Pronounced upper stratospheric and mesospheric NOx enhancements were measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) in the Southern Hemisphere (SH) polar vortex from May to August 2003, reaching average abundances of 60 ppbv at 50--60 km in July. Peak mixing ratios of around 200 ppbv were measured in the polar night, representing the highest values ever recorded in the SH. The observed NOx enhancements are attributed to production by electron precipitation in the upper mesosphere and lower thermosphere and subsequent descent with the meridional circulation. Using measured CH4 and CO distributions as dynamic tracers, the downward transport of NOx-rich air masses into the lower and middle stratosphere has been investigated. Upper atmospheric air with average NOx abundances of 15 ppbv reached the 800--1000 K potential temperature region (around 30 km) by the end of July, where it remained until the final warming in late October. The NOx descent was confined to the polar vortex, although significant mixing of tropical and NOx-rich vortex air masses began already in August above 40 km. The amount of upper atmospheric NOy measured inside of the SH vortex in late spring was 1.1 Gigamoles (GM) which is in good agreement with previous estimates from HALOE data. The global coverage of MIPAS data further allows to quantify the upper atmospheric NOx dispersed into the stratosphere during August-September, estimated in 1.3 GM. The net deposition of NOx into the stratosphere during the 2003 Antarctic winter (2.4 GM) makes up 9% of the N2O oxidation source in the SH, twice as much as estimated in previous studies. NOx and tracer distributions observed on several days during the NH winter 2002/2003 have been analyzed for comparison. We found that high planetary wave activity, resulting in the major midwinter warming had led to a rather inefficient NOx downward transport with negligible deposition of NOx into the lower and middle stratosphere.