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Tropical upper tropospheric ozone enhancements due to potential vorticity intrusions over Indian sector
Journal of Atmospheric and Solar-Terrestrial Physics 132 (2015) 147–152
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Tropical upper tropospheric ozone enhancements due to potential vorticity intrusions over Indian sector M. Sandhya a,b, S. Sridharan a,n, M. Indira Devi b, H. Gadhavi a a b
National Atmospheric Research Laboratory, Gadanki, Pakala Mandal, Chittoor District, Andhra Pradesh, India Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2014 Received in revised form 16 July 2015 Accepted 17 July 2015 Available online 18 July 2015
Influence of potential vorticity (PV) intrusions at 13.5°N over and near Indian sector (50°E–90°E) on tropical upper tropospheric ozone mixing ratio (OMR) variations is demonstrated based on two case studies. Increase of ECMWF (European Centre for Medium-range Weather Forecasting) reanalysis (ERA)interim OMR in the upper troposphere (200–500 hPa) is observed during the intrusion events consistently in both cases. The OMR also shows similar tongue like structure as PV and it even follows the spatial shift of the PV tongue. In addition, the enhancements in the upper tropospheric OMR during the intrusion events are confirmed using microwave limb sounder (MLS) ozone data at 216 hPa. It is suggested that the existence of strong downdrafts, associated with the ageostrophic circulation due to jet stream, which is inferred from longitude-height cross-section of ERA-interim vertical velocity could bring the ozone further down, though high PV tongue remains only at higher level (above 400 hPa). The importance of these results lies in demonstrating the role of PV intrusion events on the enhancement of tropical upper tropospheric ozone over Indian sector, where the impact of the PV intrusions is not well understood when compared to that over Pacific and Atlantic sectors. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Potential vorticity intrusion Upper tropospheric ozone Vertical velocity
1. Introduction Stratospheric ozone protects us from ultra-violet radiation, whereas upper tropospheric ozone is a greenhouse gas and it contributes positively towards the radiation budget by trapping global outgoing longwave radiation (OLR) and thereby increases the surface temperature (Worden et al., 2008). The photochemical reaction and downward transport from the stratosphere are the major sources for the upper tropospheric ozone (Danielsen, 1968; Logan, 1985), though the latter dominates the former (Roelofs and Lelieveld, 1997). The upper tropospheric ozone is mainly controlled by stratosphere–troposphere exchange (STE) processes at mid- and high latitudes (Barre et al., 2012), though the influence of these processes on low-latitudes is yet to be understood (Levy et al., 1985; Hsu and Prather, 2009). Besides, ozone variabilities show close connection with quasi-biennial oscillation and El-Nino Southern Oscillation (Zerefos et al., 1992). Seasonal cycle of ozone concentration at tropopause and the substantial mass flux across tropopause determines the downward flux of ozone to the upper and middle troposphere (Škerlak et al., 2014). The tropopause folds are favourable for the cross-tropopause exchange (Danielsen, 1968) and they can be identified from large potential vorticity (PV) n
Corresponding author. Fax: þ 91 8585 272018.
http://dx.doi.org/10.1016/j.jastp.2015.07.014 1364-6826/& 2015 Elsevier Ltd. All rights reserved.
value in the upper troposphere (Elbern et al., 1998; Tyrlis et al., 2014; Škerlak et al., 2015). These PV intrusions, transport trace gases like ozone from the stratosphere to the upper troposphere and further to lower levels (Sprenger et al., 2007), though major contribution to tropospheric ozone comes from photochemical production (Staehelin et al., 1994; Yenger et al., 1999). Fadnavis et al. (2010) studied seasonal variation of ozone over India and they observed that the upper tropospheric ozone is more during winter and pre-monsoon months. Consistent with this, Sandhya and Sridharan (2014) noted more PV intrusions over Indian sector during pre-monsoon and winter. Rossby wave breaking at midlatitude stratosphere leads to intrusion of high potential vorticity air from mid-latitude stratosphere to tropical troposphere in the presence of upper tropospheric westerlies, which are favourable conditions for the intrusions (Waugh and Polvani, 2000). These PV intrusions play a vital role on promotion of convection (Kiladis, 1998, Sandhya and Sridharan, 2014), inhibition of convection (Russell et al., 2008), intensification of cyclone (Browning, 1997) etc. Most of the studies on the influence of PV intrusions on surface or lower tropospheric ozone variabilities are from Northern Hemispheric mid-latitudes (Wakamatsu et al., 1967; Chung and Dann, 1967; Davies and Schuepbach, 1994; Bithell et al., 2000 ; Langford et al., 2012; Lin et al., 2012 to state a few). Not much work has been done from low-latitudes and in particular over Indian sector due probably to the less number of occurrence of PV
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intrusion events. The present study shows an evidence for the increase in ozone mixing ratio (OMR) in the tropical upper troposphere associated with a few PV intrusion events over Indian sector.
