Tropical influence independent of ENSO on the austral summer Southern Annular Mode

Abstract

AbstractA link between atmospheric variability in the tropics independent of ENSO and the Southern Annular Mode (SAM) is found based on seasonal mean data for austral summer. Variations associated with El Niño Southern Oscillation (ENSO) are removed using a linear method, and a Tropics Index (TI) is defined as the zonal average of the ENSO‐removed 500 hPa geopotential height between 10°S and 10°N. Since the detrended TI shows no link to sea surface temperature variability in the tropics, it appears to be related to internal atmospheric variability. We find that the TI can explain about 40% variance of the SAM interannual variability and about 75% of the SAM long‐term trend between 1957/1958 and 2001/2002, where here the SAM includes the ENSO signal. Positive/negative values of the TI are associated with the positive/negative SAM. A possible link between the TI and global warming is noted.