Double ITCZs

{Reference: Double ITCZs – Journal of Geophysical Research, Vol. 106 No. D11, by Chidong Zhang}

The ocean-atmospheric phenomenon called Double Inter Tropical Convergence Zone (DITCZ) investigated by the Researchers at the National Institute of Oceanography, Goa, have investigated over the Western Indian Ocean, for its meteorological characteristics.

What is DITCZ?

• A Double Inter Tropical Convergence Zone is a phenomenon featured with two ITCZs, one at each side of the equator.
• About 10 degrees north or south of the equator there forms a region of convective activity which is called the Inter Tropical Convergence Zone (ITCZ). Sometimes, on the opposite side of the equator, another ITCZ forms which is short-lived (November-December) in the western Indian Ocean and this phenomenon is called Double Inter Tropical Convergence Zone (DITCZ).

When and Where are DITCZs found?

• 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 TTCZ is found over the Atlantic Ocean.
• The existence of double ITCZs is nothing new. Those who follow closely tropical weather and climate have long recognized that a double ITCZ occurs frequently over the eastern Pacific during boreal spring.
• 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.

Why is this Study important?

• The study derives significance as former studies found weak signals of the DITCZs over the Indian Ocean in November. Yet, the temporal evolution of these in the Indian Ocean could not be assured, primarily because of dearth of data.
• Double ITCZs are significant climatological features of the tropics because of their repeating occurrence at certain longitudes and during certain seasons. Double ETCZs deserve more research attention than they have received, especially in the context of interpreting climate simulations of GCMs and theoretically understanding the ITCZ.

How did the researchers investigate?

Using a suite of sensors, including those of NASA satellites, which provided rainfall-distribution data, when and where the DITCZ existed over the Indian Ocean for the study period (1988-2005). They used these sensors, to study the different phases of the DITCZ’s life-cycle and investigated it for rainfall, fresh water flux (difference b/w evaporation and precipitation), cloud liquid water, cloud cover and relative humidity. Analysing cloud cover for November-December of the years 2002-2009 it was found that a large area was covered by clouds in the last 2 weeks of November and first 2 weeks of December. High relative humidity because of the moistening due to convection and convectively formed cirrus clouds in the upper troposphere on both sides of the equator, was observed. It was found that the values of these and the other parameters were consistent with the criteria for formation of a DITCZ. To explore the potential impacts of the El Nino Southern Oscillation (ENSO) events on DITCZs, daily rainfall data for the years 1997, 2002 and 2006 were analysed. A robust relationship between the two was found. This is in interesting contrast to the eastern Pacific Ocean (where they were absent during the ENSO years, 1983, 87, 92 and 97).

Why it is difficult to pinpoint how DITCZs are caused?

It is difficult to pinpoint whether DITCZs are caused by oceanic processes, the atmospheric dynamics or a combination of both due to the contribution of various ocean atmospheric processes and their feedbacks to a different degree in different regions.