Decadal Variation of the Annual Cycle in the Australian Dataset

During the 1970s and 1980s, elements of the atmospheric circulation over the Southern Hemisphere changed markedly from their state in the two previous decades. The change was evident in the zonal asymmetry, especially in the zonal harmonic wave 3 at higher latitudes, and in the semiannual wave in pressure and wind. The semiannual wave changed in the same sense in all three southern oceans, but by the largest amount in the South Pacific Ocean. The second harmonic in a 12-month series, by which we describe the semiannual wave, dominates the shape of the long-term mean annual curve of sea level pressure in mid- and high latitudes; this harmonic weakened during the period, and its midlatitude peak disappeared in the South Pacific Ocean. As part of the low-frequency changes during the 18 years from 1972 to 1988, the central pressure in the subantarctic trough fell, the trough moved northward, and the subtropical ridge moved south. The meridional movement of the trough and the ridge, and the concurrent trend in their central pressure, weakened the semiannual wave in midlatitudes.