Tropopause folds and cross-tropopause exchange: A global investigation based upon ECMWF analyses for the time period March 2000 to February 2001

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D12, 8518, doi: /2002jd002587, 2003 Tropopause folds and cross-tropopause exchange: A global investigation based upon ECMWF analyses for the time period March 2000 to February 2001 Michael Sprenger, Mischa Croci Maspoli, and Heini Wernli Institute for Atmospheric and Climate Science, Eidgenossische Technische Hochschule (ETH) Hönggerberg, Zurich, Switzerland Received 29 May 2002; revised 30 August 2002; accepted 30 August 2002; published 27 February [1] A new methodology is proposed to identify folds of the dynamical tropopause (taken as the 2 potential vorticity (PV) units (pvu) isosurface) from global analysis data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF). It consists of a threedimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses. The method is applied to a 1-year period starting in March Seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics, with maximum frequencies of about 30%. Pronounced maxima are located in the subtropical bands that extend from 20 E to 120 E on both hemispheres during summer. Generally, subtropical folds are comparatively shallow. Deep folds occur most frequently in midlatitudes during winter (over eastern North America and the western North Atlantic), with maximum frequencies of about 1%. Detailed investigation of the relationship between individual folds and cross-tropopause exchange events reveals that, on the average, individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics. However, due to the much higher frequency of subtropical folds, the percentage of exchange events that are associated with tropopause folds is larger near latitude (50 70%) than further poleward (20 30% during winter and 10 20% during summer). This indicates that tropopause folds are the key feature for cross-tropopause exchange in the subtropics, whereas in the extratropics other tropopause structures are at least of equal importance. INDEX TERMS: 0368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; 3364 Meteorology and Atmospheric Dynamics: Synoptic-scale meteorology; KEYWORDS: tropopause folds, stratosphere-troposphere exchange, tropopause dynamics, ECMWF analyses, potential vorticity Citation: Sprenger, M., M. Croci Maspoli, and H. Wernli, Tropopause folds and cross-tropopause exchange: A global investigation based upon ECMWF analyses for the time period March 2000 to February 2001, J. Geophys. Res., 108(D12), 8518, doi: /2002jd002587, Introduction [2] The examination of the vertical thermal structure of the atmosphere with the aid of serial radiosoundings during the early 1930s led to the identification of multiple stable layers in the tropopause region. According to the concepts of Bjerknes and Palmén [1937] these stable layers were thought to represent the same folded discontinuity surface at different heights (i.e., a tropopause fold). Later, the tropopause was defined in terms of potential vorticity (PV) [e.g., Reed and Danielsen, 1959], a quantity which is conserved under adiabatic and frictionless conditions [Ertel, 1942; Kleinschmidt, 1950] and which therefore ideally represents the concept of the tropopause as a quasi-impermeable discontinuity surface. A frequently used PV value to Copyright 2003 by the American Geophysical Union /03/2002JD002587$09.00 demark the tropopause is 2 pvu [Hoskins et al., 1985], where 1 pvu corresponds to 10 6 m 2 s 1 Kkg 1. Using this dynamical tropopause definition, tropopause folds can be defined as regions where vertical soundings reveal multiple crossings of the 2 pvu isosurface. [3] Tropopause folds are important atmospheric flow features for several reasons: (1) they are a manifestation of upper tropospheric frontogenesis [Keyser and Shapiro, 1986] during the nonlinear evolution of baroclinic waves [e.g., Hoskins, 1972; Bush and Peltier, 1994]; (2) they represent positive PV anomalies in the upper troposphere that can interact with lower tropospheric features leading to rapid cyclogenesis [e.g., Uccellini, 1990; Wernli et al., 2002]; (3) descent of stratospheric air in comparatively narrow folds goes along with significant vortex stretching and increasing rotation [Wirth, 2000], and therefore folds have the potential to instigate severe mesoscale weather events [e.g., Browning and Reynolds, 1994; Thorpe, 1997; STA 3-1

