• Introduction
• Diagnostic tools
• A case study of isentropic exchange above Reunion Island
• Conclusion
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figures 5a, 5b, 5c, 5d, 5e, 5f
• Figures 6a, 6b, 6c, 6d, 6e, 6f,
• Figure 7

1) Introduction :

The Rossby wave breaking events participate directly to the transport of air masses from the tropics to the poles, via the Brewer-Dobson circulation marked by large-scale air rising in the tropics and by sinking in the poles [Holton et al., 1995]. These wave breaking events to midlatitudes also generate air filaments born in the tropics, stretched isentropically and ultimately mixed in the surf zone [Waugh et al., 1994a, 1994b]. Conversely, other filament structures do an opposite transport, from the surf zone to the tropics. The behaviour of long lived chemical species and of potential vorticity implies the existence of a dynamic stratospheric subtropical barrier that controls and limits isentropic horizontal exchanges between tropical and extratropical stratospheres[Grant et al., 1996]

The seasonal variation of the intensity of planetary wave breakings is characterised by a maximum in the winter hemisphere. Hence, larger gradients on the Tropical Stratospheric Reservoir (TSR) edges and a more important filamentary activity are expected during this period. The seasonal variation of intensity and location of the subtropical barrier is also function of altitude and depends significantly on the QBO phase [Randel et al., 1998; Dunkerton et al., 1996 ; Schroeberl et al., 1997].

From a case study based on Reunion island data, this work presents a preliminary analysis of isentropic transport trough the subtropical stratospheric barrier. Reunion Island [21°S, 55°E] is a French territory in the south western part of Indian Ocean. Reunion is one of the few stations steadily recording atmospheric measurements in the southern tropical zone.
From ozone anomaly profiles obtained from ECC radiosoundings and from DIAL O3 lidar measurements, an example of isentropic exchange between 550K and 700K is presented. This case study exchange occurred around the 12^th July 2000. A high resolution PV contour advection code (MIMOSA) and a dynamic barrier locating code taking advantage of an area coordinate (LOBADY) are used as a diagnostoc tools. These codes are working on potential vorticity data from ECMWF fields.

2) Diagnostic tools:

1) Area coordinate code (LOBADY):

This code is based on dynamic diagnostic tools developed by Nakamura [Nakamura et al., 1996] running on PV maps from ECMWF. In this concept, the two dimensional PV mixing is diagnosed using A, the area between PV contour and a reference contour, presently corresponding to equator line. For dynamic diagnostic, two tools using an area coordinate are useful to define and identify barriers:

• the PV gradient in area coordinate . This gradient is expected display a local maximum at the barrier location.
• L²= 
  where 
  L is the equivalent length of the PV contour and determines the mixing effectiveness due to erratic movements. This equivalent length is expected to display a minimum at the barrier position.

  By combining these two criteria, it is possible to determine PV values corresponding to barriers and to infer barriers location by reversing to Cartesian coordinates.
  This method enables to make a quasi-automatic detection of dynamic barriers on instantaneous PV maps.

 

2) PV advection code (MIMOSA):

We use the high resolution contour advection code (MIMOSA) developed at the Service d'Aéronomie, CNRS, by A. Hauchecorne [Hauchecorne ,1998]. This code is based on PVdata calculated from 1.125x1.125° ECMWF fields. From these fields, PV is advected on twelve hours and regrided on the initial grid to reduce advection divergence.

3) Ozone measurements at Reunion island station:

Pressure, temperature, humidity and ozone radiosoundings have been launched routinely at Reunion island since 1992, with a bi-mensual rate until the beginning of 1999 and every week from that date. ECC cells with a 0.5% cathode solution are used for ozone measurements. More than two hundred profiles have been recorded and a monthly climatology of ozone from this Reunion data set is available. Anomaly detection on individual profiles are set against this climatology in the present study.

A stratospheric ozone DIAL Lidar has been installed at Reunion island, since May 2000. This Lidar, presented on Fig. 1, has been built by Geneva University and its scientific management is done together by the Service d'Aeronomie, CNRS, and the Laboratoire de Physique de l'Atmosphere, Reunion University. Ozone vertical profiles are obtained between 18 and 40 km altitudes.

