Simultaneous influence of the Southern Hemisphere annular mode on the atmospheric circulation of the Northern Hemisphere during the boreal winter

Abstract

This study has discovered a significant linear relationship between the Southern Hemisphere (SH) annular mode (SAM) and the mid‐latitude Northern Hemisphere (NH) circulation in February using two reanalysis data sets. Significant positive height anomalies corresponding to the positive SAM appear over the mid‐latitude region of the NH, and significant positive surface air temperature anomalies prevail over Western Europe and Northeast Asia. The realistic vorticity forcing associated with the SAM is applied in a dry linear baroclinic model, and the results demonstrate that the SAM in February can induce height anomalies with the same sign at 300 hPa over the mid‐latitudes of the NH and that these anomalies have amplitudes as large as 22% of their counterparts in the SH. Further numerical experiments show that the positive height response over the mid‐latitudes of the NH mainly comes from the influence of the realistic positive vorticity over the mid‐latitudes of the SH and that the vorticity forcing over the southern Indian Ocean seems to be the most effective at exciting the NH circulation anomalies. The numerical experiments also demonstrate that the transient eddy feedback forcing contributes to the formation of mid‐latitude NH circulation anomalies, especially over Northeast Asia. The maintenance of the negative vorticity anomalies over Europe is further diagnosed through vorticity budget analysis. In December, however, the significant vorticity anomalies associated with the SAM are not limited to the middle and high latitudes of the SH but also appear over the subtropics of the SH (approximately 30°S). The numerical experiments show that cancellation occurs between the responses to the subtropical vorticity forcing and the mid‐latitude vorticity forcing, thereby accounting for the independence of the NH circulation from the December SAM variability.