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

R. F. C. Marques Instituto Nacional de Pesquisas Espaciais, São José dos Campos, Sao Paulo, Brazil

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V. Brahmananda Rao Instituto Nacional de Pesquisas Espaciais, São José dos Campos, Sao Paulo, Brazil

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Abstract

A long-lasting blocking event occurred over the southeast Pacific Ocean near the west coast of South America from 29 July through 14 August 1986. This blocking happened in a recently found new region of blocking in the southeast Pacific. During the blocking event the transient eddies were forced to move to the north and south of the blocking. This caused precipitation over South America to increase to the north of the blocking high and diminish over southern South America.

The blocking event is analyzed in detail and its impact on the winter general circulation is discussed. Change in amplitude of several wavenumbers and the zonal wind (zero wavenumber) suggest that the blocking event is essentially a local phenomenon. Since transient eddies are prevented from moving through the blocking high the variance of meridional wind is small. However, the variance of geopotential height is maximized because of the persistence of the block. This signature of the blocking on the general circulation is similar to what is seen over the New Zealand region. Calculation of local Eliassen–Palm flux showed that in the region of the split jet both the barotropic and baroclinic components complement each other to decelerate the westerlies and maintain the block. This seems to fit into the “eddy straining” concept. The principal difference between blocking over the southeast Pacific and the New Zealand regions is in barotropic energy exchange between the eddies and zonal flow. In the present case eddies maintain two branches of the jet (subtropical and subpolar) by converting eddy kinetic energy into zonal kinetic energy. At the location of the split jet, zonal kinetic energy is converted into eddy kinetic energy, thus maintaining the split jet. A composite of four blocking events over the southeast Pacific confirmed these results. This is opposite to what was found for a blocking episode near New Zealand.

Corresponding author address: Dr. V. Brahmananda Rao, INPE, C.P. 515, São José dos Campos, São Paulo 12201-970, Brazil.

Email: vbrao@met.inpe.br

Abstract

A long-lasting blocking event occurred over the southeast Pacific Ocean near the west coast of South America from 29 July through 14 August 1986. This blocking happened in a recently found new region of blocking in the southeast Pacific. During the blocking event the transient eddies were forced to move to the north and south of the blocking. This caused precipitation over South America to increase to the north of the blocking high and diminish over southern South America.

The blocking event is analyzed in detail and its impact on the winter general circulation is discussed. Change in amplitude of several wavenumbers and the zonal wind (zero wavenumber) suggest that the blocking event is essentially a local phenomenon. Since transient eddies are prevented from moving through the blocking high the variance of meridional wind is small. However, the variance of geopotential height is maximized because of the persistence of the block. This signature of the blocking on the general circulation is similar to what is seen over the New Zealand region. Calculation of local Eliassen–Palm flux showed that in the region of the split jet both the barotropic and baroclinic components complement each other to decelerate the westerlies and maintain the block. This seems to fit into the “eddy straining” concept. The principal difference between blocking over the southeast Pacific and the New Zealand regions is in barotropic energy exchange between the eddies and zonal flow. In the present case eddies maintain two branches of the jet (subtropical and subpolar) by converting eddy kinetic energy into zonal kinetic energy. At the location of the split jet, zonal kinetic energy is converted into eddy kinetic energy, thus maintaining the split jet. A composite of four blocking events over the southeast Pacific confirmed these results. This is opposite to what was found for a blocking episode near New Zealand.

Corresponding author address: Dr. V. Brahmananda Rao, INPE, C.P. 515, São José dos Campos, São Paulo 12201-970, Brazil.

Email: vbrao@met.inpe.br

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