baroclinicity during the second half of the 20th Century ... · Increase of Upper Troposphere/Lower...
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Increase of Upper Troposphere/Lower Stratosphere wave baroclinicity during the second half of the 20th Century J.A. Añel 1,2 , J.M. Castanheira 2 ,C.A.F. Marques 2 , J.C. Antuña 3 , M.L.R. Liberato 4 , L. de la Torre 1 , L. Gimeno 1 1 EPHYSLAB, Universidade de Vigo, Fac. Ciencias de Ourense, Ourense, Spain 2 CESAM, Universidade de Aveiro, Aveiro, Portugal 3 Camagüey Lidar Station, Camagüey, Cuba 4 Departamento de Física, Universidade de Trás-os-Montes e Alto Douro, Portugal [email protected] Contribution A strengthening of the equatorward tempera- ture gradient in the upper troposphere/lower stratosphere (UTLS), at subtropics and mid- latitudes, is consistently reproduced in sev- eral modelling studies of the atmospheric re- sponse to the increase of greenhouse gas radia- tive forcing. Some of them suggest an increase of the baroclinicity in the UTLS region because of the enhanced meridional temperature gradi- ents [e.g. García and Randel, 2008]. Here we present observational evidence of an increase of UTLS wave baroclinicity, during the second half of the 20th century. The ev- idence is given by significant positive trends in the energy of baroclinic normal modes of the NCEP/NCAR reanalysis, in the eddy avail- able potential energy of the ERA-40 reanalysis and in the frequency of double tropopause (DT) events in radiosonde data. Data and Methodology Part of the study is based on NCEP/NCAR re- analysis and covers the 1958-2006 period. We analyze the Northern Hemisphere cool season (November to April) daily means of the horizon- tal wind components (u, v) and of the geopoten- tial height, available on 17 standard pressure lev- els from 1000 to 10 hPa, on a 2.5°x 2.5°horizontal grid. The global horizontal wind (u, v) and geopotential (ϕ) fields were expanded in terms of the normal modes of the NCEP/NCAR refer- ence atmosphere [Liberato et al., 2007]. Radiosonde observations from a 187-station global network, described by Añel et al. [2008], were analyzed for the 1970-2006 period. The soundings retained in the analysis satisfy the fol- lowing homogenization criteria: i) the 50 hPa (70 hPa) level must be reached in the tropics (extratropics); ii) there must be at least one re- ported level in the vicinity of each of the follow- ing mandatory pressure levels: 500, 400, 300, 200, 150, 100, 70, and 50 hPa; iii) the sounding sta- tion must report soundings satisfying condition i) and ii) at least in five different years for each of the following periods: 1970-79, 1980-89, 1990-99, and 2000-06. Figure 1 shows the radiosonde stations consid- ered in the study. The solid symbols represents the stations retained for the computations of the trends in the frequency of DT events. Results Observing the energy values in the top panel of Fig. 2, it may be concluded that it is the mode m = 4 which shows the largest trend of energy. Inci- dently, this vertical mode is the most sensitive to variability in the UTLS region. Thus, results sug- gest that most of the increase in baroclinic energy comes from the UTLS region. Figure 2: (top) Vertical spectrum of the mean en- ergy (E m ) associated with the baroclinic Rossby waves of wave numbers s = 1,..., 10. (bottom) Lin- ear trends of the November-April mean energy associated with the Rossby waves of wave num- bers s = 1,..., 10, for the first 9 baroclinic modes. The trends are given as percentages of the respec- tive mean energies in the period of 1958-2006. Figure 3 shows the time series of the November- April mean energy of Rossby waves with wave numbers s = 1,..., 10. The upper row represents the energy sum for the deeper baroclinic modes (m < 5) and shows positive energy trends. The left panel shows the time series of the energy of all frequencies and the right panel shows the en- ergy of high frequency waves. The lower row in the figure shows similar time series but for the sum of the energy of the shallower baroclinic modes (m > 5) with linear decreases. Randel et al. [2007] suggested that the occur- rence of double tropopauses is due to the excur- sions of the low latitude (tropical) tropopause to higher latitudes, overlying the lower extratropi- cal tropopause. Consequently, the occurrence of double tropopauses is typical of baroclinic circu- lation in the UTLS region. Thus, an event of dou- ble tropopause must be associated either with the generation of high frequency baroclinic waves or with the amplification of stationary baroclinic waves, with vertical structures which may ex- plain the circulation variability in the UTLS re- gion. Therefore if the wave baroclinicity of the UTLS has increased during the last five decades, it may also be expected that an increase in the frequency of double tropopause events has oc- curred. Figure 4: relative frequency of DT events in the 30°N-60°N (left panel) and 30°S-60°S (right panel) latitude belts. References 1. Añel, J. A., J. C. Antuna, L. de la Torre, J. M. Castanheira, and L. Gimeno (2008), Climatological features of global multiple tropopause events, J. Geophys. Res., 113, D00B08, doi:10.1029/2007JD009697. 2. García, R., and W. J. Randel (2008), Acceleration of the Brewer-Dobson Circulation due to Increases in Greenhouse Gases, J. Atmos. Sci., 65, 2731-2739 3. Liberato, M. L. R., Castanheira, J. M., Torre, L. de la, DaCamara, C. C., and L. Gimeno (2007), Wave Energy Associated with the Variability of the Stratospheric Polar Vortex, J. Atmos. Sci., 64, 2683-2694 4. Randel, W. J., Seidel, D. J., and L. L. Pan (2007), Observational characteristics of double tropopauses, J. Geophys. Res., 112, D07309, doi:10.1029/2006JD007904
Transcript of baroclinicity during the second half of the 20th Century ... · Increase of Upper Troposphere/Lower...
