NEW YORK CITY-The 280,000-square-foot renewal and expansion of law firm Winston & Strawn at Tishman Speyer Properties’ 200 Park Ave., and the consolidation of Natixis into 182,200 square feet at Mitsui Fudosan’s 1251 Ave. of the Americas, rank among the largest office leases seen in Manhattan this year. Yet while they’re the biggest deals announced so far this month, GlobeSt.com has already reported about 520,000 square feet of Manhattan office leases since Dec. 1 that don’t include these two transactions.

And here’s nearly 160,000 square feet more announced on Monday and Tuesday of this week: Regus for 26,112 square feet at 41 Madison Ave.; Adspace Networks in a 10,835-square-foot headquarters deal at 99 Park Ave.; the Richline Group for a 32,488-square-foot relocation from Mt. Vernon, NY to 1385 Broadway; Rizzoli International Publications and M. Booth & Associates taking 13,487 and 26,974 square feet, respectively, at 300 Park Ave. South; and a trio of deals at 499 Park Ave.: Woodbine Capital Advisors with 11,300 square feet, Halstead Property for 17,500 square feet and a 20,000-square-foot expansion by Cantor Fitzgerald.

Add all of these up, and the total for the first half of December would be nearly 1.1 million square feet, just in terms of deals large enough to be reported on GlobeSt.com. (The New York Post first reported in mid-November that a Natixis deal was imminent.)

Leasing volume across Manhattan is likely to finish 2010 somewhere north of 20 million square feet, with the year-end flurry of deals comparing favorably to 2008 and 2009’s final weeks. By comparison, Manhattan leasing totaled 16.79 million square feet in ’09 and 17.56 million square feet in ’08, according to CB Richard Ellis. Were the two-week tallies for the first half of December of this year annualized, the 12-month figure would be more than 25 million square feet, although the pressure to get deals done by the end of the calendar year certainly has something to do with it.

Moreover, despite the high overall volume, the mega-deals seen at the peak of the market haven’t staged a comeback yet. The much-discussed relocation of Conde Nast to one million square feet at 1 World Trade Center is not yet a done deal, and the 406,399-square-foot relocation of law firm Proskaeur Rose to 11 Times Square still stands as the year’s largest direct lease. Societe Generale’s move to 245 Park Ave. is larger in terms of square footage—at least 442,000 square feet—but it’s a sublease from JPMorgan Chase.

Yet there are also indications that the current pace of office leasing is sustainable, thanks to improvements in the underlying fundamentals. “Earlier this year we saw a wave of New York City office leasing, predominantly from the release of pent-up demand,” says Robert L. Freedman, chairman of the tri-state region for Colliers International, in a release announcing the firm’s 2011 forecast. “But now, thanks to increased global business, we are seeing an improving local economy and modest, organic growth.”

Although rents are showing signs of stability, Freedman adds that office tenants in New York City still have strategic opportunities. “Tenants facing lease expirations within the next two to three years should at least kick the tires to see what is available today,” he says. “If not, they risk being pressured later to sign a lease when the market is less favorable to them.”