2. Data analysis 2.1. ERA-interim data sets European Centre for Medium Range Weather Forecasting (ECMWF) data are produced on model levels (hybrid pressuresigma coordinates) and at the surface. ECMWF data are also interpolated to pressure levels and some datasets include data on isentropic levels. The ECMWF reanalysis (ERA) interim uses 60 levels with the model top at 0.1 hPa. The ERA-interim data sets are results from the analysis conducted at 6-h intervals available for latitude–longitude grids 3° 3° 0.125° 0.125° and are prepared by ECMWF using their variational data assimilation system (Berrisford et al., 2009). The data sets are currently available in the website http://apps.ecmwf.int/datasets/data/interim-full-daily// for 15 isentropic levels and 37 pressure levels. In the present study, potential vorticity, ozone mixing ratio and vertical velocity (Omega) at different pressure levels and potential vorticity and zonal wind at 350 K isentropic level and zonal wind, meridional wind, geopotential at 200 hPa pressure level are used. The ERA-Interim data is divided mainly in to three periods namely Pre GOME assimilation period (till 1995), GOME assimilation period (1996–2002) and the post GOME assimilation period (2003 onwards). Dragani (2010) studied the quality of ERA-Interim ozone data by comparing with in situ measurements. They reported that in the pre GOME assimilation period, the residuals between ozonesondes and the corresponding ERA-Interim ozone profiles were within 710% in the tropics and mid-latitudes at most levels and within 7 20% at high-latitudes. However in GOME assimilation and post assimilation periods, the level of disagreement was within 7 5% in the tropics.
2.2. MLS-Ozone data The Aura Microwave Limb Sounder (MLS) onboard Earth Observing System (EOS) launched on 15 July 2004. It is sun synchronous with altitude 705 km and with 98° inclination. ML2O3 version 3 data is used for the present study. This is a standard product for ozone derived from radiances measured by the 240 GHz radiometer. The spatial coverage is near-global ( 82° to þ82° latitude), with each profile spaced 1.5° or ∼165 km along the orbit track (roughly 15 orbits per day). The recommended useful vertical range is from 261 to 0.0215 hPa and the vertical resolution is between 2.5 and 6 km (Cheung et al., 2014).
3. Results The intrusion of high PV tongues, from high to low latitudes can be identified from latitude to longitude cross-section of PV at 350 K isentropic level, for the days 15 May, 2011 and 7 May, 2009 respectively in Fig. 1a and b. On 15 May 2011, the tongue of high PV greater than 1.4 PVU (potential vorticity unit; 1 PVU¼10 6 km2 kg 1 s 1) reaches up to 12°N over 65°E–70°E and on 7 May 2009, it crosses 13.5°N over 60°E–65°E. The latitude–longitude cross-section of magnitude of the resultant wind obtained from the zonal and meridional wind components and geopotential at 200 hPa (∼ 350 K) for the same days are shown in Fig. 2c and d. The subtropical jet flow can be inferred from the wind speed greater than 50 m/s over 20°N40°N on 15 May 2011 with the trough of subtropical jet over 65°E–70°E. The existence of trough can also be identified from the folded geopotential contour. Fig. 2d also shows the subtropical jet flow over 30°N–40°N on 7 May 2009, when the trough of the jet stream is over 60°N–65°N at latitude 13.5°N. Extratropical air with high PV can enter to the tropical troposphere through the tropopause fold at the western side of the upper level trough associated with jet streams (Kentarchos et al., 1999, Ding and Wang, 2006). In order to show the time evolution of PV intrusion shown in Fig. 1, the time-longitude cross-sections of PV and zonal wind at
Fig. 1. Latitude–longitude cross-sections of (a, b) PV at 350 K isentropic level and (c, d) resultant wind speed and geopotential at 200 hPa for the days 15 May 2011 and 7 May 2009.
M. Sandhya et al. / Journal of Atmospheric and Solar-Terrestrial Physics 132 (2015) 147–152
149
Fig. 2. Longitude-time cross-sections of (a, b) PV and (c, d) zonal wind at 13.5°N for the days 120–150 of year 2011 and 2009.
