Local meridional circulation and deserts

This paper investigates the dry climatology of Sahara and Northwest China d eserts from the viewpoint of local meridional circulation with Xie and Arki n rainfall dada and NCAR/NCEP reanalysis data. Results show that there are very dry centers with annual rainfall less than 50 mm over these two desert s while the rainy seasons are very different. In the south part of Sahara d esert center and Northwest China desert, over 70% rainfall takes place in J une, July and August (JJA). While in the north part of Sahara, rainfall mai nly concentrates in December, January and February (DJF). The local biosphe re-radiation mechanism proposed by Charney cannot explain the climatology o f such very dry centers. Neither can the monsoon-desert mechanism proposed by Rodwell and Hoskins do for the strongest descent center is much more nor thward than the driest center over Sahara in JJA. From the viewpoint of loc al meridional circulation, the dryness climatology of Sahara and Northwest China deserts is investigated and compared. It is shown that in DJF, descen t of local meridional circulation dominates the two deserts and very dry cl imate is unavoidable although the relative wet season is weak over the nort hern part of Sahara due to Mediterranean climate. While in JJA, there is as cent over the two deserts especially over Northwest China. Such ascent can explain the rainy season in south part of Sahara and Northwest China desert s. However, it is the local meridional circulation that takes strong and dr y northerly from higher latitudes. The northerly either takes little moistu re to the centers or prevents deep and strong convection over the centers. Such local meridional circulation leads to the dry climatology over the two deserts.