2 STA 3-2 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE Griffiths et al., 2000; Goering et al., 2001]; and (4) folds are favorable structures for cross-tropopause exchange associated with clear air turbulence [e.g., Danielsen, 1968; Shapiro, 1980; Vaughan et al., 2001] and/or convective and radiative erosion [Lamarque and Hess, 1994; see also Vaughan et al., 1994, and references therein]. For these reasons knowledge of the frequency and global geographical distribution of tropopause folds is important for the climatological understanding of extratropical synoptic-scale and mesoscale weather systems, and of irreversible mixing between stratospheric and tropospheric air. [4] Various methods have been developed so far to identify tropopause folds, either from vertical soundings (of temperature or ozone) [Van Haver et al., 1996; Beekmann et al., 1997] or from meteorological analysis data [Elbern et al., 1998]. The advantage of the direct use of observations from radiosondes is the high vertical resolution, whereas techniques that are applicable to analysis data are not restricted to the sparse sounding locations. Elbern et al. [1998] define tropopause folds as coincident maxima of PV and Q-vector divergence [Hoskins et al., 1978] in the upper troposphere. Using a detection algorithm based upon Q-vectors, they emphasize the frontogenetic character of the flow pattern that typically accompanies tropopause folding in midlatitudes (see also Davies and Rossa [1998]). Their climatology for the 10-year time period from December 1983 to November 1993, calculated with relatively coarsescale (T63) European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, reveals only a weak seasonal cycle of the hemispherically integrated fold frequency (with a minimum in summer) and clear midlatitude maxima in the zonal mean fold distribution (near 60 S and 55 N). [5] In this study a purely geometric tropopause fold definition is used that is in line with the early studies of the folded 2 pvu tropopause, and which includes no inherent assumptions on the dynamical origin of the folding. However, the study by Bithell et al. [1999] indicates that the topology of PV surfaces can be quite complex. For instance, tube-like structures in the lower troposphere can appear as isolated blobs of high PV in vertical cross sections although they can still be connected with the stratosphere. This calls forth the necessity to develop a refined algorithm to unambiguously identify tropopause folds from analysis data sets. Such an algorithm is introduced in section 2, and global fold distributions are presented for a 1-year time period in section 3. Section 4 then focuses on the relationship between folds and cross-tropopause exchange (the above item (4)). For instance, the contribution of folds to the total exchange across the tropopause is investigated. The final section (section 5) provides a summary and discussion of the main results. 2. Data Sets and Methodology 2.1. Data Sets [6] In this study ECMWF analysis data are used for the time period from March 2000 to February They are based upon the T319L60 spectral model until November 2000 and the T511L60 version thereafter. These comparatively high-resolution global analyses are available with a temporal resolution of 6 hours. From the 60 levels, about 19 cover the relevant region for folds from 100 to 600 hpa. (Compared to the former 31-level model, this corresponds to a slight increase in the vertical resolution in this layer from 30 to 25 hpa.) For the present study, the data was interpolated horizontally on a regular grid with a resolution of Identification of Tropopause Folds [7] A new methodology is proposed that allows identification of tropopause folds as areas with multiple crossings of the dynamical tropopause (±2 pvu) in pseudosoundings using global analysis data. A difficulty that arises when looking at the topology of the 2 pvu surface is that diabatic processes in the troposphere (latent heat release, radiative and frictional processes) can generate pronounced positive PV anomalies [e.g., Hoskins et al., 1985; Reed et al., 1992; Wernli and Davies, 1997]. If their amplitude is larger than 2 pvu, they could be erroneously identified as patches of stratospheric air. For instance the PV blob with values larger than 2 pvu in Figure 1c could, in principle, be produced by condensational heating (in which case there would be no tropopause fold present in the displayed vertical section), or it could be connected with the stratosphere outside of the section shown. This is actually the case (see below) and the structure near 8 W can be classified as a fold. [8] In order to properly distinguish between the 2 pvu surface demarcating the tropopause and diabatically produced 2 pvu features, a three-dimensional search algorithm is applied to categorize globally all volumes of air separated by 2 pvu surfaces (see Figure 2). The different categories are the troposphere (label 1), the stratosphere (label 2), blobs (frequently produced by latent heat release) with jpvj > 2 pvu that are connected neither with the stratosphere nor with the surface (label 3), surface-bound blobs (typically produced by frictional processes) with jpvj > 2 pvu (label 4) and tropospheric blobs with jpvj < 2 pvu enclosed in the stratosphere (label 