Inter-annual variations and trends in surface temperature, water vapor and precipitation and the impact of ENSO and volcanoes

Satellite-based data sets are used to study inter-annual variations and trends in surface temperature, water vapor and precipitation on both tropical and global scales with data sets spanning the last 30 years. Techniques are applied to the satellite data sets to separate out the ENSO, volcano and long-term (trend or inter-decadal) changes. During this period there has been a significant increase in temperature, an associated increase of atmospheric water vapor, but a near zero change in global precipitation. However, there is a significant increase in precipitation over tropical oceans, balanced mainly by a decrease in mid-latitudes, at least in the Northern Hemisphere.

These relations among variables are also examined at the inter-annual scale to understand how they may relate to the long-term change relations. At the inter-annual scale water vapor responds to ENSO-related global, surface temperature variations, while the variation in global precipitation is very weak. A similar result is found when looking at trends of the three variables during the last 20-30 years. At the inter-annual time scale the impact of volcanoes is shown to be different than ENSO, with a distinct change in global precipitation. Seasonal variations are also examined.

In summary, water vapor responds to surface temperature variations at all time scales, while the precipitation response is much weaker. However, while the precipitation variations (over very large scales such as the Tropics or global) are weak, there is a sharper variation in the pattern of precipitation. At the trend scale there is an increase noted in the deep tropics, and a decrease in middle latitudes. While the total precipitation remains nearly constant, this pattern of change is related to the impact of dynamics on the system and the related energy constraints.

Comparisons with a re-analysis model (MERRA) are made to understand the utility of using it for understanding the processes described. The analysis also indicates that the precipitation decreases related to volcanoes are due at least in part to aerosol effects (not simulated in the model) and not only due to surface temperature decreases.