Why Are the Climatological Zonal Winds Easterly in the EquatorialUpper Troposphere?

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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 Niño–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.