350 K isentropic level for the period 1–31 May 2011 and 2009 are shown in Fig. 2. The intrusion event is identified from the PV value greater than 1.4 PVU. The PV intrusion event can be inferred from the PV value exceeding the threshold in Fig. 2a and b on 15 May 2011 (day no. 135), and persists till 21 May, 2011 (day no. 141) over 50°E–70°E and during 7–9 May 2009 (day no. 127–129) over 50°E– 65°E respectively. The zonal wind at 350 K isentropic level (Fig. 2c and d) shows westerly flow over the intrusion region, during the days of PV intrusion. The upper tropospheric westerlies favour PV intrusion to tropical latitudes (Waugh and Polvani, 2000). On 15 May 2011 (day no. 135), the westerly maximum (410 m/s) is over the region 65°E–70°E. However as day progresses, the regime of eastward flow shifts further west to 60°E–50°E. Corresponding to the shift in the upper level westerlies, the PV intrusion also shows (Fig. 1a) westward movement. However in the case of 2009 PV intrusion event, there is no westward shift and the upper troposphere zonal wind remains westerly over the region 50°E–80°E during 1–11 May. Fig. 3a–e shows the longitude height cross-section of PV for the days 14–18 May 2011. On 14 May 2011, descent of a PV tongue can be seen, though the PV value does not reach the threshold to identify as the PV intrusion event. The occurrence of PV intrusion can be inferred on 15 May 2011 below 200 hPa over 65°E–70°E. As the day progresses, the upper PV anomaly shifts to the west of the
intrusion region consistent with the shift in upper tropospheric westerly and on 19 May 2011, it prevails over 50°E–60°E (not shown). However the tongue of PV reaches nearly 400 hPa during 15–18 May 2011 and 450 hPa on 19 May 2011 (not shown). Associated with this intrusion event, the upper troposphere ozone (Fig. 3f–j) shows enhancement. On 14 May 2011, though the PV does not reach the threshold, OMR shows same tongue structure as PV and it penetrates down to 350 hPa. The OMR is high (4115 ppb) along the downward track of the tongue, whereas OMR at other non intrusion region is below 90 ppb only. On subsequent days also, high OMR along the track of PV tongue can be inferred from the figure. The high OMR tongue descends from 400 hPa on 15 May to 450 hPa on 19 May 2011 and it can be noted that the OMR follows the westward shift of the PV anomaly. Though the PV tongue is just above 400 hPa, the OMR tongue descends further down to 400 hPa on 17–18 May 2011. Fig. 4 shows longitude height cross-sections of PV (Fig. 4a–e) and OMR (Fig. 4f–j) during 5–9 May 2009. The PV intrusion event can be inferred on 7 May 2009 from the PV value greater than 1.4 PVU over 60°E–65°E. On 5 May 2009 when there is no PV intrusion and high OMR ( 4130 ppb) exists only above 300 hPa and the tongue like structure is absent. On 6 May 2009, though the PV value does not reach the threshold to identify the PV intrusion, PV tongue with relatively higher values of PV can be seen over 50°E–
Fig. 3. Longitude-height cross sections of PV and OMR at 13.5°N from 14 May 2011 to 18 May 2011.
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Fig. 4. Longitude-height cross sections of PV and OMR at 13.5°N from 05 May 2009 to 09 May 2009.
60°E. Associated with this high PV layer, OMR ∼130 ppb reaches near to 400 hPa. On the next day, when PV crosses the threshold (1.4 PVU) over 60°E–65°E, the high ozone tongue reaches down to even 550 hPa, though the high PV tongue prevails only above 300 hPa. However, on the subsequent days (8–9 May 2009), though the descent of PV is not present to lower levels, existence of high OMR can be observed throughout the upper troposphere over 65°E–70°E. The presence of high OMR in the upper troposphereduring the PV intrusion events are confirmed with observational data taken from MLS. The MLS ozone data is less contaminated in the UTLS region (Livesey, 2007). Fig. 5 shows MLS OMR variations at 216 hPa over 10°N–16°N and 50°E–90°E for the above presented cases. The day just before intrusion is marked with a star symbol in the figure. The average values of OMR at 216 hPa for the above presented cases are 75.7 ppb and 68.6 ppb for the years 2009 and 2011. Immediately after the PV intrusion, the OMR value increases to greater than 80 ppb and reaches a maximum of 115 ppb in the case of 2009 event and 70–120 ppb in the case of 2011 event. Though there is an increase in the upper tropospheric ozone due to the PV intrusion events, which transport stratospheric ozone rich air to the tropical upper troposphere, the ozone can descend further down to lower levels due to the ageostrophic flow at the western side of upper level trough associated with a jet stream (Kentarchos et al., 1999). The ageostrophic circulation associated with jet stream might be the reason for the downward
Fig. 5. Daily variation of MLS OMR at 216 hPa averaged over 10°N–16°N and 50°E– 90°E for the years 2011 and 2009.
