Northern Hemisphere annular mode in summer: Its physical significance and its relation to solar activity variations

Abstract

We use the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis to calculate empirical orthogonal functions of summer geopotential heights in the Northern Hemisphere at all levels in the troposphere and the stratosphere. The leading patterns in summer are distinct from the patterns in winter. Also, the leading patterns in the summer stratosphere are distinct from the patterns in the summer troposphere. The summer Northern Hemisphere annular mode (NAM) in the stratosphere has the same sign everywhere but shows higher variability at low latitudes unlike the dipolar structure of the winter NAM. A physical interpretation of the summer NAM in the stratosphere is readily apparent because low (high) values of its principal component correspond to warmer (colder) than climatological mean summer conditions in the stratosphere. The summer NAM in the troposphere, on the other hand, is characterized by variability over the Asian monsoon region. Also, the summer NAM in the stratosphere and upper troposphere is correlated with the solar ultraviolet flux, such that in solar maximum conditions the stratospheric circulation is more “summer-like” than average and it is less summer-like in solar minimum conditions. The summer NAM is thus seen as a potentially useful tool in investigating the sources of variability in the summer atmosphere.