NATURAL CLIMATE FLUCTUATIONS AT DECADAL TIME SCALES
The mechanisms of several most important modes of natural climate fluctuations are considered. Pacific Decadal Oscillation (PDO), Arctic Oscillation (AO) and Atlantic Multidecadal Oscillation (AMO) are the most important decadal modes. PDO is investigated most comprehensively. It reaches high amplitude because of positive feedback between trade winds in the tropical Pacific and temperature difference between the east and west Pacific. Change of phase happens because of delayed negative feedback between the warm anomaly in tropical Pacific and advection of relatively cold water from subtropics by surface winds induced by this anomaly. Arctic oscillation is the atmospheric mode that reaches large amplitude due to the interaction of Rossby waves with zonal mean flow. Nevertheless, the origin of decadal fluctuations of AO is still poorly understood. Studies of the mechanism of AMO can be separated into two types. According to the first one there is interaction between Atlantic meridional circulation and water density anomalies. The second mechanism explains the oscillation as propagation of thermal or salinity Rossby wave, while meridional circulation plays secondary role.