Sensitivity of the QBO to Mean Tropical Upwelling under a Changing Climate Simulated with an Earth System Model

Abstract

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 CO2 run and a 4 x CO2 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 CO2 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.