5). After this labeling is completed, it is fairly simple to identify tropopause folds in vertical profiles as multiple crossings of the interface between labels 1 and 2 (see Figure 2). The vertical distance p between the upper and middle tropopause crossing is further used to classify the identified folds into deeply folded (p > 350 hpa), medium folded (p > 200 hpa), and shallow folded (p > 50 hpa) regions. Areas where p < 50 hpa are not regarded as folded. [9] A technical difficulty with the labeling arises in a few cases when a stratospheric intrusion touches the surface (or merges with a surface-bound PV anomaly). According to the above classification, the entire stratosphere would be regarded as a surface-bound blob (label 4). To avoid this erroneous labeling, in such a case the specific humidity field q is used to divide the PV structure into a stratospheric part (q < q 8 ) and a surface-bound part (q > q 8 ), with a subjectively chosen threshold value of q 8 =10 4 gkg 1. (Sensitivity studies indicate only a weak impact of the exact choice of this value.) With this additional criterion, the algorithm unambiguously identifies tropopause folds from global PV fields. Further details of the algorithm are discussed by Croci Maspoli [2002]. [10] In order to verify the algorithm, Figure 1a shows the areas where the algorithm identifies shallow (blue contour), medium (black contour) and deep (red contour) folded regions at 00 UTC 22 October At that time an

3 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE STA 3-3 Figure 1. Tropopause fold case study at 00 UTC 22 October Panel (a) indicates the pressure value of the 2 pvu tropopause (colors, scale in hpa) and the identified tropopause folds (the location of shallow folds is shown by the blue line, medium folds by the black line, and deep folds by the red line). Panels (b) and (c) show W-E oriented vertical sections (their location is indicated by the black lines in panel (a)) of PV (colors, scale in pvu, black line for 2 pvu) across the stratospheric streamer at 38 N and 46 N, respectively. In all panels, the black asterisks denote STT events. elongated tropopause fold, associated with a stratospheric streamer, extended from the Norwegian coast to North Africa. Vertical sections across the streamer (e.g., Figures 1b and 1c) confirm that the fold is correctly identified and classified as a deep fold near N (Figure 1c) and as a medium fold near N (Figure 1b) and north of 52 N. In addition, the asterisks in Figure 1 demark events of stratosphere-to-troposphere transport (STT) and reveal the pronounced exchange activity in the folded tropopause region. This relationship will be discussed quantitatively in section 4, but before, in section 3, the fold identification algorithm is applied to obtain a global 1-year climatology of tropopause folds Calculation of Cross-Tropopause Air Mass Flux [11] In order to identify individual events of STT and troposphere-to-stratosphere transport (TST), the Lagrangian method introduced by Wernli and Bourqui [2002] is used. It is based on the calculation of three-dimensional trajectories [Wernli and Davies, 1997; Stohl et al., 2001] and the tracing of PV and potential temperature along them. During the 1-year time period trajectories are started every 24

4 STA 3-4 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE Figure 2. A schematic vertical section showing the attribution of five labels to different parts of the atmosphere that are separated through the 2 pvu isosurface. Tropopause folds are identified where multiple crossings from label 1 to label 2 (i.e., from the troposphere to the stratosphere) occur in a vertical profile. hours on a global grid with a horizontal spacing of 80 km and a vertical spacing of 30 hpa in the extratropics (10 hpa in the tropics, see below) between the 80 and 600 hpa surfaces. Following the study of Holton et al. [1995] the 2 pvu tropopause is complemented in the tropics by the = 380 K isentrope. Therefore STT occurs when either the PV value along a trajectory changes from more than 2 pvu to less than 2 pvu ( 2 pvu in the Southern Hemisphere) and < 380 K, or if changes from more than 380 K to less than 380 K and jpvj < 2 pvu (vice versa for TST). To separate significant exchange events from highly transient (and potentially spurious) ones, a residence time threshold criterion is applied. It is verified that the air parcels reside at least during a time period t 8 in the troposphere and stratosphere before and after the exchange. In this study a threshold value of t 8 = 24 hours has been chosen. For a detailed discussion of the qualities and caveats of the methodology, the reader is referred to the studies of Bourqui [2001] and Wernli and Bourqui [2002]. For a selected case study of a deep stratospheric intrusion over the Alps, the method has been intercompared with other models and methods to