Increase of Upper Troposphere/Lower Stratosphere wavebaroclinicity during the second half of the 20th CenturyJ.A. Añel1,2, J.M. Castanheira2,C.A.F. Marques2, J.C. Antuña3, M.L.R. Liberato4, L. de la Torre1, L. Gimeno1
1EPHYSLAB, Universidade de Vigo, Fac. Ciencias de Ourense, Ourense, Spain
2CESAM, Universidade de Aveiro, Aveiro, Portugal
3Camagüey Lidar Station, Camagüey, Cuba
4Departamento de Física, Universidade de Trás-os-Montes e Alto Douro, Portugal
ContributionA strengthening of the equatorward tempera-ture gradient in the upper troposphere/lowerstratosphere (UTLS), at subtropics and mid-latitudes, is consistently reproduced in sev-eral modelling studies of the atmospheric re-sponse to the increase of greenhouse gas radia-tive forcing. Some of them suggest an increaseof the baroclinicity in the UTLS region becauseof the enhanced meridional temperature gradi-ents [e.g. García and Randel, 2008].Here we present observational evidence of anincrease of UTLS wave baroclinicity, duringthe second half of the 20th century. The ev-idence is given by significant positive trendsin the energy of baroclinic normal modes ofthe NCEP/NCAR reanalysis, in the eddy avail-able potential energy of the ERA-40 reanalysisand in the frequency of double tropopause (DT)events in radiosonde data.
Data and MethodologyPart of the study is based on NCEP/NCAR re-analysis and covers the 1958-2006 period. Weanalyze the Northern Hemisphere cool season(November to April) daily means of the horizon-tal wind components (u, v) and of the geopoten-tial height, available on 17 standard pressure lev-els from 1000 to 10 hPa, on a 2.5°x 2.5°horizontalgrid. The global horizontal wind (u, v) andgeopotential (ϕ) fields were expanded in termsof the normal modes of the NCEP/NCAR refer-ence atmosphere [Liberato et al., 2007].Radiosonde observations from a 187-stationglobal network, described by Añel et al. [2008],were analyzed for the 1970-2006 period. Thesoundings retained in the analysis satisfy the fol-lowing homogenization criteria: i) the 50 hPa(70 hPa) level must be reached in the tropics(extratropics); ii) there must be at least one re-
ported level in the vicinity of each of the follow-ing mandatory pressure levels: 500, 400, 300, 200,150, 100, 70, and 50 hPa; iii) the sounding sta-tion must report soundings satisfying conditioni) and ii) at least in five different years for each ofthe following periods: 1970-79, 1980-89, 1990-99,and 2000-06.Figure 1 shows the radiosonde stations consid-ered in the study. The solid symbols representsthe stations retained for the computations of thetrends in the frequency of DT events.
ResultsObserving the energy values in the top panel ofFig. 2, it may be concluded that it is the mode m= 4 which shows the largest trend of energy. Inci-dently, this vertical mode is the most sensitive tovariability in the UTLS region. Thus, results sug-gest that most of the increase in baroclinic energycomes from the UTLS region.
Figure 2: (top) Vertical spectrum of the mean en-ergy (Em) associated with the baroclinic Rossbywaves of wave numbers s = 1,..., 10. (bottom) Lin-ear trends of the November-April mean energyassociated with the Rossby waves of wave num-bers s = 1,..., 10, for the first 9 baroclinic modes.The trends are given as percentages of the respec-tive mean energies in the period of 1958-2006.
Figure 3 shows the time series of the November-April mean energy of Rossby waves with wavenumbers s = 1,..., 10. The upper row represents
the energy sum for the deeper baroclinic modes(m < 5) and shows positive energy trends. Theleft panel shows the time series of the energy ofall frequencies and the right panel shows the en-ergy of high frequency waves. The lower rowin the figure shows similar time series but forthe sum of the energy of the shallower baroclinicmodes (m > 5) with linear decreases.
Randel et al. [2007] suggested that the occur-rence of double tropopauses is due to the excur-sions of the low latitude (tropical) tropopause tohigher latitudes, overlying the lower extratropi-cal tropopause. Consequently, the occurrence of
double tropopauses is typical of baroclinic circu-lation in the UTLS region. Thus, an event of dou-ble tropopause must be associated either with thegeneration of high frequency baroclinic wavesor with the amplification of stationary baroclinicwaves, with vertical structures which may ex-plain the circulation variability in the UTLS re-gion. Therefore if the wave baroclinicity of theUTLS has increased during the last five decades,it may also be expected that an increase in thefrequency of double tropopause events has oc-curred.
Figure 4: relative frequency of DT events inthe 30°N-60°N (left panel) and 30°S-60°S (rightpanel) latitude belts.
References1. Añel, J. A., J. C. Antuna, L. de la Torre, J. M. Castanheira, and L. Gimeno (2008), Climatological features of global multiple tropopause events, J. Geophys.
Res., 113, D00B08, doi:10.1029/2007JD009697.
2. García, R., and W. J. Randel (2008), Acceleration of the Brewer-Dobson Circulation due to Increases in Greenhouse Gases, J. Atmos. Sci., 65, 2731-2739
3. Liberato, M. L. R., Castanheira, J. M., Torre, L. de la, DaCamara, C. C., and L. Gimeno (2007), Wave Energy Associated with the Variability of the StratosphericPolar Vortex, J. Atmos. Sci., 64, 2683-2694
4. Randel, W. J., Seidel, D. J., and L. L. Pan (2007), Observational characteristics of double tropopauses, J. Geophys. Res., 112, D07309, doi:10.1029/2006JD007904
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