Evolution of the Lorenz Energy Cycle in the Intertropical Convergence Zone in the South American Sector of the Atlantic Ocean

Abstract

he intertropical convergence zone (ITCZ) in the South American sector of the Atlantic Ocean is identified using outgoing longwave radiation (OLR) data in order to investigate the evolution of the Lorenz energy cycle in the region dominated by this large-scale feature. The NCEP reanalysis data are utilized to calculate the zonal and eddy components (denoted by Z and E, respectively) of kinetic energy K and available potential energy A (i.e., KZ, KE, AZ, and AE) and their conversions, on a daily basis. A wavelet decomposition of the time series is performed to detect long-term cycles/trends in the Atlantic ITCZ region. This work also investigates trends in sea surface temperature (SST) and sea level pressure (SLP) in the ITCZ region and connections between the ITCZ and the Southern Oscillation index (SOI). A strong annual cycle in all the energy components with high peaks in austral summer is observed. Approximately 91% of the zonal component of energy is contained on decadal or longer time scales. The annual and semiannual variabilities are significant and the synoptic-scale variability is also present. The zonal component of kinetic energy KZ presents a decreasing trend during the last 28 years, which means a weakening of trade winds in the region studied. The values of KZ and AE are significantly higher during the period 1982/83, indicating that the intense El Niño?Southern Oscillation (ENSO) and/or the El Chichón eruption may have affected the circulation in the ITCZ region. The 28-yr mean energy conversion and generation terms are in general weaker than in the hemispheric calculations but the energy conversions proceed in the same sense as in the hemispheric situation.