estimate cross-tropopause exchange [Meloen et al., 2003] and qualitatively validated with observations [Cristofanelli et al., 2003]. These studies indicate that the adopted Lagrangian technique can accurately identify the subsynoptic-scale exchange processes, although with a tendency to underestimate the vertical penetration of the exchange air parcels, probably due to the neglection of turbulent mixing. (It is also important to note that the accuracy of the meteorological fields is essential for the quality of both the PV structures (e.g., folds) and the trajectory calculations required for the investigation of cross-tropopause transport. Our cursory intercomparison of PV structures calculated from 31-level and more recent 60-level ECMWF analyses (that are used in the present study) reveals close agreement near the extratropical tropopause but marked differences in the subtropics (not shown). This indicates that indeed PV structures calculated from analyses are sensitive to the model resolution and the assimilation technique, and that

5 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE STA 3-5 Figure 3. Geographical fold frequency distributions (in %) for DJF (left panels) and JJA (right panels) for (a and b) shallow folds, (c and d) medium folds, and (e and f ) deep folds. also for this reason quantitative estimates of exchange mass fluxes should be interpreted with care.) [12] In contrast to the study by Wernli and Bourqui [2002], which was restricted to the extratropical Northern Hemisphere, the present study covers the entire globe including the tropics. This has technical consequences for the definition of the dynamical tropopause (see above) and for the trajectories starting grid. Every trajectory represents a certain air mass given by the dimensions of the trajectories starting grid: m = g 1 x y p. If a trajectory crosses the tropopause it is assumed that this air mass has entirely crossed the tropopause. This assumption is valid as long as the vertical dimension p of the box represented by the trajectory is smaller than the air parcels vertical displacement during the 24-hour trajectory integration time. In the extratropics vertical motions near the tropopause layer are relatively large such that a value of p = 30 hpa is sufficiently small. However, in the tropics (i.e., from 20 S to 20 N), vertical velocities at 380 K are typically much smaller than at the extratropical tropopause, and a starting grid with a vertical resolution of p = 10 hpa is required to accurately calculate cross-tropopause transport in this region. 3. A 1-Year Climatology of Tropopause Folds 3.1. The Geographical Distribution [13] Figure 3 shows the tropopause fold frequency distributions for the summer and winter seasons (June August 2000 (JJA) and December 2000 to February 2001 (DJF)). (The fold frequency at a certain location represents the percentage of time instances (6 hourly ECMWF fields have been analyzed) where a fold has been identified at this location during the considered season.) During both seasons and in both hemispheres, shallow folds are most frequent in the subtropics between 20 and 40 (Figures 3a and 3b). There is however an apparent hemispheric symmetry (asymmetry) during the JJA (DJF) season. Particularly high fold frequencies occur during DJF over China (>30%), and during JJA (15 20%) over the southern Indian Ocean and in a band that extends from the Mediterranean along the northern edge of the Tibetan Plateau to the Yellow Sea. This band coincides with the northern edge of the Monsoon anticyclone [Hoskins, 1991]. The high frequencies indicate that in these regions folds are almost permanent features. Seasonally averaged temperature fields (not shown) reveal pronounced upper tropospheric baroclinicity in these areas with frequent foldings. Comparison with the precipitation climatology of Xie and Arkin [1997] indicates that the areas with maximum tropopause fold frequencies are relatively dry, except for the beginning of the Pacific storm track over southern Japan during DJF. This indicates in a very qualitative way that these shallow folds are unlikely to be strongly affected by erosion through condensational heating. [14] Medium folds (Figures 3c and 3d) have a more widespread geographical distribution than shallow folds (in particular in the winter hemispheres) and occur with maximum frequencies of about 8%. These maxima coincide

