A New Perspective on Blocking

Transcription

1 743 A New Perspective on Blocking J. L. PELLY AND B. J. HOSKINS Department of Meteorology, University of Reading, United Kingdom (Manuscript received 6 February 2002, in final form 27 August 2002) ABSTRACT It is argued that the essential aspect of atmospheric blocking may be seen in the wave breaking of potential temperature ( ) on a potential vorticity (PV) surface, which may be identified with the tropopause, and the consequent reversal of the usual meridional temperature gradient of. A new dynamical blocking index is constructed using a meridional difference on a PV surface. Unlike in previous studies, the central blocking latitude about which this difference is constructed is allowed to vary with longitude. At each longitude it is determined by the latitude at which the climatological high-pass transient eddy kinetic energy is a maximum. Based on the blocking index, at each longitude local instantaneous blocking, large-scale blocking, and blocking episodes are defined. For longitudinal sectors, sector blocking and sector blocking episodes are also defined. The 5-yr annual climatologies of the three longitudinally defined blocking event frequencies and the seasonal climatologies of blocking episode frequency are shown. The climatologies all pick out the eastern North Atlantic Europe and eastern North Pacific western North America regions. There is evidence that Pacific blocking shifts into the western central Pacific in the summer. Sector blocking episodes of 4 days or more are shown to exhibit different persistence characteristics to shorter events, showing that blocking is not just the long timescale tail end of a distribution. The PV index results for the annual average location of Pacific blocking agree with synoptic studies but disagree with modern quantitative height field based studies. It is considered that the index used here is to be preferred anyway because of its dynamical basis. However, the longitudinal discrepancy is found to be associated with the use in the height field index studies of a central blocking latitude that is independent of longitude. In particular, the use in the North Pacific of a latitude that is suitable for the eastern North Atlantic leads to spurious categorization of blocking there. Furthermore, the PV index is better able to detect blocking than conventional height field indices. 1. Introduction Since the late 1940s it has been recognized that blocking is one of the most important aspects of the weather in middle latitudes: the usual mobile weather systems of the middle latitudes are diverted toward polar latitudes and the ambient westerly winds are replaced by easterlies. Namias (1947) discussed its role in one particular winter. Berggren et al. (1949) gave an elegant and suggestive pictorial depiction of the characteristic development of the thermal structure of the atmosphere during a blocking episode. Elliott and Smith (1949) gave examples in the North Atlantic and the North Pacific. In 1950 Rex gave his celebrated definition of blocking and produced the first climatology (Rex 1950a,b). An essential ingredient of a block is the large amplitude equivalent barotropic anticyclone on the poleward side of the anomalous easterlies. In the North Atlantic, there is usually a dipole structure with a cyclone on the equatorial side. In the North Pacific the anticy- Corresponding author address: Prof. B. J. Hoskins, Department of Meteorology, University of Reading, Earley Gate, P.O. Box 243, Reading RG6 6BB, United Kingdom. b.j.hoskins@rdg.ac.uk clone often looks more like an amplified ridge with the upstream and downstream troughs digging into its base, leading to the pictorial description block. The early studies and most in the subsequent half century focused on the blocking of the westerly jet and its associated weather systems. However, White and Clarke (1975), using monthly mean data, and Dole (1978), Charney et al. (1981), and Dole and Gordon (1983), using daily data, concentrated on persistent high pressure anomalies that are mostly associated with blocks. In order to produce a climatology of blocking from digital analyses, Lejenäs and Økland (1983) returned to the previous ideas and quantified the blocking criteria of Rex (1950a) using the well established concept of zonal index, a measure of the average westerly flow based on 500-hPa height. For a longitude to be blocked they demanded that a latitudinal average of the zonal wind be easterly; that is, Z /2 Z /2 0 (1) 0 0 at that longitude. They also demanded that the value of this index averaged with those 10 to the west and east should be negative. Lejenäs and Økland (1983) used a constant central latitude 0 c 50 N and a latitudinal width American Meteorological Society

2 744 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 For their study in which they quantified for the first time the ability of a forecast model to handle blocking, Tibaldi and Molteni (1990) used the basic criterion of Lejenäs and Økland (1983) given in (1). However in order to ensure that there were average westerlies poleward of the block they added the criterion Z /2 Z 3 /2 A. (2) 0 0 With A 10 m this ensures an average westerly flow north of the block of at least some 8 m s 1. They also allow for some variation of the blocking latitude by taking 0 c (3) where c 50 N is the central blocking latitude and 4. D Andrea et al. (1998) in their comparison of atmospheric general circulation model (AGCM) simulations of blocking later reduced the value of A in (2) to 5 m, thereby requiring an average westerly on the poleward flank of greater than some 4 m s 1. For convenience on a 3.75 grid they also slightly modified c,, and. There is general agreement among the various studies that blocking is a maximum in the eastern North Atlantic European sector. However, there is an interesting disagreement about the North Pacific. All the qualitative, synoptic studies and the quantitative persistent anticyclonic anomaly studies put the maximum in the similar jet exit region, that is, in the eastern North Pacific west coast of North America region. However, the quantitative studies based on (1) and other criteria tend to give a maximum west of the date line. Lejenäs