An Antarctic circumpolar wave (ACW) of 3- to-4-year period propagated eastward around the Southern Ocean in covarying sea surface temperature (SST) and sea level pressure (SLP) anomalies from 1983 through 1992, composed of two wavelengths that circled most of the global ocean along the midlatitude storm track near 40¿S. In this ACW, positive SST anomalies drove upward latent heat flux, positive precipitation (PCP), and mid to upper level diabatic heating anomalies in the troposphere, the latter balanced principally by the cooling tendency from deep convection (White and Chen, 2002). One hypothesis has the anomalous PCP and deep convection in this ACW arising from that in more frequent and/or stronger extratropical cyclones in the synoptic storm aggregate. Here we verify this hypothesis, finding negative SLP anomalies in this ACW collocated with positive extratropical cyclone intensity (ECI) and extratropical cyclone density (ECD) anomalies in the annual synoptic storm aggregate. Because individual extratropical cyclones are 3 to 4 times smaller than SLP anomalies in the ACW, their density and intensity appear to be modulated by anomalous zonal SST gradients in the ACW. These results are corroborated by finding greater interannual PCP anomalies over southeast Australia collocated with positive SST anomalies, occurring on the eastern side of covarying negative SLP and positive ECI and ECD anomalies in the ACW along the mid-latitude storm track. Because positive interannual PCP anomalies over southeast Australia derive from the integral of rainfall from more dense and/or intense extratropical cyclones in the aggregate, the same is true for the ACW propagating eastward along the midlatitude storm track over the remainder of the Southern Ocean.