A GCM study of volcanic eruptions as a cause of increased stratospheric water vapor

Recent general circulation model (GCM) experiments have shown that idealized climatic perturbations that increase the temperature of the tropical tropopause region can cause larger than expected surface temperature increases. This is because the extra water vapor that is transported into the stratosphere acts as a positive radiative forcing agent. Since major volcanic eruptions in the Tropics also perturb temperatures in the region of the tropical tropopause, evidence is sought for the existence of such a mechanism by comparing model simulations and observations of the period following the eruption of Mount Pinatubo in 1991. Lower-stratosphere temperature perturbations are simulated using a GCM of intermediate complexity, and increases in water vapor concentration are found in the lower stratosphere that decay slowly in the years following the peak temperature increase. Such increases are consistent with the limited observations that exist over this period, as well as earlier work done with simpler 2D models. Since the amount of extra water transported into the stratosphere is sensitive to the absolute temperature change in the tropical tropopause region, this mechanism will be modulated by the phases of ENSO and the quasi-biennial oscillation, as well as being dependent on the vertical profile of the heating due to the volcanic aerosol. The global-mean radiative forcing associated with the perturbation in stratospheric water vapor is approximately +0.1 W m, and thus slightly counteracts the negative radiative forcing effect of the volcanic aerosol itself. It is suggested that repeated volcanic eruptions might at least partially account for increases in stratospheric water vapor over the last 40 years.