A northern hemispheric climatology of cross-tropopause exchange for the ERA15 time period ( )

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D12, 8521, doi: /2002jd002636, 2003 A northern hemispheric climatology of cross-tropopause exchange for the ERA15 time period ( ) Michael Sprenger and Heini Wernli Institute for Atmospheric and Climate Science, Eidgenossische Technische Hochschule (ETH) Hönggerberg, Zurich, Switzerland Received 9 June 2002; revised 30 August 2002; accepted 9 September 2002; published 8 March [1] A 15-year climatology of cross-tropopause exchange in the extratropical Northern Hemisphere is presented based upon a previously developed Lagrangian methodology and reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) for the years For this time period, no apparent trends exist for upward (troposphere-to-stratosphere, TST) and downward (stratosphere-to-troposphere, STT) cross-tropopause transport. The seasonal cycles of hemispherically integrated TST and STT and the zonally integrated meridional exchange distributions show relatively little interannual variability. The 15-year averaged geographical distribution of STT reveals significant deviations from zonal symmetry and pronounced seasonal variability. STT occurs predominantly over the Atlantic and Pacific storm track regions during winter, spring, and autumn and also over the Mediterranean in winter and spring. In contrast, during summer, STT maxima are located over the continents, particularly over southeastern Europe and central Asia. TST is less variable and is strongest near the southern tip of Greenland and the Aleutian Islands during all seasons. Aspects of the interannual variability of the geographical exchange distributions are investigated. Over the Atlantic, STT varies significantly in line with the changing storm tracks during different phases of the North Atlantic Oscillation (NAO), whereas TST does less so. Special consideration is given to vertically deep exchange, where the exchange air parcels transfer rapidly from the lowermost stratosphere to the boundary layer (or vice versa). These processes occur mainly during winter and exhibit pronounced maxima over the North American west (deep STT) and east (deep TST) coasts. 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: stratosphere-troposphere exchange, vertical transport, tropopause dynamics, ECMWF reanalyses, North Atlantic Oscillation Citation: Sprenger, M., and H. Wernli, A northern hemispheric climatology of cross-tropopause exchange for the ERA15 time period ( ), J. Geophys. Res., 108(D12), 8521, doi: /2002jd002636, Introduction [2] In a recent study, Wernli and Bourqui [2002] introduced a Lagrangian methodology to identify cross-tropopause exchange events from meteorological analysis data and applied the technique to the extratropical Northern Hemisphere and to a 1-year time period from May 1995 to April The Lagrangian approach [Wernli and Davies, 1997; Stohl, 2001] allows consideration of the pathway of exchange air parcels, of their residence times in the troposphere and stratosphere before and after the exchange, and of deep exchange as a particularly important category of exchange events where stratospheric air descends rapidly to the atmospheric boundary layer (or vice versa). Here, the same methodology is applied to 15-year reanalysis data ( ) from the European Centre for Copyright 2003 by the American Geophysical Union /03/2002JD Medium-Range Weather Forecasts (ECMWF) in order to derive a representative climatology of downward (stratosphere-to-troposphere, STT) and upward (troposphere-tostratosphere, TST) transport in the extratropical Northern Hemisphere. Such a climatology, based upon relatively high-resolution meteorological data, attempts to build a bridge between former global-scale estimates of crosstropopause transport [Holton et al., 1995] and various synoptic-scale case studies. For a discussion of the two perspectives the reader is referred to the review by Stohl et al. [2003], Bourqui [2001], and Wernli and Bourqui [2002]. [3] Existing multiyear climatologies of the cross-tropopause mass flux are either based on the budget approach [Appenzeller et al., 1996b; Gettelman and Sobel, 2000] which only provides hemispherically integrated net exchange fluxes, on the Eulerian Wei method [Wei, 1987; Siegmund et al., 1996; Grewe and Dameris, 1996] which is highly sensitive to technical parameters [Wirth and Egger, 1999], or on isentropic trajectory calculations [Seo and STA 6-1

2 STA 6-2 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE Bowman, 2001] which neglect the impact of cross-isentropic flow. In the present study a consistent 15-year data set and a sound methodology based upon three-dimensional trajectories are used to calculate climatological geographical distributions of (deep) STT and TST and to asses their interannual variability. A complementary study for the same time period [James et al., 2003a, 2003b] uses a particle dispersion model that includes parameterizations of turbulence and convection: two processes that are neglected by our trajectory-based approach. [4] The outline of the study is as follows: section 2 briefly summarizes the data and methodology. In section 3, the results from the 15-year exchange climatology are presented, in particular the geographical distributions of STT, TST and the net mass flux, and their seasonal variations. Aspects of the interannual variability are considered in section 4, including composites of cross-tropopause exchange for the positive and negative phase of the North Atlantic Oscillation (NAO). The main conclusions of this study are summarized in section Data and Methodology [5] The ECMWF reanalysis data set (ERA15) covers the 15 years from January 1979 to December 1993 [Gibson et al., 1997]. The analysis fields required for the trajectory integration (horizontal and vertical wind components, temperature) are available every 6 hours on 31 vertical levels from the surface up to 10 hpa and were interpolated on a regular grid with 1 horizontal resolution. Secondary fields like potential temperature and potential vorticity (PV) were then calculated on the original hybrid model levels. [6] This data set, a PV-based tropopause definition (2 pvu), and the Lagrangian methodology introduced by Wernli and Bourqui [2002] are used to obtain a 15-year climatology of cross-tropopause transport (downward STT and upward TST) in the extratropical Northern Hemisphere. Analogously to the study of Wernli and Bourqui [2002], trajectories were started every 24 hours on a regular grid with a horizontal (vertical) grid spacing of 80 km (30 hpa) between 80 and 590 hpa in the entire Northern Hemisphere. For instance for STT, preliminary exchange