Sensitivity of the QBO to mean tropical upwelling under a changing climate simulated with an earth system model

The sensitivity of the equatorial quasi-biennial oscillation (qbo) to increasing greenhouse gas (ghg) concentrations over time is evaluated using a high-top (85 km) earth system model,which generates the qbo using a non-orographic gravity wave drag parameterization. based on a pre-industrial control run (1850),a 1/2 co 2 run and a 4 x co 2 run are performed as sensitivity experiments. in addition,long-term transient behaviors of the qbo in a historical climate run (1850-2005) and a future projection run (2006-2100) are investigated. the period of the simulated qbo lengthens in a ghg-rich warmer climate,changing from a quasi-regular 24 months in 1850-1980 to a variable 24-31 months in 1980-2050,although never exceeding 31 months even in the extremely warm 4 x co 2 run. such elongation of the qbo period is mostly caused by strengthening of the mean tropical upwelling of the brewer-dobson circulation,which counteracts the wave forcing that drives the qbo. simultaneous reductions in the easterly wind maximum of the qbo are also caused by this mechanism. © 2012,meteorological society of japan.