6 STA 3-6 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE Figure 4. Examples of identified tropopause folds in regions with climatologically high fold frequencies (a) at 12 UTC 24 July 2000 at 34 E and (b) at 12 UTC 3 December 2000 at 107 E. The vertical sections are S-N oriented and show the PV field (gray shading, in pvu; the 2 pvu surface is emphasized by the black line) and potential temperature (dashed-dotted lines, with a contour interval of 10 K). with the ones in the shallow fold distributions over central China (downstream of the Himalayan mountains) and western Australia in DJF, and over Turkey, Afghanistan and within an elongated belt to the east of Madagascar in JJA. Figure 4 shows two illustrative examples of pronounced tropopause folds in regions with frequent foldings, i.e., over Turkey during summer and central China during winter. In the extratropics, medium folds occur mainly during winter over North America and the North Atlantic with maximum frequencies of 1 2%. [15] Finally, deep folds (Figures 3e and 3f) are relatively rare features (with maxima of 1.5%) and occur predominantly in the extratropics. Due to the rareness of deep folds and the consideration of only a 1-year time period, the details of the geographical distribution of deep folds can not be regarded as representative in a climatological sense. However, there is clear indication that deep folds are more frequent during winter than summer. In the Northern Hemisphere, they occur preferentially over Labrador and the area downstream toward the southern tip of Greenland (Figure 3f), the region where Atlantic storm track cyclones typically attain their maximum intensity [Hoskins and Hodges, 2002, Figure 5e]. In the Southern Hemisphere, the band with maximum deep fold frequencies during JJA along 60 S (that extends from 60 E to 160 E, cf. Figure 3e) coincides with a climatological maximum of moving cyclones [Sinclair, 1994]. The two bands with frequent folds in this area along 30 S and 60 S could be related to the split-jet structure of the climatological flow field to the west of Australia [Bals-Elsholz et al., 2001]. [16] During the March May season (not shown), the fold distributions are quite similar to the ones for winter. Also not presented is the September November season, when the number of shallow and medium folds attains a minimum, and local maxima of medium folds occur over Korea, Australia and South Africa The Zonally Averaged Vertical Distribution [17] To provide a better impression of the vertical distribution of the identified folds, the zonally averaged DJF distributions are shown in Figure 5 for shallow, medium and deep folds. Here the vertical extent of a fold is defined as the distance between the highest and lowest tropopause crossing in a vertical profile (i.e., between the crossing points 1 and 3 in Figure 2). The black line marks the position of the zonally averaged seasonal mean 2 pvu, 380 K tropopause. It becomes apparent that shallow folds coincide with the subtropical regions where the climatological tropopause is steep (between 100 and 300 hpa). They are centered rather symmetrically on the height of the climatological tropopause, which is not surprising, because (due to the rather high frequency of shallow folds in this region) they themselves contribute significantly to the climatological mean. [18] In contrast, medium and deep folds occur below the climatological tropopause and constitute pronounced (and relatively rare) stratospheric anomalies in the climatological troposphere. Whereas medium folds typically reach down to the hpa level, deep folds can (at least in the winter hemisphere) extend down to 800 hpa. It becomes again apparent that deep folds occur preferentially poleward of 30, where the climatological tropopause is almost horizontal The Seasonal Cycle [19] Figure 6 shows the seasonal cycle of the hemispherically averaged tropopause fold frequencies. On both hemispheres, deep and medium folds reveal a pronounced seasonal variability with a clear winter maximum (in

7 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE STA 3-7 Figure 5. The zonally averaged and vertically resolved fold frequencies (in %) during DJF for (a) shallow folds, (b) medium folds, and (c) deep folds. The black line indicates the seasonal mean zonally averaged 2 pvu tropopause. January in the Northern Hemisphere and in June/July in the Southern Hemisphere). In contrast, the seasonal cycle of the frequency of shallow folds is weaker in the Northern Hemisphere and almost absent in the Southern Hemisphere. The diagram also illustrates that the total numbers of shallow, medium and deep folds each differ by almost an order of magnitude. 