and Økland (1983) recognized this discrepancy but ascribed it to a previous lack of good data. In his theoretical discussion of blocking, Rossby (1951) used the analogue of the hydraulic jump from the narrow strong westerly jet upstream to the broad weak flow downstream. However, following Green (1977), in later dynamical studies there has been a move toward a potential vorticity (PV) framework in which the blocking anticyclone is associated with a region of anomalously low PV values and the cyclone, if present, with anomalously high PV values. Hoskins et al. (1985) showed how a PV potential temperature ( ) perspective gives a complete description of balanced midlatitude weather phenomena, such as blocking. As discussed there and in Shutts (1983, 1986), Hoskins and Sardeshmukh (1987), Vautard and Legras (1988), Hoskins (1997), and elsewhere, the essence of the formation of a block is that a substantial mass of subtropical air, with its low PV on a surface, is advected poleward ahead of a large amplitude slow moving cyclone. As a PV anomaly, this air develops its own anticyclonic circulation and cuts off from its region of origin. It then influences the upstream weather systems to elongate meridionally and deposit more low PV air on the poleward side and high PV air on the equatorward side, thereby acting to reinforce the block. One aim of this paper, discussed in section 2, is to produce a dynamical index of blocking based on these PV ideas. Also, the major assumption that the central blocking latitude can be taken to be independent of longitude is questioned. Reliable PV diagnostics are not yet available over an extended period. However, in section 3, we present a 5-yr climatology of Northern Hemisphere blocking using the new index. In section 4 we compare the results using our index to those with a height index as discussed above. In particular, we resolve the disagreement in the longitude of North Pacific blocking. Finally, a discussion of the results and conclusions of this study are given in section A new PV blocking index Following Hoskins and Berrisford (1988), the simplest summary of the dynamical behavior in the upper troposphere is to define a dynamical tropopause at PV 2 potential vorticity units (PVU) and consider the advection of on it. In the absence of diabatic processes, on a PV surface would be conserved. In the idealization of the troposphere stratosphere used in Hoskins and Bretherton (1972), Thorpe (1985), and more recently in Ambaum and Verkley (1995) and Swanson (2000, 2001), in which the tropopause marks a discontinuity between two regions with uniform but different PV, this information along with surface and total mass information could be inverted to give the balanced flow. A cutoff warm, high (though low temperature) anomaly region on PV 2 then corresponds to an anticyclone, which is strongest in the upper troposphere and extends to the surface to a degree dependent on the horizontal dimensions of the anomaly. As an example, Fig. 1 shows the 250-hPa height and PV 2 fields for a day during a Euro-Atlantic blocking event. The geopotential field indicates the blocking anticyclone centered near the United Kingdom with easterly winds near 50 N over western Europe. As often in this region, there is an accompanying cyclonic region on the equatorward side of the easterlies, forming a meridionally oriented dipole. The on PV 2 field (Fig. 1b) generally shows a potentially cold tropopause in polar regions and a warm one in the subtropics. Each of the synoptic features in the height field is shown rather more clearly in the on PV 2 field and the Lagrangian behavior is more apparent. The mature stage of a wave-breaking event is apparent in the European region. The warm air in the anticyclonic anomaly is about to cut off. Once it has done so, the associated anticyclone can only be removed by diabatic/frictional processes or the warm anomaly moving back into the subtropics. The cyclonic region on the equatorward flank is associated with a similar process occurring in the cold air. A second dipole block is also seen in both fields over western Canada. Following the general dynamical discussion in the introduction and this particular example, it is natural to

3 745 FIG. 1. Analyses of (a) 250-hPa geopotential height (dam) and (b) (K) on PV 2 for 1200 UTC 21 Sep associate blocking with a reversal of the normal negative meridional gradient in on PV 2, and this forms the basis of the dynamical blocking index defined and used here. a. Blocking index and local instantaneous blocking Figure 2 shows a schematic representation of the relevant parameters used here for defining a blocking index B at a given longitude 0. The thick line is a representative on PV 2 contour during a blocking episode, which is centered at 0 in this case. The blocking index B at longitude 0 is defined as the difference in the average potential temperatures in the northern and southern boxes: FIG. 2. A schematic representation of the relevant parameters for calculating the PV blocking index B at a given longitude 0. The thick line is a representative on PV 2 contour during a blocking event centered at 0 in this case. 0 / B d d. (4) 0 0 /2 By this definition, B 0 in the westerly flow over Asia in Fig. 1, but B 0 in the blocking regions over Europe and western Canada. The longitude 0 could be said to be blocked if B 0, indicating that there is high potential temperature to the north and low potential temperature to the south. As was noted above, Lejenäs and Økland (1983), Tibaldi and Molteni (1990), and D Andrea et al. (1998) all used the same value, independent of longitude, for the central blocking latitude c. However, if we return to a consideration of the fundamental nature of blocking, a different choice is suggested. Since blocking is associated with the interruption of the midlatitude westerly jet and the blocking of mobile midlatitude weather systems, we could use either aspect to suggest the specification of c. Using the westerly jet has problems associated with the subtropical jet. At some longitudes, for example in the European North African sector, on average there are two jets. At other longitudes, for example in the western North Pacific, the weather systems are typically on the polar flank of the subtropical jet. Consequently, it is more convenient to use an average measure of weather system activity to define c. In order to represent the latitude longitude distribution of midlatitude weather systems, Fig. 3 shows the climatological annual mean high-pass transient eddy kinetic energy (EKE) at 300 hpa. The Northern Hemisphere maxima in synoptic activity associated with the Atlantic and Pacific storm tracks are evident at around N. These latitudes are indeed representative of

4 746 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 FIG. 3. Annual mean high-pass transient EKE (m 2 s 2 ) at 300 hpa taken from the ERA-15 dataset ( ECMWF Reanalysis). Spectral filtering has been applied at truncation T21. where we might expect to see blocking. Also, the problem with the double jet structure over Europe and Asia is not seen in this field; instead, it picks out only the poleward branch of the jet, which is most appropriate for considering midlatitude blocking. Therefore, this field is used here as a basis for the definition of the variation in c with longitude. We may draw a smooth line through the EKE maxima in the Northern Hemisphere in order to define a value for c at each longitude. The results of this process are shown by the black line on the graph in Fig. 4. The annual mean central blocking latitude can be seen to vary by about 15 around the hemisphere, with Euro- Atlantic blocking occurring at a higher latitude than its Pacific counterpart. Also shown in Fig. 4 are colored lines indicating the seasonal variations in the central blocking latitude. The largest seasonal deviation from the annual mean central blocking latitude occurs in the summer season, shown in red. There is a large northward migration of the synoptic activity over Asia at this time of year, and a sharp transition as it moves back to lower latitudes at about 130 E. This transition is associated with the two jets in the European North African sector rejoining at the beginning of the Pacific storm track. However, we will later show that the blocking episode frequency drops to zero in this region at this time of year, so the effect of the sharp transition in latitude is minimal. Except for this summer anomaly, the seasonal blocking latitudes tend to be within about 4 of latitude of the annual mean. Furthermore, the largest seasonal deviations occur in regions that are not well known for blocking, that is, E and near 260 E. There will also be interannual variability in the central blocking latitude, due to the North Atlantic Oscillation (NAO) and the El Niño Southern Oscillation (ENSO), for example. But we do not expect this variability to be larger than the seasonal variations seen in Fig. 4. Therefore, we take 0 c ( ), (5) where c ( ) is the annual mean central blocking latitude FIG. 4. The black line shows the central blocking latitude around the Northern Hemisphere calculated from the annual mean high-pass transient EKE (m 2 s 2 ) at 300 hpa taken from the ERA-15 dataset ( ECMWF Reanalysis). Colored lines show the seasonal variations in the central blocking latitude: Jun Aug (JJA; red), Sep Nov (SON; blue), Dec Feb (DJF; green), Mar May (MAM; yellow). in Fig. 4 and 4, as in Tibaldi and Molteni (1990). So, for example, at 0, B is calculated for , 56.5, and 60.5 N, and then the actual blocking index is taken as the maximum of these three values. Local instantaneous blocking is said to occur if this blocking index is positive. Hereafter, we will refer to the maximum of the three blocking index values as the actual blocking index, B. In Fig. 2 is a typical latitudinal scale for blocking. From the example shown in Fig. 1, 30 of latitude seems a reasonable value to choose for. Furthermore, given that blocking episodes interrupt the passage of the mobile midlatitude weather systems, another relevant length scale for comparison is the latitudinal width of the synoptic activity in the storm tracks, as shown in Fig. 3. Again, 30 of latitude appears reasonable. Consequently, is taken here to be 30 of latitude, though the exact value of has been found not to be crucial. The zonal width of the averaging boxes in Fig. 2 does not represent a longitudinal scale for blocking (such a scale will be considered later in section 2b). Instead, is intended to be large enough for a meaningful average of the 2 PVU potential temperature to be taken, while being small enough not to obscure the synoptic-scale details of the flow (i.e., L R ). Thus, is here taken as 5 of longitude, although again this is not a particularly sensitive parameter, and its value could be altered depending on the intended application. The blocking index is then calculated instantaneously at 5 longitude intervals around the Northern Hemisphere. This method allows the blocking index to be calculated instantaneously at any longitude. In this way, it is possible to produce graphs of blocking index against longitude for a given time. As an example, Fig. 5 shows such a graph for 1200 UTC 21 September 1998, corresponding to the analyses in Fig. 1. The regions of B

5 747 However, the blocking index defined in this section will also pick up features in individual synoptic systems, perhaps associated with PV filaments such as those seen in Fig. 1b. Consequently, in the next two sections the local instantaneous blocking index described here will be extended to account for the longitudinal extent and persistent nature of atmospheric blocking. FIG. 5. A graph of blocking index (in K) against longitude for 1200 UTC 21 Sep This graph is coincident with the analyses of 250- hpa geopotential height and on PV 2 shown in Fig in the Euro-Atlantic sector and over Canada correspond to instantaneously blocked regions in Fig. 1, with high potential temperature to the north and low potential temperature to the south. Elsewhere around the Northern Hemisphere B 0 and there is no blocking. It would, of course, be possible to define a seasonally varying c to use with our blocking index, but, in order to allow easy interpretation of any results, the index is designed to be as simple as possible while still capturing the large-scale synoptic features that we are interested in. Here, we choose to concentrate on Northern Hemisphere blocking, but this analysis technique could be equally easily applied to blocking in the Southern Hemisphere, in general circulation models (GCMs), or even blocking on other planets. One advantage of using this technique for analyzing blocking in GCMs would be that the central blocking latitudes can be determined from the model climatology, so the index will identify blocks even in a model with incorrectly positioned storm tracks. A study of Southern Hemisphere blocking should use the PV 2 surface to represent the dynamical tropopause. The stationary waves are not as pronounced in the Southern Hemisphere due to the reduced topographic variation, so the blocks tend to be less persistent in general and the blocking latitude only varies between about 50 and 55 S, as shown by the peak in high-pass transient EKE in Fig. 3. The seasonal variation is also reduced (not shown), so using the annual mean value of the central blocking latitude would be an even better approximation for Southern Hemisphere blocking. Indeed, a constant latitude may be sufficient. In order to examine blocking on other planets, a tropopause surface would have to be identified, but the basic analysis technique would not change. The term blocking is usually used when the region of the anomalous anticyclone or that of the easterly winds has only small longitudinal movement and has a longitudinal extent and a persistence in time that are greater than normally associated with synoptic systems. b. Large-scale blocking Local instantaneous blocking described in the previous section accounts for the large-scale nature of blocking in the latitudinal direction (using 30 ), but takes no account of any longitudinal extent. Largescale blocking is therefore defined to take account of this aspect of atmospheric blocking. Large-scale blocking is said to occur at a particular longitude if B is positive for at least 15 of longitude including this point. Previous studies have used values anywhere between 12 (Tibaldi and Molteni 1990) and 45 (Rex 1950a) to define a longitudinal scale for blocking L block. But, at a typical blocking latitude of 50 N, 15 of longitude corresponds to about 1100 km, so that L block L R. Thus, the Rossby radius of deformation provides a lower bound on the longitudinal scale for blocking. From Fig. 5, we see that there are two Northern Hemisphere regions where at least 15 of longitude exhibit B 0 on 21 September 1998, so large-scale blocking is occurring in conjunction with both the Euro-Atlantic and Canadian dipole blocks at this time. c. Blocking episodes As mentioned in section 1, atmospheric blocking is a quasi-stationary pattern which may persist for days or even weeks in some cases. The concept of large-scale blocking may therefore be extended to define blocking episodes that take into account this temporal persistence of blocking. Here, we use a timescale of 4 days to define a blocking episode, following the work of Tibaldi and Molteni (1990). Rex (1950a) used a much longer timescale of 10 days (together with his much larger spatial scale), but, as we will see in section 3, a 4- or 5-day timescale is more appropriate. Thus, a blocking episode is said to occur at a particular longitude when large-scale blocking occurs within 10 of longitude of that point for at least 4 consecutive days. This definition enables the blocking features identified from day to day to move around longitudinally to some extent, but the 10 limitation ensures that blocking episode features identified on consecutive days are almost always associated with the same blocking event. d. Sector blocking From previous studies and as will be seen below, blocking tends to occur more frequently in particular

6 748 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 longitudinal sectors. To discuss such blocking, it is very convenient to define the occurrence of sector blocking and sector blocking episodes in a manner similar to the pointwise definitions of large-scale blocking and blocking episodes. A sector is considered to be a region spanning 65 of longitude. Sector blocking is then defined to occur if B is positive for at least 15 of longitude within the sector. A sector blocking episode is said to occur if sector blocking occurs for at least 4 consecutive days. Having defined the concept of a sector blocking episode, the onset date of a particular episode is defined as the first day of the episode, and, similarly, the decay date is defined as the first day of nonblocking after the episode. e. Features identified by the index From Fig. 5, the large-scale blocking index certainly captures the Euro-Atlantic dipole block shown in Fig. 1. In fact, the persistence of this feature is such that it is part of a Euro-Atlantic sector blocking episode. Fig. 6 shows four further snapshot examples of recent dipole blocks and blocks, which have been identified as sector blocking episodes by the index. Shown are the analyses of 250-hPa geopotential height, on PV 2, and blocking index versus longitude within a relevant sector. The first two examples are dipole blocks in the Euro- Atlantic sector (20 W to45 E). These two blocks occur at different times of year and are situated at quite different blocking latitudes. This is despite the fact that the PV blocking index uses the annual mean blocking latitude (shown in Fig. 4). The blocking index graphs show that the blocking signature often extends over much more than 15 of longitude, certainly 30 ( 2000 km) is not uncommon. However, on individual days, the longitudinal extent can drop back down to around 15 of longitude just prior to reinforcement of the block by synoptic eddies, supporting the use of this value. The other two snapshot sector blocking episode examples in Fig. 6 show blocks in the Pacific. The block during August 2000 occurs in the central Pacific which, as will be seen in section 3, is where most Pacific blocking occurs in summer. The other block occurs further east, near the end of the Pacific storm track, which is a prime location for blocking episodes. 3. A 5-yr climatology of blocking a. Data used Here we present a 5-yr climatology of Northern Hemisphere blocking from June 1996 to May This climatology has been constructed using the objective blocking index described in the previous section in conjunction with 5 yr of daily 1200 UTC European Centre for Medium-Range Weather Forecasts (ECMWF) analyses of potential temperature on the PV 2 surface. Analyses of potential temperature on PV 2 were not routinely archived at ECMWF until 18 July 2000, so the analyses prior to this date were obtained using a program written by E. Klinker at ECMWF. This program calculates PV from archived temperature and wind fields on 15 isobaric surfaces between 10 and 1000 hpa. The potential temperatures for PV 2 are then found by linear interpolation in the vertical. Only minor differences are seen between the two types of on PV 2 analyses. b. Northern Hemisphere blocking frequency Figure 7 shows annual mean blocking frequencies against longitude between June 1996 and May The solid curve shows the local instantaneous blocking frequency, the dashed curve the large-scale blocking frequency, and the dotted curve the blocking episode frequency. All three curves pick out two distinct blocking regions in the Northern Hemisphere, which may be identified with the geographical blocking preferences noted by Rex (1950b). The first region extends from the eastern North Atlantic, across Europe and well into Asia, with most blocking occurring in the European region. The second, smaller region of Northern Hemisphere blocking activity is located over the eastern North Pacific and the west coast of North America. These blocking regions are ones where the upper-level stationary waves already exhibit weak ridges in the annual mean and are located downstream of the climatological maxima in synoptic activity associated with the Atlantic and Pacific storm tracks, so they might well be expected to be preferential for blocking. However, as discussed in the introduction, the agreement with the North Pacific blocking location given by the synoptic studies means disagreement with the modern quantitative height index studies. This point will be returned to in section 4. Notably, there is also a considerable amount of blocking over Asia; but both the jet and the synoptic activity are relatively weak in this area, so these Asian blocking episodes tend to be much weaker than their Euro-Atlantic and Pacific counterparts. By definition, the frequency of large-scale blocking must be less than or equal to that of local instantaneous blocking. The difference between the two frequencies is a measure of the frequency of small-scale reversals in the gradient of on PV 2 gradient probably associated with small synoptic systems or PV filaments/ streamers (Appenzeller and Davies 1992). In contrast to the larger scales, the frequency of these is seen to be fairly constant around the Northern Hemisphere at a value of about 4% in the annual mean. The definition of blocking episodes demands temporal persistence but gives more freedom in longitudinal position than that for large-scale blocking. The frequencies of the two events are generally similar. The overall similarity between the three blocking frequencies is because, except

7 749 FIG. 6. Four snapshot examples of Northern Hemisphere sector blocking episodes. The first two examples are Euro-Atlantic dipole blocks, and the other two examples are central Pacific blocks. Shown are the analyses of 250-hPa geopotential height (dam), (K) on PV 2 (see color scale in Fig. 1), and blocking index (K) vs longitude within the blocked sector. The 0 meridian is oriented downward for the Euro-Atlantic blocks and upward for the Pacific blocks.

8 750 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 FIG. 7. Annual mean blocking frequency against longitude between Jun 1996 and May The solid curve shows the local instantaneous blocking frequency, the dashed curve shows the large-scale blocking frequency, and the dotted curve shows the blocking episode frequency. for the small-scale blocking events, which are fairly independent of longitude, most local instantaneous blocking is associated with large-scale blocking, and most large-scale blocking occurs within a blocking episode. Thus, blocking frequencies defined locally and instantaneous generally provide a good proxy for blocking episode frequencies. c. Blocking variation with season Figure 8 shows the seasonal variability of Northern Hemisphere blocking episode frequency. Euro-Atlantic blocking episode activity (from 20 W to45 E) is high in summer, peaks in autumn, and then diminishes in winter and spring. The summer and autumn centers of blocking activity are located over Europe at approximately 20 E. In winter, the peak broadens and it shifts into the Atlantic ( 15 W) in spring. There is a different and particularly pronounced annual blocking cycle in the Pacific. Autumn is the most inactive time of year for Pacific blocking, with the blocking episode frequency dropping to zero over the western Pacific during the 5-yr period examined here. In winter and spring, the blocking episode frequency is fairly constant across the Pacific at between 5% and 10%. Then, in summer, blocking activity peaks in the central Pacific sector, but is much reduced over the west coast of North America. Thus, Northern Hemisphere blocking activity varies seasonally, and different regions exhibit different annual blocking cycles. These annual cycles are dependent on regional and seasonal variations in the strength of the jet, the amount of synoptic activity, and the stationary wave patterns; however, of these three factors, the stationary wave patterns appear to be most closely associated with the seasonal blocking episode frequencies. There is also considerable interannual blocking variability, especially in the Pacific where the Pacific North American (PNA) pattern plays a large role. However, FIG. 8. Blocking episode frequency against longitude for JJA (red), SON (blue), DJF (green), and MAM (yellow) between Jun 1996 and May Also shown are the longitudinal positions of the sectors defined in the text. the main Northern Hemisphere blocking regions can still be identified in most individual years, so that it is probable that the general nature of our climatological results are robust based on only 5 yr of data. d. Sector blocking event duration Six Northern Hemisphere blocking sectors, each spanning 65 of longitude, have been defined as shown in Fig. 8. The Euro-Atlantic sector coincides with the annual mean peak in blocking activity in this region. The central Pacific sector corresponds to the summer peak in blocking activity in the Pacific, and the west coast of North America sector spans the remainder of the Pacific blocking region. The other three sectors have been defined to fill the gaps between the regions covered by the three blocking sectors mentioned above (however, no sector encompasses the small region from 265 to 275 E). Using our blocking index, we have identified all the sector blocking events occurring in these six sectors over the 5-yr period under consideration. The durations of each of these events have also been noted, and Fig. 9 is a histogram of the total number of sector blocking events in the six Northern Hemisphere sectors lasting at least a given number of days. Events lasting 4 days or more are sector blocking episodes (by the definition in section 2c), whereas shorter events correspond to other blocking phenomena, such as cutoff lows. At first sight, the distribution in Fig. 9 looks like a simple exponential. In fact, following the work of Dole and Gordon (1983), we might expect the distribution to be exponential. In their study of persistent anomalies of the extratropical Northern Hemisphere wintertime circulation, they found that, for anomaly durations beyond about 5 days, there was an approximately constant probability of the anomaly lasting at least 1 more day. This may be formalized as follows. Let N( ) be the number of blocking events lasting at least days and set

9 751 FIG. 9. Total number of sector blocking events in the six Northern Hemisphere sectors between Jun 1996 and May 2001 lasting at least a given number of days. N( 1) N( ), (6) where is the proportion of episodes of duration days that last another day (0 1). For constant, N( ) N(0) exp( / 0), and (7) lnn / lnn(0), (8) 0 where 0 is a characteristic timescale in days related to by exp( 1/ 0 ). Thus, if (6) is valid for constant,, a logarithmic plot of the number of events N against the episode duration should yield a straight line with a constant negative gradient that gives 0 and. With this framework in mind, the Northern Hemisphere sector blocking event data from Fig. 9 has been plotted with a logarithmic scale in Fig. 10. From an exponential distribution of blocking event durations, we would expect the logarithmic plot to yield a straight line. However, there is a distinct change in behavior around 4 or 5 days such that a much better fit is given by two straight lines, with 0 being smaller for 4 or 5 days. Thus, is larger for longer blocking events, so that a higher proportion of blocking events last at least another day once the events have reached 4 or 5 days long. It has often been questioned whether there is something special about atmospheric blocking, or whether it is just the long timescale tail end of a distribution. Charney et al. (1981) found that frequency versus persistence diagrams for positive height field anomalies showed indications of a shoulder at 7 8 days. Here, we have been able to show that sector blocking episodes exhibit different persistence characteristics to the shorter timescale sector blocking events: a characteristic timescale for a sector blocking episode is 4 days, whereas the corresponding timescale for shorter sector blocking events is about half that. A timescale of about 2 days is reasonable for the synoptic events, such as midlatitude cyclones, which are identified as short blocking events. The 4-day timescale for sector blocking episodes is comparable to a typical spindown timescale for a block, and to the timescale over which a block might be expected FIG. 10. Number of sector blocking events lasting at least a given number of days, as in Fig. 9, but the numbers of events are now plotted on a log scale (to base e). The dashed lines show linear regressions for the short sector blocking event data (duration 1 3 days) and the midrange sector blocking episode data (duration 6 16 days). Only the midrange episode data is used to avoid the slightly more noisy tail end of the distribution. The corresponding timescale 0 is indicated for each of these regressions. to decay diabatically. Also, 4 days could perhaps be thought of as a large-scale advection timescale, measuring the time taken for a large-scale high (low PV) cutoff at high latitudes to be advected and absorbed back into its latitude of origin. In addition, we might expect atmospheric blocking to be more persistent than midlatitude cyclones, for example, due to the reinforcement of blocks by synoptic eddies. Therefore, the dynamically based PV index shows that there is something special about atmospheric blocking such that sector blocking episodes are more persistent than their shorter timescale counterparts. 4. Comparison with a standard 500-hPa geopotential height blocking index a. Northern Hemisphere blocking frequency using the Tibaldi and Molteni (1990) index A climatological picture of the Northern Hemisphere annual mean local instantaneous blocking frequency against longitude is shown in Fig. 11. The solid curve shows this blocking frequency as calculated using the PV blocking index (as in Fig. 7) and the dashed curve is calculated using the local, instantaneous Tibaldi and Molteni (1990) index. The other curves will be considered later. One main difference between the solid and dashed curves in Fig. 11 is that the PV index generally identifies more local instantaneous blocking events than the Tibaldi and Molteni index. This is to be expected since PV fields give a more detailed view of atmospheric structure than the coarse-grained view given by height pressure fields. A weak reversal of the meridional gradient of on a broad latitudinal scale used may not correspond to a reversal of the zonal wind field on a similar scale. It is possible that the local instantaneous

