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

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 Niño-3 SST anomalies, consistent with a well-known coherence between the angular momentum and El Niño. While this suggests that TAM can be forced effectively by El Niño events, data analyses and an AGCM experiment with climatological SSTs show that TAM appears even in the absence of El Niños, indicating that its origin lies in the atmospheric internal dynamics. A leading singular vector for the zonal mean operator of the linear model, a mode closest to neutrality, indeed resembles well the observed TAM. Furthermore, a considerable part of zonally asymmetric anomalies associated with TAM can be reproduced as a linear, stationary wave response to the anomalous advection of stationary waves by the zonal mean flow associated with the near-neutral mode. Both the neutral mode diagnosis and TAM of the AGCM reveal that the tropical westerly anomalies are associated with the weakened Hadley cell. The dynamics responsible for persistence of such a mode is mainly the balance between transport of the basic-state vorticity and the Coriolis force in the zonal momentum budget. Thus, these results indicate that TAM is essentially identified as a near-neutral mode of the zonally symmetric atmosphere.