4. The Relationship Between Folds and Exchange [20] In this section, a qualitative and quantitative link is established between the identified tropopause folds and cross-tropopause exchange events A Qualitative Comparison [21] A qualitative but intuitive comparison considers the seasonal mean geographical distributions of tropopause folds (Figure 3) and the STT and TST mass fluxes (Figure 7). (Note that the mass flux values in Figure 7 are larger by about a factor of 3 than the ones shown in the studies of Wernli and Bourqui [2002, Figures 7 and 8] and Sprenger and Wernli [2003]. This is due to the choice of a different residence time threshold value for the identification of exchange events: here t 8 = 24 hours has been chosen compared to t 8 = 96 hours in the two other studies.) [22] During JJA, several maxima in the exchange climatology (labels A G in Figure 7) match well with maxima in the corresponding climatology of shallow and/or medium folds. For instance, during JJA, some subtropical maxima of STT (Figure 7b, A: southern Pacific; B: west coast of Australia; C1 and C2: Turkey and western Turkestan) and of TST (Figure 7d, D: southern tip of Africa and further east; E1 and E2: Turkey and western Turkestan) coincide with regions of frequent shallow folds (Figure 3b). Some of these features (B, C1, C2, E1, E2) are also evident in the distribution of medium folds (Figure 3d), and the maxima over Turkey (C1, E1) even for deep folds (Figure 3f). Further common structures in the exchange and fold distributions become discernible only for deep folds, namely the STT maxima over Labrador (F) and near the coast of Antarctica at 100 E (G). [23] During DJF, a striking match occurs over the Himalayan mountains (label H in Figure 7c), where a very distinct maximum is found in TST and the shallow and medium fold climatologies (Figures 3a and 3c). Other regions with local exchange maxima that coincide with frequent shallow folds are the southwestern tip of Australia (TST, label I) and central Chile and Argentina (J for TST and K for STT). Finally, the pronounced exchange max- Figure 6. Annual variation of the hemispherically averaged frequency (in %) of shallow, medium, and deep folds in the Northern Hemisphere (upper panel) and Southern Hemisphere (lower panel). The values for medium folds are multiplied by a factor 10 and the ones for deep folds by a factor 100.

8 STA 3-8 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE Figure 7. Geographical cross-tropopause mass flux distributions (in kg km 2 s 1 ) for DJF (left panels) and JJA (right panels) for (a and b) STT and (c and d) TST. imum in the western North Atlantic (label L) is associated with a maximum of deep folds. To a lesser degree, such a correspondence also prevails for the end of the Pacific storm track region south of Alaska (label M) A Detailed Quantitative Investigation [24] The comparison in the last section provided qualitative evidence for a significant link between cross-tropopause transport and tropopause folds on a seasonal timescale. However, the mere matching of seasonal mean patterns does not necessarily imply that the individual events which contributed to the means are also temporally linked. Therefore, a more refined comparison will be performed here which takes into account the individual exchange and fold events. [25] To this end a temporal and spatial neighborhood is defined for every exchange and fold event. Exchange events (i.e., the locations where trajectories cross the tropopause) are well defined points in space and time that normally do not correspond exactly to grid points and data times of the ECMWF analyses. In contrast, folds are not considered in such a point-wise fashion, but rather as an area that can extend over several data points (cf. the identified fold areas in Figure 1a). Folds can only be identified at a data time of the ECMWF analyses. An exchange point and a fold area are defined to be neighbored (i.e., linked) if the minimum geographical distance between the two is smaller than 300 km and if the temporal separation is smaller than 1 hour. No explicit criterion is necessary for the vertical separation since folds and exchange both occur near the tropopause. [26] Using this definition for neighbored exchange and fold events, two questions will be investigated. The first one focuses on individual exchange events and asks whether they are related to a fold or not, whereas the second question puts the focus on individual folds and asks how much exchange (STT and TST) occurs in their close environment. [27] Figure 8 illustrates the results of the first question for the JJA and DJF seasons. It shows the probability as a function of latitude that an individual STT (solid) and TST (dashed) event occurs in the neighborhood of a fold. Most striking is the large percentage (50 70%) of STT and TST in the subtropics which are associated with shallow folds (red lines). The values are slightly larger for the winter hemisphere than for the summer hemisphere, in accordance with the increased frequency of folds in winter. Note also that STT and TST are almost equally associated with folds in the subtropics. In the extratropics, the probabilities for an exchange event to occur near a shallow fold amounts to 20 Figure 8. Probability that an individual exchange event (solid lines for STT, dashed lines for TST) in a latitude band occurs in the direct vicinity of a tropopause fold (red lines for all folds, blue lines for medium, and green lines for deep folds). Panel (a) is for JJA and (b) for DJF.

