Wave-breaking characteristics of midlatitude blocking

Transcription

1 Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 138: , July 2012 A Wave-breaking characteristics of midlatitude blocking G. Masato, a *B.J.Hoskins b and T. J. Woollings a a Department of Meteorology, University of Reading, UK b Grantham Institute, Imperial College, London, UK *Correspondence to: G. Masato, Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK. g.masato@rdg.ac.uk In this article, Northern Hemisphere winter midlatitude blocking is analysed through its wave-breaking characteristics. Rossby wave breaking is identified as a key process in blocking occurrence, as it provides the mechanism for the meridional reversal pattern typical of blocking. Two indices are designed to detect the major properties of wave breaking, i.e. the orientation (cyclonic/anticyclonic direction of breaking or DB index) and the relative contribution of air masses (warm/cold relative intensity or RI index). The use of the DB index differentiates between the anticyclonic cases over Europe and Asia and the cyclonic events over the oceanic basins. One of the three regions displaying cyclonic type was found over the Atlantic Ocean, the other two being over the Pacific Ocean. The first of these is located over the western side of the Pacific and is dominated by warm air extrusions, whereas the second is placed northward of the exit region of the jet stream, where the meridional θ gradient is much weaker. Two European blocking types have been detected using the RI index, which separates out the cases dominated by warm and cold air masses. The latter cases in particular exhibited a well-structured dipole, with associated strong anomalies in both temperature and precipitation. Copyright c 2011 Royal Meteorological Society Key Words: Rossby waves; blocking types Received 20 June 2011; Revised 8 November 2011; Accepted 10 November 2011; Published online in Wiley Online Library 12 December 2011 Citation: Masato G, Hoskins BJ, Woollings TJ Wave-breaking characteristics of midlatitude blocking. Q. J. R. Meteorol. Soc. 138: DOI: /qj Introduction Since the beginning of modern meteorology, atmospheric blocking has been an object of intense study (Berggren et al., 1949). It has often been described as the counterpart of the strong westerly flow regime. Many theories have been based on the dualism of the two weather regimes (e.g. Charney and DeVore, 1979), one described by an intense jet stream and a strong meridional temperature (and pressure) gradient, the other characterised by a much weaker westerly flow and a strong meridional wind component. The characteristic structure of blocking, particularly evident during the mature phase of its life cycle, exhibits a strong anticyclone located on the poleward side and often a cyclone on the equatorward side. Such a spatial distribution is associated with a large anomaly in wind direction, with the westerly flow strongly reduced and usually replaced by easterlies. It has also been noted since the early studies that blocking dramatically affects the surface weather, both in terms of temperature and precipitation (e.g. Rex, 1950a,b). Its occurrence is quite often associated with strong heatwaves and dry spells over the summer season and conversely, with cold outbreaks during the winter period (e.g. Trigo et al., 2004; Sillmann and Croci-Maspoli, 2009; Bühler et al., 2011). Over the years, much effort has been made to describe the principal characteristics and understand the mechanisms of the formation and development of blocking. The different approaches can be classified as in Barriopedro et al. (2010) into two main types, the absolute field and the departure field. Lejenäs and Økland (1983) and Dole and Gordon (1983), can be considered respectively the progenitors of the two observational approaches. While the latter Copyright c 2011 Royal Meteorological Society

2 1286 G. Masato et al. identified blocking as anticyclonic anomalies in the middle troposphere (in the 500 hpa geopotential height), the former constructed an index, as a function of longitude, that searched for the reversal of the meridional geopotential height gradient at 500 hpa, according to the formula I(λ) = Z 40N (λ) Z 60N (λ), (1) with λ representing longitude values and Z the geopotential at 500 hpa. Pelly and Hoskins (2003) (PH03 hereafter) followed the idea of Lejenäs and Økland, but they introduced an important novelty with the usage of the potential vorticity (PV) field (Hoskins et al., 1985). It is assumed that the extratropical synoptic development can often be considered with a good approximation to behave as if it is frictionless and adiabatic. In this case PV is a very good candidate to study the synoptic development of blocking as it is materially conserved in time, providing an excellent tracer for the air masses contributing to blocking formation, and can be inverted to give the balanced component of the flow. The PV approach to study atmospheric blocking was used in its quasi-geostrophic form in the theory presented in Shutts (1983), where the mechanism of eddy straining is considered to be crucial for the onset and maintenance of blocking, an idea developed in Hoskins et al. (1983) with the introduction of the E vector. The full PV approach was eventually developed in Hoskins et al. (1985). Hoskins (1997) and later PH03 described the mechanism in terms of the extrusion of low-pv air masses along potential temperature (θ) surfaces from the Tropics towards northern latitudes, or equivalently as high-θ air masses along PV surfaces near the tropopause. The anticyclonic overturning of such an air mass and the subsequent cut-off from its original region leads to the transport of high-pv air equatorward in the downstream region and the additional transport of low-pv air in the upstream region, resulting in a reinforcement of the block. In PH03, a Local Instantaneous Blocking (LIB) event is identified, similar to Lejenäs and Økland, when for a single day and for a single point in longitude the meridional gradient of θ on a particular PV surface (PV=2 units or PV2; the dynamical tropopause) is reversed. The reversal of the meridional gradient is therefore strictly related to the overturning mechanism just described. Such dynamics are associated with Rossby wave-breaking, which is easily identified in the PV approach (e.g. PH03). The other novelty introduced in PH03 was to look for reversals around a central blocking latitude, taken to be a reference latitude for the climatological storm track. This compares with the constant latitude of 50 N in Eq. (1), which is not representative of the jet stream everywhere. In the last decade, the PV approach has been used in various ways to study blocking behaviour (e.g. Croci- Maspoli et al., 2007; Gabriel and Peters, 2008). In particular, Tyrlis and Hoskins (2008b) (TH08 hereafter) provided an extensive analysis of the Northern Hemisphere blocking activity, using the ERA-40 reanalysis coverage ( to ) and applying the blocking index (B) introduced in PH03. They composited blocking in several regions of the Northern Hemisphere and, by analysing their onset and the θ anomalies along the dynamical tropopause prior to the blocking onset, they investigated the possible reasons for blocking occurrence. However, apart from the index calculated first in PH03, no other specific tools associated with the wave-breaking characteristics were derived. In Figure 1, four cases that exhibit different wave-breaking characteristics on the dynamical tropopause are displayed. Cases (a) and (c) both exhibit a cold air extrusion upstream of the warm air extrusion, whereas (b) and (d) both show the opposite, with the cold air extrusion downstream of the warm extrusion. It is also clear the cases differ in the relative amplitude of the associated air mass extrusions. In cases (a) and (b) the subtropical warm θ air extrusion is stronger, whereas in cases (c) and (d) the polar low-θ air extrusion dominates. In this study, two new indices are introduced to describe these wave-breaking properties. The aim is to provide more information using the PV approach, in order to better discriminate the different evolution of the Rossby wave-breaking responsible for blocking occurrence. At the same time, this will also serve as the basis for an objective classification of blocking according to its main dynamical characteristics. After introducing the dataset and methodology in section 2, the overall results will be illustrated in section 3. Sections 4 and 5 will investigate more deeply the blocking types derived from the analysis of section 3. In particular, in section 4 the blocking events classified according to the direction of breaking (DB) index will be analysed. The results will broadly confirm and quantify those in TH08, in which cyclonic blocking was mainly observed over the oceanic basins, while anticyclonic blocking was found to occur in the downstream regions, and in particular over the Euro-Asiatic continent. Section 5 will be dedicated to the second index, the relative intensity (RI) index, that shows which of the two air mass extrusions dominates in blocking formation. While the orientation of the overturning (here identified with DB) has often been discussed in blocking studies, the second characteristic is rarely found in the literature, although some authors have examined it, mainly in terms of wave-breaking dynamics in general (e.g. Gabriel and Peters, 2008). This is probably because it discriminates between systems dominated by the poleward blocking high and those dominated by the equatorward cut-off low, while blocking has usually been associated only with the former. It is therefore interesting that the two different blocking types observed over Europe (described in section 5) are only found once the RI index is taken into account. Section 6 will conclude the article, providing a summary of the PV approach and underlining the most important results obtained with its application. 2. Data and methodology 2.1. Data and indices definitions The data used in this study are from the ERA-40 reanalysis (Uppala et al., 2005), from December 1957 to February Only events for the winter period, December February (DJF), are examined. The dataset used here has the full ERA-40 resolution of (Gaussian grid; N80). The θ field on the 2 PVU potential vorticity surface (1 PVU= 10 6 Km 2 kg 1 s 1 ) has been analysed. As in TH08, the daily values of the indices have been derived by averaging the 6-hourly data into daily means. The same approach has been followed for the other fields used, i.e. geopotential at 500 hpa (Z500), mean sea level pressure (MSLP), 2 m temperature (2mT) and precipitation (precip). An algorithm is employed based on the set of equations at longitudinal

3 Wave-Breaking Characteristics of Midlatitude Blocking 1287 Figure 1. Four wave-breaking examples taken from the archive generated by using the methodology described in section 2. The plotted field is θ on PV2. The date in the title indicates the day (out of the 90 days of DJF) and the year (e.g means the winter season ). The box with the black contours is centred around the Central Blocking Latitude and it comprises the longitude and latitude points used to calculate DB and RI indices. grid point i: θ n i = 2 φ0 + φ/2 θ i dφ, θ s i φ = 2 φ0 θ i dφ, (2) φ 0 φ φ 0 φ/2 B i = θ n i θ s i, (3) where φ is latitude. The B index is therefore the difference between the mean θ on the 2 PVU surface to north and south (Figure 2 in PH03, where θ n and θ s are shown). Because the interference with the eastward movement of weather systems is considered to be an essential aspect of blocking, at each longitude, following PH03 and TH08, the reversal has been sought in the neighbourhood of a Central Blocking Latitude (CBL, φ 0 in Eq. (2)) determined as the location of the maximum in latitude of the 300 hpa synoptic time-scale transient eddy kinetic energy. The calculation of the integral is then repeated (at each point in longitude) for the CBL value, and for ( points) to the north and to the south of the CBL; B at this longitude is defined to be the maximum of the three values. A filter corresponding to a running mean of 15 in longitude has been applied to θ n i and θ s i. This smoothing was not used in either PH03 or TH08 and has been added to reduce the influence of small-scale features on the DB and RI indices. The resulting B field is calculated for 80 points in longitude (every 4.5 ), which makes it very similar to that calculated in TH08 (where the B field is every 5, for a total of 72 points along longitude). Two new indices are introduced here: DB (direction of breaking), which detects the wave-breaking orientation, and RI (relative intensity), which shows the relative strength Figure 2. Schematic for (left) warm-anticyclonic and (right) cold-cyclonic wave-breaking, with the associated DB and RI indices. The green line represents θ, which is the average of θ n and θ s, calculated from Eqs (2). θ represents the climatology, calculated for the winter season (DJF) over the 44 years of reanalysis. associated with the air mass extrusions that will form a blocking event. The classification into cyclonic anticyclonic overturning has been already taken into account in some work (e.g. Schwierz et al., 2004; Gabriel and Peters, 2008), nevertheless in this article a new algorithm will be proposed in order to nest it consistently within the PV framework of PH03. Instead of representing blocking solely by its high geopotential anomalies (e.g. Dole and Gordon, 1983),

4 1288 G. Masato et al. Figure 3. Schematic for the DB RI plot. DB is on the x-axis, while RI is on the y-axis. Positive values of the first index are for anticyclonic wavebreaking and positive values for the second one indicate warm air extrusion dominance. As sketched, a unique wave-breaking type is identified for each quadrant of this plot. implying that the anticyclone must be the leading feature of such a phenomenon, in this study we opt for the more classical definition of blocking (e.g. Berggren et al., 1949), that recognises the possibility of a dipole (anticyclone to the north and cyclone to the south). Using the reversal of the meridional gradient to identify wave-breaking (and consequently blocking), and furthermore introducing the RI metric, should allow equal importance to both the air extrusions and their associated (warm and cold) cut-off features. Figure 2 illustrates how the two indices are derived to give such information for a wave-breaking development. A warm-anticyclonic and cold-cyclonic case are illustrated respectively on the left- and right-hand sides of the figure. For the first case the longitudinal temperature gradient is negative (the transition is from high to low values of θ). If the difference of θ along longitude is taken between points i 1andi + 1, as illustrated in Figure 2, and we define where DB = θ i 1 θ i+1, (4) θ i = θ n i + θ s i, (5) 2 then the anticyclonic orientation is associated with a positive DB index. Similarly, for cyclonic wave-breaking (right-hand side of Figure 2), the longitudinal gradient is positive and the associated DB index is negative. For the relative intensity index, RI, the potential temperature averaged over the northern and southern sectors combined is compared to its climatological value. We define RI = θ i θ i, (6) where θi is the 44-year climatological value for DJF for a given grid point i. When the index is positive the warm air extrusion dominates (left-hand side of Figure 2) and θ i is greater than θi, and when it is negative the cold air dominates (right-hand side of Figure 2). In this paper the three indices (B, DB and RI) are all calculated only for a single point in latitude at each of the 80 points in longitude. This point has been chosen from the CBL, selecting this latitude or the one to north or south at which B is a maximum. The new indices have been designed to provide essential properties associated with the wave-breaking phenomenon. They can be displayed as in Figure 3, with the DB and RI indices as abscissa and ordinate respectively to give a DB RI phase-space plot. On the right-hand side of the plot are the anticyclonic events, with those dominated by warm air extrusions at the top and those characterised by cold θ extrusions below. On the left-hand side of the plot are the corresponding cyclonic wave-breakings. Idealised wavebreakings as depicted in Figure 3 may be compared with the individual case-study events in Figure 1, which have been arranged in the same manner Blocking events and their classification In PH03 and TH08 two more definitions were included to refine the B index. This is primarily a wave-breaking index, and it provides the mechanism through which blocking is formed. However, blocking events are more complex structures that require further definitions in both space and time domains. The aim is to detect an event that is clearly different from a simple synoptic ridge. The two definitions introduced in PH03 have been used here; the Sector Blocking (SB), which accounts for the spatial constraint and blocking stationarity and the Sector Blocking Episode (SBE), which accounts for time persistence (PH03 provide more details). In particular,in Masatoet al. (2009) the time constraint has been tested with the aid of a simple statistical Markov model. In that study, following from TH08, it was indeed observed that the Markov model underestimated the frequency of SBE, namely those events that persist longer than 5 d. Therefore, it has been decided to take the same threshold for the SBE characterisation and both definitions in space and time to derive full blocking events. The result is a time series of 3960 SB values (44 winter seasons times 90 days per season) for all the 80 points in longitude. The index is labelled as 1 when it meets the criteria of the SB definition and 0 when it does not. Each nonzero point is then marked as cyclonic or anticyclonic and as warm- or cold-air dominated by retaining the corresponding DB and RI values. A subsequent step has dealt with the classification of the whole episode, considering that a SB event evolves in time. Therefore, the definition for the SBE has been applied (following TH08) for each SB event. If, for a given point in longitude, SB exhibits at least five consecutive non-zero values in time, then a SBE is identified. The separation of the archived SBE events into three different classes, depending on RI values, is rather straightforward. The mean of the RI values for the days the event lasts is taken and indicates whether an event is overall warm, cold or unclassified. It is not always obvious whether a blocking episode is clearly dominated by warm or cold extrusions. If the associated RI value is close enough to zero, then the event is defined as unclassified. This close enough statement has to be defined. It is here recalled that the indices were already normalised by their standard deviation (the reference mean and standard deviation have been based on those winter days when B was greater than zero) before applying the SB and SBE definitions. A threshold whose value has been set to 0.2 has been taken. Those events characterised by a RI index falling between 0.2 and +0.2 were therefore labelled as unclassified. Although such a threshold is rather arbitrary, it is noted that varying it by a few decimals gives only slight changes in the final frequency distributions.

5 Wave-Breaking Characteristics of Midlatitude Blocking 1289 A similar approach can be used when considering the DB index. However, as it is associated with the wave-breaking orientation, DB has to be either positive or negative, at least for the onset stage of blocking. As also shown in Figure 2, the wave-breaking that generates a blocking episode must occur with a definite signature, either anticyclonically or cyclonically. The event is then labelled as anticyclonic or cyclonic based only on the DB value associated with the onset of blocking. Although the DB signature can quickly change its sign in a few days, it has been noted that once the first wave (the one responsible for blocking onset) breaks with a given signature, then the following wave-breakings very likely overturn with the same orientation, leading the whole blocking event to maintain the same signature. There are a few exceptions to this, mostly over Asia (50 90 E). In these cases an anticyclonic cut-off dominates at the mature phase of blocking, with weak or non-existent anomalies to the south. Lacking a strong dipole structure, these events often decay relatively fast by advection downstream (not shown). This behaviour could be explained by the low level of Rossby wave activity in this region as well as by the structure of the blocking (Altenhoff et al., 2008). Therefore, the classification of blocking with the DB index could be better represented by relying on the onset day only and choosing the same threshold of 0.2 to identify the unclassified events. In the next section, the SBE are calculated and the events subsequently classified according to the DBand RI indices, providing evidence for the different Northern Hemisphere blocking types. 3. General results The results of the DB and RI classification are shown in Figure 4 as a function of longitude. The frequencies are plotted for the classification of both the onset day only (Figure 4(a, c)) and the events in their entirety (Figure 4(b,d)). Let us focus first on DB at onset (Figure 4(a)). The anticyclonic signature exhibits the highest peak in frequency, covering the entire Eurasian continent (10 W 110 E). Cyclonic blocking is generally dominant over the oceanic basins (75 30 W and 140 E 140 W). This is in general accordance with TH08 (their Figure 3), and with the hypothesis that the horizontal shear of the jet, which during the winter season is substantially tilted along the southwest northeast direction, strongly influences the morphologic structure of blocking. Over the downstream regions of the oceans (with the highest peak over Europe), where u/ y is positive on the equatorward flank of the jet, the main signature is anticyclonic, whereas upstream (both over the central and west oceanic portions), where u/ y is negative on the poleward flank of the jet, the dominant signature becomes cyclonic. It is worthwhile to highlight the local maximum for the cyclonic class over Asia, although it is lower in frequency than its anticyclonic counterpart, and the double peak over the Pacific Ocean. It appears that there are two different sources of blocking therein, one at E, the other at W. The former is associated with the entrance region of the jet stream, while the latter seems to be linked to its exit region. Generally, it can be seen that the unclassified frequency is almost null and not longitudinally dependent, confirming that the onset of blocking is indeed either cyclonic or anticyclonic. It is also evident that the features of the SBE classification into the three categories do not change significantly when the entire events (Figure 4(b)) are taken into consideration. This supports the hypothesis that blocking maintains its wave-breaking signature once the onset has occurred. A general decrease in frequency for the leading categories (anticyclonic over Eurasia and cyclonic over the oceans) can be observed when compared to the onset of blocking, but they all remain dominant. The cyclonic events over Asia (60 E) are the only cases that undergo a significant drop, with the associated frequency more than halved. Asia is the region (together with Europe) where the unclassified category is significantly higher if compared with its onset counterpart. This signal is associated with less persistent events which show a lack of subsequent wave-breaking and a neutral DB signature overall. The frequency of the three categories for the RI behaviour is shown for the onset only (Figure 4(c)) and for the whole blocking events (Figure 4(d)). Generally, it can be noticed that the frequency of warm blocking is on average higher than that of the cold events. The only exception is Eurasia, where the blocking onset is dominated by cold air extrusions. However, when the whole event is considered, both warm and unclassified blocking have their frequency increased. Therefore, it appears that some Eurasian cold events might exhibit a transition between their onset and mature phases, with the initial cold air extrusion being on average progressively balanced or even replaced by warm air masses. Focusing now on the classification of the events considered in their entirety (Figure 4(b, d)), over the western Pacific Ocean blocking events are only characterised by warm air extrusions, while this is not the case for the other sectors. Euro-Atlantic and Asian events are subject to both warm and cold air dominance, although the first type is higher in frequency than the latter. The two main maxima for warm blocking can be associated with two of the three DB maxima, the first over Europe and Asia (anticyclonic blocking), and the second over the Pacific Ocean (cyclonic blocking). It is interesting to observe that two RI peaks, although lower in frequency, are present for the cold and unclassified blocking categories as well. The first, over Europe and Asia, is in phase with the warm RI peak, whereas the second, over the Pacific Ocean, is clearly shifted downstream. In summary, the West Pacific Ocean is dominated by warm, cyclonic blocking events. The Central Pacific and the Atlantic Ocean exhibit a cyclonic signature with warm events occurring twice as often as cold ones, even though over East Pacific (140 W) the three classes (warm, cold and unclassified) show equal absolute frequencies (whole events, Figure 4(b, d)). Over Europe and Asia, blocking is mainly anticyclonic, although a cyclonic contribution cannot be neglected, especially over Asia. Both warm and cold RI values are apparent over Europe, so that one could expect the presence of two different blocking types in that region. On a final note, it has been decided to use the composite technique in sections 4 and 5 relying on the onset day classification for both indices (Figure 4(a, c)). It has already been said that this is the better approach to follow for the DB index (section 2.2). Further, it is observed that the onset day classification for the RI index (Figure 4(c)) allows effective separation between cold and warm blocking events over Europe.

6 1290 G. Masato et al. (a) # events Events Onset Longitude Cylonic Anticyclonic Unclassified (b) # events Whole events Longitude Cylonic Anticyclonic Unclassified (c) # events Events Onset Longitude Cold Warm Unclassified (d) # events Whole events Longitude Cold Warm Unclassified Figure 4. Frequency of SBE as a function of longitude. In (a, b), the classes are derived by considering the DB signature only, and in (c, d) by considering the RI signature only (see text for further explanation). Blocking classification relies on the indices (a, c) of the onset day only, and (b, d) of the entire events, considering the mean of all days involved in each blocking event. 4. The DB index 4.1. Eurasian events The general behaviour described in section 3 can now be analysed in more detail. To do this, a composite analysis has been performed, based on the classification of the onset day only, and in this section focusing on the DB index only. This will serve to underline the main characteristics of the blocking types as identified in section 3. Figure 5 shows the result for the European region, where 20 E has been chosen as a representative longitude for the area. The temporal evolution of the anticyclonic breaking is nicely represented. The clockwise rotation of the two air masses illustrated by the anomalies of θ on PV2 which occurs from day 2 to day +2 is in good accordance with the anticyclonic example given in Figure 2. The evolution of the associated full field (green contours) shows a pronounced overturning, with the maximum curvature along the 310 K and 320 K isentropes. In Figure 5(e h), the anomalies of four different fields are shaded. Z500, MSLP and precipitation are shown at the onset of blocking (day 0), while 2mT is composited 2 d after the onset. This is because the temperature advection at the surface produces its major effects a few days after blocking is established (i.e. after the onset of wavebreaking). The low θ anomalies over Southern Europe, clearly visible along the dynamical tropopause, are barely seen in the mid-troposphere and completely disappear at the ground, where the blocking dipole as a result is dramatically biased towards positive pressure anomalies. The temperature is lower than average over the whole of Central and Eastern Europe. The distribution of such anomalies is consistent with the pressure pattern already described and with the direction of the anticyclonic wavebreaking, which generates strong northeasterly winds in the area. This is one of the most prominent features identified by Rex (1950a). Low temperatures can also be observed over Northeastern Canada, probably associated with the low pressure system just northwest of the region. Positive anomalies in precipitation are not expected, as low pressure values at the surface are not identified (MSLP in Figure 5(f)). Dryer conditions are instead observed for the UK and for Western Europe in general, in good accordance with the ridge that covers the area. Very similar results in terms of precipitation were found in Sillmann and Croci-Maspoli (2009) (their Figure 3c compared with Figure 5(h) here), where a general analysis on Euro-Atlantic blocking and its impact on European extremes was conducted using a PVbased blocking indicator. These results are also in agreement with those of Trigo et al. (2004) (cf. their Figures 4c, 5c and 6c with Figure 5(g, h) here). The temporal evolution of the anomalies of θ on PV2 for the blocking events over Asia (taken at 90 E; Figure 6) is more noisy, mainly because fewer events are available for the composites. In general terms it can be noted that this blocking type is less intense than its European counterpart. The θ anomalies are visible only at the actual onset of the event, and the overturning of the full field (represented by the green contours) is less enhanced. Interestingly, the maximum curvature is observed for the 300 K and 310 K isentropes. This behaviour can explain the limited meridional extension of the cold air mass, which is generally confined north of N. As for the European events, the northern lobe of the blocking dipole seems

7 Wave-Breaking Characteristics of Midlatitude Blocking 1291 Figure 5. (a) (d) Composites of the full field (contours) and anomalies (shading) of θ on PV2 for blocking events at 20 E (93 events) classified as anticyclonic (dashed line in Figure 4(a)). The composites start 2 d before the onset and finish 4 d after, with the onset day marked as day 0. The isentropes for the full field start from 330 K (southern contour) and are drawn at 10 K intervals. The thin solid black line represents the CBL. (e) (h) Composites of anomalies (shading) of different fields for the events: Z500, MSLP, 2mT and precipitation, in m, hpa, C and mm, respectively. The composites are all shown for the onset day apart from the 2mT anomalies, which are shown 2 d after the onset. The contours represent the associated θ on PV2 full field. Figure 6. As Figure 5, but for blocking events at 90 E (51 events). to dominate, both for the upper and mid-tropospheric levels. Interestingly, the 2 m temperature does not seem to exhibit such behaviour; the cold region is detected to the southeast of the blocking ridge, where the northeasterly anomalies are the most intense. A very warm area is also present on the northern flank of the high pressure system at the ground. The temperature panel in particular illustrates to what extent a blocking occurrence can modify the weather for a large region such as Eastern Asia, where the temperature anomalies reach ±4 C. In contrast to Europe, the precipitation field does not show any significant anomaly. The result is not surprising, as the storm track is not present over this region, therefore it is difficult to expect large departures from the climatology Oceanic events The maxima detected in Figure 4 indicated three different regions of cyclonic blocking, two over the Pacific Ocean

8 1292 G. Masato et al. and one over the Atlantic. Figure 7 shows the composite analysis performed for the events at 150 E. The resulting θ anomalies are rather noisy in this case also, due to the limited sample size. This, together with the smoothing effect resulting from the compositing technique, obscures the warm air dominance that characterises this blocking type. Nevertheless, the cyclonic breaking is clearly visible for the temporal evolution of both the anomalies and the full field of θ on PV2. In particular, the overturning process is completed at day 4, where two isolated contours at 310 K and 300 K (full field) are observed respectively over the high and low θ anomalies. The composites of the other fields show generally large departures from the climatology. Of interest is the westward vertical tilt of the southern lobe. It is associated with an intense trough, where the meridional gradient of θ on PV2 (green contours) is very strong, i.e. near 35 Nwhere the storm track location is usually identified. The occurrence of this blocking type to the north of the jet stream illustrates very well the crucial contribution played by the horizontal shear of the jet itself, which characterises the cyclonic nature typical of such blocking events. High temperature anomalies are detected downstream and between the lobes of the blocking dipole, exactly opposite to the behaviour of the anticyclonic events. This is due to the cyclonic wavebreaking that typically builds along the southeast northwest direction. The result is an extrusion of warm and moist air from the Subtropics, which is associated with the positive anomalies in precipitation observed in Figure 7(h). Also of interest are the very low temperature anomalies observed over Alaska, which are probably linked to the extended ridge that dominates the subpolar regions of the North Pacific. An increase in precipitation is detected for a longitudinal band, along 35 N, possibly associated with the southward shift of the storm track, while a decrease is observed for a vast area over Central Pacific, just southward of the ridge system. The other cyclonic events over the Pacific Ocean are composited at 160 W (Figure 8), where the second maximum in frequency is identified (cf. Figure 4 for reference). The cyclonic breaking is once again clearly visible. The overturning mainly involves the 310 K and 320 K isentropes and, unlike its upstream counterpart (and despite roughly the same number of composited events), it does not produce any clear θ contour cut-off along the dynamical tropopause (cf. Figure 7(d)). Generally, the tilt of this wave-breaking is less zonally oriented and more meridionally stretched. These blocking events occur where the meridional gradient of θ on PV2 is quite weak, and the Rossby wave that will subsequently break exhibits in turn a larger meridional displacement. This contrasts with the wave-breaking at 150 E which showed a smaller meridional displacement, due to the intense meridional θ gradient, and was damped much more quickly. The ridge is centred over Alaska, while the trough is located over the Central Pacific, roughly at the exit region of the jet. The temperature dipole is zonally oriented, with warm and cold anomalies respectively on the western and eastern side of the high pressure system. Much of North America exhibits very low temperatures (more than 10 C below climatology), apart from a region southwest of the Rockies, which is sheltered from the northerly winds. At the same time, the decrease in precipitation over the western coast of North America is consistent with such a configuration in which the typical westerlies coming from the ocean are replaced by drier, northerly winds. Conversely, the enhanced precipitation over the Eastern Pacific (30 45 N) could be directly linked to the southward shift of the storm track due to blocking. The cyclonic events composited over the Atlantic Ocean (at 30 W) are displayed in Figure 9 and in many ways resemble those at 160 W. They develop in a region of weak meridional θ gradient, and the associated Rossby wavebreaking mainly extends along the meridional direction. The cyclonic overturning is observed at day 2 and day 0 (the onset of blocking), both in the anomalies and the full field of θ on PV2. However, at day +2 and day +4 the DB signature for the full θ on PV2 field is partially lost, and the evolution of blocking bears some resemblance to the well-known structure, in particular following the 310 K isentrope. This shape could be a result of the compositing technique, especially given the tendency for anticyclonic European blocks to shift upstream to become cyclonic Atlantic blocks (Nakamura and Wallace, 1993; Woollings and Hoskins, 2008). The DB algorithm classifies blocking as either cyclonic or anticyclonic using the local gradient only, so by construction the B and DB indices cannot identify (or inverted ) structures. An alternative approach has been tested, which identifies blocking regimes as depicted in a Hovmöller plot, similar to Barnes et al. (2011). In this framework (not shown), it is possible to identify blocking events by comparing the DB values on the western and eastern sides of a block. With this approach, blocking events are observed, in particular over the Eastern Atlantic and Western Europe, although their characteristics (such as persistence or intensity) do not seem significantly different from those of a purely anticyclonic or cyclonic event. A tripole in the temperature anomalies is detected for the Atlantic Ocean composite, with low high low features respectively over North America, Greenland and Northern Europe (and Siberia). The temperature contrast is greatest between Greenland and Scandinavia, where the strongest meridional winds occur. Perhaps unsurprisingly, the temperature pattern is similar to that of the North Atlantic Oscillation (Croci-Maspoli et al., 2007; Woollings et al., 2008). The pattern for the precipitation field is rather complex. A dipole of low high precipitation, extending from 30 to 45 N, is observed respectively upstream and downstream of the trough that forms the southern lobe of blocking. It is also interesting to note the intense, but rather confined, region of increased precipitation that is displayed along the southeastern side of Greenland, probably exposed to the strong southeasterly winds. Negative anomalies are instead detected between the UK and Iceland, where the ridge is centred. 5. The RI index It has been acknowledged that blocking has been widely studied taking into account its cyclonic and anticyclonic nature (e.g. Martius et al., 2007; Gabriel and Peters, 2008; Kunz et al., 2009), however very little has yet been said on the magnitude of the air masses involved and their importance in classifying different blocking types, as clearly stressed in the early studies (e.g. Rex, 1950b). Gabriel and Peters (2008), following Thorncroft et al. (1993) and Peters and Waugh (1996), differentiated Rossby wave-breakings into four different types, known as P1/2 (poleward cyclonic/anticyclonic) and LC1/2 (equatorward anticyclonic/cyclonic) breakings. However, the metric derived in our paper is not focused on

9 Wave-Breaking Characteristics of Midlatitude Blocking 1293 Figure 7. As Figure 5, but for blocking events at 150 E (40 events) classified as cyclonic (solid line in Figure 4(a)). Figure 8. As Figure 5, but for blocking events at 160 W (46 events) classified as cyclonic (solid line in Figure 4(a)). the poleward equatorward evolution of a Rossby wavebreaking, but rather on the relative influence of the warm and cold air mass extrusions involved in blocking formation and evolution. As mentioned in section 3, Europe seems to experience two major blocking types, driven respectively by warm and cold air extrusions. The composite analysis provided in this section will then focus on Europe using the classification of Figure 4(c), considering once again the onset of the events only. The temporal evolution of the θ on PV2 anomalies for the warm events (Figure 10) clearly shows the prevalence of warm air masses. Cold anomalies are visible only at day +4, once blocking is already well established, and even then are not comparable with their warm counterpart. The mid and lower troposphere mirror this feature, with a strong ridge observed for both the Z500 and MSLP fields, whereas no trough is detected as the southern lobe of the blocking system. A cyclonic anomaly occurs over Greenland. Although this does not extend to upper levels, it has considerable amplitude at low levels and combined with the downstream ridge leads to strong southerly winds. These in turn impact on the southern side of Greenland, causing the increase of precipitation over the area. The main temperature anomalies seem to occur on the northwestern

10 1294 G. Masato et al. Figure 9. As Figure 5, but for blocking events at 30 W (47 events) classified as cyclonic (solid line in Figure 4(a)). and southeastern flanks of the high pressure system centred on southern Scandinavia. The magnitude of the warm anomalies is larger, but confined in a rather limited area (Northern Scandinavia and Barents Sea). Conversely, low temperatures are observed over a large region between Eastern Europe and Western Russia, even though their magnitude is lower than 2 C. The temporal evolution of θ on PV2 for the cold events (Figure 11) tells a very different story. The cold and warm anomalies that contribute to blocking formation are already visible 2 days before the onset. Their clockwise rotation as a function of time (typical anticyclonic behaviour; Figure 2), is more clearly observed, despite the similar number of events used. In contrast to the warm events, the overturning observed along the 310 K and 320 K isentropes is quite large and elongated meridionally. Therefore, from all these factors, it can be inferred that the Rossby wave associated with cold European blocking is more meridionally elongated, it exhibits a larger amplitude and hence the subsequent breaking effectively produces a much better structured blocking dipole. This is verified with the midand lower-tropospheric maps. Both Z500 and MSLP show the typical blocking dipole, with the ridge over the Eastern Atlantic and the trough centred over Central Europe. Consistent effects are also observed for the 2mT and the precipitation fields. Cold air anomalies are spread over the whole European area, with values peaking between 4 and 5 C. Despite the noisy precipitation field, a dipole of high and low anomalies can still be noted. A dryer region is observed at the centre of the high pressure system, whereas the only European area with increased precipitation corresponds to the surface cut-off low, centred between the Black Sea and Central Europe. It has been demonstrated that the two blocking regimes over Europe are indeed associated with very different synoptic situations. This shows that the application of the RI index can be valuable for the identification of diverse blocking types. It is interesting to compare with recent work by Rennert and Wallace (2009). In their study, they filtered the Z500 field into three different frequency bands, L, M and H, respectively for 1 5, 6 30 and 30 d. Interestingly, the empirical orthogonal function (EOF) analysis they applied to the intermediate band M (Figure 2 of Rennert and Wallace, 2009) produced very similar patterns to those found here for the European Atlantic region. In particular, their first and second EOFs resemble respectively the warmand cold-anticyclonic composites in this study (e.g. Z500 or MSLP anomalies in Figures 10 and 11), even though for the latter the main dipole differs somewhat in the location of the centre of action. Here the main dipole is established between the Northeast Atlantic ridge and the Central Europe trough. Such intermediate frequencies might be expected to include blocking time-scales, so the similarity of the findings between the two studies in a blocking region is perhaps not surprising. Rennert and Wallace found that the negative polarity of the North Atlantic Oscillation with above-normal heights over Greenland was indeed associated with enhanced M variability over Greenland. 6. Concluding remarks This study has introduced a comprehensive methodology for the analysis of blocking properties. This work is based on the PV approach introduced by PH03, which has been tested and developed over the years (Tyrlis and Hoskins, 2008a,b). The introduction of two new indices which describe the main wave-breaking characteristics can provide a novel classification of blocking, which sheds more light on the complexity of the phenomenon. Accordingly, blocking has been classified according to its wave-breaking orientation (DB) and to the intensity of the air mass extrusions (RI). Five main blocking groups have been identified, two over Europe and Asia and three over the oceanic basins. The former are driven by anticyclonic breaking, the latter instead by cyclonic breaking, confirming previous literature (e.g. Martius et al., 2007; Gabriel and Peters, 2008, TH08).

11 Wave-Breaking Characteristics of Midlatitude Blocking 1295 Figure 10. As Figure 5, but for blocking events at 20 E (52 events) classified as warm (dashed line in Figure 4(c)). Figure 11. As Figure 5, but for blocking events at 20 E (66 events) classified as cold (solid line in Figure 4(c)). The Asian cases were weaker than their counterpart over Europe, yet they still exhibited a noticeable temperature anomaly at the ground with peaks of ±4 C. The east-based Pacific events were associated with the existence of another peak in frequency for cyclonic events, northward of the exit region of the jet stream, where the meridional θ gradient is much weaker. In contrast to the Atlantic, the jet in the Pacific has a relatively weak southwest northeast tilt, leading to a dominance of cyclonic wave-breakings on its northern flank (negative horizontal shear). While the temperature anomalies at the ground are the most distinctive impact of anticyclonic blocking, the main characteristic of the cyclonic events is the change in precipitation. This could be because the cyclonic cases occur mainly over the ocean surface, which can act as a source of moisture and convection. The anticyclonic blocking is instead mainly present over land, where, for instance, the cold air advection occurring downstream of the blocking dipole can considerably reduce surface temperatures. The other main finding is the distinction between two different blocking types over Europe, using the RI index which discriminates between events dominated by warm and cold air extrusions. Unlike any other region of the winter Northern Hemisphere, Europe exhibits rather high numbers

12 1296 G. Masato et al. of both blocking types. A composite analysis has revealed the existence of a warm-anticyclonic blocking type, whose prominent feature is a strong ridge centred over Southern Scandinavia, and cold-anticyclonic events that exhibit a well-structured dipole, whose northern lobe extends from the Eastern Atlantic to Iceland and whose southern one is centred between the Black Sea and Central Europe. In particular for the second type, strong temperature and precipitation anomalies are detected. Finally, the two blocking types found over Europe and the use of the RI index raise the possibility for a systematic search of other blocking regimes over the entire Northern Hemisphere. This has been done by extending the described approach (B, DB and RI indices) into a two-dimensional form (latitude versus longitude), focusing on the linkage between blocking and modes of low-frequency variability; the results will be presented in a future paper. 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