Tropospheric convergence

The availability of worldwide gridded reanalysis (NCEP, ECMWF, NASA-DAO) has allowed assessments of the water vapor transport into Amazonia, and in fact some recent works have shown contradictory results in terms of trends in input moisture into the Amazon basin. For instance, Costa and Foley (1999) identified a statistically significant decreasing trend in the atmospheric transport of water both into and out of the Amazon basin, based on 20 years (1976-96) of the NCEP reanalysis. On the other hand, Curtis and Hastenrath (1999) have identified statistically significant upward trends of lower tropospheric convergence, upward flow, convergence of atmospheric water vapor transport, and precipitable water over the Amazon basin, based on the analysis of 40 years (1958-97) of the NCEP reanalysis. 

Figure 17.5 also shows the track of flight 10 during PEACE-B on 14 May 2002. A mid-latitude cyclone was located over central China on 13 May. Warm and moist southerly winds in the lower troposphere intensified ahead of this surface cyclone over southern China, converging into a fi ont extending between 20°N, 100°E and 30°N, 115°E. Deep convective clouds, with tops above 10 km around 30°N, 105-120°E, are found in an infrared (IR) image obtained by the Geostationary... 

The exchange of air between the northern and southern troposphere is caused to some extent by eddy diffusion in the equatorial upper troposphere, and by the seasonal displacement of the interhemispheric tropical convergence zone which lies north of the equator in July and south of the equator in January. The displacement is greatest in the region of the Indian Ocean, where the ITCZ is relocated over the Indian subcontinent in July. By tracing the transport of fission products from Chinese and French weapons tests, Telegadas (1972) has demonstrated the importance of monsoon systems for the interhemispheric air exchange in the Indian Ocean. 

A simple depiction of a tropospheric air column with surface-level convergent flow is given in Figure 21.8, wherein horizontal convergence at the surface is matched by horizontal divergence at upper levels. The situation sketched in Figure 21.8 especially represents that in cloudy weather systems. 

---