"Northumberland" rejoined the fleet at the Trafalgar 200 celebrations, then embarked for a period of sea training, starting with BOST (Basic Operational Sea Training) in January 2006, straight after the Christmas leave period. For a time during 2006 "Northumberland" accompanied the submarine on her deployment to the US AUTEC (Acoustic Undersea Testing and Evaluation Centre) which is based on Andros Island in the Bahamas. In 2007 she deployed to the Mediterranean as part of the UK's contribution to NATO maritime forces.