The Tropospheric Seasonally Varying Mean Climate over the Western
Hemisphere (1979-1995)
A comparison of the Reanalysis vertical motion and precipitation rates with OLR and merged satellite estimates and rain gauge observations of precipitation is shown in Figs. 113-128. Rather than perform an exhaustive comparison, we present 17-year mean time-latitude sections for selected longitudes. The sections are for the Atlantic (30 °W), South America (60 °W) and the eastern Pacific (90 °W and 120 °W). We focus our discussion on the tropics (20 °N-20 °S).
The Reanalysis vertical motion and precipitation rates (Figs. 113 and 114) do not show the same seasonal cycle over the Atlantic as is displayed in the merged rainfall and OLR analyses (Figs. 115 and 116). The latter indicate that the Atlantic ITCZ remains north of the equator throughout the year, and that it is strongest from May through November. The Reanalysis vertical motion and precipitation rates indicate that the ITCZ is south of the equator from February through May, and has a double structure during June and July. In the Reanalysis the ITCZ is weakest from June through September and strongest in March-April. Thus, the position and intensity of the Atlantic ITCZ do not appear to be well represented in the Reanalysis data.
Over South America (60 °W), the merged rainfall analysis and the OLR display a similar seasonal cycle (Figs. 119 and 120). The heaviest rainfall and lowest OLR occur over the southern Amazon Basin from January through March. Subsequently, there is a northward shift in the region of maximum rainfall and minimum OLR to north of the equator during June-August. Both analyses indicate decreasing convective activity over northern South America from July through September and an increase in convective activity near 10 °S from September through January. The Reanalysis vertical motion and precipitation rates indicate a maximum in rising motion and rainfall near 5 °S throughout the year (Figs. 117 and 118). From May through November a second maximum appears near 12 °N, with a relative minimum at 5 °N. The Reanalysis does not display the meridional shifts in the rainfall maximum that are evident in the OLR and merged precipitation analyses. In addition, the Reanalysis precipitation rates are too low over the southern Amazon Basin (near 10 °S) during January-March and too high between the equator and 5 °S and near 10 °N from May through December.
In the eastern Pacific (90 °W), the patterns of Reanalysis vertical motion and precipitation rates (Figs. 121 and 122) agree quite well with the merged precipitation and OLR analyses (Figs. 123 and 124). These analyses show a marked seasonality in the intensity of the ITCZ, which is strongest from May through October and remains between 5 and 10 °N throughout the year. The Reanalysis and merged rainfall analyses show a tendency for a weaker Southern Hemisphere ITCZ from February through April.
At 120 °W, the Reanalysis patterns (Figs. 125 and 126) do not correspond quite as well with the merged precipitation and OLR analyses (Figs. 127 and 128). From November through February the Reanalysis rainfall maximum is north of the merged rainfall maximum. The seasonal cycles are quite different, with the merged and OLR analyses indicating the heaviest rainfall from April through December and the Reanalysis indicating the heaviest rainfall from November through April. The analyses are consistent in indicating a Southern Hemisphere ITCZ during March and April.
