A quasi-biennial signal in western US hydroclimate and its global teleconnections

On interannual time scales, quasi-cyclic activity in the troposphere is generally thought to arise from chaotic feedback processes, and therefore to be unpredictable in nature. Here, evidence is presented for periodic 'clockwork' climate behavior, seen as a stable quasi-biennial (QB) oscillation in Western US winter precipitation, estimated at a period of 2.16 +/- A similar to 0.05 years. In the Western US, the QB precipitation mode has an influence comparable to that of ENSO, with strongest effects observed at central latitudes of the region. The oscillation displays systematic phase-shifting with respect to the annual cycle, signifying a stable, nonseasonal and strongly periodic QB mechanism. The cycle in precipitation results proximally from a meridional standing pressure wave over the North American Pacific coast. Analysis of monthly pressure data confirms the nonseasonal, periodic character of the oscillation, which appears to have maintained phase from the mid-20th century to recent years. The regional QB pressure cycle is traced to a quasi-synchronous pulsation of meridional pressure waves in the extratropics of both hemispheres, termed a Biennial Annular Mode Oscillation (BAMO). The BAMO oscillates in quadrature with a weaker similar to 26-month signal in the tropical Southern Oscillation. Due to a shorter period, both oscillations operate independently of the stratospheric Quasi-biennial Oscillation (QBO).