The role of transient tropospheric forcing in stochastic low-order models of sudden stratospheric warmings

The amplitude of tropospherically forced planetary waves is known to be of first-order importance in producing sudden stratospheric warmings (SSWs). This forcing amplitude is observed to undergo strong temporal fluctuations. Characteristics of the resulting transient forcing leading to major stratospheric warmings are investigated in a highly truncated [zero dimensional] model of stratospheric wave-mean flow interaction, as well as the [one dimensional] Holton-Mass model. Modeled sudden warmings are shown to be highly sensitive to the time scale of the prescribed tropospheric forcing in both systems. Transient momentum forcing due to quasi-random gravity wave activity is also taken into account by means of an additive noise term in the zonal momentum equation. This small-scale forcing needs to be parameterized in climate models and is usually considered to be of little importance in driving SSWs. It is shown that this noisy small-scale forcing can strongly affect the nature of stratospheric wave-mean flow interaction and the occurrence of SSWs in the truncated models.