Seasonal and Subseasonal Variability of Potential Energy Injected into the Tropical UTLS by Convective and Tropical Cyclone Outflow

Abstract

Abstract This study employs 40 years of ERA-Interim data to quantify the periodic behavior of the integral amount of potential energy stored in the tropical upper UTLS (upper troposphere and lower stratosphere) originally placed there by high-entropy outflow of deep convection and tropical cyclones (TCs). The upper UTLS is defined to be isentropic layers above the level of zero net radiation (LZNR) in the tropics. Once there, the trapped high-entropy air mass must flow into the extratropics where radiative loss will allow it to subside back to lower-entropy levels. Mean poleward fluxes of isentropic mass are prevented by an inertial wall associated with Earth’s rotation. This causes the mass of these isentropic layers to build up, creating a bubble of high potential energy. Periodic releases of this mass into the extratropics in the form of tropical plumes (TPs) help drain the bubble of its mass buildup over time. We present a metric to quantify the energy trapped and stored in this bubble to be the jet available potential energy (JAPE), in reference to the availability of this energy to fuel the kinetic energy of the subtropical jet stream (STJ), which bounds the tropical JAPE bubble (TJB) on its poleward extremities. We also calculate the isentropic mass M, which measures the isentropic thickness. Results of a 40-yr-time-series analysis of upper-UTLS JAPE and M show that the TJB features periodicity in the buildup and release of JAPE and M.