transport of ozone further down to the PV tongue. The longitudeheight cross-section of pressure velocity for the days of intrusion (15 May 2011 and 7 May 2009), along with 1 PVU and 1.5 PVU PV isolines at 13.5°N is shown in Fig. 6a and b. The PV isolines descend to 350 hPa over 65°E–70°E and to 250 hPa over 55°E–65°E on 15 May 2011 and 7 May 2009 respectively, indicating the tropopause fold over that region. The wind is downward with value greater than 0.1 Pa/s at the height region 250–600 hPa over 60°E– 70°E and the downward wind persists till 1000 hPa over 60°E–65° E on 15 May 2011. Similarly, the vertical velocity is downward over 50°E–65°E from 300 to 900 hPa on 7 May 2009 also. This downward wind might have transported OMR further down in the troposphere, though high PV tongue is restricted to higher level and it gets diffused over time.
4. Discussion and conclusions Two cases are presented (May 2011 and May 2009) in which there is a clear evidence of the influence of PV intrusions at 13.5°N over and near Indian sector (50°E–90°E) on tropical upper tropospheric OMR variations. The latitude–longitude cross-section of PV at 350 K and upper level wind at 200 hPa on the days of intrusion show that the mid-latitude air enters to the low latitudes along the trough of subtropical jet stream. Near jet streams, PV gradient over an isentropic level takes the form of a narrow tube of enhanced PV gradient, which can act as a waveguide for Rossby waves (Schwierz et al., 2004). Wave breaking occurs when the Rossby waves having sufficiently large amplitudes propagating into the westerly duct region and it produces intrusions of stratospheric air with high PV into the tropical upper troposphere (Waugh and Polvani 2000). In the present study, the upper tropospheric westerly at 350 K isentropic level favours the intrusion to the low latitudes in both cases. Based on a climatological study, Waugh and Polvani (2000) demonstrated the importance of upper tropospheric westerly winds during Northern winter over Pacific and Atlantic sectors for the PV intrusions to low-latitudes. Sandhya and Sridharan (2014) also noted that the PV intrusions over Indian sector during pre-monsoon time were also favoured by upper tropospheric westerlies. Here, the PV intrusion during the 2011 case shows a westward shift as day progresses. The westward movement of PV is found to be consistent with the westward shift of the upper tropospheric westerly winds.
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Fig. 6. Longitude-height cross section of vertical velocity (omega) at 13.5°N for the days 15 May 2011 and 7 May 2009. The isolines of PV at 1 and 1.5 PVU are also plotted.
Besides, the longitude-pressure variation of PV during the intrusion days shows the vertical extension of PV penetrating up to 400 hPa in both the cases. Enhancements of ERA-interim OMR in the upper troposphere (200–500 hPa) are consistently observed during the intrusion events in both the cases. The OMR also shows similar tongue like structure as that of PV and it even follows the spatial shift of the PV. In addition, the enhancements in the upper tropospheric OMR during the intrusion events are confirmed using the MLS ozone data at 216 hPa. The upper tropospheric ozone enhancement continues for a long time (∼8 days) in the case of 2011 event, whereas, the OMR enhancement persists only for 5 days in the 2009 event. This is found to be consistent with the life cycle of PV intrusion event in both the years. Earlier studies reported that the STE was a major source for tropospheric ozone (Danielsen, 1968). Fadnavis et al. (2010) noted enhancement in the monthly mean upper tropospheric ozone over Indian sector during winter and premonsoon months and attributed it to be due to the stratospheric intrusions, which are more during these months. In the present study, two case studies are shown relating the occurrence of PV intrusions and upper OMR enhancements directly. The vertical velocity on intrusion days shows downward motion over the intrusion region. However the vertical velocity is upward to the east of intrusion. It is suggested that the downward vertical velocity associated with the ageostrophic circulation due to the jet flow might have transported ozone further down to the levels of troposphere, though the PV tongue remains at higher levels and gets diffused by mixing with surrounding air. Increase of upper tropospheric ozone can impact air quality if it is transported to the boundary layer (Fiore et al., 2002). The impact of tropospheric ozone on the weather and climate will be the subject of our future study.
Acknowledgements The authors gratefully acknowledge ECMWF for providing the ERA-interim data sets used in this study. The author wish to thank the Editor and the two Reviewers for their comments and suggestions, which greatly helped them to improve the manuscript to its present form.
