Annular modes in global daily surface pressure

Annular modes are patterns characterized by synchronous fluctuations in sur face pressure of one sign over the polar caps and the opposite sign at lowe r latitudes. The Southern Annular Mode (SAM) and Northern Annular Mode (NAM , also called the Arctic Oscillation) patterns are the leading empirical or thogonal functions (EOFs) of slowly-varying, hemispheric, cold-season, sea- level pressure anomalies (deviations from climatology). Daily indices of th e SAM and NAM are a measure of the similarity between surface pressure anom aly patterns and the annular modes. Here it is shown that the first two EOF time series of daily, global, year-round, zonally-averaged surface pressur e are nearly identical to the SAM and NAM indices. Together they account fo r more than 57% of the daily variance of zonally-averaged surface pressure. The SAM and NAM patterns extend through the tropics, well into the opposit e hemispheres. Fluctuations of the SAM and NAM indices are accompanied by i nterhemispheric transfer of mass.