Two meteorological equators

Thu 10 May 2012

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

The study gains importance as earlier studies found weak signals of the DITCZs over the Indian Ocean in November. However, till now, the temporal evolution of these in the Indian Ocean could not be ascertained, mainly because of paucity of data.

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 Intertropical Convergence Zone (DITCZ).

The study was conducted by Dr. M.R. Ramesh Kumar (Senior Scientist, Physical Oceanography Division) and colleagues and has been published in the International Journal of Remote Sensing.

The researchers determined, 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 between 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).

Due to the involvement of several ocean atmospheric processes and their feedbacks to a different degree in different regions, it is difficult to pinpoint whether DITCZs are caused by oceanic processes, the atmospheric dynamics or a combination of both, say the authors.

Meteorologists who have studied the ITCZs also call it the earth's meteorological equator.

This is because it forms the rising arm of the Hadley circulation.

The Hadley circulation is characterised by winds which rise near the equator, flow pole wards after reaching the upper troposphere, and then descend around 30 degrees north and south latitude and flow towards the equator.

The westerly shift of the equator-ward winds, due to the Corialis effect causes the Trade winds.

[source: TheHindu]

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