Extended abstract - to appear in the Proceedings of the 1st General Assembly of the WCRP Project 'Stratospheric Processes and their Role in Climate' (SPARC), Melbourne, Australia, 2-6 December 1996.

LOWER STRATOSPHERIC WATER VAPOUR AT LOW LATITUDES AS OBSERVED BY HALOE

D.R Jackson1, S.J Driscoll1,*, E.J Highwood2, J.E Harries1 and J.M Russell III3

ABSTRACT

In this paper we present observations of water vapour in the lower equatorial stratosphere made by the Halogen Occultation Experiment (HALOE) instrument, which flies on the Upper Atmosphere Research Satellite (UARS). These data make an important new contribution to the observational knowledge of this region, since lower stratospheric water vapour has largely been observed to date using in-situ instruments, and the only global longitudinally-resolving satellite observations of this region to be reported are those from the Stratospheric Aerosol and Gas Experiment II (SAGE II). Here, we use the HALOE data to provide insight into the amount of cross-isentropic flow in the Asian monsoon region. In addition, the interannual variability in the water vapour distribution in boreal winter is examined, and linked to the long-lived 1990-1995 El Nino Southern Oscillation (ENSO).

1. INTRODUCTION

It is widely accepted that most water vapour enters the stratosphere near the tropical tropopause, and hence tropical tropopause conditions have a strong impact on the stratospheric water vapour distribution. However, it is not exactly clear how the cross-tropopause transport of water vapour occurs. This has been the subject of much recent research (see Potter and Holton, 1995, and references therein). A problem with the study of such troposphere to stratosphere transport is the paucity of observations. The dehydration of the lower stratosphere has largely been investigated using in situ measurements, which, although usually very accurate, suffer from limited temporal and spatial coverage. Much more extensive coverage of water vapour in this region has come from Limb Infrared Monitor of the Stratosphere (LIMS) (see eg Jones et al, 1986) and Stratospheric Aerosol and Gas Experiment II (SAGE II) (Rind et al, 1993) satellite observations. However, the LIMS dataset is short (Oct. 1978 - May 1979) and of low accuracy near the tropopause, whilst Rind et al did not in general discuss interannual variability in the SAGE II retrievals. Further observations of middle atmosphere water vapour have recently become available via the Halogen Occultation Experiment (HALOE), which flies on the Upper Atmosphere Research Satellite (UARS). HALOE can observe at lower stratospheric levels located very close to the tropical tropopause, and in addition is highly suitable for studying interannual variability, since it is still in operation at the time of writing. Therefore, the HALOE dataset extends the observational knowledge of lower stratospheric water vapour, and in particular provides valuable new information about its interannual variability.

2. INSTRUMENT DESCRIPTION

HALOE views solar infrared radiation in the 2.5 to 10.0 µm region by means of solar occultation, and measures a variety of atmospheric constituents, plus temperature and aerosol extinction (see Russell et al, 1993). The water vapour channel is at 6.60 µm. HALOE is capable of observing water vapour from the tropopause up to the upper mesosphere with a vertical resolution of approximately 2 km, and with an accuracy of +_ 10 % between 0.1 and 100 mb, rising to +_ 30 % at the boundaries of the observational range (Harries et al, 1996). Because HALOE is a solar occultation experiment, observations are made only at sunset and sunrise. 15 observations at both sunset and sunrise are made each day, and are clustered around two rings of latitude (one for sunrise, one for sunset). As the orbit drifts, these latitudes slowly change and may pass each other and reverse, with a period of order one month. Details of the calculation of monthly and seasonal fields appear in Jackson et al (1996).

3. LOWER STRATOSPHERIC WATER VAPOUR IN THE ASIAN SUMMER MONSOON REGION

Three years of HALOE data, from December 1992 to November 1995, were used to create seasonal plots of water vapour. A full comparison of these fields with the seasonal climatology of SAGE II data reported by Rind et al (1993) appears in Jackson et al (1996). Here, we concentrate on the region of the lower stratosphere affected by the Asian summer monsoon. In June - July - August (JJA) at 129 mb (Figure 1a) the area of the highest water vapour values (greater than 6.0 ppmv) is located between 15oS and 30oN, with a distinct region of high water vapour located over Asia, which is caused by strong tropospheric convection linked with the Asian Monsoon. This region of high water vapour is also observed in JJA at both 100 mb (not shown) and 83 mb (Figure 1b). At both 129 and 100 mb, water vapour values are a maximum at low latitudes. However, at 83 mb this pattern has reversed, with lowest values now found over the equatorial region. This differing pattern occurs firstly because the Asian monsoon acts to hydrate the lower stratosphere in JJA at both 129 and 100 mb, and secondly because the annually varying pattern in water vapour present at 100 mb (with a minimum in December - January - February (DJF)) and a maximum in JJA) propagates slowly upwards with time (see Mote et al, 1996). Therefore, it is likely that much of the low equatorial water vapour seen at 83 mb in JJA entered the stratosphere in the previous season and propagated slowly upwards.

Figure 1 - Three-year mean of HALOE water vapour mixing ratio in JJA. Contour interval: 0.4 ppmv. Values less than 3.2 ppmv and greater than 6.0 ppmv are shaded. Shade lines run from bottom left to top right for values less than 3.2 ppmv, and from bottom right to top left for values greater than 6.0 ppmv. Values less than 2.4 ppmv and greater than 8.8 ppmv have denser shading. Regions not sampled by HALOE are left blank. a) 129 mb; b) 83 mb.

