  All-sky maps of the cosmic microwave background temperature fluctuations are
usually represented by a spherical harmonic decomposition involving modes
labelled by their degree l and order m (where -l < m < +l). The zonal modes
(i.e those with m = 0) are of particular interest because they vary only with
galactic latitude; any anomalous behaviour in them might therefore be an
indication of erroneous foreground substraction. We perform a simple
statistical analysis of the modes with low l for sky maps derived via different
cleaning procedures from the Wilkinson Microwave Anisotropy Probe (WMAP) and
show that the zonal modes provide a useful diagnostic of possible systematics.
