On the semiannual variation in the northern hemisphere extratropical height field

A qualitative examination of the spatial distribution of the maximum semiannual oscillation (SAG) amplitudes in the Northern Hemisphere (NH) extratropical 500-hPa height field suggests that the SAO has a very dominant zonally asymmetric east-west structure. A comparison between the NH stationary eddies and the NW SAO shows that the NH midlatitude SAO can be explained almost entirely as being a result of the spatial and temporal asymmetries in the annual variation of the stationary eddies. It is also shown that the polar area SAO maxims over Siberia and Alaska are related to the east-west stationary eddy distribution. It is suggested that, ultimately, the mechanism for the SAO in the NH extratropics is simply a result of land-sea contrasts, similar to the mechanism proposed by van Loon for the Southern Hemisphere (SH) extratropical SAG. The chief difference, however, is that the NH extratropics are dominated by east-west land-sea contrasts duc to the large continental land masses in the NH, while the SH land-sea contrast reflects the north-south differential heating between Antarctica and the surrounding ocean.