The occurrence rate of great geomagnetic storms displays a pronounced semiannual variation. Of the forty-two great storms during the period 1940-1990, none occurred during the solstitial months of June and December, and 40% (17) occurred during the equinoctial months of March and September. This suggests that the semiannual variation found by averaging indices is not the result of some statistical effect superposed on the effects of random storm occurrence but rather is dominated by the storms themselves. Recent results indicate that the intense southward interplanetary magnetic fields (IMFs) responsible for great storms can reside in the postshock plasma preceding the driver gas of coronal mass ejections (CMEs) as well as in the driver gas itself. Here we propose that strong southward fields in the postshock flow result from a major increase in the Russell-McPherron polarity effect through a systematic pattern of compression and draping within the ecliptic plane. Differential compression at the shock increases the Parker spiral angle and, consequently, the azimuthal field component that projects as a southward component onto Earth's dipole axis. The resulting prediction is that southward fields in the postshock plasma maximize at the spring (fall) equinox in CMEs emerging from toward (away) sectors. This pattern produces a strong semiannual variation in postshock IMF orientation and may account at least in part for the observed semiannual variation of the occurrence of great geomagnetic storms. ¿ American Geophysical Union 1992