10 752 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 FIG. 11. Annual mean, local instantaneous blocking frequency against longitude between Jun 1996 and May The solid curve is calculated using the PV blocking index described in section 2a. For comparison, the dashed curve is calculated using the Tibaldi and Molteni (1990) index, the dotted curve is calculated using the PV blocking index with a constant blocking latitude ( 0 50 N), and the dot dashed curve is calculated using the Tibaldi and Molteni (1990) index with a variable blocking latitude. blocking criterion permits too many small-scale events. However, as was seen in Fig. 7, the large-scale blocking and blocking episode criteria both give frequencies typically only a few percent less, still generally significantly more than given by the Tibaldi and Molteni index. We have looked at a considerable number of the events selected by our criteria and not found any that we believe a synoptician could not describe as blocking. As discussed above, another major difference between the two curves is that they identify different blocking regions in the Pacific: the PV index identifies an east Pacific peak at about 225 E similar to that identified by the synoptic studies mentioned in section 1, whereas the Tibaldi and Molteni index identifies a peak in the western Pacific at about 170 E. This discrepancy must be due to the different formulations of the two indices: our index uses PV diagnostics and a variable blocking latitude, whereas the Tibaldi and Molteni index uses 500-hPa height and a fixed blocking latitude of 50 N around the entire Northern Hemisphere. The implications of these two differences are now examined. To illustrate the impact of assuming a fixed blocking latitude, the dotted curve in Fig. 11 shows the blocking frequency calculated using the PV index with a constant blocking latitude of 50 N: the differences between the solid and dotted curves are therefore entirely due to the different blocking latitudes used in each calculation. In the Pacific, the difference between the two curves is considerable: using a variable blocking latitude, the Pacific blocking frequency peaks in the eastern Pacific at about 225 E, whereas, using a fixed blocking latitude of 50 N, this peak shifts west to about 170 E in the western Pacific, coincident with the peak in blocking activity given by the Tibaldi and Molteni index. Thus, due to the use of a fixed blocking latitude, the Tibaldi and Molteni index predicts that Pacific blocking peaks in the sector of the strongest mean westerly tropospheric jet and upstream of the maximum in the storm track. These Pacific blocking events identified by Tibaldi and Molteni are examined in more detail in section 4b. As we will show, many such events would not be described as blocked by a synoptician; instead they correspond to low pressure minima on the northern flank of the Pacific jet. Therefore, the relatively high blocking frequencies identified by the Tibaldi and Molteni blocking index over the western Pacific are entirely spurious, and the main Pacific blocking region is actually located over the eastern ocean basin as suggested by Rex (1950b) many years previously. In order to complete the picture, the dot dashed line in Fig. 11 is calculated using the 500-hPa blocking index of Tibaldi and Molteni in conjunction with the variable blocking latitude as shown in Fig. 4. Comparing this with the blocking frequency calculated using the local, instantaneous Tibaldi and Molteni index, it is seen that, allowing the latitude for the blocking criterion to reflect the latitude at which synoptic eddies has almost completely removed the identification of blocking in the western Pacific. The differences between the solid line and the dot dashed line are primarily due to the use of different fields by the two indices. The 500-hPa height index identifies very little blocking in the Pacific. This is probably because Pacific blocks most often take the form of blocks, which do not exhibit the same strong easterly flow as Euro-Atlantic dipole blocks (see Fig. 6). The PV index, on the other hand, does identify a considerable amount of Pacific blocking, which suggests that this dynamical formulation is more suitable for identifying blocks. b. Features identified by the Tibaldi and Molteni index It is of interest to compare the blocking episodes identified by Tibaldi and Molteni s index to those identified by our PV index. First, let us consider whether or not Tibaldi and Molteni s index identifies those PV index episodes in section 2d. Of the two Euro-Atlantic sector blocking episodes shown in Fig. 6, Tibaldi and Molteni s index only identifies the episode in May The episode in November 2000 is too far north to be captured by their index. Thus, despite the fact that this episode blocks a substantial amount of the upper-level flow, it is not identified as a block by Tibaldi and Molteni s index. Of the two blocks shown in Fig. 6, again, only one is identified by Tibaldi and Molteni s index. The episode in August 2000 is not identified despite the fact that it was one of the most notable Pacific blocks in the year starting 1 August Therefore, this leads to the question what sorts of features Tibaldi and Molteni s index does identify in the Pacific. Figure 12 shows two examples of events identified

11 753 FIG. 12. Snapshot 500-hPa height analyses of two events identified as Pacific blocking episodes by Tibaldi and Molteni s (1990) index. The blocked longitude range is given for each example. Neither of these events were identified as blocking episodes by the PV blocking index. Note that 180 is oriented downward. as Pacific blocking episodes by Tibaldi and Molteni s 500-hPa height index. One example is taken from winter, and one from summer, and neither was identified by the PV blocking index. The blocked features identified in Fig. 12 both correspond to pressure minima situated on the northern flank of the jet. Many other examples of similar features have been found, mainly in the western Pacific, at all times of year. These features are not interrupting the jet and the passage of weather systems at all, and would not synoptically be described as blocks. Hence, the western Pacific peak in blocking activity identified using the Tibaldi and Molteni index in Fig. 11 is indeed spurious, and the real peak is farther east, as shown by the PV index blocking frequency. The failure of Tibaldi and Molteni s blocking index in the Pacific can be explained in terms of the constant blocking latitude of 50 N that they use around the entire