Why are the climatological zonal winds easterly in the equatorial upper troposphere?

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Abstract

Using 16 years of NCEP-NCAR reanalysis data on the 200-mb surface, it is shown that in the deep Tropics, the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind. This acceleration is mainly due to transient eddies of intraannual and interannual timescales, and to those associated with the Madden-Julian oscillation (MJO). The interannual timescale eddy fluxes are dominated by eastward-propagating disturbances with zonal wavenumber 2 and a period of 2-4 yr, suggesting that these eddy fluxes may be tied to the El Nino-Southern Oscillation. In the deep Tropics, the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the Hadley cells. The deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies. This result indicates that a nonzero obliquity of the earth is crucial for maintaining the present climate's equatorial easterlies. Consistent with the above findings, in an idealized GCM with a fixed equinox insolation and a sea surface temperature field symmetric across the equator, the tropical upper-tropospheric zonal wind is westerly. This is in part because such a GCM does not retain a transient meridional circulation arising from the seasonal cycle and because the horizontal eddy momentum flux convergence due to the MJO is stronger than that in the observations.

Original languageEnglish (US)
Pages (from-to)1353-1364
Number of pages12
JournalJournals of the Atmospheric Sciences
Volume56
Issue number10
StatePublished - May 15 1999

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zonal wind
troposphere
eddy
meridional circulation
momentum
Madden-Julian oscillation
general circulation model
timescale
Hadley cell
obliquity
insolation
westerly
El Nino-Southern Oscillation
sea surface temperature
divergence
disturbance

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

Cite this

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title = "Why are the climatological zonal winds easterly in the equatorial upper troposphere?",
abstract = "Using 16 years of NCEP-NCAR reanalysis data on the 200-mb surface, it is shown that in the deep Tropics, the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind. This acceleration is mainly due to transient eddies of intraannual and interannual timescales, and to those associated with the Madden-Julian oscillation (MJO). The interannual timescale eddy fluxes are dominated by eastward-propagating disturbances with zonal wavenumber 2 and a period of 2-4 yr, suggesting that these eddy fluxes may be tied to the El Nino-Southern Oscillation. In the deep Tropics, the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the Hadley cells. The deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies. This result indicates that a nonzero obliquity of the earth is crucial for maintaining the present climate's equatorial easterlies. Consistent with the above findings, in an idealized GCM with a fixed equinox insolation and a sea surface temperature field symmetric across the equator, the tropical upper-tropospheric zonal wind is westerly. This is in part because such a GCM does not retain a transient meridional circulation arising from the seasonal cycle and because the horizontal eddy momentum flux convergence due to the MJO is stronger than that in the observations.",
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Why are the climatological zonal winds easterly in the equatorial upper troposphere? / Lee, Sukyoung.

In: Journals of the Atmospheric Sciences, Vol. 56, No. 10, 15.05.1999, p. 1353-1364.

Research output: Contribution to journalArticle

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AU - Lee, Sukyoung

PY - 1999/5/15

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N2 - Using 16 years of NCEP-NCAR reanalysis data on the 200-mb surface, it is shown that in the deep Tropics, the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind. This acceleration is mainly due to transient eddies of intraannual and interannual timescales, and to those associated with the Madden-Julian oscillation (MJO). The interannual timescale eddy fluxes are dominated by eastward-propagating disturbances with zonal wavenumber 2 and a period of 2-4 yr, suggesting that these eddy fluxes may be tied to the El Nino-Southern Oscillation. In the deep Tropics, the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the Hadley cells. The deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies. This result indicates that a nonzero obliquity of the earth is crucial for maintaining the present climate's equatorial easterlies. Consistent with the above findings, in an idealized GCM with a fixed equinox insolation and a sea surface temperature field symmetric across the equator, the tropical upper-tropospheric zonal wind is westerly. This is in part because such a GCM does not retain a transient meridional circulation arising from the seasonal cycle and because the horizontal eddy momentum flux convergence due to the MJO is stronger than that in the observations.

AB - Using 16 years of NCEP-NCAR reanalysis data on the 200-mb surface, it is shown that in the deep Tropics, the horizontal transient eddy momentum flux accelerates the zonal mean zonal wind. This acceleration is mainly due to transient eddies of intraannual and interannual timescales, and to those associated with the Madden-Julian oscillation (MJO). The interannual timescale eddy fluxes are dominated by eastward-propagating disturbances with zonal wavenumber 2 and a period of 2-4 yr, suggesting that these eddy fluxes may be tied to the El Nino-Southern Oscillation. In the deep Tropics, the single most important factor that decelerates the zonal mean zonal wind is the horizontal momentum flux divergence due to the transient meridional circulation associated with seasonal cycle of the Hadley cells. The deceleration by the transient meridional circulation is much greater than the acceleration due to the transient eddies. This result indicates that a nonzero obliquity of the earth is crucial for maintaining the present climate's equatorial easterlies. Consistent with the above findings, in an idealized GCM with a fixed equinox insolation and a sea surface temperature field symmetric across the equator, the tropical upper-tropospheric zonal wind is westerly. This is in part because such a GCM does not retain a transient meridional circulation arising from the seasonal cycle and because the horizontal eddy momentum flux convergence due to the MJO is stronger than that in the observations.

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