Cargese International School, COST ACTION 723, UTLS, 3-15 Oct., 2005 Water Vapour Variation in the Upper Troposphere and Lower Stratosphere over the Asian Summer Monsoon Nawo Eguchi National Institute for Environmental Studies (NIES), Japan Abstract : Abstract : The water vapour variation over the Asian summer monsoon, especially with a focus on the vertical structure, was investigated using the Upper Atmosphere Research Satellite (UARS) / Microwave Limb Sounder (MLS) water vapour data (version 7.02) during the summer The wetter air intrudes into the tropical lowermost stratosphere (LS) over the convective region (70-180ºE, 25-35ºN) and extends to the extra-tropical LS along the isentropic surfaces below 380 K around the Asian subtropical jet (ASJ). In order to estimate both water vapour fluxes passing through and along the isentropic surfaces, the water vapour budget is calculated diagnostically using the conservation equation of water vapour for the isentropic coordinate. The time (seasonal) mean water vapour flux divergence, amounting to the maximum value in the ASJ, is the main term; and the structure resembles to a stationary Rossby wave which has the wave train along the ASJ. The term of vertical flux divergence on the 380 K isentropic surface over the convective region is large; and it is about the same value as the seasonal mean flux divergence in the ASJ on the 350 K isentropic surface. I came to a conclusion that the diabatic effects and the stationary Rossby wave associated with the Asian summer monsoon play an important role in both the water vapour variation and the exchange process between the upper troposphere (UT) and LS in the tropical and extra-tropical region. 1

There are two main entrances of mass and chemical species from the troposphere to the stratosphere ( ①, ② in Figure A). ① Stratosphere-Troposphere Exchange (STE) between the tropical upper troposphere (also known as the Tropical Tropopause Layer; TTL) and the tropical lower stratosphere passing through the isentropic surface (the tropical tropopause). ② STE between the tropical upper troposphere (or TTL) and the extra-tropical lower stratosphere along isentropic surfaces. 1. Introduction [Holton et al.,1995] OverworldMiddle world (Tropopause ～ 380 K) (>380 K) Tropical Tropopause Layer (TTL) Figure A : Schematic latitude-height cross-section showing the basic ideas of STE processes. The previous studies showed that the Asian summer monsoon impacts the STE process around the tropopause region, and the distributions of water vapour and other trace gases. STE is associated with intraseasonal variations of the Asian subtropical jet (ASJ), especially at the northeast side of the Asian monsoon high [Terao,1998; Dethof et al.,1999; Dunkerton,1995]. However, there is less understanding about the STE process and water vapour variation around the tropopause region during the Asian summer monsoon. 2 ① ②

2. Purpose What is the water vapour variation like, especially the vertical direction, in the tropopause region over the Asian summer monsoon ? What is the dominant STE process which affects the water vapour variation in the tropopause region, and what kind of time scales is dominated? 3. Data Water Vapour data from Upper Atmosphere Research Satellite (UARS) / Microwave Limb Sounder (MLS) (Version 7.02) [cf. Read et al., 2004] MLS measures the atmosphere in the limb direction with a vertical resolution of nearly 3km. Water vapour mixing ratio [ppmv or g/kg], relative humidity with respect to ice [%] between hPa (11 levels) May – September 1992 Cirrus Clouds Frequency data from UARS / Cryogenic Limb Array Etalon Spectrometry (CLAES) At 100 and 146 hPa, cirrus clouds are inferred from CLAES aerosol (revision 9) using a threshold extinction value 3.3e-3 km -1 [cf. Mergenthaler et al., 1999]. Meteorological data from ECMWF 40 Years Re-Analysis (ERA-40) Temperature, horizontal wind, vertical wind, potential vorticity 3