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Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Tropical upper tropospheric ozone enhancements due to potential vorticity intrusions over Indian sector M. Sandhya a,b, S. Sridharan a,n, M. Indira Devi b, H. Gadhavi a a b
National Atmospheric Research Laboratory, Gadanki, Pakala Mandal, Chittoor District, Andhra Pradesh, India Department of Physics, Andhra University, Visakhapatnam, Andhra Pradesh, India
art ic l e i nf o
a b s t r a c t
Article history: Received 20 October 2014 Received in revised form 16 July 2015 Accepted 17 July 2015 Available online 18 July 2015
Influence of potential vorticity (PV) intrusions at 13.5°N over and near Indian sector (50°E–90°E) on tropical upper tropospheric ozone mixing ratio (OMR) variations is demonstrated based on two case studies. Increase of ECMWF (European Centre for Medium-range Weather Forecasting) reanalysis (ERA)interim OMR in the upper troposphere (200–500 hPa) is observed during the intrusion events consistently in both cases. The OMR also shows similar tongue like structure as PV and it even follows the spatial shift of the PV tongue. In addition, the enhancements in the upper tropospheric OMR during the intrusion events are confirmed using microwave limb sounder (MLS) ozone data at 216 hPa. It is suggested that the existence of strong downdrafts, associated with the ageostrophic circulation due to jet stream, which is inferred from longitude-height cross-section of ERA-interim vertical velocity could bring the ozone further down, though high PV tongue remains only at higher level (above 400 hPa). The importance of these results lies in demonstrating the role of PV intrusion events on the enhancement of tropical upper tropospheric ozone over Indian sector, where the impact of the PV intrusions is not well understood when compared to that over Pacific and Atlantic sectors. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Potential vorticity intrusion Upper tropospheric ozone Vertical velocity
1. Introduction Stratospheric ozone protects us from ultra-violet radiation, whereas upper tropospheric ozone is a greenhouse gas and it contributes positively towards the radiation budget by trapping global outgoing longwave radiation (OLR) and thereby increases the surface temperature (Worden et al., 2008). The photochemical reaction and downward transport from the stratosphere are the major sources for the upper tropospheric ozone (Danielsen, 1968; Logan, 1985), though the latter dominates the former (Roelofs and Lelieveld, 1997). The upper tropospheric ozone is mainly controlled by stratosphere–troposphere exchange (STE) processes at mid- and high latitudes (Barre et al., 2012), though the influence of these processes on low-latitudes is yet to be understood (Levy et al., 1985; Hsu and Prather, 2009). Besides, ozone variabilities show close connection with quasi-biennial oscillation and El-Nino Southern Oscillation (Zerefos et al., 1992). Seasonal cycle of ozone concentration at tropopause and the substantial mass flux across tropopause determines the downward flux of ozone to the upper and middle troposphere (Škerlak et al., 2014). The tropopause folds are favourable for the cross-tropopause exchange (Danielsen, 1968) and they can be identified from large potential vorticity (PV) n
Corresponding author. Fax: þ 91 8585 272018.
http://dx.doi.org/10.1016/j.jastp.2015.07.014 1364-6826/& 2015 Elsevier Ltd. All rights reserved.
value in the upper troposphere (Elbern et al., 1998; Tyrlis et al., 2014; Škerlak et al., 2015). These PV intrusions, transport trace gases like ozone from the stratosphere to the upper troposphere and further to lower levels (Sprenger et al., 2007), though major contribution to tropospheric ozone comes from photochemical production (Staehelin et al., 1994; Yenger et al., 1999). Fadnavis et al. (2010) studied seasonal variation of ozone over India and they observed that the upper tropospheric ozone is more during winter and pre-monsoon months. Consistent with this, Sandhya and Sridharan (2014) noted more PV intrusions over Indian sector during pre-monsoon and winter. Rossby wave breaking at midlatitude stratosphere leads to intrusion of high potential vorticity air from mid-latitude stratosphere to tropical troposphere in the presence of upper tropospheric westerlies, which are favourable conditions for the intrusions (Waugh and Polvani, 2000). These PV intrusions play a vital role on promotion of convection (Kiladis, 1998, Sandhya and Sridharan, 2014), inhibition of convection (Russell et al., 2008), intensification of cyclone (Browning, 1997) etc. Most of the studies on the influence of PV intrusions on surface or lower tropospheric ozone variabilities are from Northern Hemispheric mid-latitudes (Wakamatsu et al., 1967; Chung and Dann, 1967; Davies and Schuepbach, 1994; Bithell et al., 2000 ; Langford et al., 2012; Lin et al., 2012 to state a few). Not much work has been done from low-latitudes and in particular over Indian sector due probably to the less number of occurrence of PV
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intrusion events. The present study shows an evidence for the increase in ozone mixing ratio (OMR) in the tropical upper troposphere associated with a few PV intrusion events over Indian sector.
2. Data analysis 2.1. ERA-interim data sets European Centre for Medium Range Weather Forecasting (ECMWF) data are produced on model levels (hybrid pressuresigma coordinates) and at the surface. ECMWF data are also interpolated to pressure levels and some datasets include data on isentropic levels. The ECMWF reanalysis (ERA) interim uses 60 levels with the model top at 0.1 hPa. The ERA-interim data sets are results from the analysis conducted at 6-h intervals available for latitude–longitude grids 3° 3° 0.125° 0.125° and are prepared by ECMWF using their variational data assimilation system (Berrisford et al., 2009). The data sets are currently available in the website http://apps.ecmwf.int/datasets/data/interim-full-daily// for 15 isentropic levels and 37 pressure levels. In the present study, potential vorticity, ozone mixing ratio and vertical velocity (Omega) at different pressure levels and potential vorticity and zonal wind at 350 K isentropic level and zonal wind, meridional wind, geopotential at 200 hPa pressure level are used. The ERA-Interim data is divided mainly in to three periods namely Pre GOME assimilation period (till 1995), GOME assimilation period (1996–2002) and the post GOME assimilation period (2003 onwards). Dragani (2010) studied the quality of ERA-Interim ozone data by comparing with in situ measurements. They reported that in the pre GOME assimilation period, the residuals between ozonesondes and the corresponding ERA-Interim ozone profiles were within 710% in the tropics and mid-latitudes at most levels and within 7 20% at high-latitudes. However in GOME assimilation and post assimilation periods, the level of disagreement was within 7 5% in the tropics.
2.2. MLS-Ozone data The Aura Microwave Limb Sounder (MLS) onboard Earth Observing System (EOS) launched on 15 July 2004. It is sun synchronous with altitude 705 km and with 98° inclination. ML2O3 version 3 data is used for the present study. This is a standard product for ozone derived from radiances measured by the 240 GHz radiometer. The spatial coverage is near-global ( 82° to þ82° latitude), with each profile spaced 1.5° or ∼165 km along the orbit track (roughly 15 orbits per day). The recommended useful vertical range is from 261 to 0.0215 hPa and the vertical resolution is between 2.5 and 6 km (Cheung et al., 2014).
3. Results The intrusion of high PV tongues, from high to low latitudes can be identified from latitude to longitude cross-section of PV at 350 K isentropic level, for the days 15 May, 2011 and 7 May, 2009 respectively in Fig. 1a and b. On 15 May 2011, the tongue of high PV greater than 1.4 PVU (potential vorticity unit; 1 PVU¼10 6 km2 kg 1 s 1) reaches up to 12°N over 65°E–70°E and on 7 May 2009, it crosses 13.5°N over 60°E–65°E. The latitude–longitude cross-section of magnitude of the resultant wind obtained from the zonal and meridional wind components and geopotential at 200 hPa (∼ 350 K) for the same days are shown in Fig. 2c and d. The subtropical jet flow can be inferred from the wind speed greater than 50 m/s over 20°N40°N on 15 May 2011 with the trough of subtropical jet over 65°E–70°E. The existence of trough can also be identified from the folded geopotential contour. Fig. 2d also shows the subtropical jet flow over 30°N–40°N on 7 May 2009, when the trough of the jet stream is over 60°N–65°N at latitude 13.5°N. Extratropical air with high PV can enter to the tropical troposphere through the tropopause fold at the western side of the upper level trough associated with jet streams (Kentarchos et al., 1999, Ding and Wang, 2006). In order to show the time evolution of PV intrusion shown in Fig. 1, the time-longitude cross-sections of PV and zonal wind at
Fig. 1. Latitude–longitude cross-sections of (a, b) PV at 350 K isentropic level and (c, d) resultant wind speed and geopotential at 200 hPa for the days 15 May 2011 and 7 May 2009.
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Fig. 2. Longitude-time cross-sections of (a, b) PV and (c, d) zonal wind at 13.5°N for the days 120–150 of year 2011 and 2009.