events were selected with the criterion that the trajectory s PV value is larger (smaller) than 2 pvu at the beginning (end) of this 24- hour period (vice versa for TST). For all preliminary exchange events the trajectories are extended 4 more days both backward and forward in time. It is then verified that the air parcel residence time on either side of the tropopause exceeds a certain threshold value t 8. It is only these air parcels that are considered as significant exchange events, and parcels that recross the interface on shorter timescales are eliminated. Exchange mass fluxes have been calculated for different values of t 8 (24, 48, 72, and 96 hours), but in the following only results for t 8 = 96 hours will be presented. (See the study of Wernli and Bourqui [2002] for a detailed discussion of the sensitivity to this threshold parameter.) [7] This methodology, which is based on the analysis of PV along 9-day air parcel trajectories, allows separate identification of STT and TST, and to distinguish between vertically shallow and deep exchange events. The latter category is particularly important because these events bring together air masses with potentially strongly differing chemical compositions on a short timescale of a few days. A more thorough discussion of the approach, its characteristics and caveats can be found in the studies of Wernli and Bourqui [2002] and Bourqui [2001] Year Climatology [8] In this section, the 15-year averaged distributions of cross-tropopause transport are presented. Special focus is given to the geographical patterns of climatological downward (STT), upward (TST), and net (STT TST) exchange mass fluxes (section 3.1), their seasonal variability (section 3.2), the potential temperature distributions of STT and TST (section 3.3), and deep exchange (section 3.4) Year Mean Exchange Distributions [9] Figure 1 shows the 15-year averaged northern hemispheric distributions of STT and TST, and of the net and two-way exchange mass fluxes. These multiyear climatological patterns reveal preferred areas of exchange and strong zonal variability. STT mass fluxes (Figure 1a) are largest in the midlatitudes (from 40 N to65 N) over the North Pacific and Atlantic oceans and over central and southeastern Europe. A weaker but clearly identifiable maximum is also located over central Asia to the north of the Tien Shan mountains (near 80 E/45 N). TST (Figure 1b) is generally weaker than STT, in particular south of 60 N. It attains local maxima in the western part of the Bering Sea and to the south and north of Greenland. The difference of the two fields (STT TST) (Figure 1c), i.e., the net exchange, reflects the general dominance of STT over TST in the extratropics with pronounced maxima of net downward exchange over the western United States and central Europe, i.e., downstream of the Pacific and Atlantic storm track areas. However, within the storm track areas (in particular over the Pacific) the net exchange is remarkably weak. Other prominent features are the maxima over Turkey and central Asia, and the minimum (i.e., net upward exchange) in the region near the Queen Elisabeth Islands and northern Greenland. In the latter area the climatological tropopause pressure is exceptionally high ( hpa), except for summer [Zängl and Hoinka, 2001]. Also, in the other Arctic regions, the net mass flux is (weakly) upward, in agreement with theoretical considerations [Juckes, 1997] and our previous diagnostic study [Wernli and Bourqui, 2002]. Finally, Figure 1d illustrates the two-way exchange distribution, which is the crosstropopause exchange mass flux that goes with equal amplitude in both directions (calculated at every grid point as min(stt,tst)). It resembles strongly the TST distribution and looks strikingly different from the net exchange pattern. This indicates that the preferred regions of maturing extratropical cyclones over the oceans are associated with vigorous exchange in both directions (and a moderate net downward flux). In contrast, the downstream continents, where baroclinic waves typically break, are characterized by strong STT and very weak TST Seasonal Variability [10] In order to get an impression of the seasonal variability, the seasonal mean exchange distributions for winter

3 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-3 Figure 1. Geographical distribution of the annual mean (a) downward (STT), (b) upward (TST), (c) net (STT TST), and (d) two-way (min(stt,tst)) cross tropopause mass fluxes based upon exchange trajectories with a threshold residence time of 96 hours. Values are in kg km 2 s 1. and summer are presented in Figure 2. The distributions for spring and autumn (not shown) closely resemble the 15-year mean patterns (Figure 1). [11] For STT, the winter and summer patterns differ substantially, in certain areas by up to a factor of 3. During winter, STT (Figure 2a) is particularly strong in the Pacific and Atlantic storm track regions and over the Mediterranean, and weak over Siberia and downstream of the Rocky Mountains. Quantitative comparison of STT in the Atlantic and Pacific storm tracks yields comparable values with a slightly more pronounced maximum over the Atlantic. During summer (Figure 2d), STT maxima occur over the continent, in particular over Turkey and Kazakhstan. This zone of enhanced STT corresponds to a band of particularly strong upper tropospheric baroclinicity (not shown) indicating the possible importance of upper-level frontogenesis and associated tropopause folding for the exchange processes in this area. In the oceanic regions, which are characterized by a strongly reduced cyclonic activity compared to winter, STT is also significantly weaker. [12] For TST, the seasonal differences are much smaller. The same geographical maxima can be found year-round, and correspond quite well to the northern areas of frequent cyclone occurrences. An exception is the summer maximum over the eastern Mediterranean (Figure 2e) just south of the even stronger maximum of STT (Figure 2d) which is a notable feature as it constitutes the single well localized and large amplitude TST maximum between 30 N and 50 N. According to a global investigation of tropopause fold occurrence [Sprenger et al., 2003] it coincides with a preferred area of summertime tropopause folding in the entrance region of the Asian jet stream which marks the northern edge of the large-scale monsoon anticyclone [Hoskins, 1991]. Other seasonal variations occur over the

