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Downloaded 10/02/21 07:24 PM UTC 2718 MONTHLY WEATHER REVIEW VOLUME 146 of a Few Days SEPTEMBER 2018 Y U E T A L . 2717 On the Linkage among Strong Stratospheric Mass Circulation, Stratospheric Sudden Warming, and Cold Weather Events YUEYUE YU Key Laboratory of Meteorological Disaster, Ministry of Education, and Joint International Research Laboratory of Climate and Environment Change, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China MING CAI Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, Florida CHUNHUA SHI Key Laboratory of Meteorological Disaster, Ministry of Education, and Joint International Research Laboratory of Climate and Environment Change, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China RONGCAI REN Key Laboratory of Meteorological Disaster, Ministry of Education, and Joint International Research Laboratory of Climate and Environment Change, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, and State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China (Manuscript received 25 March 2018, in final form 21 June 2018) ABSTRACT It is well established that sudden stratospheric warming (SSW) events tend to be accompanied by continental- scale, surface cold-air outbreaks (CAOs) in midlatitudes in boreal winter. However, SSW events occur at most onetotwotimesperwinter,whereasCAOsoccurthreeto seven times over each of the North American and Eurasian continents. Using the ERA-Interim dataset for 37 winters (November–March) from 1979 to 2016, we reveal that SSW events correspond to a large-amplitude or long-lasting subset of pulse-like, anomalously strong, stratospheric mass circulation events. The anomalously strong, stratospheric mass circulation events (referred to as PULSE events) occur more than nine times in an average winter. The ‘‘displacement’’ versus ‘‘split’’ types of SSWs tend to correspond to the ‘‘wavenumber 1’’ versus ‘‘wavenumber 2’’ types of PULSEs, though the re- lationship between split-type SSWs and wavenumber-2-type PULSEs is weaker. Like SSW events, PULSEs also have a close relationship with CAOs. The robust relationship with CAOs still holds for the PULSE events not accompanied by SSW events. Using PULSE events, we determine that more than 70% of CAOs in the 37 winters occur in the week before and after a PULSE event, with a false alarm rate of CAO occurrence of about 25.7%. SSW events, however, are associated with only about 5.7% of CAOs, with a false alarm rate of 21.7%. Therefore, the linkage between individual continental-scale CAOs and PULSE events represents a more generalized re- lationship between the stratospheric circulation anomalies and surface weather. PULSE signals should also be considered as a potentially useful stratospheric indicator of the occurrence of individual CAO events. 1. Introduction Supplemental information related to this paper is available at Sudden stratospheric warming (SSW) is the most the Journals Online website: https://doi.org/10.1175/MWR-D-18- drastic dynamic process in the stratosphere, character- 0110.s1. ized by a decelerated or reversed westerly jet sur- rounding the polar vortex and a reversed temperature Corresponding author: Yueyue Yu, [email protected] gradient between high and midlatitudes over the course DOI: 10.1175/MWR-D-18-0110.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/02/21 07:24 PM UTC 2718 MONTHLY WEATHER REVIEW VOLUME 146 of a few days. During major warming events, a remark- the extreme vortex events category. Some studies able two-way coupling between the stratosphere and have investigated the stratosphere–troposphere cou- troposphere has been well documented (Eliassen and pling processes during extreme planetary wave heat flux Palm 1961; Matsuno 1970; Butchart et al. 1982; Kodera events (Shaw and Perlwitz 2013, 2014; Tripathi et al. et al. 1990; Holton et al. 1995; Baldwin and Dunkerton 2015). Cai et al. (2016) pointed out that a robust rela- 1999, 2001; Shepherd 2002; Kushner and Polvani 2004; tionship with CAOs exists for PULSE events, not lim- Polvani and Waugh 2004; Limpasuvan et al. 2004, 2005; ited to extremely large-amplitude events. There is a McDaniel and Black 2005; Haynes 2005; Garfinkel et al. higher probability of CAO occurrence over the mid- 2010). Large regions in the midlatitudes tend to be latitude regions of Eurasia and North America within anomalously cold during the 1–2 months after the cen- the 1–2 weeks before and after the peak dates of PULSE tral dates of SSW events (Thompson et al. 2002; Kenyon