(Adds exporter comment, current arrivals estimates)
ABIDJAN Jan 3 (Reuters) - Ivory Coast expects its cocoa output to dip 13 percent this season to 1.289 million tonnes due mainly to a lack of investment in ageing plantations, a top official at the Coffee and Cocoa Council said in an interview on Thursday.
The expected decline in supply from the world's top cocoa grower is likely to reinforce market expectations of a possible global deficit in the October 2012/September 2013 season that could bolster prices.
"According to four bean counts, the last of which was completed in November, we expect a main crop harvest of around 955,000 tonnes," the official said, adding that the lighter mid-crop harvest during the last part of the season was expected to amount to 334,000 tonnes.
Output during the 2011/12 season was 1.486 million tonnes.
The CCC official, who asked not to be named, attributed the anticipated decline to "ageing trees and a lack of maintenance" at the country's plantations.
Ivory Coast has endured more than a decade of political turmoil, which culminated in a civil war in 2011 that killed more than 3,000 people and sparked ethnic unrest in the country's western cocoa-growing regions.
Farmers have said the turmoil, a lack of government support, and low farmgate cocoa prices have made it impossible to upgrade their plantations. Since the end of the war, the government has adopted a reform plan aimed at boosting farmer incomes, which has helped lead to higher farmgate prices this year.
Exporters had initially expected Ivory Coast's main crop harvest, which runs from October to March, to break just over 1 million tonnes, but they were in the process of downgrading their forecasts.
About 624,000 tonnes of cocoa arrived at the country's two ports from the start of the season to Dec. 30, down from 702,127 tonnes a year ago, according to an exporters' estimate based on the number of truck deliveries.
Official figures from the CCC were not immediately available.
"Looking at the current numbers, we can no longer hold that forecast. The gap has been between 70,000 and 90,000 tonnes for more than a month, and arrivals typically fall in January," said the director of a European exporter firm based in Abidjan, asking not to be named.
He said a deterioration in weather conditions compared with last year was contributing to the shortfall. (Reporting by Ange Aboa; Writing by Richard Valdmanis; Editing by Alison Birrane and Jane Baird)