The Tropical Tropopause Layer, the Quasibiennial Oscillation, and Stratospheric Dehydration

James R. Holton

University of Washington

The tropical tropopause layer (TTL) is the region of the tropical atmosphere that lies between the top of the main cumulus outflow layer (~12 km) and the thermal tropopause (~16 km). This layer is a transition layer between dynamical control of the vertical mass flux by tropospheric convection, and by the stratospheric Brewer-Dobson circulation, and is crucial to understanding the dehydration of air entering the stratosphere. The annual cycle in transport of water vapor into the stratosphere is influenced by the seasonal variation of the Brewer-Dobson circulation and also by the annual cycle in TTL temperatures. The coldest temperatures in the TTL occur over the equatorial West Pacific during Northern Hemisphere winter. Horizontal transport through this "cold trap" region causes air parcels that enter the TTL at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean tropopause temperature. Although the annual cycles in tropopause temperature and stratospheric pumping are the major controls on stratospheric water, the equatorial quasi-biennial oscillation (QBO) also plays a role.

The influence of the QBO on water vapor fluxes occurs both through its modulation of the Brewer-Dobson circulation, and through its modulation of the temperatures in the TTL. A simple two-dimensional (longitude-height) model of the water vapor budget at the equator in the 14-20 km altitude range is used to study the role of the QBO in stratospheric dehydration. The influence of the QBO depends strongly on the phase of the QBO tropopause temperature signal relative to the annual cycle. Lowest water vapor mixing ratios occur when easterly shear is a maximum below the 20 km level during Northern Hemisphere winter so that the negative temperature anomaly of the QBO near the tropopause enhances the cold phase of the annual cycle.

About the Lecturer

James R. Holton is a Professor and Chair of the Atmospheric Sciences Department at the University of Washington in Seattle. He holds a B. A. degree in physics from Harvard College (1960), and a Ph.D. in meteorology from MIT (1964). He is a Senior Fellow of the Joint Institute for the Study of the Atmosphere and Oceans (JISAO) at the University of Washington, and was the Acting Director of JISAO during 1981-1982.

Professor Holton is a Fellow of the American Meteorological Society, a Fellow of the American Geophysical Union, and a member of the U.S. National Academy of Sciences. He has served on numerous advisory committees and NASA science teams, including service as a Principal Investigator on the Upper Atmosphere Research Satellite (UARS) Science Team. Professor Holton is currently a co-investigator on the High-Resolution Dynamic Limb Sounder (HIRDLS) scheduled for launch on the Aura Satellite in June 2003. He is the author of An Introduction to Dynamic Meteorology, coauthor of Middle Atmosphere Dynamics, and numerous journal articles. Professor Holton's research area is large-scale dynamics, with emphasis on the dynamics of the stratosphere including stratospheric effects of gravity waves generated by convective storms, transport of trace constituents in the stratosphere, and stratosphere-troposphere exchange of mass and constituents.

James Holton