Dec 20 (Reuters) - Goldman Sachs senior investment strategist Abby Joseph Cohen, whose upbeat stock market forecasts made her the face of the 1990s bull market, will retire at the end of 2021 to join the Columbia University faculty, the company said on Monday. Cohen, who joined Goldman Sachs in 1990, will become professor of business at Columbia University"s Graduate School of Business on Jan. 1, the firm said. In a statement, the company said Cohen played a critical role in expanding its global investment research franchise, with leadership positions including chief U.S. strategist and president of the Global Markets Institute, Goldman"s public policy center. "She has been an influential presence at the firm and across the financial industry," said Jan Hatzius, head of the global investment research division and the firm’s chief economist.