4 STA 6-4 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE Figure 2. Geographical distribution of the downward (STT, left), upward (TST, middle), and net (STT TST, right) cross tropopause exchange mass fluxes for (a c) winter (December, January, and February) and (d f ) summer (June, July, and August) based upon exchange trajectories with a threshold residence time of 96 hours. Values are in kg km 2 s 1. Pacific (where the maximum of TST moves about 20 further east from winter to summer), and in the subtropics where a band with significant TST only appears in summer. [13] The net exchange (Figures 2c and 2f ) also reveals clearly the strong seasonal variations of cross-tropopause transport, in particular of STT. During winter the regions with pronounced net downward mass flux extend from the central Pacific and Atlantic oceans inland towards central North America and Europe. In contrast, during summer, STT and TST almost cancel each other over the oceans and the net downward flux is restricted to the continental parts of the N latitude band. Regions with enhanced (in comparison to the 15-year mean) net upward exchange occur in the Arctic during winter and in the subtropics during summer, which might be associated with the monsoon systems The Potential Temperature Distributions of Exchange [14] It is worthwhile considering the potential temperature () distributions of the identified STT and TST events (Figure 3) which undergo quite a strong seasonal cycle (as already noted in the study of Wernli and Bourqui [2002]). During all seasons, STT occurs within a relatively narrow band (Figure 3a). In winter, the peak activity is at 300 K and this peak is shifted to about 20 K higher values in summer. About 90% of the STT events take place in the range K in winter, K in spring, K in summer, and K in autumn. TST is characterized by broader, and during summer and autumn by bimodal, distributions (Figure 3b). The isentrope with maximum TST changes from 307 K in winter to 325 K in summer, and is generally some 8 K above the isentrope with maximum STT. During summer (and with reduced amplitude also during autumn) a secondary maximum of TST occurs near 360 K. This is associated with the region of intense TST over Turkey (cf. Figure 2e), as confirmed by a localized examination of the distribution of TST in the region extending from E, N (not shown). Another special region of interest is the southern tip of Greenland (20 60 W, N) where the largest northern hemispheric two-way exchange occurs (Figure 1d). In this region wintertime STT is most intense on fairly low isentropes (295 K). This is remarkable because it indicates that STT takes place mainly on lower isentropic levels than the K surfaces that are typically investigated in case studies of wintertime synoptic-scale weather systems [e.g., Hoskins et al., 1985; Appenzeller et al., 1996a; Rossa et al., 2000]. Also the net exchange is relatively small near 315 K during winter (Figure 3c), but is strongly positive on the levels from 285 to 305 K. During summer, the net exchange distribution is shifted to higher values and has relatively strong negative values near 360 K. [15] This marked seasonal cycle of the distributions of cross-tropopause transport is mainly a consequence of the seasonal variations of the location of the isentropic surfaces

5 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-5 themselves, and not of a strong shift in the latitudinal distribution of the exchange activity [Wernli and Bourqui, 2002]. One implication is that a single isentropic surface can not be regarded as representative for cross-tropopause exchange. For instance on 300 K, STT and TST are both very intense during winter but totally absent during summer (because in the warm season the 300 K isentrope hardly reaches the stratosphere), whereas above 325 K the summer season dominates for both upward and downward exchange. Figure 3. Potential temperature distributions of (a) STT, (b) TST, and (c) net cross-tropopause transport (STT TST) for the four seasons based upon exchange trajectories with a threshold residence time of 96 hours. The seasonally integrated values are in kg (2 K) Vertically Deep Exchange Events [16] An important aspect of STT and TST is the amplitude of the associated vertical transport in the troposphere. According to the terminology of Wernli and Bourqui [2002], deep exchange denotes events where air parcels are rapidly transported from the stratosphere down to levels below the 700 hpa level (deep STT), or from the lowermost troposphere ( p > 700 hpa) to the stratosphere (deep TST). Historical case study examples of deep STT are documented for instance by Staley [1962] over the western United States, and more recently over northern Europe with a subsequent descent to the Arabian Sea [Wernli and Davies, 1997], east of Newfoundland with a descent towards the UK [Wernli, 1997], and over Central Europe with a descent to the eastern Mediterranean [Stohl and Trickl, 1999]. As discussed by Wernli and Bourqui [2002], the tropospheric part of deep TST events is reminiscent of the concept of warm conveyor belts [Browning, 1990; Wernli, 1997; Stohl, 2001], i.e., of moist ascending airstreams that occur typically ahead of intense midlatitude cold fronts. From a chemical point of view it is a central aspect of cross-tropopause transport to know the boundary layer origins of deep TST and the destinations of deep STT. The former indicate for instance the likelihood for rapid transport of pollutants into the lowermost stratosphere, and the latter point to areas where downward transport of stratospheric ozone is most likely to affect the surface ozone budget. [17] So far all exchange events (that fulfill the 4-day residence time criterion) were taken into account, but now Figure 4 focuses on deep STT and TST during the winter season only. (During the other seasons deep exchange is comparatively rare, see later in section 4.1.) Deep STT [18] Deep STT (Figure 4a) is strongest in the N latitude band over the Atlantic and Pacific storm track regions. A pronounced maximum is discernible at the end of the Pacific storm track in the Gulf of Alaska. The North Atlantic maximum of deep STT occurs near the southern tip of Greenland, and a further (albeit weaker) maximum is situated over Central Europe. Comparison of the deep with all STT events (Figure 2a) reveals similarity of the two geographical distributions with a tendency for deep STT to occur further north. Quantitatively, the ratio of deep STT to total STT (not shown) is largest (0.3) over Canada, southern Siberia/northern China and the Alps (0.25). In the oceanic storm track regions this ratio amounts to typically 0.2 over the Pacific and 0.16 over the Atlantic. (Note that all these values are for winter, during summer this ratio hardly exceeds 0.03.) [19] The low tropospheric destination points of deep STT (Figure 5a) show three major maxima near 30 N: along Baja California and the southern U.S. west coast, the U.S.

