Cold air rises in the tropics. Does cold air rise in CMIP6?

Abstract:

Conventional wisdom suggests that warm air rises, and cold air sinks. However, in the tropical free troposphere, rising air is in fact colder than subsiding air, as shown by observations and reanalysis (Yang and Seidel 2020). We attribute this phenomenon to well-established physics. First, the molecular weight of water vapor is less than that of dry air. Therefore, humid air is lighter than dry air at the same temperature and pressure (the vapor buoyancy effect). Second, the small effect of Earth’s rotation means that density is horizontally homogeneous in the tropical free troposphere (the weak buoyancy gradient). When the atmosphere is organized into a moist ascending region and a dry subsiding region, the dry-region air must be warmer in order maintain equal density with the moist region. That is, rising air is colder than subsiding air.

This is more than a curiosity. When climate warming enhances the amount of atmospheric water vapor, this amplifies the moist-dry temperature difference. There is greater warming in the dry region. Using numerical experiments in a cloud resolving model, we found that this dry-region warming gives rise to a negative climate feedback, similar to the lapse rate feedback (Seidel and Yang 2020).

Here, we test whether this moist/cold dry/warm temperature structure and its associated vapor buoyancy feedback are simulated in CMIP6 models. One would expect that all models would simulate this phenomenon, as they consider the vapor buoyancy effect in their atmospheric dynamics. However, we find considerable spread in how CMIP6 models simulate the moist/cold dry/warm temperature structure in the tropics. We investigate how this affects tropical climate sensitivity through the vapor buoyancy feedback.

References:

Yang, D. and S. Seidel, 2020: The Incredible Lightness of Water Vapor. Journal of Climate. doi: 10.1175/JCLI-D-19-0260.1

Seidel, S. and D. Yang, 2020: The Lightness of Water Vapor Helps to Stabilize Tropical Climate. Science Advances. doi: 10.1126/sciadv.aba1951