Temperature.
Engine draws in cold air in winter, cold air more dense, more dense more oxygen, more oxygen more efficient use of suck-squeeze-bang-blow. Principal of turbo-supercharging. Compressing (charging air) makes it warm, loses thermal efficiency so add intercooler to cool charge down.
The speed record breaking inter-war cars often had a compartment next to the air intake which before a run was attempted was filled with crushed ice in an attempt to cool the air down.
The answers all in air temperature.