Figure 1: schematic representation of the Reunion stratospheric ozone DIAL Lidar

 

3) A case study of isentropic exchange above Reunion Island:

The radiosounding ozone profile versus potential temperature, obtained the 12^th July 2000, at Reunion island, is presented on figure 2. It is compared with the monthly climatologic ozone profile of July. The 12^th July, ozone concentration shows an abnormal behaviour above 500K by comparison with the climatologic evolution. This anomaly is important and significative between 550K and 750K. It can be noted that it is positive below 625K and negative above. It can reach 15 to 20% around 680K.

Figure 2: comparison of the 12-07-2000 ozone radiosounding profile with the July climatologic profile.

 

In order to confirm this observation on the radiosoundings ozone profile, we have also a Lidar sounding for the 12^th of July night, that is to say 12 hours after the radiosounding. The Lidar profile displays fluctuations very closed to the radiosounding ones, as we can see on Fig 3. The differences, less than 8% above 20 km, can be explained by the fact that the two measurements were not simultaneous and that the two instruments have different vertical resolutions. Yet, the anomaly observed on radiosounding is clearly apparent on Lidar measurement, between 24 and 28 km.

Figure 3: Comparison between Lidar and radiosoundings profiles obtained the 12th of July, at Reunion island (left box). Statistic error on DIAL profile (medium box). Relative error between the two measurements (Right box)."

In order to understand the origin of this anomaly on the ozone profile, it is interesting to consider the potential vorticity behaviour above Reunion Island for the month of July 2000. All the potential vorticity data shown in this study have been calculated from ECMWF dynamic fields. Its evolution above Reunion Island during July 2000 at 600K is presented on Fig. 4. The average of this parameter correponding to whole austral winter 2000, and 2s upper and lower limits are also presented (dotted line) in this Figure. It is to be noted that vorticity above Reunion Island, displays an important anomaly, significative at 2s on 12^th of July 2000. This anomaly is positive and corresponds to the arrival of very high PV air masses in this zone.

Figure 4: PV evolution at 600 K above Reunion island during July 2000 compared with average PV value on all winter 2000.

The results computations by LOBADY code of dynamic barriers location, for dates around 12^th July 2000 at 600K altitude, are presented on Fig. 5. It is to be noticed that Reunion island (West from Madagascar) is rather under the influence of equatorial air masses before the 10^th July. The subtropical dynamic barrier is around Reunion Island on the 10^th July. From the 11^th July, the stratosphere above this site is under the influence of midlatitudes air masses . It is noteworthy that although Reunion is under the influence of midlatitudes from the 11^th July, the major influence on PV (Fig. 4) is actually taking place since the 12^th.

It can clearly be observed, on Fig. 5, that the subtropical barrier is initially present around 25°S on average. It loses its shape from the 10^th of July above South Africa, creating a low PV large tongue that stretches, the following days, towards the south of Indian Ocean area. This way down of low PV to the south is going simultaneously together with , on its front, a way up of midlatitudes air to the north. It is this high PV back tongue that influences Reunion island from the 12^th of July.

Figure 5: south subtropical dynamic barrier location, around 12^th July 2000, calculated with LOBADY code.

 

In order to confirm these observations, we have computed with MIMOSA code high resolution PV contour advections. The PV fields advected for dates around the 12^th of July are presented on Fig. 6. The much better resolution of contour advection method enables to more occuratly observe the filament coming above Reunion. By zooming in the southwestern part of the Indian Ocean, we can more clearly observe the arrival of a high PV air mass on the 12^th, above Reunion island, coming from midlatitudes. This model shows again the formation of a low PV air filament around the 9^th of July in the southern part of African continent. This filament stretches to the south-east and is phased with the way up of midlatitudes air masses to Reunion island.

Model outputs, for the following days, show that Reunion island stays under the influence of this high PV core only until the 15^th of July. The low PV tongue further stretching, after this date, to the south-east. It will be completely mixed in the surf zone around the 22^th of July.