350 K isentropic level for the period 1–31 May 2011 and 2009 are shown in Fig. 2. The intrusion event is identified from the PV value greater than 1.4 PVU. The PV intrusion event can be inferred from the PV value exceeding the threshold in Fig. 2a and b on 15 May 2011 (day no. 135), and persists till 21 May, 2011 (day no. 141) over 50°E–70°E and during 7–9 May 2009 (day no. 127–129) over 50°E– 65°E respectively. The zonal wind at 350 K isentropic level (Fig. 2c and d) shows westerly flow over the intrusion region, during the days of PV intrusion. The upper tropospheric westerlies favour PV intrusion to tropical latitudes (Waugh and Polvani, 2000). On 15 May 2011 (day no. 135), the westerly maximum (410 m/s) is over the region 65°E–70°E. However as day progresses, the regime of eastward flow shifts further west to 60°E–50°E. Corresponding to the shift in the upper level westerlies, the PV intrusion also shows (Fig. 1a) westward movement. However in the case of 2009 PV intrusion event, there is no westward shift and the upper troposphere zonal wind remains westerly over the region 50°E–80°E during 1–11 May. Fig. 3a–e shows the longitude height cross-section of PV for the days 14–18 May 2011. On 14 May 2011, descent of a PV tongue can be seen, though the PV value does not reach the threshold to identify as the PV intrusion event. The occurrence of PV intrusion can be inferred on 15 May 2011 below 200 hPa over 65°E–70°E. As the day progresses, the upper PV anomaly shifts to the west of the
intrusion region consistent with the shift in upper tropospheric westerly and on 19 May 2011, it prevails over 50°E–60°E (not shown). However the tongue of PV reaches nearly 400 hPa during 15–18 May 2011 and 450 hPa on 19 May 2011 (not shown). Associated with this intrusion event, the upper troposphere ozone (Fig. 3f–j) shows enhancement. On 14 May 2011, though the PV does not reach the threshold, OMR shows same tongue structure as PV and it penetrates down to 350 hPa. The OMR is high (4115 ppb) along the downward track of the tongue, whereas OMR at other non intrusion region is below 90 ppb only. On subsequent days also, high OMR along the track of PV tongue can be inferred from the figure. The high OMR tongue descends from 400 hPa on 15 May to 450 hPa on 19 May 2011 and it can be noted that the OMR follows the westward shift of the PV anomaly. Though the PV tongue is just above 400 hPa, the OMR tongue descends further down to 400 hPa on 17–18 May 2011. Fig. 4 shows longitude height cross-sections of PV (Fig. 4a–e) and OMR (Fig. 4f–j) during 5–9 May 2009. The PV intrusion event can be inferred on 7 May 2009 from the PV value greater than 1.4 PVU over 60°E–65°E. On 5 May 2009 when there is no PV intrusion and high OMR ( 4130 ppb) exists only above 300 hPa and the tongue like structure is absent. On 6 May 2009, though the PV value does not reach the threshold to identify the PV intrusion, PV tongue with relatively higher values of PV can be seen over 50°E–
Fig. 3. Longitude-height cross sections of PV and OMR at 13.5°N from 14 May 2011 to 18 May 2011.
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Fig. 4. Longitude-height cross sections of PV and OMR at 13.5°N from 05 May 2009 to 09 May 2009.
60°E. Associated with this high PV layer, OMR ∼130 ppb reaches near to 400 hPa. On the next day, when PV crosses the threshold (1.4 PVU) over 60°E–65°E, the high ozone tongue reaches down to even 550 hPa, though the high PV tongue prevails only above 300 hPa. However, on the subsequent days (8–9 May 2009), though the descent of PV is not present to lower levels, existence of high OMR can be observed throughout the upper troposphere over 65°E–70°E. The presence of high OMR in the upper troposphereduring the PV intrusion events are confirmed with observational data taken from MLS. The MLS ozone data is less contaminated in the UTLS region (Livesey, 2007). Fig. 5 shows MLS OMR variations at 216 hPa over 10°N–16°N and 50°E–90°E for the above presented cases. The day just before intrusion is marked with a star symbol in the figure. The average values of OMR at 216 hPa for the above presented cases are 75.7 ppb and 68.6 ppb for the years 2009 and 2011. Immediately after the PV intrusion, the OMR value increases to greater than 80 ppb and reaches a maximum of 115 ppb in the case of 2009 event and 70–120 ppb in the case of 2011 event. Though there is an increase in the upper tropospheric ozone due to the PV intrusion events, which transport stratospheric ozone rich air to the tropical upper troposphere, the ozone can descend further down to lower levels due to the ageostrophic flow at the western side of upper level trough associated with a jet stream (Kentarchos et al., 1999). The ageostrophic circulation associated with jet stream might be the reason for the downward
Fig. 5. Daily variation of MLS OMR at 216 hPa averaged over 10°N–16°N and 50°E– 90°E for the years 2011 and 2009.
