Seasonal Variations of Tropospheric Wind over Indonesia: Comparison between Collected Operational Rawinsonde Data and NCEP Reanalysis for 1992-99

Abstract

Seasonal variations of wind in the troposphere are analyzed based on operational rawinsondes at 11 stations over Indonesia for 1992-1999 and the results are compared with NCEP reanalysis. For meridional wind, an annual oscillation caused by north-south shift of a meridional (Hadley-type) circulation is clearly found. Winter-poleward flow of the upper-tropospheric meridional circulation is stronger than summer-poleward flow. The winter (southern) hemispheric cell in northern summer has a larger invasion across the equator (as suggested in the zonal-mean Hadley cell), and weaker meridional flows than the winter (northern) hemispheric cell in northern winter. Along the boundary of twin meridional circulation cells, a zonal (Walker-type) circulation seems to exist, and has easterly (westerly) wind in the upper (lower) troposphere in the Indonesian region. The zonal circulation is also shifted from the equator associated with the meridional circulation cell boundary. However, a semiannual oscillation, which has easterly maxima in January-February and July-August is more clearly observed in the upper-tropospheric zonal wind, which is consistent with a consideration on conservation of absolute angular momentum for the upper-tropospheric winter-poleward (equatorward) air mass transport associated with the winter-hemispheric meridional circulation cell, invading the summer hemisphere across the equator. In comparison with the seasonal-vertical variations based on the operational rawinsonde data, the NCEP reanalysis seems to overestimate the upper-tropospheric northerly and easterly maxima during northern summer. In other words, the NCEP reanalysis produces a winter hemispheric side meridional circulation cell with similar intensities between both solsticial seasons, in spite that in the operational rawinsonde data the winter hemispheric side meridional circulation cell in northern summer is weaker than that in northern winter. The overestimation of easterly may also be related with an insufficient reproduction of the winter-hemispheric meridional circulation cell-such as too large northward invasion.