9 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE STA 3-9 Figure 9. Histograms showing the accumulated probability (in %) that the environment of a tropopause fold is associated with a certain cross-tropopause mass flux (in kg km 2 s 1 ) for DJF (left panels) and JJA (right panels) for (a and b) shallow and (c and d) medium folds. Solid lines are for STT and dashed lines for TST. Bold lines are for the extratropical Northern Hemisphere (40 90 N) and thin lines for the subtropical Northern Hemisphere (20 40 N). 30% during winter and 10 20% during summer. These numbers are comparable to the percentage of exchange events that are associated with stratospheric streamers and cutoffs [Sprenger et al., 2001]. [28] If only medium and deep folds are considered, then the percentage of exchange events that are associated with these folds becomes significantly smaller. For instance, only about 2 5% of the exchange events in the wintertime Northern Hemisphere occur in the vicinity of deep folds. Note that such small numbers are not surprising given the restrictive definition and the resulting rareness of deep folds. [29] Results of the second question (How much exchange occurs in a tropopause fold?) are shown in Figure 9 for two different zonal bands in the Northern Hemisphere. Plotted is the accumulated probability that the environment of a fold (i.e., the fold area itself plus a 300 km wide band) is associated with a certain area-averaged cross-tropopause mass flux. This quantity is a useful measure for the fold s exchange activity, and it allows comparison of the climatological exchange mass fluxes (Figure 7) with the ones that actually occur in fold environments. [30] General results that are independent of the season and hold equally for shallow and medium folds are that (1) midlatitude folds are associated with larger exchange mass fluxes than subtropical folds (for both STT and TST) and (2) within folds STT is more intense than TST. The quantitative numbers are such that, for instance for STT during winter, 50% of all shallow folds are associated with an exchange mass flux of at least 800 kg km 2 s 1 (Figure 9a), whereas this value is even larger for medium folds (1050 kg km 2 s 1 ) (Figure 9c). During summer (Figures 5a and 5c) the typical exchange mass fluxes are smaller. [31] In the subtropics, there are much less shallow and medium folds that are associated with very large exchange fluxes (>4000 kg km 2 s 1 ) than in the extratropics. For instance for medium folds during winter, 10% of all extratropical folds exceed this value but only about 1% of the subtropical ones (Figure 9c). This points to the different nature and dynamical evolution of folds in the two areas: subtropical folds are often quasi-stationary whereas in the midlatitudes, folds are transient features that rapidly develop and decay in association with a synoptic-scale dynamical system. Comparison of the exchange activity for shallow

10 STA 3-10 SPRENGER ET AL.: TROPOPAUSE FOLDS AND CROSS-TROPOPAUSE EXCHANGE and medium folds shows that slightly more exchange occurs in the vicinity of medium folds, which is in agreement with the fact that medium folds represent considerably stronger anomalies of the dynamical tropopause. 5. Discussion and Final Remarks [32] A new method has been introduced to identify tropopause folds from global meteorological data sets. In contrast to the approach of Elbern et al. [1998] the definition and identification of folds is based purely upon the topology of the dynamical tropopause. The algorithm is easily applicable to large (reanalysis) data sets and offers the possibility to distinguish between shallow and deep folds. A caveat of this geometrical approach is the obvious sensitivity to the definition of the dynamical tropopause. For instance the sample fold structures shown in Figures 1b and 1c reveal that a tropopause value of 1.5 pvu would yield a significantly larger fold, whereas with a tropopause value of 3 pvu folds would become quite rare. The chosen value of 2 pvu is typically used in dynamical studies of extratropical weather systems and is consistent with our investigations of cross-tropopause exchange [Wernli and Bourqui, 2002; Sprenger and Wernli, 2003]. [33] A global 1-year climatology of tropopause folds has been established which reveals clear annual variations in the frequency of folds (with a maximum in winter and a minimum in summer) and distinct geographical distributions that vary considerably from winter to summer. Shallow folds are most frequent in the subtropics, in the winter Northern Hemisphere with comparatively little zonal variability apart from a maximum over China, and in JJA with clear maxima to the north of the monsoon anticyclone and over the southern Indian