Abstract
After the recent release of the historical runs by community Earth system model version 2–the whole atmosphere community climate model (CESM2-WACCM), the major sudden stratospheric warming (SSW) events in this model and in its previous version (CESM1-WACCM) are compared based on a modern reanalysis (JRA55). Using the World Meteorological Organization (WMO) definition of SSWs and a threshold-based classification method that can describe the polar vortex morphology, SSWs in models and the reanalysis are further classified into two types, vortex displacement SSWs and vortex split SSWs. The general statistical characteristics of the two types of SSW events in the two model versions are evaluated. Both CESM1-WACCM and CESM2-WACCM models are shown to reproduce the SSW frequency successfully, although the circulations differences between vortex displacement SSWs and vortex split SSWs in CESM2-WACCM are smaller than in CESM1-WACCM. Composite polar temperature, geopotential height, wind, and eddy heat flux anomalies in both the two models and the reanalysis show similar evolutions. In addition, positive Pacific–North America and negative Western Pacific patterns in the troposphere preceding vortex displacement and split SSWs are observed in both observations and the models. The strong negative North Atlantic oscillation-like pattern, especially after vortex split SSW onset, is also identified in models. The near-surface cold Eurasia–warm North America pattern before both types of SSW onset, the warm Eurasia–cold North America pattern after displacement SSW onset, and the cold Eurasia–cold North America pattern after split SSW onset are consistently identified in JRA55, CESM1-WACCM, and CESM2-WACCM, although the temperature anomalies after the split SSW onset in CESM2-WACCM are somewhat underestimated.
Highlights
Major sudden stratospheric warming (SSW) is a radical event mostly observed in the Arctic stratosphere associated with rapid and large stratospheric variability during wintertime except the September 2002 and 2019 SSWs over the Antarctic [1,2]
The polar temperature rises dramatically in several days when westerlies reverse to easterlies and the polar stratospheric vortex becomes displaced from the Arctic or even breaks into two sister vortices [3], known as vortex displacement SSWs and Atmosphere 2019, 10, 679; doi:10.3390/atmos10110679
The Pacific–North America (PNA) and western Pacific (WP) patterns largely the SSW onset (Figures 8a3 and 9a3), and positive height anomalies appear over the high-latitude weaken during the SSW onset (Figure 8a3 and 9a3), and positive height anomalies appear over the of the North Atlantic for vortex split SSWs, which project onto a negative North Atlantic oscillation (NAO) pattern (Figure 9a3)
Summary
Major sudden stratospheric warming (SSW) is a radical event mostly observed in the Arctic stratosphere associated with rapid and large stratospheric variability during wintertime except the September 2002 and 2019 SSWs over the Antarctic [1,2]. Atmosphere 2019, 10, 679 vortex split SSWs [4] Ever since this phenomenon was found by Richard Scherhag [5], much work has been conducted, including SSW theories [6,7], influences on surface weather and climate [8,9], simulations and predictions by models [10,11,12], and classifications [4,13,14,15,16,17,18]. The questions we attempt to answer in this study are as follows: (1) How well do the two model versions simulate the frequencies of vortex displacement and split SSWs? A comparison of different types of SSWs between the reanalysis and models is conducted in Section 3 in various aspects, including statistics, stratospheric and tropospheric circulation evolutions, upward planetary waves, and their downward impacts.
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