Measurements of stratospheric carbon dioxide and water vapor at northern midlatitudes: Implications for troposphere-to-stratosphere transport

Simultaneous in situ measurements of CO₂ and water vapor in the lower stratosphere were obtained in November 1992 and May 1993 up to 21 km at midlatitudes. Seasonal changes in both species were observed up to pressure-altitudes of 19 km (∼440 K), a result of vertical propagation of seasonal oscillations in CO₂ and H₂O mixing ratios for air entering the stratosphere in the tropics followed by horizontal transport to midlatitudes. The inferred phase difference between their seasonal cycles at the tropical tropopause (2 months) remained coherent at midlatitudes, although the amplitudes were reduced, implying a mean transport time of 4-6 months from the tropical tropopause (∼16 km) to ∼18.5-19 km at midlatitudes in both November and May. The measurements indicate that air enters the stratosphere throughout the year and suggest that vertical propagation of the seasonal minimum in water vapor, rather than deep convective overshoot, is both sufficient and necessary to explain the minimum in water (“hygropause”) observed several kilometers above the tropopause.