![]() ![]() The former are characterized by a several week long pulse of troposphere-induced upward wave activity fluxes, which is absorbed in the stratosphere leading to increased temperatures over the polar cap and an overall weakening of the zonal-mean flow. 8 classified SSWs according to the different coupling mechanism between vertical wave activity and the stratospheric polar vortex and distinguished between so-called absorbing and reflecting SSWs. However, these metrics describe the stratospheric extreme events itself but do not necessarily capture differences in the tropospheric response. SSWs have been extensively studied and have been classified by their spatial properties 4 (split versus displaced events), by the dominant type of wave forcing 5 (wave 1 versus wave 2) or by their intensity 6 (major versus minor warmings). 3 Understanding the exact coupling mechanisms between the troposphere and the stratosphere is hence central to improve S2S predictions in the mid-latitudes including cold spells. 1, 2 Particularly, extremely weak polar vortex states, such as sudden stratospheric warmings (SSW), can be impactful to societies as they are often associated with large-scale cold-air outbreaks in the densely populated mid-latitudes. Variability in the stratospheric polar vortex in boreal winter can influence the tropospheric circulation and is an important source of predictability on sub-seasonal to seasonal (S2S) timescales. Identifying the causal pathways that operate on weekly to monthly timescales can pave the way for improved sub-seasonal to seasonal forecasting of cold spells in the mid-latitudes. ![]() Moreover, our findings suggest the reflective mechanism to be sensitive to the exact region of upward wave-activity fluxes and to be state-dependent on the strength of the vortex. Causal effect network (CEN) analyses confirm the atmospheric pathways associated with this asymmetric pattern. The second pattern is zonally asymmetric and linked to downward reflected planetary waves over Canada followed by a negative phase of the Western Pacific Oscillation (WPO) and cold-spells in Central Canada and the Great Lakes region. This coupling mechanism is well-documented in the literature and is consistent with the downward migration of the northern annular mode (NAM). The first pattern is zonally symmetric and associated with absorbed upward-propagating wave activity, leading to a negative phase of the North Atlantic Oscillation (NAO) and cold-air outbreaks over northern Eurasia. Both patterns represent a weak polar vortex but they are associated with different wave mechanisms and different regional tropospheric impacts. Using cluster analysis, we show that there are two dominant patterns of increased polar cap heights in the lower stratosphere. Weak vortex states, often associated with sudden stratospheric warmings (SSW), have been shown to increase the risk of cold-spells especially over Eurasia, but its role for North American winters is less clear. ![]() The stratospheric polar vortex can influence the tropospheric circulation and thereby winter weather in the mid-latitudes. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |