Abstract

In this study, the atmospheric conditions for the December 2013 Middle East snowstorm are examined from a case study perspective and by performing a composite analysis of extreme winter events from 1950 to 2013 using reanalysis data. It is revealed that this snowstorm arises from the occurrence of an omega (Ω)-type European blocking (EB) with a strong downstream trough that is associated with a southward-displaced positive-phase North Atlantic Oscillation (NAO+) event. In the anomaly field, the EB exhibits a northeast–southwest (NE–SW)-tilted dipole structure. The Ω-type EB transports cold air into the Middle East and produces snowfall within the trough over the Middle East.

The composite analysis shows that the location of cold temperatures depends strongly on the tilting direction and strength of the EB dipole anomaly. The NE–SW [northwest–southeast (NW–SE)]-tilted EB dipole occurs with a southward (northward)-displaced NAO+ event. The NE–SW-tilted EB dipole anomaly is associated with an arching-type low-frequency wave train that spans the North Atlantic, Europe, and the Middle East. This tilting has the most favorable structure for cold air outbreaks over the Middle East and southeastern Europe because this tilting leads to an intense downstream trough over this region. In contrast, a NW–SE-tilted EB dipole anomaly leads to cold temperatures over northwestern Africa and southwestern Europe. The analyses herein also suggest that a strong jet over the North Atlantic may be a precursor for a southward-displaced NAO+ event that is usually associated with an Ω-type EB with a NE–SW-tilted dipole in the anomaly height field that favors a cold air outbreak over the Middle East.

1. Introduction

During 9–15 December 2013, a severe winter snowstorm hit the Middle East, affecting parts of Israel, Jordan, Syria, Lebanon, Turkey, Iran, Egypt, and the Palestinian territories. In particular, this was the first snowfall seen in Egypt in more than 100 years, as reported by local news (http://earthsky.org/earth/rare-snow-storm-hits-middle-east). Extreme cold events like this one have been an important research topic (Yiou and Nogaj 2004; Cattiaux et al. 2010; Seager et al. 2010) because these events often cause enormous economic consequences.

Many studies have revealed that the occurrence of extreme cold events over Europe is related to low-frequency modes with time scales of 2–3 weeks of atmospheric variability such as the North Atlantic Oscillation (NAO) and atmospheric blocking in the Euro-Atlantic sector (Scaife et al. 2008; Buehler et al. 2011; Sillmann et al. 2011). It has been recognized that the phase of the NAO and the establishment of European blocking (EB) are the two most important contributors to the variability of European weather and climate (Hurrell 1995; Sillmann et al. 2011; Luo et al. 2014; Diao et al. 2015). The negative (positive) phase of the NAO, denoted as the NAO (NAO+), is usually associated with cold (warm) temperatures over northern Europe and warm (cold) temperatures over southern Europe (Hurrell 1995; Diao et al. 2015). These temperature anomalies result from weakened (increased) meridional pressure gradients and a more northerly (southerly) storm track over the Atlantic Ocean during NAO (NAO+) events (Hurrell 1995).

Over the past few decades, many studies have found that severe winter storms and extreme precipitation over the eastern Mediterranean Sea and the Middle East are associated with explosively deepening cyclones or mobile short-wave troughs (Petterssen 1956; Alpert and Reisin 1986; Lee et al. 1988; Trigo et al. 2002) and low-frequency teleconnection patterns (Kutiel and Paz 1998; Eshel and Farrell 2000; Krichak and Alpert 2005; Ziv et al. 2006; Feldstein and Dayan 2008). In particular, the case study of Lee et al. (1988) showed that the interactions among planetary, synoptic, and mesoscale circulations led to the severe winter weather in the Middle East from 1 to 3 March 1980. They found that a confluent flow pattern that emerges over North Africa and southern Europe downstream of a block west of the Greenwich meridian provided a planetary-scale circulation that was favorable for this snowstorm, while the southeastward movement of traveling short wave troughs led to the progressive cooling of the troposphere confined in lower and middle layers over the eastern Mediterranean Sea region and the Middle East. However, the physical processes underlying the outbreak of the 2013 Middle East snowstorm are unclear. The aim of this paper is to reveal what mechanism and what type of large-scale circulation led to the outbreak of this type of snowstorm.

This paper is organized as follows: The data and method are described in section 2. In section 3, we present a case analysis of the 2013 Middle East snowstorm. In section 4, we present a composite analysis based on the classification of NAO+ events in terms of their meridional position as well as the strength and type of EB events. It is shown that southward-displaced EB-related NAO+ events or omega (Ω)-type EB events associated with NAO+ events are important for Middle East cold events. The possible cause of why the 2013 Middle East snowstorm is so unusual is presented in section 5. Conclusions and further discussion are given in section 6.

2. Data and method

a. Data

We used the daily 500-hPa geopotential height and near-surface (0.995 sigma level) air temperature from December 1950 to February 2014 on a 2.5° × 2.5° grid from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis dataset. The daily precipitation data over Europe were obtained from the European Climate Assessment and Dataset (http://eca.knmi.nl/download/ensembles/download.php#datafiles) and they are on a 0.5° × 0.5° grid for the same time period.

The normalized daily NAO index based on a rotated empirical orthogonal function (REOF) analysis was obtained from the National Oceanic and Atmospheric Administration/Climate Prediction Center (NOAA/CPC; ftp://ftp.cpc.ncep.noaa.gov/cwlinks/). We computed daily 500-hPa geopotential height and surface air temperature (SAT) anomalies at each grid point as the deviations of their daily fields from their climatological mean (1950–2014) for each day in winter of the year. This removes the mean annual cycle. Here, we only analyze data for the boreal winter from December to February (DJF).

b. Definition of NAO+ events

An NAO+ event is defined to have taken place when the amplitude of the daily NAO index exceeds one standard deviation (STD) for at least three consecutive days. The lifetime of an NAO+ event is defined as the time period that the daily NAO index increases from zero to its maximum value and then decreases back to zero, rather than the time that the NAO index remains above +1 STD. The period of an NAO event defined here cannot exceed 30 days, as in Luo et al. (2012), because the observed NAO events typically have a 10–20-day time scale (Feldstein 2000). Tests showed that the main results presented below are insensitive to the NAO definition (not shown).

c. Blocking identification method

To identify EB events, here we use the one-dimensional blocking index of Tibaldi and Molteni (1990, hereinafter TM). The TM index is based on the reversal of the 500-hPa geopotential height gradient, and requires that the following conditions be satisfied for a blocking event to occur:

 
formula
 
formula

where GHGS and GHGN represent the 500-hPa geopotential height gradients at each longitude at lower and higher latitudes, respectively. Note that Z denotes the daily 500-hPa geopotential height at a given longitude λ; ϕN = 80°N + Δ, ϕ0 = 60°N + Δ, and ϕS = 40°N + Δ represent three latitudes in the blocking region, respectively (TM). Moreover, Δ = −5°, 0°, and 5° are used rather than Δ = −4°, 0°, and 4° as in TM, because of the 2.5° × 2.5° reanalysis grid.

At a given time and longitude, a blocking event is identified if conditions (1a) and (1b) are satisfied for any one of the three values (−5°, 0°, and 5°) for Δ. A 5-day moving average is applied to the daily 500-hPa geopotential height data at each grid cell prior to calculating the blocking index. This partly removes high-frequency noises according to NOAA/CPC (http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking/index/index.nh.shtml). A blocking event is required to persist for more than three consecutive days and the lag 0 day represents the day that the blocking amplitude is strongest during the blocking life cycle. The definition for the period and strength of the blocking event can also be found at the NOAA/CPC website.

3. A case study of the Middle East snowstorm in December 2013

a. Downstream European blocking

Figure 1 shows the evolution of the daily 500-hPa geopotential height, the 850-hPa wind anomaly, the SAT anomaly, and areas of precipitation from 8 to 15 December 2013. It is found from the zonal wind anomaly field (the arrow in Fig. 1) that there is an intensified zonal flow at 850 hPa over the North Atlantic (90°–30°W). On 8 December, a ridge emerges over northwestern Europe, slightly downstream of the NAO region, that undergoes rapid growth during the following several days. On 11 December, the ridge evolves into an omega-type EB with an intensified cutoff low to its southeast, that is, a dipole anomaly with northeast–southwest (NE–SW) tilt.1 From 10 to 14 December, snowfall is seen in the Middle East (red box in Fig. 1). This period corresponds to the mature stage of this EB event. On 12 December, the EB begins to decay and the cutoff low is no longer present after 14 December. During the entire EB event, the low SAT over the Middle East and southeastern Europe remains persistent. This temperature decline appears to be closely associated with the strengthening of this cutoff low. The maximum SAT anomalies from −5° to −15°C over the Middle East and southeastern Europe are seen from 12 to 13 December. A similar blocking event to the east of the Greenwich meridian was noted by Lee et al. (1988) in their March 1980 Middle East snowstorm case study. As shown in Fig. 1, the cold air was transported to lower latitudes through the southwestward winds associated with the cutoff low. Consistently, Ziv et al. (2006) noted that extreme rainfall over Israel is closely associated with a NE–SW-tilted surface low over Cyprus (often referred to as a Cyprus low) downstream of a blocking ridge. The moisture advection induced by this trough is crucial for extreme precipitation in Israel. More recently, de Vries et al. (2013) found that the majority of extreme precipitation events associated with active Red Sea troughs are also characterized by a NE–SW-tilted trough over the Middle East located downstream of a ridge over Europe. As demonstrated below, the latitudinal position of the upstream NAO+ event seems to determine the horizontal tilt and strength of this trough through the tilting of the EB dipole anomaly.

Fig. 1.

Horizontal fields of the daily 500-hPa geopotential height (gpm), 850-hPa wind anomaly (the unit of wind anomaly vectors: m s−1) and surface air temperature anomalies (°C) from 8 to 15 Dec 2013. The height contours are drawn from 5400 to 5700 gpm with a 100-gpm interval and the area of precipitation is indicated with green dots. Only the negative temperature anomalies are plotted with the blue shading. The red box represents the Middle East and the red dashed line in the Atlantic region indicates the region with 500-hPa zonal wind speed exceeding 40 m s−1 (the jet stream). The wind vector is plotted with an arrow whose value is marked in each panel.

Fig. 1.

Horizontal fields of the daily 500-hPa geopotential height (gpm), 850-hPa wind anomaly (the unit of wind anomaly vectors: m s−1) and surface air temperature anomalies (°C) from 8 to 15 Dec 2013. The height contours are drawn from 5400 to 5700 gpm with a 100-gpm interval and the area of precipitation is indicated with green dots. Only the negative temperature anomalies are plotted with the blue shading. The red box represents the Middle East and the red dashed line in the Atlantic region indicates the region with 500-hPa zonal wind speed exceeding 40 m s−1 (the jet stream). The wind vector is plotted with an arrow whose value is marked in each panel.

Moreover, Fig. 1 (green shading) shows that the precipitation field is split into two branches around the blocking region: one branch over the northern side of the blocking region and the other over the Middle East associated with the cutoff low. As shown by Luo et al. (2014), the blocking-related precipitation field often exhibits a double-branched structure because of the splitting of synoptic-scale eddies around the block. Thus, the existence of an omega-type block with a strong downstream trough or cutoff low over the European continent appears to be crucial for the Middle East snowfall event of December 2013.

Figure 2 shows the evolution of the daily NAO index in December 2013. It is seen that the NAO index is mostly positive during that month. The EB event occurs during the period from 9 to 15 December. Because the location of this EB event extends from the eastern Atlantic to western Europe, the daily NAO index is expected to decline during the EB event, because the block has a negative projection onto the canonical NAO over the eastern part of the NAO region (shown on the NOAA/CPC website). Even so, the height field in Fig. 1 shows that the flow field projects onto the NAO+ pattern farther upstream over the northwestern Atlantic during the period from 9 to 13 December despite the NAO index being small. The combination of the westward-displaced NAO+ pattern and eastern Atlantic blocking results in a weakly positive and sometimes negative NAO index value from 9 to 13 December. Such a NAO+ event is the so-called EB-related NAO+ event (as defined in section 4a). Thus, the above result suggests that the EB-related NAO+ event may play an important role in the Middle East snowstorm of December 2013. Eshel and Farrell (2000) also noted that there is a close link between changes in the NAO and eastern Mediterranean precipitation. However, Ben-Gai et al. (2001) found that the correlation between Israel precipitation and the NAO is not statistically significant. In contrast, correlations of the SAT and surface pressure over Israel with the NAO are statistically significant. Feldstein and Dayan (2008) noted that the east Atlantic/western Russia (EA/WR) dipole teleconnection pattern and the circumglobal wave packet of Branstator (2002) are important for Israeli winter precipitation anomalies. The positive-phase EA/WR pattern is characterized by a ridge over Europe and a trough that extends from western Russia to the Middle East. This pattern resembles the EB–Middle East trough shown in Fig. 1. There are many papers that link the EA/WR and similar teleconnection patterns such as the North Sea–Caspian pattern to positive precipitation anomalies in the Middle East (e.g., Kutiel and Paz 1998; Eshel and Farrell 2000, 2001; Krichak et al. 2000, 2002; Kutiel and Benaroch 2002; Kutiel et al. 2002; Paz et al. 2003; Xoplaki et al. 2004; Krichak and Alpert 2005; Ziv et al. 2006). Although these studies allude to the importance of a trough over the Middle East in conjunction with an EB for Middle East precipitation, what mechanism determines the occurrence of the EB event together with the downstream trough remains unclear. In particular, the question of which particular type of EB-related NAO+ events contributes to extreme precipitation events in the Middle East, including the snowstorm in December 2013, needs to be elucidated. This problem is investigated in detail in the following sections.

Fig. 2.

Time series of the normalized daily NAO index (solid line) from 1 to 31 Dec 2013, based on the REOFs of 500-hPa geopotential height obtained from the NOAA/CPC website. The dashed line represents the 9-point moving average.

Fig. 2.

Time series of the normalized daily NAO index (solid line) from 1 to 31 Dec 2013, based on the REOFs of 500-hPa geopotential height obtained from the NOAA/CPC website. The dashed line represents the 9-point moving average.

b. Spatial structure of the EB dipole

To better understand the spatial structure of the omega-type block shown in Fig. 1, we show the 500-hPa daily geopotential height anomalies from 8 to 15 December 2013 in Fig. 3. It is evident that there exists a negative-over-positive dipole anomaly pattern over the northwestern Atlantic (90°–30°W) that resembles a westward-displaced NAO+ pattern. On 8 December, a positive anomaly appears over the eastern North Atlantic and western Europe and a negative anomaly is seen on its downstream side over Europe. The positive anomaly intensifies and moves very slowly eastward. A very weak positive anomaly remains over the eastern North Atlantic along with a negative anomaly near Greenland from 8 to 12 December. As a result, as discussed above, the NAO index is weakly positive and sometimes even negative during the period from 8 to 12 December (Fig. 2). Moreover, we see that the downstream negative anomaly over Europe intensifies slightly and moves southwestward as the blocking anticyclonic anomaly intensifies. The combination of the intensified positive anomaly and southwestward-displaced downstream negative anomaly leads to the NE–SW tilting of the dipole anomaly. Although the large-scale circulation anomaly pattern over the North Atlantic basin has the form of a NAO+ pattern, the entire wave field, including lower latitudes, resembles a low-frequency wave train that propagates from the North Atlantic basin, across Europe, and into the Middle East, as in Feldstein and Dayan (2008). This wave train has an arching structure during the period from 10 to 12 December (Fig. 3). Thus, we conclude that it was this NE–SW-tilted EB dipole together with the upstream NAO+ pattern that provided the large-scale environment that produced the Middle East snowstorm in December 2013. In the next section, statistical and composite analyses are conducted to examine what types of NAO+-related EB circulations may lead to the extreme snowfall over the Middle East. In our composite analysis, the precipitation over an area with large cold SAT anomalies likely corresponds to snowfall according to the statistical analysis of Dai (2008), although we do not have snowfall data to show this.

Fig. 3.

Time sequences of daily 500-hPa geopotential height anomalies (gpm) from 8 to 15 Dec 2013. The contours are drawn from −300 to 300 gpm with 100-gpm interval.

Fig. 3.

Time sequences of daily 500-hPa geopotential height anomalies (gpm) from 8 to 15 Dec 2013. The contours are drawn from −300 to 300 gpm with 100-gpm interval.

4. A composite analysis of NAO+ and EB events

a. Classification of NAO+ events

As revealed by the case analysis presented in the above section, the Middle East snowstorm in December 2013 is related to the presence of the upstream NAO+ and EB events. Here, we consider an NAO+ event as being associated with an EB event if the largest amplitude of the EB event occurs within the lifetime of the NAO+ event. Such NAO+ events are referred to as EB-related NAO+ events, following Luo et al. (2007). Otherwise, the NAO+ events without EB are referred to as EB-unrelated NAO+ events. Using the TM index, we found a total number of 88 NAO+ events during 1950–2014, of which 42% (37 cases) are EB-related NAO+ events. To understand the impact of EB-related NAO+ events on the temperature in the Middle East, we computed the time-mean SAT anomaly field averaged from lag −5 to lag 5 days for all EB-related and EB-unrelated NAO+ events (with lag 0 being at the peak of the NAO+ event, based on the amplitude of the NAO index). Figure 4 shows that, on average, anomalously low SAT occurs over the Middle East, southeastern Europe, and northwestern Africa during the EB-related NAO+ events. However, during the EB-unrelated NAO+ events, low SAT values occur mainly over central North Africa, with a weak SAT decline seen over the Middle East. Figure 4 also shows that the positive SAT anomalies over northern Europe and the high-latitude North Atlantic are larger during the EB-related NAO+ events than during the EB-unrelated NAO+ events.

Fig. 4.

Time-mean surface air temperature anomalies from lag −5 to lag +5 days for (a) EB-related and (b) EB-unrelated NAO+ events, and (c) the (a),(b) difference. Only regions with surface air temperature anomalies above the 95% confidence level for a two-sided Student’s t test are drawn. Lag 0 day denotes the day that the NAO+ amplitude is largest and the red box denotes the Middle East region.

Fig. 4.

Time-mean surface air temperature anomalies from lag −5 to lag +5 days for (a) EB-related and (b) EB-unrelated NAO+ events, and (c) the (a),(b) difference. Only regions with surface air temperature anomalies above the 95% confidence level for a two-sided Student’s t test are drawn. Lag 0 day denotes the day that the NAO+ amplitude is largest and the red box denotes the Middle East region.

To understand whether EB events without the concurring NAO+ events can significantly affect the SAT over the Middle East, we examine the SAT difference between EB events with and without concurring NAO+ events. An EB event is defined to be associated with a NAO+ event if the largest amplitude of the NAO+ event occurs within the lifetime of the EB event. Such an EB event is referred to as a NAO+-related EB event. All other EB events are called NAO+-unrelated EB events. It is clear that the number of the NAO+-related EB events is not identical to that of the EB-related NAO+ events because the periods of the NAO+ and EB events are not the same. Using these definitions, we found 46 NAO+-related EB events and 190 NAO+-unrelated EB events during 1950–2014. Figure 5 shows the time-mean SAT anomalies for the NAO+-related and NAO+-unrelated EB events and their difference. It is clear that for the NAO+-related EB events the SAT field shows negative anomalies over a widespread region over central and southern Europe and parts of the Middle East (Fig. 5a). However, for NAO+-unrelated EB events, negative SAT anomalies appear only over a narrow region to the northwest of the Middle East (Fig. 5b). This indicates that the NAO+-related EB events are more favorable for a low SAT over the Middle East than NAO+-unrelated EB events. This is broadly consistent with the result for EB-related NAO+ events (Fig. 4a). Therefore, we will focus on the EB-related NAO+ events, even though the number of EB-related NAO+ events is slightly smaller than that of NAO+-related EB events. Nevertheless, the basic results are consistent between EB-related NAO+ and NAO+-related EB events, although there is a small difference between these two types of events (Figs. 4 and 5). Because our aim is to look for upstream conditions associated with EB, we place our attention on the EB-related NAO+ events rather than the NAO+-related EB events even though it is the EB events per se, and not the NAO+ events, that directly cause the extreme snowstorms in the Middle East.

Fig. 5.

Time-mean surface air temperature anomalies from lag −5 to lag +5 days for (a) NAO+-related and (b) NAO+-unrelated EB events, and (c) the (a),(b) difference. Only regions with surface air temperature anomalies above the 95% confidence level for a two-sided Student’s t test are drawn. Lag 0 day denotes the day that the EB amplitude is largest and the red box denotes the Middle East region.

Fig. 5.

Time-mean surface air temperature anomalies from lag −5 to lag +5 days for (a) NAO+-related and (b) NAO+-unrelated EB events, and (c) the (a),(b) difference. Only regions with surface air temperature anomalies above the 95% confidence level for a two-sided Student’s t test are drawn. Lag 0 day denotes the day that the EB amplitude is largest and the red box denotes the Middle East region.

On the other hand, although the EB-related NAO+ events are associated with a marked decline in the SAT over the Middle East, southeastern Europe, and northwestern Africa (Fig. 4a), what particular type of EB-related NAO+ events determine the region of the SAT decline is unclear. To distinguish the NAO+ event in December 2013 from the features of the composite EB-related NAO+ events during 1950–2014, in Fig. 6, we compare the time-mean 500-hPa zonal winds of the 1950–2014 EB-related NAO+ composite for the lag −2 to lag 2 day interval with those during December 2013 averaged over the Atlantic region (90°–30°W) and European region (0°–40°E). During the NAO+ period, the latitude of the jet core over the Atlantic can be used as an approximate measure of the position of the center of the NAO+ dipole anomaly, as denoted by the zero line between its positive and negative anomalies (Luo et al. 2008). Following this approach, it can be seen that the NAO+ dipole on December 2013 (the dashed line in Fig. 6a) is located far to the south of the mean position for EB-related NAO+ events (the solid line in Fig. 6a). This suggests that the southward displacement of the EB-related NAO+ event may have played an important role in the temperature decrease over the Middle East and southeastern Europe during the December 2013 snowstorm.

Fig. 6.

Time-mean 500-hPa zonal winds over (a) Atlantic region (90°–30°W) and (b) European region (0°–40°E) from lag −2 to lag +2 days for all EB-related NAO+ events (solid line) and from 10 to 14 December for an NAO+ event case (dashed line) in December 2013. In (a) [(b)], lag 0 day denotes the day of the strongest NAO+ (EB) amplitude for all EB-related NAO+ events (solid line).

Fig. 6.

Time-mean 500-hPa zonal winds over (a) Atlantic region (90°–30°W) and (b) European region (0°–40°E) from lag −2 to lag +2 days for all EB-related NAO+ events (solid line) and from 10 to 14 December for an NAO+ event case (dashed line) in December 2013. In (a) [(b)], lag 0 day denotes the day of the strongest NAO+ (EB) amplitude for all EB-related NAO+ events (solid line).

We further classify EB-related NAO+ events into northward- and southward-displaced NAO+ events, based on the latitudinal position of the Atlantic jet core. Furthermore, Fig. 6b shows that the zonal wind is weaker south of around 50°N over the European continent for the EB-related NAO+ event in December 2013 than the composite zonal wind of all the EB-related NAO+ events. From a simplified perspective, a block corresponds to a region of weak zonal wind (Rex 1950). If the blocking anomaly exhibits a dipole structure, then the position of the minimum zonal wind corresponds essentially to the position of the center of the blocking dipole anomaly in the north–south direction, as denoted by the zero line between its positive and negative anomalies (Luo and Chen 2006, their Fig. 5). Because the EB dipole anomaly exhibits clear horizontal tilting along the NE–SW direction (Fig. 3), we define the mean latitude of the zero line between the positive and negative anomalies of the EB dipole anomaly as the latitudinal position of the EB dipole anomaly. Thus, if the minimum zonal wind in the block is shifted to the south, then the mean position of the EB dipole anomaly is also displaced southward. As can be seen for the December 2013 event (Fig. 6b), the EB dipole anomaly is located at a relatively low latitude compared to that of the composite EB-related NAO+ events. Since the December 2013 event is associated with a southward displacement of both the NAO+ and the EB, and because a Middle East snowstorm is more likely to take place when the block is located farther southward, it is plausible that a southward shift of the NAO+ is crucial for the occurrence of Middle East snowstorms.

Here, we use the latitude of the maximum zonal wind at 500 hPa averaged over the region 90°–30°W and over the period from lag −2 to lag 2 days (mature stage) during the NAO+ life cycle to specify the meridional position of the Atlantic jet core for describing the position of a NAO+ event. This jet latitude index is referred to as the latitude position index of the NAO+ event. An NAO+ event is defined as northward (southward) displaced if the jet latitude index is greater (less) than zero. Based on this definition, of the 37 EB-related NAO+ events during 1950–2014, 17 are northward displaced and 20 are southward displaced.

b. Different impacts of southward- and northward-displaced EB-related NAO+ events

We computed composites of the daily anomalous 500-hPa geopotential height, SAT, and precipitation fields from lag −2 to +2 days for the northward- and southward-displaced EB-related NAO+ events (Fig. 7). It is evident that southward (northward)-displaced EB-related NAO+ events are associated with strong (weak) blocking events. For the northward-displaced EB-related NAO+ events, the negative temperature anomalies are weak and located over North Africa because the cutoff low is located over western Europe (Fig. 7a). During the decay of this block, a small region of negative SAT anomalies appears over the Middle East (Fig. 7a, at lag 1 and 2 days). However, for the southward-displaced EB-related NAO+ events, an intensified cutoff low emerges on the downstream side of the blocking region (solid line in Fig. 7b). This low is located over the Middle East and southeastern Europe. The southward winds on the upstream side of this cutoff low bring cold air into the Middle East, resulting in large temperature decreases over a wide region from southeastern Europe to the Middle East (blue colors in Fig. 7b). During the decaying stage of these events, negative SAT anomalies also emerge over northwestern Africa, while very cold air persists over the Middle East. These results suggest that the southward-displaced EB-related NAO+ events may be more important for the Middle East cold events than northward-displaced EB-related NAO+ events.

Fig. 7.

Evolution of composite daily 500-hPa geopotential height field (gpm) and surface air temperature anomaly (°C) from lag −2 to lag 2 days for (a) northward- (17 cases) and (b) southward-displaced (20 cases) EB-related NAO+ events. The color shading corresponds to the regions of surface air temperature anomaly above the 95% confidence level for a two-sided Student’s t test. The height contours are drawn from 5300 to 5700 gpm with 50-gpm intervals and precipitation area is indicated with green points. Lag 0 day represents the day that the EB is strongest.

Fig. 7.

Evolution of composite daily 500-hPa geopotential height field (gpm) and surface air temperature anomaly (°C) from lag −2 to lag 2 days for (a) northward- (17 cases) and (b) southward-displaced (20 cases) EB-related NAO+ events. The color shading corresponds to the regions of surface air temperature anomaly above the 95% confidence level for a two-sided Student’s t test. The height contours are drawn from 5300 to 5700 gpm with 50-gpm intervals and precipitation area is indicated with green points. Lag 0 day represents the day that the EB is strongest.

To further investigate why the block associated with southward-displaced NAO+ events can lead to a significant decrease in the SAT over the Middle East, we show the composite daily 500-hPa geopotential height anomalies in Fig. 8 for the northward- and southward-displaced EB-related NAO+ events. Clearly, the EB anomaly exhibits an asymmetric dipole structure, in which the blocking anticyclone is much stronger than the cyclonic circulation to its south. Although the EB dipole is almost stationary for the northward-displaced events, this dipole exhibits a distinct eastward shift for the southward-displaced events, probably because of the strengthening of the midlatitude zonal winds over Europe (not shown). An interesting point is that the EB dipole exhibits NE–SW tilting for the southward-displaced EB-related NAO+ events (Fig. 8b), while it shows little tilting for the northward-displaced EB-related NAO+ events and its negative anomaly is very weak, as can be seen for lag 0 (Fig. 8a). This suggests that the Middle East cold events are likely to depend on the spatial tilting of the EB dipole and its strength. However, as we will demonstrate below, the large SAT decrease over the Middle East and southeastern Europe results primarily from the spatial tilting of the EB dipole, rather than its strength.

Fig. 8.

Composite daily 500-hPa geopotential height anomaly fields (gpm) of EB-related NAO+ events for (a) northward- and (b) southward-displaced NAO+ events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) northward- and (d) southward-displaced NAO+ events, respectively. In (a),(b), shading denotes the region above the 95% confidence level for a two-sided Student’s t test. In (c),(d), lag 0 (vertical dashed line) denotes the day of the largest EB amplitude.

Fig. 8.

Composite daily 500-hPa geopotential height anomaly fields (gpm) of EB-related NAO+ events for (a) northward- and (b) southward-displaced NAO+ events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) northward- and (d) southward-displaced NAO+ events, respectively. In (a),(b), shading denotes the region above the 95% confidence level for a two-sided Student’s t test. In (c),(d), lag 0 (vertical dashed line) denotes the day of the largest EB amplitude.

The theoretical result of Luo et al. (2007) showed that EB often follows the decay of an NAO+ event. This hints that the EB events should lag NAO+ events. However, it is unclear whether the EB-related NAO+ events here obey such a lead–lag relationship. To examine this problem, we define the maximum of the negative center of the 500-hPa NAO+ dipole anomaly for each day as the daily NAO+ intensity index, and the maximum of the positive center of the 500-hPa EB dipole anomaly for each day as the daily EB intensity index. These indices are further normalized based upon their daily deviation from the climatological-mean value for that day. The composites of the daily NAO+ and EB intensity indices are shown in Figs. 8c and 8d for northward-displaced and southward-displaced EB-related NAO+ events. It is clear that the maximum strength of the NAO+ events leads that of the EB events by about 1 day for both cases. Therefore, the EB-related NAO+ events satisfy the lead–lag relationship noted above.

Although cold events over the Middle East are more likely to be related to the spatial tilting of the EB dipole, it is unclear whether the blocking shape in the planetary-scale field with synoptic-scale eddies excluded can significantly affect negative temperature anomalies over southern Europe. This motivates our further exploration in the next subsection.

c. Impact of EB patterns

In the planetary-scale field, blocking flows often show dipole- and Ω-type patterns. Thus, we classify blocking flows into dipole- and Ω-type patterns based on the shape of the blocking field at its peak time. Here, a blocking flow is defined to be a dipole-type block if the flow field at its peak (lag 0) has a trough with the large amplitude to the south of a blocking anticyclone (a clear splitting flow). Otherwise, it is defined as an Ω-type block.

The 37 EB-related NAO+ events are classified into 18 dipole-type and 19 Ω-type EB events. Figure 9 shows the composite daily 500-hPa geopotential height, SAT anomaly, and areas of precipitation from lag −2 to +2 days for the dipole- and Ω-type EB events. The dipole-type EB events exhibit a westward displacement of the dipole structure with a trough to the south of the blocking anticyclone (solid line in Fig. 9a). During the growth stage of the dipole-type EB events, the negative SAT anomaly is intensified and shifts toward North Africa (blue region in Fig. 9a), while a weak negative SAT anomaly appears near the Middle East only during the decay phase. For the Ω-type EB events, strong negative SAT anomalies occur mainly over the Middle East (blue region in Fig. 9b), even though the composite height field shows a relatively weak Ω-type block (solid line in Fig. 9b). It is the NE–SW-tilted trough over the Middle East that leads to southward cold advection and large SAT decreases in this region. In contrast, such an intense trough does not exist over the Middle East for the dipole-type EB events. This is the main reason of why large negative SAT anomalies do not occur over the Middle East during the dipole-type EB events.