Northern Hemisphere. From Fig. 4, the blocking latitude suggested by the 300-hPa high-pass transient EKE in the Pacific is south of 50 N for most of the year. Indeed, the annual mean blocking latitude used by the PV blocking index is nearer 45 N across the Pacific. Therefore, it is not surprising that the examples shown in Fig. 12 are associated with features to the north of the jet. 5. Discussion A new approach to quantifying the occurrence of blocking has been presented. Using the insight provided by dynamical research on blocking, it seeks to identify broad-scale reversals in the meridional gradient of the potential temperature on a dynamical tropopause identified by its potential vorticity value. Based on this blocking index we have defined the occurrence of largescale blocking, blocking episodes, and sector blocking and sector blocking episodes. The onset and decay dates of blocking episodes can then also be defined. It is hoped that this will provide a framework for many studies of the blocking phenomenon that is so important in many midlatitude regions. A new climatology of Northern Hemisphere blocking has been presented by using our PV blocking index in conjunction with 5 yr of on PV 2 analyses. Most Northern Hemisphere blocking is seen to occur in the Euro-Atlantic sector. The seasonal blocking variability in this region is relatively small, but there is a suggestion of a previously undocumented westward movement of the blocking activity from Europe to the Atlantic in the springtime. This shift is strongly associated with the stationary wave patterns at this time of year. Pacific blocking peaks over the eastern Pacific and the west coast of North America. This is farther east than suggested by most modern, quantitative studies but is consistent with the older synoptic research. In summer, there is a westward shift of this Pacific blocking activity into the central Pacific associated with a change in the stationary wave patterns. There is also a considerable amount of blocking over Asia, but both the jet and the synoptic activity are relatively weak in this region, so these Asian blocking episodes tend to be much weaker than their Euro-Atlantic and Pacific counterparts. This climatological picture of blocking differs significantly from previous height field index blocking climatologies (e.g., Lejenäs and Økland 1983; Tibaldi and Molteni 1990; D Andrea et al., 1998) in terms of both the main blocking regions and the seasonal variations

12 754 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 60 in blocking frequency. These differences are largely due to the use of a variable blocking latitude around the Northern Hemisphere, which is necessary for the correct identification of blocking events by a blocking index. In addition, we have found that the use of PV diagnostics is more appropriate than 500-hPa height for identifying Pacific blocks. One of the major findings of this study is that atmospheric sector blocking episodes lasting 4 days or more exhibit different persistence characteristics to shorter blocking events. Therefore, we have been able to show, perhaps for the first time, that atmospheric blocking is a distinct phenomenon rather than just being the long timescale tail end of a distribution. Perhaps the largest limitation of the blocking climatology presented in this study is the use of only a 5- yr dataset. We have been able to show that our results are relatively robust, but 5 yr is certainly too short a period to examine the influences of, for example, ENSO on atmospheric blocking. However, a study of this kind will soon be undertaken given the imminent availability of 40 yr of on PV 2 analyses in the ECMWF reanalysis dataset (ERA-40). Using this dataset it will be possible to more thoroughly investigate blocking in different sectors at different times of year. Such a study could also address questions concerning the relationship between the North Atlantic Oscillation and Euro-Atlantic blocking, or between the PNA and Pacific blocking. Furthermore, ERA-40 data could be used to compare characteristics of blocking episodes with those of shorter blocking events. In this way, it might be possible to elucidate the reasons for the different persistence characteristics between these two phenomena. The approach introduced used in this study also has implications for studying blocking in general circulation models. The methodology used in section 2a to determine the blocking latitudes around the Northern Hemisphere could be easily applied to the climatology of a GCM. Thus, if the jet and the synoptic activity were too far north in a given GCM, this would not necessarily affect the characterization of model blocking. Such an approach would be appropriate for comparing the current representation of blocking in GCMs in the context of the Atmospheric Model Intercomparison Project (AMIP). Furthermore, a similar type of analysis could be performed to study the impacts of climate change on blocking. Changes in the blocking climatology, which would be crucial for the climate of the region concerned, could be discerned with the aid of climate change simulations from a GCM that represents present-day conditions sufficiently well. It is planned to perform some of these data and GCM analyses in future research. Based on sector blocking episode diagnostics, a study has already been performed of the predictability and dynamics of blocking using the ECMWF Ensemble Prediction System. The predictability component of this will be presented in Pelly and Hoskins (2003). Acknowledgments. Thanks to Paul Berrisford for producing Fig. 3. Thanks also to Tim Palmer and Ken Mylne for many interesting discussions during the course of this work. The comments of the reviewers, Stephano Tibaldi, John Green and two others, have also been very helpful. REFERENCES Ambaum, M. H. P., and W. T. M. Verkley, 1995: Orography in a countour dynamics model of large-scale atmospheric flow. J. Atmos. Sci., 52, Appenzeller, C., and H. C. Davies, 1992: Structure of stratospheric intrusions into the troposphere. Nature, 358, Berggren, R., B. Bolin, and C.-G. Rossby, 1949: An aerological study of zonal motion, its perturbations and break-down. 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