Double ITCZs

The phenomenon of double ITCZs has not received much research attention perhaps because of its infrequent occurrence and relatively weak signals. Spurious double ITCZs simulated by GCMs, however, are not rare. It is argued that understanding the nature of double ITCZs in reality is necessary to advance our knowledge of the ITCZ in general as well as to evaluate GCM simulations. Using Xie and Arkin CMAP precipitation data, Reynolds' SST data, ERS surface wind data, and TOGA TAO surface meteorology observations, climatological conditions for Pacific double ITCZs are documented. It is shown that in the eastern Pacific, a double ITCZ is a climatological phenomenon in March and April, whose most prevailing condition is relatively low SST along the equator flanked by relatively high SST at both side of the equator. In the western Pacific, signals of a double ITCZ are the most frequent in July, whose main conditions are a strong cold tongue to the east and strong westward surface advection of low theta-e due to the equatorial easterlies penetrating into the western Pacific warm pool. The implications of these findings to current ITCZ theories are discussed.