Share 'Roof Air Vents Above Bend Oregon Garage Attic'

My home, built in 1990, has two vents (plus a hot water tank exhaust pipe vent and various leaks to the outside) located in the roof over an attic space above the connected-to-house garage. I know these vents (I think) should be kept open during the summer months but how about temporarily closing them or at least reducing their size during the cold fairly dry winter months.   Winter-time temperat…