Abstract Expressionism is the term applied to new forms of abstract art developed in the 1940s and 1950s by a group of painters in America, primarily in New York. It is characterized by large scale and the impression of spontaneity. Painters associated with Abstract Expressionism include Arshile Gorky, Helen Frankenthaler, Clyfford Still, Jackson Pollock, Mark Rothko, Robert Motherwell, Sam Francis, Lee Krasner, Willem de Kooning, Joan Mitchell, and Franz Kline. See also: action painting, color field painting. Web resource here.
Joan Mitchell. To the Harbormaster. Oil on canvas. 1957. Private collection.