The Seasonal-scale Persistence of Tropical Tropospheric Temperature Associated with the El Niño/Southern Oscillation

Abstract

From an examination of the analyses of the NCEP/NCAR reanalysis data for the years 1979 to 2000, it is found that the persistence of tropical 200 hPa geopotential heights, representing tropospheric mean temperatures, exhibits a ‘barrier’ during the boreal autumn, especially between September and October. On the other hand, the persistence of indices associated with the El Niño/Southern Oscillation (ENSO), which is intimately related to variations of the tropical tropospheric mean temperature, have been shown to decrease during the spring. >From this feature, a tropical climatic year that starts in October is proposed. The ‘annual’ average of the tropical zonal mean 200 hPa heights for the tropical climatic year introduced above is presently de-fined as the tropical year index (TYI).
Associated with the TYI, a zonally uniform tropical temperature anomaly is prominent at the 700-150 hPa levels over the period from November to the following September. On the other hand, the tropical temperatures at the 1000-850 hPa levels are not zonally uniform, but vary almost simultaneously with the underlying tropical sea surface temperatures. The temperature anomalies are found to be larger in the upper troposphere (0.4-0.6 K), than in the lower troposphere (0.2-0.3 K). The tropical tropospheric warming (or cooling) is not directly connected with the increase (or decrease) in the amount of the precipitation over the entire tropics. The mean mass of tropical water vapor, within the 700-300 hPa levels, gradually increases after the peak of ENSO events, unlike the tropospheric temperatures.