The length scale of poleward moisture transport

Atmospheric transport critically links polar climate and chemistry to the mid-latitudes. Our ability to describe this transport in simple yet physical ways is essential for gaining insight into how sensitively polar regions respond to climate forcing. One framework of potential appeal for describing the average poleward moisture transport is a moist isentropic representation of Earth's atmospheric flow. In this framework, air masses trade sensible and latent heat for potential energy as they move along surfaces of constant equivalent potential temperature. Because the geometry and, specifically, length of the surfaces is largely defined by the amount of rainout, the moist isentropic framework lends itself to identifying source regions of polar moisture and predicting how those source regions shift as moisture length scales vary between climate states. 

Using water tags and water vapor isotope ratios simulated in the Community Earth System Model, this presentation demonstrates that poleward moisture transport is indeed largely moist isentropic in the zonal mean. The link between water vapor isotope ratios and moisture length scales is further demonstrated using simple scaling arguments based on principals of water mass balance. The results of the two analyses suggest the following: 

1) changes in moisture source regions are tightly linked to meridional gradients in temperature and effective rainout;

2) for a given rise in temperature, the inverse of the moisture length scale will scale with accelerations/decelerations of the meridional wind; and

3) assessing relative variations in the isotopic composition of neighboring regions may be a promising technique for studying changes in moisture length scale today and in the past.