Fig. 9.

As in Fig. 7, but for (a) dipole- (18 cases) and (b) omega-type (19 cases) EB events.

Fig. 9.

As in Fig. 7, but for (a) dipole- (18 cases) and (b) omega-type (19 cases) EB events.

The composite EB dipole 500-hPa geopotential height anomaly exhibits NW–SE (NE–SW) tilting and slow westward (eastward) displacement for the dipole-type (Ω-type) EB events. For these two cases, the negative anomaly to the south of the blocking anticyclone is relatively weak (dashed line in Figs. 10a,b). Although the blocking anticyclone is strong for the dipole-type EB events, their impact on the SAT over the Middle East is weak because the lack of tilt of the block suppresses the advection of cold air into this region (Fig. 10a). In contrast, the impact of Ω-type EB events on the Middle East temperature decline is large although the blocking anticyclone is relatively weak (Fig. 10b). We can also see that the EB event (Fig. 10d) lags the NAO+ event (Fig. 10c) by about 2 days for dipole-type EB events (Fig. 10c), but by approximately 1 day for the Ω-type EB events (Fig. 10d). This supports the abovementioned lead–lag relationship between NAO+ and EB events. Thus, the impact of the EB-related NAO+ events on Middle East SAT depends strongly upon whether the EB dipole anomaly exhibits NE–SW tilting.

Fig. 10.

As in Fig. 8, but for (a) dipole- and (b) omega-type EB events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) dipole- and (d) omega-type EB events, respectively.

Fig. 10.

As in Fig. 8, but for (a) dipole- and (b) omega-type EB events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) dipole- and (d) omega-type EB events, respectively.

d. Impact of the EB strength

While the above results indicate that the NE–SW-tilted EB dipole anomaly is important for the temperature decrease over the Middle East, it is not clear how the strength of the block affects the temperature in the Middle East. This issue is examined here. We define the intensity of an EB event by the largest value of the TM index at lag 0. A strong (weak) EB event is defined to have taken place if the intensity of the EB event is greater (less) than the mean strength of all EB events. As a result, the 37 EB-related NAO+ events are classified into 16 strong and 21 weak EB events.

Figure 11 shows the composite daily 500-hPa geopotential height, SAT anomalies, and areas of precipitation from lag −2 to +2 days for the strong and weak EB events. For the strong EB events, most of the negative SAT anomalies occur over southeastern Europe. The negative SAT anomalies increase in amplitude as the EB strengthens, extending to northwestern Africa during the decay phase of the strong EB events. As can be seen from its corresponding anomaly field (Fig. 12a), the EB dipole over Europe shows little tilting before it peaks (lag 0 day). During the decay phase of the EB dipole, the impact of the EB dipole on the SAT anomaly over southeastern Europe weakens (Fig. 11a from lag 1 to 3 days) because the EB dipole is weakened and shows northwest–southeast (NW–SE) tilting (Fig. 12a). In contrast to the strong EB events, large negative SAT anomalies for weak EB events are mostly concentrated in the Middle East (Fig. 11b). This is mainly because the EB dipole anomaly for this case exhibits NE–SW tilting (Fig. 12b at lag 0). Also, Figs. 12c and 12d show that the EB dipole lags the NAO+ dipole by about two (one) days for the strong (weak) EB events. Thus, strong EB events tend to have a longer delay time (relative to the NAO+ event) than weak EB events. While the strong EB events are associated with SAT decreases over southeastern Europe, the temperature decrease induced by the weak EB events is mainly concentrated over the Middle East resulting from the NE–SW tilting of the weak EB dipole. This further indicates the importance of the NE–SW-tilted EB dipole anomaly for the occurrence of persistent low temperatures over the Middle East.

Fig. 11.

As in Fig. 7, but for (a) strong and (b) weak EB events.

Fig. 11.

As in Fig. 7, but for (a) strong and (b) weak EB events.

Fig. 12.

As in Fig. 8, but for (a) strong and (b) weak EB events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) strong and (d) weak EB events, respectively.

Fig. 12.

As in Fig. 8, but for (a) strong and (b) weak EB events. The normalized intensity indices of composite NAO+ (solid) and EB (dashed) dipole anomalies for (c) strong and (d) weak EB events, respectively.

e. Optimal patterns of the EB dipole tilting for Middle East cold events

Figure 13 shows the idealized common patterns of the EB dipole anomaly. As shown from the above results, the EB dipole anomaly associated with NAO+ events that exhibits NE–SW tilting (Fig. 13c) is the optimal large-scale circulation pattern for the outbreak of Middle East cold events, followed by an EB dipole anomaly without tilting (Fig. 13b) and then an NW–SE-tilted EB dipole (Fig. 13a). A comparison among Figs. 8, 10, and 12 shows that if the EB dipole anomaly exhibits NE–SW tilting, then an arching-type low-frequency wave train becomes more distinct in the Euro-Atlantic sector (Figs. 8b, 10b, and 12b). Over Europe and the Middle East, this wave train appears to resemble the positive EA/WR teleconnection pattern discussed in section 3. However, a quadrupole low-frequency anomaly structure is dominant if the EB dipole shows no tilting or NW–SE tilting (Figs. 8a, 10a, and 12a). That is to say, the arching-type wave train is only associated with NE–SW tilting of the EB dipole anomaly. Thus, it appears that an arching-type low-frequency wave train contributes to the marked decline of the Middle East SAT. However, it is unclear what factors determine the strength and spatial tilting of the EB dipole. This question will be examined in another study.

Fig. 13.

Idealized sketch diagrams of the (a) NW–SE-tilted blocking dipole, (b) blocking dipole without tilting, and (c) NE–SW-tilted blocking dipole.

Fig. 13.