transport of ozone further down to the PV tongue. The longitudeheight cross-section of pressure velocity for the days of intrusion (15 May 2011 and 7 May 2009), along with 1 PVU and 1.5 PVU PV isolines at 13.5°N is shown in Fig. 6a and b. The PV isolines descend to 350 hPa over 65°E–70°E and to 250 hPa over 55°E–65°E on 15 May 2011 and 7 May 2009 respectively, indicating the tropopause fold over that region. The wind is downward with value greater than 0.1 Pa/s at the height region 250–600 hPa over 60°E– 70°E and the downward wind persists till 1000 hPa over 60°E–65° E on 15 May 2011. Similarly, the vertical velocity is downward over 50°E–65°E from 300 to 900 hPa on 7 May 2009 also. This downward wind might have transported OMR further down in the troposphere, though high PV tongue is restricted to higher level and it gets diffused over time.
4. Discussion and conclusions Two cases are presented (May 2011 and May 2009) in which there is a clear evidence of the influence of PV intrusions at 13.5°N over and near Indian sector (50°E–90°E) on tropical upper tropospheric OMR variations. The latitude–longitude cross-section of PV at 350 K and upper level wind at 200 hPa on the days of intrusion show that the mid-latitude air enters to the low latitudes along the trough of subtropical jet stream. Near jet streams, PV gradient over an isentropic level takes the form of a narrow tube of enhanced PV gradient, which can act as a waveguide for Rossby waves (Schwierz et al., 2004). Wave breaking occurs when the Rossby waves having sufficiently large amplitudes propagating into the westerly duct region and it produces intrusions of stratospheric air with high PV into the tropical upper troposphere (Waugh and Polvani 2000). In the present study, the upper tropospheric westerly at 350 K isentropic level favours the intrusion to the low latitudes in both cases. Based on a climatological study, Waugh and Polvani (2000) demonstrated the importance of upper tropospheric westerly winds during Northern winter over Pacific and Atlantic sectors for the PV intrusions to low-latitudes. Sandhya and Sridharan (2014) also noted that the PV intrusions over Indian sector during pre-monsoon time were also favoured by upper tropospheric westerlies. Here, the PV intrusion during the 2011 case shows a westward shift as day progresses. The westward movement of PV is found to be consistent with the westward shift of the upper tropospheric westerly winds.
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Fig. 6. Longitude-height cross section of vertical velocity (omega) at 13.5°N for the days 15 May 2011 and 7 May 2009. The isolines of PV at 1 and 1.5 PVU are also plotted.
Besides, the longitude-pressure variation of PV during the intrusion days shows the vertical extension of PV penetrating up to 400 hPa in both the cases. Enhancements of ERA-interim OMR in the upper troposphere (200–500 hPa) are consistently observed during the intrusion events in both the cases. The OMR also shows similar tongue like structure as that of PV and it even follows the spatial shift of the PV. In addition, the enhancements in the upper tropospheric OMR during the intrusion events are confirmed using the MLS ozone data at 216 hPa. The upper tropospheric ozone enhancement continues for a long time (∼8 days) in the case of 2011 event, whereas, the OMR enhancement persists only for 5 days in the 2009 event. This is found to be consistent with the life cycle of PV intrusion event in both the years. Earlier studies reported that the STE was a major source for tropospheric ozone (Danielsen, 1968). Fadnavis et al. (2010) noted enhancement in the monthly mean upper tropospheric ozone over Indian sector during winter and premonsoon months and attributed it to be due to the stratospheric intrusions, which are more during these months. In the present study, two case studies are shown relating the occurrence of PV intrusions and upper OMR enhancements directly. The vertical velocity on intrusion days shows downward motion over the intrusion region. However the vertical velocity is upward to the east of intrusion. It is suggested that the downward vertical velocity associated with the ageostrophic circulation due to the jet flow might have transported ozone further down to the levels of troposphere, though the PV tongue remains at higher levels and gets diffused by mixing with surrounding air. Increase of upper tropospheric ozone can impact air quality if it is transported to the boundary layer (Fiore et al., 2002). The impact of tropospheric ozone on the weather and climate will be the subject of our future study.
Acknowledgements The authors gratefully acknowledge ECMWF for providing the ERA-interim data sets used in this study. The author wish to thank the Editor and the two Reviewers for their comments and suggestions, which greatly helped them to improve the manuscript to its present form.
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