A half-yearly variation of the circumpolar surface drift in the Southern Hemisphere

The annual march of the zonal geostrophic wind at sea level in middle and high southern latitudes includes a marked half-yearly wave whose phase reverses near 50 S. South of this latitude the maxima are in the transitional seasons and north of it in the extreme seasons. Surface-current velocitites at middle latitudes in the western Indian Ocean suggest that the circumpolar current may vary semiannually in the same sense as the wind.