Idealized sketch diagrams of the (a) NW–SE-tilted blocking dipole, (b) blocking dipole without tilting, and (c) NE–SW-tilted blocking dipole.

f. Variation of Atlantic jet and latitudinal position of NAO+ events

As mentioned above, we define the latitude of the maximum zonal wind averaged over the region 90°–30°W and over the period from lag −2 to lag 2 days as the latitudinal position of an NAO+ event (lag 0 denotes the day that the NAO+ peaks). To examine the relationship between the latitudinal position of NAO+ events and the variation of Atlantic jet, in Fig. 14, we show the evolution of the meridional profile for the composite daily zonal wind averaged over the Atlantic region (90°–30°W) during the period from lag −5 to 4 days for southward- and northward-displaced EB-related NAO+ events, as defined above. Figure 14 shows that at lag −5 days, the meridional position of the Atlantic jet is almost identical for southward- and northward-displaced NAO+ events, which is particularly evident before lag −5 days (not shown). This can also be seen from the meridional profile of the zonal wind averaged from lag −15 to −5 days (Fig. 14b). For the northward-displaced NAO+ events, retrogression of the EB dipole anomaly toward the eastern North Atlantic is seen (Fig. 8a). This causes the northward shift of the Atlantic jet core (Fig. 14a from lag −4 to 0 days). For this reason, the center of action of the NAO+ dipole shows a northward displacement. In contrast for the southward-displaced EB-related NAO+ events (solid line in Fig. 14a), the EB dipole anomaly exhibits an eastward shift (Fig. 8b). Thus, the southward (northward) position of the Atlantic jet core associated with the NAO+ event is linked to whether the EB dipole anomaly shifts westward or eastward (Figs. 8a,b). Furthermore, because the southward (northward)-displaced Atlantic jet is strong (weak), the EB dipole associated with the NAO+ anomaly undergoes a slow eastward (westward) displacement. This point is consistent with the theoretical study of Luo and Cha (2012).

Fig. 14.

(a) Composites of daily 500-hPa zonal winds over the Atlantic region (90°–30°W) for northward-displaced (17 cases, dashed line) and southward-displaced (20 cases, solid line) EB-related NAO+ events from lag −5 to lag 4 days, where lag 0 day represents the day that the NAO+ amplitude is largest. (b) The composite 500-hPa zonal wind from lag −15 to −5 days, in which the solid (dashed) line represents southward (northward)-displaced EB-related NAO+ events.

Fig. 14.

(a) Composites of daily 500-hPa zonal winds over the Atlantic region (90°–30°W) for northward-displaced (17 cases, dashed line) and southward-displaced (20 cases, solid line) EB-related NAO+ events from lag −5 to lag 4 days, where lag 0 day represents the day that the NAO+ amplitude is largest. (b) The composite 500-hPa zonal wind from lag −15 to −5 days, in which the solid (dashed) line represents southward (northward)-displaced EB-related NAO+ events.

On the other hand, we see that the Atlantic jet prior to the NAO+ onset is stronger for southward-displaced EB-related NAO+ events (solid line in Fig. 14b) than for the northward-displaced events (dashed line in Fig. 14b). This suggests that the strength of the Atlantic background jet prior to the NAO onset may be important for determining the north–south shift of the NAO+ dipole as well as the strength and spatial tilting of the associated EB dipole anomaly. This problem will be investigated further in a separate study.

5. Why is the 2013 Middle East snowstorm an exceptional event?

To understand why the 2013 Middle East snowstorm is so unusual, it is interesting to examine the vertically integrated moisture fluxes both for this particular storm and for a composite of various types of NAO+ and EB events. Based on Krichak et al. (2014), we define the zonal and meridional components of the vertically integrated moisture flux (VIMF) between 1000 and 700 hPa as

 
formula
 
formula

where q is the specific humidity, g is the gravitational acceleration, p is the air pressure, and u and υ are the zonal and meridional wind components, respectively.

For six of the classifications of EB–NAO+ events presented above, we show composites of the vertically integrated moisture flux vector along with the anomalous temperature and precipitation on lag 0 day in Fig. 15. For northward-displaced EB-related NAO+ events (Fig. 15a), dipole-type EB events (Fig. 15b), and strong EB events (Fig. 15c), even though there is an eastward moisture flux from the eastern Mediterranean Sea toward the Middle East, it is difficult for a snowstorm to take place as these events do not accompany a strong temperature decline over the region (Figs. 7a, 9a, and 11a for lag 0 day). However, for southward-displaced EB-related NAO+ events (Fig. 15d), omega-type EB events (Fig. 15e), and weak EB events (Fig. 15f), there is a notable temperature decrease over the Middle East because of the cold advection associated with a trough over this region (Figs. 7b, 9b, and 11b for lag 0 day). In this case, snowstorms can more easily occur, as there is a strong flux of water vapor from the eastern Mediterranean Sea into the Middle East (Figs. 15d–f).

Fig. 15.

Composite vertically integrated moisture flux vectors at lag 0 for six classifications of EB events: (a) northward- and (b) southward-displaced EB-related NAO+ events, (c) dipole- and (d) omega-type EB events, and (e) strong and (f) weak EB events. In each panel, the green (blue) region denotes precipitation (negative temperature anomaly). The arrows indicate the magnitude of the flux vector scaled by 1000 kg m−1 s−1 in each panel.

Fig. 15.

Composite vertically integrated moisture flux vectors at lag 0 for six classifications of EB events: (a) northward- and (b) southward-displaced EB-related NAO+ events, (c) dipole- and (d) omega-type EB events, and (e) strong and (f) weak EB events. In each panel, the green (blue) region denotes precipitation (negative temperature anomaly). The arrows indicate the magnitude of the flux vector scaled by 1000 kg m−1 s−1 in each panel.

We also calculated the vertically integrated moisture fluxes associated with this event (Fig. 16). It appears for this event that the water vapor source comes mainly from the Mediterranean Sea. For example, beginning on 11 December 2013 and extending until 14 December 2013, a strong southeastward moisture flux from the eastern Mediterranean Sea toward the Middle East can be seen. In particular, the moisture flux convergence over the Middle East appears to reach its peak on 13 December, and weakens afterward. We also see that the negative SAT anomaly intensified during the period from 8 to 14 December and underwent widespread expansion reaching to the south of 15°N. This combination of a large region of very cold air, together with a strong moisture flux from the eastern Mediterranean Sea, both being driven by the low-latitude trough on the eastern side of the block, likely accounted for the extreme snowfall over the Middle East nearby lower-latitude locations such as Cairo, Egypt.

Fig. 16.

The daily vertically integrated moisture flux vectors for the blocking case during the period from 8 to 15 Dec 2013. In each panel, the green (blue) region denotes precipitation (negative temperature anomaly). The arrow indicates the magnitude of the flux vector scaled by 1000 kg m−1 s−1 in each panel.

Fig. 16.

The daily vertically integrated moisture flux vectors for the blocking case during the period from 8 to 15 Dec 2013. In each panel, the green (blue) region denotes precipitation (negative temperature anomaly). The arrow indicates the magnitude of the flux vector scaled by 1000 kg m−1 s−1 in each panel.

