The Relationship between Water Vapor and ENSO Precipitation

El Niño Southern Oscillation (ENSO) events change the distribution of precipitation across the tropics in a way that leads to a modulation of weather patterns around the world. The root cause of these changes in precipitation during El Niño/La Niña events has not been thoroughly documented. Previous observational work on tropical precipitation indicate that there is an exponential relationship between water vapor and precipitation. These observations lend itself to the questions whether there is relationship between water vapor and ENSO-related precipitation, and if this relationship can be used to understand ENSO evolution.

To explore this relationship, regression analysis was done using the Nino 3.4 time series, ERA5 reanalysis data, and the GPCP precipitation data set. Linear regressions of column integrated specific humidity and anomalous precipitation reveal a robust nonlinear correlation, as in observations. Estimates of precipitation based on this nonlinear relationship robustly captures the horizontal distribution of ENSO-related rainfall. A small residual between the two fields exists along the Maritime Continent and equatorial South America. Linear regression of moist static energy (MSE) reveals that changes in MSE are mostly due to changes in column water vapor along the equatorial cold tongue region of the Pacific. These results reveal that changes in water vapor dictate changes in the energy (MSE) available in ENSO-related rainfall.