09/07/2000                                                                10/07/2000

11/07/2000                                                                12/07/2000

13/07/2000                                                                14/07/2000

Figure 6: Map of advected PV around Reunion Island at 600 K, calculated with MIMOSA high resolution transport model.

 

The zonal wind at 800K, for this period (data not shown), shows the formation of a very important westerly core on the south of Madagascar, around 35°S. Its evolution is directly correlated to the filament development described above. This core is born around the 10^th, reaches its maximum on the 12^th and moves, the following days, to the east. This suggest that this westerly core could be at the origin of the filamentation. This agrees with the fact that planetary waves can enter from below in the stratosphere only when winds are from the west. A strong transport out of the tropics occurs when there are westerlies in midlatitudes [Waugh et al., 1996].

In order to further understand the signatures observed on ozone profiles, we have compared annual ozone profiles obtained from HALOE instrument on board of UARS platform. Three characteristic climatologic profiles have been calculated: the first one corresponds to Reunion island latitude, the second to latitudes close to 10°S and the third to midlatitudes around 40°S. They are presented on Fig. 7.

Assuming that ozone is conservative during a transport of relatively short time scale, the Fig. 7 suggests that the arrival at Reunion island of air masses coming from midlatitudes should implicate a positive signature on ozone, if the transport is occurring at an altitude inferior to about 625K, and a negative anomaly if the transport is taking place above.

Figure 7: comparison of annual climatologic ozone profiles corresponding to equatorial latitudes (10°S), midlatitudes (40°S), and Reunion island latitude. These profiles are obtained from HALOE measurements.

 

This last inference is in complete agreement with the anomalies observed on 12^th July ozone profiles. An increase of concentration is observed below 625K and a decrease above.

Conclusion:

DIAL Lidar observations and the radiosounding observations on stratospheric ozone obtained at Reunion island offer the opportunity to analyse a case study of quasi-horizontal stratospheric transport, between 550K and 700K, around the 12^th July 2000. This transport has been diagnosed from the dynamic barrier detection LOBADY code and the high resolution PV advection MIMOSA code. It appears that the air masses that interest Reunion island from the 12^th of July, and that induce a high PV increase, come from midlatitudes. This isentropic way up is consecutive to the formation of an air filament coming from equatorial latitudes, characterised by low PV, stretched to the south east and isentropically mixed in the midlatitudes surf zone. This transport from midlatitudes are perfectly according with the anomalies observed on Reunion island ozone profiles on the 12^th of July.

The installation of a stratospheric DIAL lidar at Reunion Island, and the implementation of performant diagnostic tools for dynamic analysis, enable today to think of making more complete studies on stratospheric transport in this tropical region. Such studies should document the transport through the subtropical stratospheric barrier and the long term ozone evolution.

Acknowledgement : I would like to thank all the LPA radiosoundings team, the technical and scientific LIDAR team from Service d'Aéronomie of CNRS. I would like to thank particularly Jean-Luc Baray for helpful comments and suggestions. HALOE data are issued from NASA Langley Research Centre.

 

References :

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Hauchecorne, A., "A high resolution advection model for interpretation of ozone filaments observed in lower stratosphere ozone lidar profiles at midlatitudes", Proc. of Mesoscale Processes in the lower stratosphere, Bad Toelz, Germany, 1998.

Grant, W.B., E.V. Browell, C.S. Long, L.L. Stowe, R.G. Grainger, and A. Lambert, "Use of volcanic aerosols to study the tropical stratospheric reservoir", J. Geophys. Res., 101, 3973-3988, 1996.

Holton, J.R., P.H. Haynes, M.E. McIntyre, A.R. Douglass, R.B. Rood, and L. Pfister, "Stratosphere-troposphere exchange." Rev. Geophys., 33, 403-439, 1995.

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Schoeberl, M.R., A.E. Roche, , J.M. Russell III, D. Ortland, P.B. Hays and J. W. Waters, "An estimation of dynamical isolation of the tropical lower stratosphere using UARS wind and trace gas observations of the quasi-biennal oscillation", Geophys. Res. Let., 24, 1, 53-56, 1997.

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