Dynamic Linkage Between Cold Air Outbreaks and Intensity Variations of the Meridional Mass Circulation
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3214 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 Dynamic Linkage between Cold Air Outbreaks and Intensity Variations of the Meridional Mass Circulation YUEYUE YU State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China RONGCAI REN State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, and Key Laboratory of Meteorological Disaster (KLME), Nanjing University of Information Science and Technology, Nanjing, China MING CAI Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida (Manuscript received 29 December 2014, in final form 24 April 2015) ABSTRACT This study investigates the dynamical linkage between the meridional mass circulation and cold air out- breaks using the ERA-Interim data covering the period 1979–2011. It is found that the onset date of continental-scale cold air outbreaks coincides well with the peak time of stronger meridional mass circulation 2 events, when the net mass transport across 608N in the warm or cold air branch exceeds ;88 3 109 kg s 1. 2 During weaker mass circulation events when the net mass transport across 608N is below ;71.6 3 109 kg s 1, most areas of the midlatitudes are generally in mild conditions except the northern part of western Europe. Composite patterns of circulation anomalies during stronger mass circulation events greatly resemble that of the winter mean, with the two main routes of anomalous cold air outbreaks being along the climatological routes of polar cold air: namely, via East Asia and North America. The Siberian high shifts westward during stronger mass circulation events, opening up a third route of cold air outbreaks through eastern Europe, where lies the poleward warm air route in the winter-mean condition. The strengthening of the Icelandic low and Azores high during stronger mass circulation events acts to close off the climatological-mean cold air route via western Europe; this is responsible for the comparatively normal temperature there. The composite pattern for weaker mass circulation events is generally reversed, where the weakening of the Icelandic low and Azores high, corresponding to the negative phase of the North Atlantic Oscillation (NAO), leads to the reopening and strengthening of the equatorward cold air route through western Europe, which is responsible for the cold anomalies there. 1. Introduction Konrad 1996). For instance, cold air outbreaks over North America have been found to be closely related Various anomalous synoptic- and planetary-scale to positive sea level pressure anomalies along the patterns have been identified as precursor signals for Alaska–Yukon border (Walsh et al. 2001)ortheco- cold air outbreaks over different regions (e.g., Wexler existence of a ridge over the Arctic and a trough over 1951; Colucci and Davenport 1987; Walsh et al. 2001; the Great Lakes region (Konrad 1996). Elsewhere, in East Asia, the intensification and expansion of the Siberian high are known to be the triggering mecha- Corresponding author address: Dr. Rongcai Ren, LASG, In- stitute of Atmospheric Physics, CAS, P.O. Box 9804, Beijing nism for cold air surges (Ding 1990; Zhang et al. 1997; 100029, China. Gong and Ho 2004; Takaya and Nakamura 2005). E-mail: [email protected] Palmer (2014) pointed out that intensifying Rossby DOI: 10.1175/JAS-D-14-0390.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/30/21 11:46 PM UTC AUGUST 2015 Y U E T A L . 3215 waves within the jet stream, excited by the latent heat the tropical heating source to the polar heating sink via a release over the warming tropical west Pacific may poleward warm air branch in the upper troposphere have contributed to the extremely cold 2013–14 winter (and also the stratosphere in the winter hemisphere) and in the United States. an equatorward cold branch in the lower troposphere.1 There is also ample evidence indicating a robust re- Cold air outbreaks can be directly related to an anom- lationship between continental-scale cold air outbreaks alous strong meridional mass circulation, with more cold and the leading oscillation modes in the winter extra- air discharged from the northern polar region into tropics. It is well recognized that the negative phase of the lower latitudes within the cold air branch. The the North Atlantic Oscillation (NAO) coincides with strengthening of the cold air branch is connected with cold anomalies over Europe and warming in the north- the strengthening of the warm air branch in the upper west Atlantic (Rogers and van Loon 1979; Hurrell and atmosphere and is driven by the amplification of large- van Loon 1997). The studies by Walsh et al. (2001) and scale waves in the midlatitudes. Therefore, the meridi- Cellitti et