Water vapor control at the tropopause by equatorial Kelvin waves observed over the Galápagos

Soundings of frost‐point hygrometers, ozonesondes, and radiosondes at San Cristóbal Island (0.9°S, 89.6°W) in September 1998 provide an observational evidence that equatorial Kelvin waves around the tropopause act as a dehydration pump for the stratosphere. During the downward‐displacement phase of a Kelvin wave, dry and ozone‐rich stratospheric air is transported into the upper troposphere. During the upward‐displacement phase, on the other hand, higher specific‐humidity air moves up in the tropopause region, but at the same time, this upward motion causes cooling of the air that limits the water vapor amount entering the stratosphere. Also, wave breaking contributes to the irreversible transport of ozone across the tropopause. Considering their omnipresence at the equatorial tropopause, we suggest that Kelvin waves may be one of the important agents for maintaining the dryness of the tropical lower stratosphere.