Observed Interannual Variability of Tropical Troposphere Relative Humidity

Abstract

Relative humidity fields from the High-Resolution Infrared Radiation Sounder (HIRS) flown on NOAA series satellites since 1979 have been used to study the seasonal aspects of the interannual variability of relative humidity in the tropical troposphere. The El Niño?Southern Oscillation (ENSO) is the only statistically identifiable physical mechanism of such variability. Boreal winter (December?February) relative humidity variations during an ENSO event follow patterns of anomalous convection and large-scale upper-level circulation. During El Niño (La Niña) regions of large negative (positive) relative humidity anomalies exist at subtropical latitudes over the Pacific Ocean. These are not always balanced by increases (decreases) in humidity near the equator. NCEP? NCAR reanalysis temperatures are used to separate observed changes in relative humidity into contributions from tropospheric temperature versus the contribution from changes in water vapor content. The authors find that at subtropical latitudes variations in temperature contribute between 50% and 70% of the observed change in relative humidity. It is also shown that large relative humidity anomalies exist over the equatorial Indian, Atlantic, and far east Pacific Oceans during the summer season (June?August) following an ENSO event. Ocean? atmosphere dynamics coupled with the seasonal cycle of relative humidity explain the existence of the long-lasting effects of ENSO in the atmosphere. The authors argue that observed linear trends in regional and tropical mean relative humidity are unlikely to be due solely to ENSO or a simple intensification of the hydrological cycle.