The Relationship between a Zonal Index and Blocking Activity

Abstract

The degree to which Northern Hemisphere blocking activity is controlled by variations in zonal mean conditions is investigated. A set of Northern Hemisphere winter season 500-hPa analyzed fields is examined for blocks using an objective index defined solely from the eddy fields. With this blocking index, it is shown that in most regions enhanced blocking activity is associated with relatively strong zonally averaged winds around 30°N and weak winds around 50°?60°N. Also, the preferred zonal positions of blocks are related to the state of the zonal mean flow. A similar analysis is carried out using data from a perpetual January GCM simulation and the same relationship between blocking activity and zonally averaged conditions is found to be valid to an even stronger degree for these data. To investigate whether anomalous zonal mean flows are actually controlling the associated level of blocking activity, two experiments with the GCM are performed. In one experiment the zonal mean state of the GCM is forced toward a configuration that is statistically associated with enhanced blocking activity in the control simulation. In the other, the zonal mean is forced toward a state associated with suppressed blocking activity. Blocking frequency is enhanced in the first experiment and weakened in the second. Furthermore, the preferred locations for blocking in the experiments match the locations found to be associated with zonal mean anomalies in the control. This suggests the zonal mean state is influencing blocking activity. Results from a steady barotropic linear model indicate that adjustments made by the planetary waves in reaction to anomalies in the zonal mean flow are partly responsible for the relationship between blocking and the zonal mean state.