Relationships between Thermodynamics and Vertical Motion Profile Shape during Otrec

Large-scale vertical motion profiles influence the energetics of the overturning circulation, as well as how convection evolves over time, and they need to be simulated correctly in order to correctly represent relationships between large-scale circulation and convection. Specifically, the top-heaviness, or shape of the vertical motion profile is important. However, observations are very limited, reanalyses often disagree on the shape of the vertical motion profiles and satellite retrievals also do not consistently agree with reanalyses.

A valuable set of observations of vertical motion profiles was provided by the Organization of Tropical East Pacific Convection (OTREC) field campaign that took place August and September 2019. This campaign provided in situ gridded observations of collocated vertical motion and thermodynamic profiles. In this work, we present an analysis of these observations to understand thermodynamic controls of the shape of the large-scale vertical motion profile.

We find that moisture and temperature anomalies are related to our measure of the vertical motion profile, the top-heaviness angle. Specifically, at the levels where vertical motion is upward, there tend to be negative temperature anomalies and positive moisture anomalies. To characterize the moisture profile variability, we introduce a new metric called the moisture dipole coefficient (MDC). We use it in conjunction with the saturation fraction to show they describe the amount of vertical motion and top-heaviness of vertical motion. We also demonstrate a correlation between the MDC and a measure of dry static stability. These point to a process we call vertically resolved moisture quasi-equilibrium explaining the observed correlation of vertical motion, moisture, and temperature. We describe our hypothesis for how this process works.