Figure 1. Horizontal map of (a) water vapour mixing ratio [ppmv] (shade), relative humidity with respect to ice [%] (black contour with 10 [%] intervals) and Outgoing Longwave Radiation (OLR ) [W/m 2 ] (blue contour with 10 [W/m 2 ] intervals from 190 [W/m 2 ]), (b) temperature [K] (shade and black contour larger than 204 [K] with 10 [K] intervals), cirrus clouds frequency [%] (yellow contour, interval is with 10 [%] from 10 [%]) at 100 hPa in August Vectors show the horizontal wind [m/s]. At 100 hPa (the lowermost stratosphere), the moist area exist at the north side of the convective regions, such as the Indian subcontinent and the western Pacific around 25ºN. The dry air extends from the middle-latitude to the equatorial region in the central Pacific. The dry region also exists at the south of the equator in the western Pacific and Indian Ocean. The anti-cyclonic circulation is located at the northwest side of the convective region (the center is 80ºE, 30ºN), the Asian subtropical jet (ASJ) forms the northern part of the circulation. The cirrus clouds occur frequently over the cold region (60ºE~120ºE, 20ºN). (a) (b) 4. Water vapour distribution around the tropopause over the Asian summer monsoon The Asian subtropical jet (ASJ) The Asian mosoon high 4

Figure 2. Latitude-pressure section of monthly mean water vapour mixing ratio [ppmv] (shade), temperature [K] (blue dotted contour with 10 [K] interval from 190 [K]), potential temperature [K] (red line) and zonal wind [m/s] (yellow contour, the dotted line indicates negative value (easterly wind).) averaged between 70ºE and 120ºE in August Black dot-dash- line indicates the lapse rate tropopause obtained from NCEP reanalysis. The wet air (> 4.5 ppmv) extends up to 83 hPa passing through the tropopause (and 380 K potential temperature) around 25ºN without crossing the coldest region (< 190 K) where the cirrus clouds occur frequently. At the north side of westerly wind (the Asian subtropical jet) core (>40ºN), wet region spreads along the isentropic surfaces (below 380 K). [ppmv] 5

Figure 3. Same as Figure 1, but for longitude-pressure section averaged between 15ºN and 30ºN. Orange marks at 100 and 147 hPa indicate the frequency of cirrus clouds larger than 10 [%], red marks at bottom of this figure show the convective region (lower than 230 [W/m 2 ] OLR from NOAA). Vectors show the zonal and vertical wind components [m/s, hPa/s]. The isentropic surfaces above 365 K tilts westward increasing height between 60ºE and 200ºE; and the easterly wind related with the anti-cyclonic circulation is dominated over there. The wet air intrudes into the lowermost stratosphere passing through the 380 K isentropic surface especially over the Asian monsoon region. 6

5. Calculation of water vapour flux divergence C in equation (1) indicates condensation which is supposed to be ignored in this study. Horizontal flux divergence is divided into three time scales, seasonal mean, intraseasonal (20-80 days) and synoptic (8-20 days) time scales (Equation (2)). R is defined as the vertical flux divergence passing through the isentropic surfaces which is associated with the sensitive heating of the convective clouds and radiative heating release (Equation (3)). In order to estimate both fluxes across and along the isentropic surfaces in the tropics and middle latitude shown in Figure 2, the water vapour budget is calculated diagnostically using the conservation equation of water vapour for the isentropic coordinate. (1) (2) (3) q : water vapor mixing ration [kg/kg] v : horizontal wind [m/s] : Seasonal mean : Intraseasonal time scales (20-80 days) : Synoptic time scales (less than 20 days) 7

Figure 4. Map for water vapour flux (vectors), (a) the stream function (shade) and (b) potential function at 100hPa in August The stream function (rotational component) of water vapour flux is about four times as large as the potential function (divergence component) over the Asian monsoon region. The divergence component is enhanced at the south side of the Asian monsoon high (< 30ºN) especially over the western Pacific. divergence convergence 8

