Is Tropical Expansion Related to Climate Sensitivity? Lessons from CMIP5 Models

Abstract:

It is commonly assumed that the expansion of the tropics closely follows global-mean surface temperature warming: i.e., 1) in models with a higher climate sensitivity, the tropics will widen more, and that 2) during periods of greater global-mean surface temperature rise, the tropics will widen more. In this study, we re-examine these ideas using two sets of model experiments from the CMIP5 archive: 1) an abrupt quadrupling of atmospheric CO₂, and 2) a 1% per year increase in atmospheric CO₂. We examine various metrics of the width of the tropics, including the 500-hPa streamfunction, precipitation minus evaporation, and the surface winds. In the Southern Hemisphere, higher climate sensitivity models generally expand the tropics more during most seasons. However, in the Northern Hemisphere, the expansion of the tropics is only robustly correlated with climate sensitivity during the winter season, and models with higher climate sensitivity actually contract the tropics during the summer season. The relationships between climate sensitivity and the width of the tropics are most apparent under high CO₂ conditions and are much less apparent in the transient response. Interestingly, not all metrics of the width of the tropics monotonically increase with global-mean surface temperature, meaning that some metrics might be more appropriate than others for detecting the expansion of the tropics during a period of transient climate change. Implications of these results will be discussed.