Determining Periodic Behavior of Globally Averaged 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 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 will indicate the variability associated with the buildup and loss of JAPE with time. Interestingly, the results suggest that significant spectral variance is found on both diurnal and annual cycles despite the global-scale budget being taken. Seasonal cycles are also found, although this is somewhat more expected. Of key importance, a significant broad spectral peak centered at a 45-day period appears to reflect the period of the Madden-Julian Oscillation, again despite the global averages taken. These cycles will be discussed and explanations for their existence will be proposed.