A modified zonal index and its physical sense

A modified zonal index (ZI) for the Northern Hemisphere (NH) general circulation is defined as the normalized difference in zonal-averaged sea level pressure anomalies between 35degreesN and 65degreesN. The ZI is a measure of hemispheric-wide fluctuations in air mass between two annular belts of action (ABAs) over middle and high latitudes, centered near 35degreesN and 65degreesN, respectively. The spatial structure of the NH general circulation represented by the ZI is a zonally symmetric pattern, similar to the NH annular mode. Some physical features associated with the ZI are discussed and summarized as a concept model, and the analysis indicates that the Ferrel cell stands out as a dominant signal in the zonal-mean circulation anomalies related to the ZI, implying a strong dynamical property of the general circulation in the mid-high latitudes.