In summary, the marked southward shift of the trough over the Middle East downstream of the Ω-type EB event likely provided the large-scale conditions for the temperature decline for the 2013 Middle East snowstorm, and the moisture flux convergence over the Middle East coming from the eastern Mediterranean Sea provided the precipitation for this event.

6. Conclusions and discussion

In this paper, we have examined the atmospheric conditions that led to the outbreak of the Middle East snowstorm in December 2013 using NCEP–NCAR reanalysis data. It is found that an Ω-type European blocking (EB) with an intense downstream trough over the Middle East occurred during this snowstorm. This large-scale circulation transports cold air into the Middle East and produces snowfall in this region, with the moisture source coming from the eastern Mediterranean Sea. A crucial feature of this trough is its northeast–southwest (NE–SW) tilt, which is ideally position to advected cold air into the Middle East. The establishment of this EB and downstream trough are found to be related to the southward shift of the Atlantic jet core during an NAO+ event that concurred with the EB event in December 2013.

Using NCEP–NCAR reanalysis daily data, we found 37 EB-related NAO+ events during DJF from 1950 to 2014. These 37 EB-related NAO+ events were classified into three types based on the latitudinal position of the NAO+ events in terms of the Atlantic jet core position (type 1), the shape of the associated EB event (type 2), and its intensity (type 3) at the mature stage. For type 1, the NAO+ events were classified into southward- and northward-displaced events. For type 2 NAO+ events, the EB events were classified into dipole- and Ω-type EB events. For type 3, the EB events were divided into strong and weak EB events.

The composite results show that for the southward-displaced EB-related NAO+ events, the composite height field exhibits an Ω-type EB block. This blocking pattern is crucial for cold outbreaks over the Middle East because there is an intense downstream NE–SW-tilted trough over the Middle East. This circulation pattern transports cold air from high-latitude eastern Europe into the Middle East and often produces precipitation (or snowfall) in the trough over the Middle East. On the other hand, during the northward-displaced EB-related NAO+ events, weak cold temperature anomalies are seen over northwestern Africa and southwestern Europe (but not the Middle East) because the associated EB dipole anomaly shows NW–SE tilting.

Analyses of the type 2 events further confirmed the important role of the Ω-type EB events for cold air outbreaks over the Middle East, although the composite EB anomaly is stronger for the dipole-type EB events than for the Ω-type EB events. This is because there is a strong downstream trough over the Middle East and southeastern Europe during the Ω-type EB events, whereas such a trough does not exist during the dipole-type EB events. Furthermore, analyses of the type 3 events showed that strong EB events are more important for cold temperatures over southeastern Europe than over the Middle East. In contrast, during weak EB events cold temperature anomalies are seen mostly over the Middle East. In addition, the time lag of the EB behind an NAO+ event implies a potential for predicting Middle East snowstorms using the NAO+ event 1–2 days ahead.

According to our results, the idealized patterns of the EB anomaly tilting can be summarized in a schematic diagram (Fig. 13). For cold outbreaks over the Middle East, the NE–SW-tilted EB dipole anomaly (Fig. 13c) is the most optimal circulation pattern, followed by the EB dipole anomaly without tilting (Fig. 13b). The NW–SE-tilted EB dipole anomaly (Fig. 13a) is detrimental to cold events over the Middle East. In particular, when the EB dipole anomaly exhibits NE–SW tilting, an arching-type low-frequency wave train emerges over the Euro-Atlantic sector, which includes anomalies that resemble the positive east Atlantic/western Russia (EA/WR) teleconnection pattern over Europe and the Middle East (Kutiel and Paz 1998; Ziv et al. 2006; Feldstein and Dayan 2008). These results are consistent with the findings of Krichak et al. (2002), who showed that the largest precipitation anomalies in the eastern Mediterranean Sea region occur when both the NAO and EA/WR are in their positive phases. Different from previous studies, here we have emphasized the role of the horizontal tilting of the EB dipole anomaly in the Middle East temperature decline and the conditions that favor the horizontal tilting of the EB dipole anomaly. It is also shown from the present study that the NE–SW tilting of the EB dipole anomaly is an important factor that contributes to the positive EA/WR pattern associated with the NAO+ pattern.

In conclusion, the Ω-type European blocking with a NE–SW-tilted dipole anomaly associated with the southward-displaced NAO+ event is the most favorable atmospheric condition for Middle East winter snowstorms. This is because there usually exists an intense downstream trough over southeastern Europe and the Middle East during Ω-type EB events that transports cold air from high-latitude eastern Europe into the Middle East and produces snowfall in the trough over the Middle East with moisture transported from the eastern Mediterranean Sea. The southward-displaced NE–SW-tilted trough over the Middle East located downstream of an Ω-type EB appears to be the most important factor for the 2013 Middle East snowstorm, along with an eastward moisture flux from the eastern Mediterranean Sea.

Our results show that the Middle East snowstorm in December 2013 was associated with an intense Atlantic jet during the NAO+ episode [see the dashed line in Fig. 6a, which represents a time average of the 500-hPa zonal winds averaged over the Atlantic region (90°–30°W) from 10 to 14 December]. The composite results also show that the Atlantic jet prior to the onset of the southward-displaced NAO+ event, which is crucial for Middle East cold events, is relatively strong (Fig. 14b). These results imply that a strong Atlantic background jet may be an important precursor for cold weather outbreaks over the Middle East. This issue will be further examined in another study. Furthermore, what factors affect the tilting and strength of the EB dipole anomaly will also be examined.

Acknowledgments

Luo and Yao acknowledge the support from the National Science Foundation of China (Grants 41375067 and 41430533). We would also like to thank three anonymous reviewers whose comments improved this paper. Dai acknowledges the funding support from the U.S. National Science Foundation (Grant AGS-135374) and the U.S. Department of Energy’s Office of Science (Award DE-SC0012602). Feldstein would like to offer his gratitude for support through National Science Foundation Grant AGS-1401220.

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Footnotes

1

Here, a dipole block is defined to exhibit no tilting if the core line between its positive and negative anomalies is along the west–east direction. However, as in Benedict et al. (2004), the blocking dipole is defined to have a NE–SW or northwest–southeast (NW–SE) tilting if the core line is oriented along the NE–SW or NW–SE direction, respectively. Clearly, the Ω-type block seen during the period from 8 to 15 December 2013 corresponds to an anticyclonic wave breaking with NE––SW tilt.