al. (2006) show that the NAO tends to be in its onal mass circulation perspective not only allows us to negative phase 3–6 days prior to cold air outbreaks over capture the preferred routes of cold air outbreaks di- different regions of the United States and Europe. Luo rectly but also can help us to investigate the precursory et al. (2014) related the cold air outbreak event in changes in various circulation fields for cold air January–February 2012 to a positive-to-negative phase outbreaks. transition of NAO. As a broader anomaly pattern than Iwasaki and Mochizuki (2012) and Iwasaki et al. the NAO, the Arctic Oscillation (AO), or the tropo- (2014) identified two major routes of cold air from the spheric northern annular mode (NAM), also tends to be northern polar region to lower latitudes: namely, the in its negative phase when there are more frequent ‘‘East Asian stream’’ and the ‘‘North American strong cold air outbreaks in the midlatitudes of Eurasia stream.’’ Shoji et al. (2014) conducted a comprehensive and North America (Thompson and Wallace 1998, 2001; isentropic diagnosis of East Asian cold air outbreaks Wettstein and Mearns 2002; Cohen et al. 2010). Ac- within the cold air branch of the meridional mass cir- companying these leading modes is the oscillation of culation. Yu et al. (2015) constructed a mass circulation extratropical zonal mean zonal wind in the troposphere, index (denoted as WB60N) to measure the intensity of which was termed the ‘‘index cycle’’ to link variations of the warm air branch of the meridional mass circulation the westerly jet to cold air outbreaks in the pioneering and showed that changes of the index can be a precursor work of Namias (1950). for the cold air outbreaks in midlatitudes. They also Several promising precursors to winter cold air out- showed that there exist two dominant geographical breaks have also been found in the stratosphere. The patterns of temperature anomalies during the cold air works of Baldwin and Dunkerton (1999), Wallace discharge period (or 1–10 days after a stronger mass (2000), and Thompson et al. (2002) indicate that cold air circulation across 608N). One represents cold anomalies outbreaks tend to occur more frequently over the mid- mainly in the midlatitudes of both North America and latitudes in the period of 1–2 months after a weak Eurasia, and the other represents cold anomalies mainly stratospheric polar vortex event. Cai (2003) found that over only one of the two continents accompanied with cold surface temperature anomalies tend to take place abnormal warmth over the other continent. Yu et al. underneath the intrusion zone of high isentropic po- (2015) mainly focus on statistical evidence of the robust tential vorticity (IPV) into the troposphere from the relation between the strengthening of meridional mass stratosphere. Kolstad et al. (2010) found that cold tem- circulation across 608N and cold air outbreaks in the perature anomalies over the southeastern United States midlatitudes but do not examine the spatiotemporal tend to appear within 1–2 weeks after the peak dates of patterns of anomalous meridional mass circulation and weak vortex events, whereas the cold anomalies over Eurasia seem to appear at the inception of weak vortex events. In addition, continental-scale cold temperature 1 The poleward and equatorward branches of the meridional anomalies are correlated with the easterly phase of the mass circulation are defined in terms of the zonally integrated mass equatorial stratospheric quasi-biannual oscillation (e.g., fluxes. Since the zonally integrated mass fluxes are poleward in the Thompson et al. 2002; Cai 2003). upper isentropic layers and equatorward in lower isentropic layers, The pioneering work of Johnson (1989) and many they are also referred to as the warm and cold air branches in the subsequent studies [e.g., Cai and Shin (2014) and ref- literature (e.g., Cai and Shin 2014). Within each of the two branches, there exist both poleward warm airmass fluxes in some erences therein] established a hemisphere-wide single- longitudinal sectors and equatorward cold airmass fluxes in others. cell model for meridional mass circulation. The mass This is particularly true in the extratropics, where the meridional circulation is a thermally direct circulation that connects mass circulation is mainly driven by baroclinically amplifying waves. Unauthenticated | Downloaded 09/30/21 11:46 PM UTC 3216 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72 TABLE 1. A list of the derived indices and variables. Derived indices and variables Variables needed for calculation Reference Isentropic mass u; us, Ps Pauluis et al. (2008) MF* y, u; ys, us, Ps Cai and Shin (2014) u_ u u_ u VF , ; s, s, Ps Yu et al. (2014) CMI y, u; ys, us, Ps Iwasaki et al. (2014) WB60N index MF at 608N Yu et al.