Ocean. These maxima also occur for medium folds. Of particular interest are the maxima in the distributions of shallow and medium folds over Turkey and Afghanistan during Northern Hemispheric summer. They correspond to distinct maxima in the distribution of upward and downward cross-tropopause transport (see also the results from a 15-year exchange climatology [Sprenger and Wernli, 2003]). This indicates qualitatively the strong link between folds and cross-tropopause transport and hints at the importance of an appropriate tropopause fold climatology for the dynamical understanding of the climatological exchange distributions. [34] A detailed quantitative analysis of the relationship between folds and exchange has shown that the majority of individual exchange events in the subtropics are associated with tropopause folds. In the extratropics the totality of folds contributes less to the total cross-tropopause mass flux, although individual folds are associated with significant exchange activity. Here the conclusion is, that folds (in the sense defined in the original literature [Bjerknes and Palmén, 1937] and in the present study) are relatively rare outside the subtropics and that there other dynamical processes like for instance the breakup of stratospheric streamers [Appenzeller et al., 1996] and the diabatic erosion of cutoff lows [Hoskins et al., 1985; Wirth, 1995] play at least an equally important role for the actual cross-tropopause exchange. [35] Qualitative comparison with the previous global fold climatology by Elbern et al. [1998], who used a dynamical criterion based upon Q-vector divergences and PV maxima on 400 and 500 hpa, yields a similar, although in our case stronger, annual cycle and significant differences in the geographical distribution of folds. Elbern et al. [1998] found maximum fold activity in the midlatitudes, in particular during winter over the North Atlantic and North Pacific storm tracks, and almost no folds during summer in the subtropics. Their climatological distribution resembles the present one for deep folds, but their definition does not account for the frequently identified shallow and medium folds in the subtropics. This is due to the fact that they applied their identification criterion to the hpa layer, whereas shallow folds occur near 200 hpa (cf. Figure 5a). It might also play a role that quasi-permanent and relatively shallow subtropical folds are not associated with strong upper-level Q-vector divergence, in contrast to deep extratropical folds, that are highly transient phenomena evolving and decaying rapidly in association with nonlinear baroclinic wave life cycles. [36] It is further interesting to compile the regions and time periods of recently documented tropopause fold case studies and compare them with our climatology. Folds were investigated for instance over Turkey in June 1998 [Zahn et al., 2000], France in November 1998 [Ehret et al., 1999], the Alps in November 1999 [Liniger and Davies, 2003], east of Newfoundland in November 1992 [Wernli, 1997; Davies and Rossa, 1998], over the Atlantic and eastern United States in October and November 1997 [Fuelberg et al., 2000], the midwestern United States in February 1979 [Lamarque and Hess, 1994], November 1998 [Olsen et al., 2000] and April 1999 [Goering et al., 2001], over southern Greenland in March 1994 [Sørensen and Nielsen, 2001], near the Canarian Islands in March 1999 [Kowol-Santen and Ancellet, 2000], and over the southern Indian Ocean in July 1998 [Baray et al., 2000]. All these regions occur as frequent fold areas in our analysis, except for central Europe, where almost no folds have been identified during the considered 1-year period. [37] It remains for future work to apply the technique to longer data sets with an appropriate spatial resolution (for instance, the ECMWF 40-year reanalyses) and to derive a robust climatology. The present investigation also pointed out the predilection of shallow and medium folds to occur in the vicinity of the elevated Asian topography and, during Northern Hemispheric summer, north of the Asian monsoon anticyclone. These climatological folding regions have not yet been discussed in the literature, and the structure and dynamical evolution of these folds deserves further investigation. [38] Acknowledgments. We thank the ECMWF and MeteoSwiss for providing access to the meteorological data. This study was partially supported through the EC project STACCATO under contract BBW resp. 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Wernli, Institute for Atmospheric and Climate Science, Eidgenossische Technische Hochschule (ETH) Hönggerberg, CH-8093, Zurich, Switzerland. (mischa@atmos.umnw. ethz.ch; sprenger@atmos.umnw.ethz.ch; henry@atmos.umnw.ethz.ch)