Transient Tropospheric Forcing of Sudden Stratospheric Warmings

Abstract

Abstract The amplitude of upward-propagating 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 SSWs are studied in reanalysis data and in highly truncated simple models of stratospheric wave–mean flow interaction. It is found in both the reanalysis data and the simple models that SSWs are preferentially generated by transient forcing of sufficiently long time scales (on the order of 1 week or longer). The time scale of the transient forcing is found to play a stronger role in producing SSWs than the strength of the forcing. In the simple models it is possible to fix the amplitude of the tropospheric forcing but to vary the time scale of the forcing. The resulting frequency of occurrence of SSWs shows dramatic reductions for decreasing forcing time scales.