events, consistent with the relationship between CAOs and Hegerl 2008; Tomassini et al. 2012). Kolstad et al. and SSW events reported by Kolstad et al. (2010). (2010) and Woo et al. (2015) found that cold anomalies Follow-up work by Yu et al. (2018b) classified PULSE tend to occur over the southeastern United States within events into different types, according to whether 1–2 weeks after the central dates of SSW events but over wavenumber-1 or wavenumber-2 waves were dominant, Eurasia during the 2 weeks before. Mitchell et al. (2013), and pointed out that the specific spatiotemporal evolu- Kidston et al. (2015), and Lehtonen and Karpechko tion of surface CAOs is dependent on the type of (2016) pointed out that the evolution of surface air PULSE event. Cai et al. (2016) also emphasized that temperature reported by Kolstad et al. (2010) often not only the slowly varying signals in the stratosphere, occurs in vortex displacement events. In splitting events, but also the PULSE signal, have a longer predictability cold anomalies tend to occur over the midlatitudes of limit than surface continental CAOs in operational both North America and Eurasia in the 1–2 months numerical models; thus, the linkage between PULSE around the central dates. Displacement events are events and CAOs may open up a new opportunity for mainly due to stronger wavenumber-1 waves, whereas subseasonal forecasts of CAOs in a hybrid (dynamical splitting events are the result of stronger wave activi- plus statistical) framework. ties of wavenumber-2 preceded by strengthened The global meridional mass circulation was system- wavenumber-1 waves (Liberato et al. 2007; Martius et al. atically documented by the pioneering work of Johnson 2009; Castanheira and Barriopedro 2010; Kuttippurath (1989) and his collaborators and followed by many and Nikulin 2012). subsequent studies [e.g., Cai and Shin (2014), and ref- As operational models have overall useful prediction erences therein]. The meridional mass circulation can be skill for the timing and structure of vortex breakup regarded as a modified version of general Lagrangian processes beyond 1–2 weeks (Christiansen 2005; meridional circulation (Andrews et al. 1987) using a Charlton and Polvani 2007; Yoden et al. 2014; Tripathi quasi-conservative variable, potential temperature, as et al. 2015), although this is still dependent on the the vertical coordinate. The meridional mass circulation methods and initial conditions, the relationship between connects the tropics to the poles and the troposphere to the surface air temperature anomaly pattern and the the stratosphere via the poleward warm air branch in the major SSW of the two types has great importance in upper troposphere and stratosphere and the equator- practical applications for subseasonal forecasts of cold ward cold branch in the lower troposphere. The net mass air outbreaks (CAOs) in the midlatitudes of the transport across the polar circle in the midlatitudes Northern Hemisphere in winter. However, major SSW is driven by anomalously westward-tilting and large- events occur, at most, once in most winters, whereas amplitude waves with a deep structure (Johnson 1989). CAOs occur several times per winter; thus, using SSW On the one hand, the meridional mass transport into the as an indicator of CAOs would result in ‘‘missing fore- polar stratosphere, which was used to define PULSE casts’’ of CAOs. events by Cai et al. (2016), tends to be coupled with the Recent studies have begun to notice another short- equatorward transport of polar air mass at lower levels lived signal in the stratosphere, which has a life cycle of (i.e., the lower branch of meridional mass circulation) in 10–20 days. This signal is the stratospheric ‘‘PULSE’’ the midlatitudes, which is one of the physical causes of events, defined as pulse-like rapid increases in the me- CAOs (Yu et al. 2015a,b). An investigation of the me- ridional mass transport across the polar circle in the ridional mass transport into the polar stratosphere and stratosphere above 400 K (Cai et al. 2016) or extreme its associated surface temperature anomalies could help planetary wave heat flux events (Shaw and Perlwitz us obtain a better understanding of the relationship 2013, 2014). Tripathi et al. (2015) included such events, between PULSE events and CAOs, which has not been together with SSW and stratospheric final warming, in satisfactorily revealed in previous studies. On the other Unauthenticated | Downloaded 10/02/21 07:24 PM UTC SEPTEMBER
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