Double ITCZ observed over the tropical Indian Ocean: characteristics derived from cloud properties and OLR

Abstract

Simultaneous occurrence of two bands of inter tropical convergence zone (ITCZ) on either side of the equator, generally known as double-ITCZ (DITCZ), over the Equatorial Indian Ocean (EIO) is investigated using the cloud characteristics derived from NOAA-14/16-AVHRR data (1996-2003), the monthly mean cloud characteristics obtained from the International Satellite Cloud Climatology Project (ISCCP-D2) (1984-2004) and the monthly mean outgoing long wave radiation (OLR: 1974-2004) data obtained from NOAA. A well discernible signature of DITCZ could be observed over the EIO in terms of total as well as high cloud amount and OLR. The doubling of ITCZ occurs mainly in the western part of EIO between 50°E and 80°E. The frequency of occurrence of DITCZ over the Indian Ocean is largest in November (percentage of occurrence ~85%) and December (~62%), which is significantly larger than that reported from earlier studies. The most preferred latitude for the northern and southern bands of DITCZ is ~5°N and ~7.5°S respectively in November and ~5°N and ~10°S in December. The amplitude of the DITCZ, defined as the difference between the total cloud fraction in the equatorial region of minimum cloudiness and that in the respective bands of the DITCZ, is 0.05 to 0.25 for the southern branch and 0.05 to 0.15 for the northern branch. The corresponding amplitude in terms of OLR is 10Wm -2 to 15Wm -2 in the southern band and 5Wm -2 to 10Wm -2 in the northern band.