Diagnosis of a Long-Lasting Blocking Event over the Southeast Pacific Ocean

A long-lasting blocking event occurred over lhe southeast Pacific Ocean near lhe west coast of South America from 29 July through 14 August 1986. This blocking happened in a recently found new region of blocking in lhe southeast Pacifico During lhe blocking event lhe transient eddies were forced to move to lhe north and south of lhe blocking. This caused precipitation over South America to increase to lhe north of lhe blocking high and diminish over southem South America. The blocking event is analyzed in detail and its impact on lhe winter general circulation is discussed. Change in amplitude of several wavenumbers and lhe zonal wind (zero wavenumber) suggest that lhe blocking event is essentially a local phenomenon. Since transient eddies are prevented from moving through lhe blocking high lhe variance of meridional wind is small. However, lhe variance of geopotential height is maximized because of lhe persistence of lhe block. This signature of lhe blocking on lhe general circulation is similar to what is seen over lhe New Zealand region. Calculation of local Eliassen-Palm ftux showed that in lhe region of lhe split jet both lhe barotropic and baroclinic components complement each other to decelerate lhe westerlies and maintain lhe block. This seems to fit into lhe "eddy straining" concept. The principal difference between blocking over lhe southeast Pacific and lhe New Zealand regions is in barotropic energy exchange between lhe eddies and zonal ftow. In lhe present case eddies maintain two branches of lhe jet (subtropical and subpolar) by converting eddy kinetic energy into zonal kinetic energy. At lhe location of lhe split jet, zonal kinetic energy is converted into eddy kinetic energy, thus maintaining lhe split jet. A composite of four blocking events over lhe southeast Pacific confirmed these results. This is opposite to what was found for a blocking episode near New Zealand.Pages: 1761-177