Gov. David Paterson said in a statement Thursday that Shorris' departure would take effect April 24, when the agency's board meets. Shorris said in the memo he expects his successor to be named then.
Shorris, 49, was appointed by former Gov. Eliot Spitzer in January 2007 to take over the agency, which also operates New York Harbor and the bridges and tunnels connecting New York City and New Jersey.
Spitzer resigned last month in a prostitution scandal. Paterson has replaced several Spitzer appointees since taking office in March.
"I am enormously proud of the work we have done together over the past 16 months. We have advanced every goal I have tried to set for this agency," Shorris wrote to his staff. He noted the agency's efforts to become more environmentally sustainable, upgrading the PATH commuter rail system, buying a fourth airport for the region and making progress rebuilding the trade center site.
Paterson praised Shorris in a statement for the acquisition of Stewart Airport in upstate New York and said that Shorris "made substantial strides at ground zero, with real progress throughout the site, including the raising of steel for the Freedom Tower."
In a speech last week to New York business leaders, Paterson indicated he was unhappy with progress in building the office towers, a Sept. 11 memorial and transit hub at the site.
"We've got to go back and revisit the issue at ground zero," he said then. "As we stand right now, it will be September 11, 2011 before anything is actually built, and estimates are that that may be two or three years off. We can do better than that."
Paterson is looking at several candidates to replace Shorris and has not made an offer to anyone, communications director Risa Heller said Thursday. The New York governor is charged with choosing the agency's executive director, while the governor of New Jersey appoints a board chairman.
Shorris, who earned $277,000 as executive director, had served as the Port Authority's first deputy executive director in the 1990s and has had jobs at the city Board of Education and as former mayor Edward Koch's finance commissioner.
