Zonal index and circulation change in the north Atlantic area, 1873–1972

Monthly and annual values are calculated of a zonal index based on surface pressure at 35°N and 55°N between 60°W and 10°W. The period under study, 1873‐1972, is divided into three sub‐periods which approximate with three epochs proposed by Lamb (1972). A comparison of our indices with Lamb's results is followed by a discussion of our monthly index, and the rarity of easterly flow in the 100‐year period is noted. The results are subjected to a trend and a power spectrum analysis and the latter indicates a number of periodicities some of which are not easily explicable.