of cool (La Niña) and warm (El Niño) phases of El Niño Southern Oscillation (ENSO) on seasonal variations in atmospheric humidity which has impact on hydrological cycle is studied using global positioning system radio occultation (GPSRO), constellation observing system for meteorology ionosphere and climate (COSMIC) derived water vapor profiles after ensuring its accuracy with National Ocean and Atmospheric Administration (NOAA)/Earth System Research Laboratory (ESRL) radiosonde observations (r2=0.91 at 850 hPa and r2=0.90 at 500 hPa) and National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data (r2=0.86). Relative and absolute humidity are calculated using GPS RO data and their variation with ENSO phases is studied. Variation in humidity is linked with phase of ENSO, and NCEP/NCAR humidity shows positive deviation during months with La Niña phase. The analysis highlights the changes in the vertical profile of atmospheric humidity (both relative and atmospheric humidity) during El Niño/La Niña phases occurring between 2006 and 2012. Maximum difference in monthly deviation (>96%) in vertical profile of humidity between both phases is observed at middle troposphere (7–8 km) in the month of September followed by June (93–96%) between 6.5 and 7 km. The difference between percent deviation during both phases of ENSO is the minimum (1.32%) in August in middle troposphere indicating less effect of ENSO phases. The effect of phases of ENSO on boundary layer height (BLH) is more prominent during the nonrainy season when, owing to low humidity, higher BLH is expected. Increase and decrease of atmospheric humidity with La Niña and El Niño phase, respectively, lowers/increases the BLH prominently during nonrainy season months of October–May. It was observed that there is delay in onset of monsoon during El Niño year, and the atmospheric vertical humidity profile by GPSRO also showed the high humidity at 5–7 km altitude one day prior to onset of monsoon and lowering of humidity on the day of monsoon. The results of the study show that seasonal variations in atmospheric humidity are induced by ENSO phases even over small geographical area and are discernible with COSMIC GPS RO data.