6 STA 6-6 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE Figure 4. Geographical distribution of the exchange mass flux during winter associated with only deep exchange events for (a) STT and (b) TST. Values are in kg km 2 s 1. Note the different scale compared to Figures 1 and 2. east coast and western Atlantic, and within an elongated band over northern Africa. It is at these locations where the stratospheric contribution to the near-surface ozone budget during winter is expected to be strongest, whereas for instance over Central Europe this contribution is smaller and over China almost zero. As expected from quasiisentropic descent from the tropopause intersection point to the lower troposphere, deep STT destinations are about 30 further south than the exchange events themselves. Figure 6 provides a more detailed analysis of the link between the exchange location and the destination area in the lower troposphere. Deep STT events that affect the U.S. west coast and Baja California cross the tropopause mainly over Oregon, Washington, and British Columbia, and less frequently over the eastern North Pacific (Figure 6a). Events reaching the U.S. east coast also cross the tropopause preferentially over the northern Rocky Mountains just north of the events that move southwards along the American Figure 5. Geographical distribution of the (a) low tropospheric destinations of deep STT and (b) low tropospheric origins of deep TST events during winter. Values are in % and correspond to the probability that a low tropospheric air parcel ( p > 700 hpa) was transported downward from the lowermost stratosphere during the previous four days (deep STT) or will be transported upwards into the lowermost stratosphere during the next four days (deep TST).

7 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-7 Figure 6. Cross-tropopause mass fluxes associated with deep exchange events during winter (colors, in units of kg km 2 s 1 ) that are linked with low tropospheric destinations and origins in a prespecified domain (indicated by the black lines). (a) Deep STT with destinations along the U.S. west coast, (b) deep STT with destinations along the U.S. east coast, (c) deep STT with destinations over the Mediterranean Sea, and (d) deep TST with origins over the western Pacific and Atlantic. west coast (compare Figures 6a and 6b). Finally, tropopause crossings associated with deep STT that affect the Mediterranean lower troposphere occur either in the central North Atlantic south of Iceland or over Central Europe (Figure 6c). The detailed investigation of such central European deep STT events was one of the main objectives of the STACCATO project [Stohl et al., 2003; Meloen et al., 2003; Cristofanelli et al., 2003] Deep TST [20] The geographical distribution of deep TST (Figure 4b) is similar to the one for all TST events (Figure 2b) with a clear maximum over the Labrador Sea and southern Greenland, and a weaker one slightly to the west of the date line. The ratio of deep TST to total TST (not shown) attains maximum values (0.25) in the aforementioned areas that correspond to the northern parts of the principal storm tracks. In the region of the Queen Elisabeth Islands (where total TST is largest), this ratio is lower (0.16). [21] In agreement with our earlier 1-year investigation [Wernli and Bourqui, 2002], the distribution of the low tropospheric origins of deep TST reveals two clear maxima at the beginning of the oceanic storm tracks (Figure 5b). These regions are downstream of the American and Asian east coasts and are potentially influenced by the emission of air pollutants from the industrial coastal regions. Figure 6d indicates that for both storm track regions the nearsurface origins lie about 20 south of the locations where the TST events actually cross the tropopause. This is in accordance with the concept of conveyor belts that constitute an integral Lagrangian feature of midlatitude cyclone life cycles

8 STA 6-8 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE Figure 7. Time series for (top) STT and TST, (middle) net (STT TST), and (bottom) deep STT and TST mass flux across the tropopause integrated over the Northern Hemisphere (20 90 N). Values are in units of kg s 1. The left panels show the curves in the course of the 15-year ERA period. In the right panels, the yearly curves are overlayed in order to emphasize the seasonal cycle. and rapidly transport moist air northward and upward along moist isentropes from the boundary layer to the upper troposphere. Interestingly, deep TST over the Pacific is confined to its western part and the events associated with low tropospheric origins near Japan mainly enter the stratosphere to the west of the date line. This is a further indication for the segmentation of the Pacific storm track into two sections to the west and the east of the date line, respectively, as previously identified by Hoskins and Hodges [2002] based upon a cyclone track analysis. 4. Aspects of the Interannual Variability [22] A 15-year climatology allows a limited investigation of the interannual variability of cross-tropopause exchange. First, 15-year time series of hemispherically integrated cross-tropopause mass fluxes are considered in section 4.1. Then, in section 4.2, the month-to-month variability of the zonally integrated mass fluxes is analyzed, and finally, in section 4.3, the variability of (deep) STT and TST in the North Atlantic and European sector in association with positive and negative phases of the NAO is discussed Time Series of the Hemispherically Integrated Exchange [23] The time series of the monthly mean and hemispherically integrated exchange mass fluxes (STT, TST and net STT TST) are shown in Figure 7. The left panels illustrate the year-to-year variability over the entire ERA15 period, and the right panels emphasize the seasonal cycle (by

9 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-9 Figure 8. Zonally integrated net cross tropopause mass fluxes in units of 10 6 kg s 1 km 1 for (a) winter and (b) summer. The different curves correspond to the individual months for the whole ERA15 period and the bold white line marks the 15-year seasonal mean. overlaying the 15 individual curves for the years ). The fluxes are integrated over the extratropical Northern Hemisphere (from 20 N to 90 N), where the net transport is expected to be downward throughout the year [Appenzeller et al., 1996b; Wernli and Bourqui, 2002]. This extratropical net downward flux is compensated by net upwelling in the tropics, which is not considered in this study. [24] STT (Figures 7a and 7b) shows a distinct seasonal cycle with a flat maximum from December to April and a pronounced minimum in August and September. The amplitude varies by 25% from its maximum to its minimum value. This seasonal cycle is remarkably robust over the 15-year period (Figure 7b), but nevertheless shows a moderate interannual variability. For instance, the peak value in 1987 is about 10% larger than the maximum in The variability is substantial if considering the month with strongest STT: during some years the maximum is in early spring (1981, 1984, 1989, 1990, and 1993), 1987 has a clear February maximum and other years (1980, 1982, 1983, and 1985) are characterized by a double-peak structure. In contrast, there is no systematic seasonal cycle for TST, where the hemispherically averaged values are fairly constant during the year, and as expected, lower than the corresponding STT mass fluxes. As a consequence, the seasonality of the net (STT TST) exchange mass flux (Figures 7c and 7d) is very similar to the one of STT alone, with largest values during winter and early spring and a distinct minimum in August and September. At that time the net mass flux amounts to about 35% of the wintertime maximum value. Again, the interannual variability is largest from December to May with variations up to 20%. [25] Visual inspection of the curves in Figures 7a and 7c and a statistical linear trend analysis reveal no significant trends for STT, TST and the net exchange mass flux. There is an increase in both STT and TST after 1990 (Figure 7a), but the 15-year time period is too short to identify this increase as a statistically robust trend. A refined analysis yields that the North Atlantic and North Pacific sector do exhibit the same increase after Further investigations based upon even longer-term exchange climatologies are required to discuss the issue of trends in cross-tropopause exchange during the last decades more thoroughly. [26] Pronounced seasonal variability can also be discerned for deep exchange events (Figures 7e and 7f ). In fact, the seasonal cycle has a much larger amplitude than the corresponding cycle for all exchange events (Figures 7a and 7b) [see also James et al., 2003a, 2003b]. The maximum of deep STT in January contrasts with the lack of deep STT in summer. A similar cycle exists for deep TSE, although with a smaller amplitude. The differences of deep STT between 2 years can amount to a factor of about 1.5 in winter. This illustrates that the interannual variability of deep exchange is larger than for the total exchange mass fluxes. Also, for deep exchange events, the hemispherically integrated time series do not show a trend, and the increase of total STT and TST after 1990 does not pertain to the category of deep exchange events The Meridional Distribution of the Net Exchange [27] In Figure 8, the zonally integrated net (STT TST) exchange mass flux is plotted for the winter and summer seasons, whereby in each plot the curves for the individual (15 3) months are overlayed. This provides a qualitative measure for the month-to-month variability of the meridional net exchange distribution. In addition to the individual monthly curves the 15-year seasonal mean is indicated by bold white lines. [28] The 15-year seasonal mean distributions are in reasonable agreement with the earlier results by Wernli and Bourqui [2002, Figure 3c]. The strongest net downward fluxes occur during winter near 40 N; during summer the peak is less intense and shifted slightly northwards (see also Figures 2c and 2f). In the subtropics (in particular during summer) and in the Arctic region (north of about 70 N) the

10 STA 6-10 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE Figure 9. Composites for five selected winter months during with maximum (left panels) and minimum (middle panels) NAOI of the cyclone frequency (a and b in %), anticyclone frequency (d and e in %), and tropopause height (g and h in hpa). The right panels show the correlation of the respective field with the NAOI. net mass fluxes are negative (i.e., upward) as already noted in the discussion of Figure 2. [29] Figure 8 indicates on the one hand the robustness of the general pattern and on the other hand remarkable quantitative differences between individual months. Except for winter, the net mass flux in the subtropics is upward for all months. However, the latitude where the net flux changes sign varies between 20 and 35 N. In the Arctic the variability is also quite large and during certain months the net flux is downward, in opposition to the 15-year average Geographical Variability and the NAO [30] Figure 7d indicates that the net mass flux shows little interannual variation if it is integrated over the extratropical Northern Hemisphere ((0.75 ± 0.1) kg s 1 during winter) and Figure 8 shows that for its meridional distribution larger relative variations can occur ((3 ± 1) 10 6 kg s 1 km 1 near 40 N in winter). Here an attempt is made to diagnose the variability of the geographical patterns of (deep) STT and TST, with a focus on the winter season and the Atlantic and European sector. [31] One measure of the interannual variability would be to compare the exchange distributions for the individual seasons with the long-term average. For instance a qualitative comparison of the wintertime STT pattern for the season 1995/1996 [Wernli and Bourqui, 2002, Figure 6a] with the 15-year mean (Figure 2a) reveals pronounced differences over the Atlantic ocean with maximum STT just to the west of Spain in the winter 1995/1996 which was characterized by a very low NAO index (NAOI = 3.9). The difference of this STT distribution with the longer-term climatology might reflect variations of cross-tropopause transport during different phases of the NAO. [32] This issue is now further analyzed with the aid of composites for the five selected winter months with the most positive and negative NAO values, respectively. The NAO represents one of the major teleconnection patterns of the winter Northern Hemisphere [Defant, 1924; Wallace and Gutzler, 1981] and takes the form of a dipole anomaly in the surface pressure field between Iceland and the region near the Azores. Here we use the NAOI defined as the difference of normalized sea level pressure between Lisbon, Portugal and Reykjavik, Island [Hurrell, 1995]. The five winter months with the highest NAOI during the ERA15 period (NAOI > 3.2) are January 1984, January/February 1989 and January/February 1990; and the ones with the lowest index (NAOI < 2.7) are January 1985, February 1986, January 1987, December 1987, and December [33] First, in order to present some characteristics of the positive and negative NAO phases, Figure 9 shows the frequency distributions of surface cyclones and anticyclones and the pressure of the 2 pvu tropopause. (Anti)- cyclones were determined as regions bounded by the outermost closed contour of the sea level pressure field. For all fields the composites for the two NAO phases are depicted (left and middle columns), and the correlation of the monthly averaged field with the monthly NAOI (right column). It is only where this correlation exceeds a value of 0.3 that the link of the NAO with the respective field can be regarded as statistically significant (on the 95% level). For all the fields that will be shown correlations larger than 0.3

11 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-11 Figure 10. Composites for five selected winter months during with maximum (left panels) and minimum (middle panels) NAOI of the STT mass flux (a c), deep STT mass flux (d f), low tropospheric destinations of deep STT (g i), and low tropospheric origins of deep TST (j l). The right panels show the correlation of the respective field with the NAOI. Fluxes are in kg km 2 s 1 and low tropospheric destinations and origins in %. exist only within the region that extends from the central United States to about 80 E. Therefore the diagrams are confined to this area. [34] Not surprisingly, the frequencies of cyclones ( f c ) and anticyclones ( f a ) differ significantly between the two NAO phases. During the positive phase, f c is largest between Iceland and the southern tip of Greenland, and the band with high values of f c extends into the Norwegian and Barentsz Sea (Figure 9a). During the negative NAO phase f c has three maxima: in the central North Atlantic to the east of Newfoundland, in the Mediterranean, and near Novaya Zemlya (Figure 9b). Most of these anomalies compare well with the results from the cyclone tracking analysis by Sickmöller et al. [2000]. Surface anticyclones occur very frequently over the Atlantic in the N latitude band during the positive NAO phase (Figure 9d). In the other phase, the subtropical maximum of f a is much weaker and slightly shifted to the south, and in the region between the UK, Iceland, and Scandinavia f a attains values of about 10% (Figure 9e). The latter feature is strongly anticorrelated with the NAOI (Figure 9f ) and is possibly related to the phenomenon of North Atlantic blocking [Schwierz, 2001]. [35] Tropopause pressure corresponds quite well to the lower tropospheric vortex structures: it is anomalously low above regions of frequent surface anticyclones (the Azores in Figure 9g, and the region near Iceland and the UK in Figure 9h) and high above maxima of f c (to the west of Iceland in Figure 9g and to the east of Newfoundland in Figure 9h). As a consequence, the polar jet extends towards the UK during the positive NAO phase (Figure 9g; note that a steep tropopause generally corresponds to high wind speed), and during the negative phase the polar jet is particularly strong (weak) in the western (eastern) North Atlantic (Figure 9h). In addition, during the positive NAO phase the tropopause is particularly steep (i.e., the subtropical jet exceptionally intense) over northern Africa and the Middle East. These inferences correspond well to the analysis of the 250 hpa zonal wind and its link with the NAO by Ambaum et al. [2001, Figure 4c]. In the following the corresponding composites for the exchange fields will be investigated (Figure 10) and qualitatively compared to the

12 STA 6-12 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE NAO-related variability of the baroclinic wave patterns discussed here STT and TST [36] During the two phases of the NAO, STT experiences strong differences in the Atlantic region that concur with the associated differences in the cyclone frequency field. In the positive phase a maximum of STT occurs in a band between Newfoundland and northern Scandinavia (Figure 10a) very similar to the structure of f c (Figure 9a). In the negative NAO phase the band with maximum STT extends zonally from Newfoundland to western Europe, in agreement with f c (Figure 9b) and the zonal orientation of the steep tropopause region near 40 N (Figure 9h). Furthermore, there is pronounced STT over the Middle East during the positive NAO phase, which might be related to the strongly intensified jet stream just south of this area (Figure 9g). And finally, for positive NAOI, STT is also enhanced in the subtropical central and eastern North Atlantic, where the composite tropopause pressure pattern (Figure 9g) qualitatively indicates frequent anticyclonic Rossby wave breaking to the south of the intense climatological anticyclone (Figure 9d). In all these regions with strong differences of STT between the two NAO phases, STT is significantly correlated with the NAO (Figure 10c). Maximum correlation values of ±0.5 occur over the central Atlantic and over Iraq. In contrast, TST correlates only weakly with the NAO (not shown), although the correlation pattern does not look unlike the one for STT. In section 3.2, it was already noted that the seasonal variations are smaller for TST than for STT, and at least if considering the link with the NAO, the same applies for the interannual variability. The only significant difference occurs over Labrador where TST is strongly enhanced during the negative NAO phase (not shown). This is just upstream of, and potentially related to, the pronounced cyclonic activity near Newfoundland in the negative NAOI composite (Figure 9b) Deep STT and TST [37] Also the distributions of deep exchange and the associated low tropospheric destinations (deep STT) and origins (deep TST) vary considerably between the two phases of the NAO. However, deep STT is only weakly correlated with the NAOI (Figure 10f), except over the eastern Mediterranean and Iraq where deep STT occurs almost exclusively during the positive NAO phase (Figures 10d and 10e). Accordingly, a pronounced maximum of stratospheric intrusions into the lower troposphere extends from northern Egypt to the Gulf of Oman (Figure 10g). Interestingly, in the Atlantic storm track region the composites for deep STT (Figures 10d and 10e) differ quite strongly from the total STT composites (Figures 10a and 10b). Regions with maximum deep STT (to the west of Iceland in the positive and over Newfoundland in the negative NAO phase) are characterized by strong total STT, anomalously high tropopause pressure and surface cyclone frequencies. Other regions with intense total STT (for instance the region near 45 N south of Iceland during the negative NAO phase) have almost no deep STT, weak cyclone activity and a comparatively flat tropopause structure. This leads to the hypothesis that different processes contribute to the almost uniform zonally extending band of strong STT during the negative NAO phase. In the western North Atlantic STT appears strongly associated with deep vertical intrusions of stratospheric air upstream of developing cyclones; further east STT might be more related to the breakup and decay of stratospheric streamers that form downstream of the relatively frequent North Atlantic anticyclones. [38] Finally, deep TST differs mainly in amplitude between the two NAO phases (not shown). In the region extending from Newfoundland to Labrador and southern Greenland deep TST is strongly enhanced during the negative phase of the NAO, when surface cyclones are particularly frequent close to this area. This is also reflected in the distribution of the low tropospheric origins of deep TST (Figures 10j and 10k): in the negative NAO phase the probability for rapid transport from the boundary layer into the lowermost stratosphere is significantly larger, and the source region is slightly further south and has larger amplitude over the U.S. east coast. This indicates on the one hand that conveyor belts during the negative NAO phase are potentially more vigorous (due to their origin in an area of higher sea surface temperature), and on the other hand that during the same phase there is enhanced likelihood of transport of emissions from the eastern United States into the stratosphere. 5. Discussion and Conclusions [39] It was the principal aim of this study to establish for the first time a 15-year climatology of the geographical distributions of upward and downward cross-tropopause transport based upon a sound Lagrangian methodology and a consistent set of reanalysis data with a sufficiently high resolution to capture the (sub)synoptic-scale processes associated with the development and breaking of baroclinic waves. The applied methodology to identify exchange events has been introduced recently by Wernli and Bourqui [2002]. It takes into account the residence time of exchange air parcels and their origin and destination over a time period of a couple of days. The present study is complemented by the particle dispersion model calculations by James et al. [2003a, 2003b] who investigated also longer residence times and assessed the contribution of STT to the tropospheric ozone budget. Together, these Lagrangian studies constitute an important link between detailed single case analyses and coarser-scale global estimations of crosstropopause transport. Furthermore, they both emphasize the importance of vertically deep STT whose seasonal cycle is characterized by a clear winter peak and a larger amplitude compared to the total STT mass flux. [40] The results confirm the conclusions from our previous 1-year investigation [Wernli and Bourqui, 2002] and reveal important additional characteristics of the northern hemispheric exchange climatology, in particular the geographical distributions of (deep) STT and TST and their seasonal cycle, the potential temperature distribution of exchange, and aspects of the interannual variability during the considered time period from 1979 to These findings will be summarized and discussed in the following paragraphs, but first consideration is given to the residence time sensitivity of the calculated mass fluxes (an aspect that was discussed in detail by Wernli and Bourqui [2002] and Bourqui [2001]). [41] In this study results have been presented only for a threshold residence time of 96 hours, this means that for

13 SPRENGER AND WERNLI: ERA15 CLIMATOLOGY OF CROSS-TROPOPAUSE EXCHANGE STA 6-13 instance a STT event was considered as significant only if it remained at least 4 days in the stratosphere (troposphere) before (after) crossing the 2 pvu tropopause. Shorter-term exchange events have been neglected, on the basis of the assumption that irreversible mixing of the exchange air parcel with its new environment is not possible if it recrosses the tropopause too rapidly. This is supported by sparse observations of well defined ozone layers in the troposphere that persisted for several days [e.g., Bithell et al., 2000]. However, representative timescales for irreversible mixing are unknown and therefore our choice of a 4- day threshold is somehow arbitrary. Additional calculations for thresholds of 1, 2, and 3 days (not shown) are in line with the results shown by Wernli and Bourqui [2002, Figures 2 and 6] and indicate that the gross STT and TST mass fluxes are strongly sensitive to this parameter, whereas the net mass fluxes and the geographical distributions of STT and TST are less so. The issue of mixing certainly constitutes one of the central topics for further research of cross-tropopause exchange and its chemical implications. [42] Now we turn to a brief discussion of the main findings of this 15-year exchange climatology. The geographical distribution of STT reveals strong zonal variations and, except for summer, pronounced maxima in the northern and downstream areas of the North Pacific and North Atlantic storm track regions and over the Mediterranean (Figures 1a and 2a). During summer STT is comparatively weak over the oceans and has pronounced maxima over the Eurasian continent. TST shows a weaker seasonal cycle and less zonal variability. Nevertheless, there are clear maxima near Greenland and the Aleutian Islands (Figures 1b, 2b and 2c). Consequently the net exchange (which is generally downwards in midlatitudes) maximizes during winter in the eastern North Pacific and central North Atlantic, and during winter over Europe and central Asia. This geographical and seasonal variability is remarkable and indicates on the one hand that cross-tropopause mass fluxes can not be accurately parameterized based upon zonal mean exchange calculations, and on the other hand that different processes are likely to be responsible for the peak STT activity during summer over the continents and during the rest of the year over the oceans. Deep exchange is even more variable (Figures 4 6). These events occur preferentially during winter, and the geographical distributions of their origin (deep TST) and destination (deep STT) reveal well defined areas where frequent interaction through rapid vertical transport is taking place between the stratosphere and the boundary layer. [43] Consideration of the potential temperature at the exchange location of all individual events yields almost Gaussian distributions with peak exchange activity varying from about 300 K (winter) to 320 K (summer) for STT, and from 305 K (winter) to 325 K (summer) for TST (Figure 3). During summer and autumn an additional peak of TST occurs at 360 K. Again, these seasonal variations are of significance and reveal that single isentropes are not representative to asses the seasonal cycle of processes near the tropopause. For instance, Seo and Bowman [2001] considered isentropic exchange on the 320 and 350 K isentropes and found a summer maximum of STT. This is in agreement with the present study if considering only these isentropes (Figure 3). However, exchange processes on lower isentropes shift the maximum of total hemispheric STT to winter and early spring (Figure 7b). It is also worth noting that a substantial part of STT occurs on fairly low isentropes (for instance in winter on K), that are usually not looked at in detailed case studies of midlatitude synoptic systems. [44] For the considered 15 years no significant trends in the northern hemispheric exchange activity could be identified. Also, the zonal mean net exchange distributions are relatively robust. However, the interannual variability of the geographical exchange patterns can be substantial as indicated by composites of (deep) STT and TST for the Atlantic and European sector for the positive and negative phases of the NAO. In particular the geographical distribution of STT differs significantly between the two extreme phases, similar to the cyclone frequency (Figures 9 and 10). Other significant features of variability associated with the NAO are the maximum of (deep) STT in the eastern Mediterranean and subsequent downward transport towards the Persian Gulf. This area near N, extending from Greece over Turkey to the western Himalayas appears as a very interesting (and not yet intensively studied) area of tropopause dynamics and associated cross-tropopause transport. During summer, it is a region with frequent tropopause foldings [Sprenger et al., 2003], strong STT and TST (Figure 2) and with a very high tropospheric ozone column as indicated by a global chemistry model study for July 1997 [Li et al., 2001]. [45] It remains for future studies to analyze quantitatively the link between cross-tropopause transport in the extratropics and the dominant meteorological structures and processes (e.g., cyclones, anticyclones, PV streamers, and PV cutoffs). Case studies indicated that these features can be associated with pronounced exchange and the present climatology confirms this qualitatively, at least for the autumn, winter and spring seasons. A detailed investigation is underway to quantify the exchange associated with individual cyclone life cycles and Rossby wave breaking events. Finally, it is important to note that the currently processed ERA40 data set will provide the possibility to extend the present climatology and to examine in more detail the issue of long-term trends and interannual variability. Comparison of exchange mass fluxes based on ERA15 and ERA40 data (that are processed using different model resolutions and assimilation techniques), will also allow to assess the sensitivity of quantitative cross-tropopause exchange estimates on the quality of the meteorological data. [46] Acknowledgments. We thank the ECMWF and MeteoSwiss for providing access to the meteorological data. Discussions with and comments from Andreas Stohl, Cornelia Schwierz, Huw Davies, Brian Hoskins, and the STACCATO community were highly appreciated. This research was supported through the EC project STACCATO under contract BBW resp. EVK2-CT References Ambaum, M. H. P., B. J. Hoskins, and D. B. Stephenson, Arctic oscillation or North Atlantic oscillation?, J. Clim., 14, , Appenzeller, C., H. C. Davies, and W. A. Norton, Fragmentation of stratospheric intrusions, J. Geophys. Res., 101, , 1996a. Appenzeller, C., J. R. Holton, and K. H. Rosenlof, Seasonal variation of mass transport across the tropopause, J. Geophys. Res., 101, 15,071 15,078, 1996b.