(a) At both isentropic surfaces, the seasonal mean water vapour flux divergence becomes the main component, and as a result, the distribution of R term (diabatic effects) is quite similar to the seasonal mean water vapour flux divergence field. The seasonal mean water vapour flux divergence has a quasi-Rossby wave structure which has the wave train along the Asian subtropical jet ( Ⓐ shown in Fig. 5 (b)). The upward water vapour flux divergence (R) is large over the convective region ( Ⓑ in Fig. 5 (a)), where the isentropic surfaces tilts westward increasing height as shown in Fig. 3. Ⓐ Ⓑ (b) Ⓐ : -2.32e -5 [10 -9 g/kg/s] Ⓑ : -4.76e -6 [10 -9 g/kg/s] Figure 5. Map for (a) R and horizontal wind (vectors) on the 380 K isentropic surface and (b) seasonal mean water vapour flux (vectors) and the divergence (shade) on the 355 K isentropic surface. The contour indicates potential vorticity from 2 to 4 [PVU; 10 6 Km 2 /kg/s]. (a) (b) 9

The water vapour variation over the Asian summer monsoon was investigated with a focus on the vertical direction using the UARS MLS water vapour data (version 7.02) during the summer The latitude (longitude)-height sections of seasonal mean water vapour show that the wetter air intrudes into the tropical lowermost stratosphere over the convective region (70-180ºE,25- 35ºN) passing through the isentropic surfaces (e.g., 380 K), and extends to the mid-latitude lower stratosphere passing through the subtropical tropopause along the isentropic surfaces (e.g.,350 K). This suggests that both vertical and horizontal stratosphere - troposphere exchanges existed potentially around the Asian summer monsoon region. In order to estimate both water vapour fluxes passing through the isentropic surfaces (over the convective area) and along the isentropic surfaces at the north side of the monsoon high, the water vapour budget is calculated diagnostically by using the conservation equation of water vapour for the isentropic coordinate. The time (seasonal) mean water vapour flux divergence is larger than the other components, such as intraseasoanl and synoptic time scales flux divergences. The horizontal structure of the seasonal mean water vapour flux divergence resembles a quasi-stationary Rossby wave structure which has the wave train along the Asian subtropical jet (ASJ), especially between the north side of the Asian monsoon high and the easternmost Pacific. The isentropic surfaces larger than 365 K tilt westward increasing height over the convective region (the southeast side of the anticyclone) where the easterly wind is enhanced. The water vapour vertical flux divergence associated with the diabatic effects is large over the convective region. I came to a conclusion that the diabatic effects and the stationary Rossby wave associated with the Asian summer monsoon play an important role in both the water vapour variation and the exchange between the upper troposphere; and lower stratosphere in the tropical and extra- tropical region. 6. Summary and Discussion : 10

References : Acknowledgements : The author would like to thank Dr. Toru Terao at Osaka Kyoiku University, Dr. Hatsuki Fujinami at Nagoya University, Dr. Yoshiyuki Kajikawa at University of Hawaii, Prof. Masato Shiotani, Drs. Hisahiro Takashima and Noriyuki Nishi at Kyoto University for their beneficial comments and valuable suggestions. I also thank UARS MLS science staffs, especially William G. Read for their giving us key data. I acknowledge the UARS CLAES science team, the European Centre for Medium-Range Weather Forecasts (ECMWF) for the datasets. Most figures of this study were drawn by using the GFD-DENNOU Library. Bannister et al., Q. J. R. Meteorol. Soc., 130, , Dethof,A, et al., Q. J. R. Meteorol. Soc., 125, , Dunkerton,T.J., J. Geophy. Res. Atmosphere, 100, D8, 16,675-16,688, Mergenthaler, J..L. et al., J. Geophy. Res. Atmosphere, 104, D18, 22,183-22,194, Park M. et al.,: J. Geophy. Res. Atmosphere, 109 (D3):Art. No. D Randel,W. and M. Park : Read, W.G. et al., J. Geophy. Res. Atmosphere, 109, D6, doi: /2003_JD004056, Terao, T., J. Meteor. Soc. Japan, 77, ,