An Exploration of Tropical Jet Available Potential Energy

Available Potential Energy (APE) accumulates in the Upper Troposphere-Lower Stratosphere (UTLS) of the tropics from the integrated outflow mass of tropical convection and storms. Due to inertial and radiative trapping, APE leaves the tropical UTLS primarily through periodic mass exchange with the extratropics. The resulting budget of tropical APE therefore serves as an indicator of both total tropical convective activity and total extratropical interaction with the tropical UTLS. As the subtropical jet typically bounds the tropical APE and the associated elevated tropical tropopause on its extratropical boundaries, we define the tropical UTLS APE to be the difference of APE across the subtropical jet, which we call the Jet Available Potential Energy (JAPE).

In this study, we examine the periodic behavior of globally integrated tropical JAPE in the UTLS based on a four times daily ERA-I reanalysis over 36 years. The intent of this study is to determine the prevailing frequencies of fluctuations of JAPE over the tropical belt and within an isentropic layer of the defining the UTLS. Results of this analysis indicate variability associated with the buildup and loss of JAPE with time. The results suggest that significant spectral variance is found on both diurnal and annual cycles, presumably associated with zonal variability and convective processes. Seasonal cycles are also found, which are thought to be associated with the seasonal variability of global convective processes. A significant broad spectral peak centered at a 45-day period appears to reflect the period of the Madden-Julian Oscillation and the modulation of its amplitude in conjunction with zonally stationary circulations and geographic location. Evidence will be presented suggesting that the fluctuation of the intensity of the tropical JAPE surplus results in a similar fluctuation of tropical-extratropical mass exchange in the UTLS.