Dynamical interpretation of the JJA Asian monsoon is easier if the data are expressed on isentropic surfaces, since in the middle atmosphere parcels of air are transported approximately along such surfaces. The 375 K isentropic surface is located between the 100 and 129 mb pressure levels under mean conditions in the region of the JJA monsoon. Figure 2a shows that in JJA on the 375 K isentropic surface the region of high water vapour over Asia is still present, indicating that significant cross-isentropic transport of moist air takes place at that level. However, at stratospheric levels more distant from the tropopause, this pattern changes; at 420 K (Figure 2b) this region of moist air has almost completely disappeared. Since under mean conditions the 420 K isentropic level is located near 83 mb, this suggests that the region of moist air to the north and west of India at 83 mb (Figure 1b) is due to a localised upward bulge in the isentropic surfaces, rather than to any transport across the 420 K isentrope.

Figure 2 - Three-year mean HALOE water vapour for JJA, expressed on an isentropic surface. Contour interval: 0.4 ppmv. Regions of shading are as described in Figure 1.a) 375 K; b) 420 K.

4. INTERANNUAL VARIABILITY

Figures 3a and 3b show HALOE water vapour on the 375 K isentropic surface for DJF 93/94 and DJF 94/95, respectively. In both years there is a belt of low water vapour at low latitudes located between the Indian Ocean and the central Pacific. An interesting difference is that in 93/94 the water vapour is a minimum near Indonesia, whereas in 94/95 there is a local maximum near Indonesia, with lower water vapour mixing ratios located to the east and to the west of there. Examination of individual monthly fields shows that such features are persistent. Sea surface temperature (SST) anomalies (calculated by Reynolds and Smith, 1994) for January 1994 and 1995 show that cold SST anomalies often coincide with local maxima in the water vapour fields in the Indonesian region. Therefore, if it is assumed that cooler SSTs lead to less deep convection, and hence a greater tropopause temperature minimum, then the SST anomalies may explain the behaviour of the water vapour field near Indonesia in DJF 93/94 and 94/95. We recognise that, although this assumption is often true, closer inspection shows that the relationship between deep convection and SSTs can often be more complicated (see eg Zhang (1993) and references therein). However, the results do suggest that interannual variability in SSTs can influence water vapour mixing ratios near the tropopause. Since such fluctuations in Pacific SSTs are associated with the long-lived 1990-1995 El Nino Southern Oscillation (ENSO), this means that El Nino signals may be being seen in equatorial water vapour fields at levels just above the tropopause.

Figure 3 - HALOE water vapour on the 375 K isentropic surface. Contour interval: 0.4 ppmv. Regions of shading are as described in Figure 1. a) DJF 93/94; b) DJF 94/95.

Acknowledgements

We would like to acknowledge the support and assistance of the HALOE Project and Science Teams. The HALOE data were obtained through the British Atmospheric Data Centre (BADC) facility. DRJ, SJD and EJH received financial support from the UK Natural Environment Research Council.

References

Harries, J.E, Russell, J.M III, Tuck, A.F, Gordley, L.L, Purcell, P, Stone, K, Bevilacqua, R.M, Gunson, M, Nedoluha, G and Traub, W.A, 1996: Validation of measurements of water vapour from the Halogen Occultation Experiment (HALOE). J. Geophys. Res., 101, 10205-10216

Jackson, D.R, Driscoll, S.J, Highwood, E.J, Harries, J.E, and Russell, J.M III, 1996: Lower stratospheric water vapour at low latitudes as observed by HALOE, 1992-1995. Submitted to Quart. J. Roy. Meteor. Soc.

Jones, R.L, Pyle, J.A, Harries, J.E, Zavody, A.M, Russell, J.M, and Gille, J.C, 1986: The water vapour budget of the stratosphere studied using LIMS and SAMS satellite data. Quart. J. Roy. Meteor. Soc., 112, 1127-1143

Mote, P.W, Rosenlof, K.H, McIntyre, M.E, Carr, E.S, Kinnersley, J.S, Pumphrey, H.C, Harwood, R.S, Holton, J.R, Russell, J.M III, Waters, J.W, and Gille, J.C, 1996: An atmospheric tape recorder: the imprint of tropical tropopause temperatures on stratospheric water vapor. J. Geophys. Res., 101, 3989-4006

Potter, B.E and Holton, J.R, 1995: The role of monsoon convection in the dehydration of the lower tropical stratosphere. J. Atmos. Sci., 52, 1034-1050

Reynolds, R. W. and Smith, T. M, 1994: Improved global sea surface temperature analyses. J. Clim., 7, 929-948

Rind, D, Chiou, E-W, Chu, W, Oltmans, S, Lerner, J, Larsen, J, McCormick, M.P and McMaster, L, 1993: Overview of the Stratospheric Aerosol and Gas Experiment II water vapor observations: method, validation and data characteristics. J. Geophys. Res., 98, 4835-4856

Russell, J.M, Gordley, L.L, Park, J.H, Drayson, S.R, Hesketh, W.D, Cicerone, R.J, Tuck, A.F, Frederick, J.E, Harries, J.E, and Crutzen, P.J, 1993: The Halogen Occultation Experiment. J. Geophys. Res, 98, 10777-10797

Zhang, C.D, 1993: Large-scale variability of atmospheric deep convection in relation to sea surface temperature in the tropics. J. Clim., 6, 1898-1913

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