The Greenland Gap.
Why didn't submarine patrol planes fly out from Greenland in order to close the "Greenland Gap" (aka The Black Pit, the open area between the ASW air patrols between Iceland and Newfoundland)? Bad weather? Difficulty in operating airplanes out of Greenland? Difficulty in building air bases there? The gap would not be closed until 1943 when long-range ASW patrol planes were made and assigned to it and escort carriers were extensively used.

The USA had permission to build bases there. In April 1940, Nazi Germany occupied Denmark on its way to an invasion of Norway, and almost a year later, the United States signed the US-Danish Agreement on Greenland, which permitted the United States to establish military bases in Greenland. The US had a similar agreement with Iceland, built bases there and flew extensive patrols from there.

- The Mid-Atlantic gap was an area outside the cover by land-based aircraft, those limits are shown with black arcs (map shows the gap in 1941). Bluish dots show destroyed ships of the Allies.