What Controls the Amplitude and Phase of the Extratropical Seasonal Cycle?

The seasonal cycle of mid to high latitude surface air temperature is prime example of a climate response to external forcing. Here we examine what controls the amplitude and phase of this cycle. By comparing the seasonal cycle in an numerical model of the atmosphere and ocean mixed layer where the large seasonal variation in humidity is allowed to affect longwave radiation to one where it is not, we examine the impact of water vapor feedback on seasonal cycle amplitude and phase. Water vapor feedback is thought to be the most powerful positive feedback to climate variations. However, it has a surprisingly small amplifying affect of about 10-20% on the overall amplitude of the extratropical seasonal cycle. Its effect on phase is also small. This is mainly because the extratropical seasonal cycle is damped not only by radiative processes, but also by heat exchange between the extratropics and tropics. When temperatures are cold (warm) during winter (summer), an anomalously large (small) amount of heat is transported from the tropics into the extratropics. On a Wm basis, this damping mechanism is slightly more effective than radiative damping, substantially diluting the effects of the classical radiative climate feedbacks such as water vapor feedback. Because the seasonal cycle is so effectively damped by both radiative fluxes and horizontal heat exchange, it is in equilibrium with the solar forcing to a surprisingly large degree, with a much smaller amplitude than would be expected if radiative feedbacks alone were responsible for the damping. We also show that if there were no lateral damping, the amplitude of the mid to high latitude seasonal cycle would increase by approximately 50% in both hemispheres, while its lag behind the solar forcing would increase from one to about two months.