The life cycle of meridional heat flux peaks.

Covariance between meridional wind and air temperature in the lower troposphere quantifies the poleward flux of dry static energy in the atmosphere; in the midlatitudes, this is primarily realised by baroclinic weather systems. It is shown that strong covariance between temperature and meridional wind results from both enhanced correlation and enhanced variance, and that the two evolve according to a distinct temporal structure akin to a life-cycle. Starting from a state of low correlation and variance, there is first a gradual build-up to modal growth at constant, high correlation, followed by a rapid decay at relatively low correlation values. This life-cycle evolution is observed most markedly over oceanic regions, and cannot be explained on purely statistical grounds. We find that local peaks of meridional heat flux are not exclusively linked to the action of individual weather systems and can affect the atmospheric circulation on larger length-scales through wave propagation along waveguides.