Ferrari ended months of speculation when they confirmed Alonso would be leaving the Italian manufacturer after five years, with four-time world champion Sebastian Vettel to replace the departing Spaniard.
The 33-year-old, who won the drivers' championship in 2005 and 2006, is now tipped to return to McLaren, where he spent the 2007 F1 season.
While Alonso's new team has not been confirmed, he revealed his next destination will be home for more than a season.
"It's time to look to the future with optimism and prepare for 2015 in a better way," Alonso told Sky Sports after finishing ninth at the season-ending Abu Dhabi Grand Prix on Sunday.
"We need to see how things go for everyone next year. Mercedes definitely have a big advantage and they will probably carry this advantage into next year.
"But I will join a project that is not for next year only, it is a long-term relationship and it is a project which will hopefully challenge Mercedes soon and this is why I have made this decision."