Tropical Axisymmetric Mode of Variability in the Atmospheric Circulation: Dynamics as a Neutral Mode*

Abstract

Abstract There is an observational evidence that the variability in the global upper-tropospheric flow fields on a monthly basis is dominated by a zonally uniform pattern symmetric about the equator. This fluctuation is characterized by the zonal mean zonal wind anomaly having the same sign within the Tropics, and thus has been related to changes in the global atmospheric angular momentum. In this paper, the mechanism of this variability, referred to as the tropical axisymmetric mode (TAM), was investigated using the NCEP–NCAR reanalysis, an atmospheric general circulation model (AGCM), and a linear baroclinic model. An index of the TAM, defined by the leading principal component of the observed 300-hPa streamfunction anomalies, shows quite significant correlation with the Nino-3 SST anomalies, consistent with a well-known coherence between the angular momentum and El Nino. While this suggests that TAM can be forced effectively by El Nino events, data analyses and an AGCM experiment with climatological SS...