Double ITCZs

This study examines where and when double InterTropical Convergence Zones (ITCZs) exist and whether double ITCZs, defined in terms of monthly precipitation distributions, are significant climatological features in the tropics. The most identifiable double ITCZ is found over the eastern Pacific during boreal spring, mainly in March and April. Weak signals of a double ITCZ exist over the Indian Ocean during November, but only infrequently. Over the western and central Pacific, signatures of a double ITCZ can often be found, but mostly during June through September. No double ITCZ is found over the Atlantic Ocean. The boreal - spring double ITCZ over the eastern Pacific fails to exist only during El Nino - Southern Oscillation warm phases, when it is replaced by a single ITCZ at the equator as the cold tongue disappears and the maximum sea surface temperature emerges at the equator. It is therefore concluded that double ITCZs are significant features of the tropical climate even though they undergo spatial and temporal variations. Simple composites of double ITCZs and associated surface conditions indicate that double ITCZs, at least over the Pacific Ocean, exist when there are local minima in surface humidity and temperature along the equator. On the basis of the limited observations, it is proposed that double ITCZs in the Pacific are more causally related to surface thermal conditions than atmospheric internal dynamics alone. During boreal spring a special distribution in sea surface temperature with its local minimum at the equator (i.e., the cold tongue) and two maxima at each side is necessary for the double ITCZ over the eastern Pacific. The westward advection of cold and dry surface air by the easterlies from the cold - tongue region is instrumental in the double ITCZ over the central and western Pacific.