Climate Science: The annual signal comes mainly from the growth/decay cycles in the mid-high northern latitudes. Evidence that the annual cycle signal derives from high northern latitudes comes from the observation that the amplitude of the annual signal is latitude dependent. At high latitudes, the growing season is telescoped into 3-4 months, and so the CO2 amplitude is large. The amplitude decreases in the sub-tropics as the growing season lengthens. In the southern hemisphere, the large expanse of oceans yields milder seasons and small amplitudes to the CO2 annual cycle. The mixing time of CO2 within a hemisphere is 3 months.
There are seasonal variations of CO2 in the tropics. The variations are not regular – as photosynthesis is both light- and water-limited, and some observations show higher photosynthesis during the dry season than the wet season.
The mixing time of CO2 within a hemisphere is 3 months.