Nicola Patmore & Ralf Toumi (contact: nicola.patmore@imperial.ac.uk)

1. A Tropopause Moist Pool over Arid Asia Minor Nicola Patmore & Ralf Toumi (contact: nicola.patmore@imperial.ac.uk) Space and Atmospheric Physics Group, Imperial College London Abstract -PV Analysis using ERA-40 Satellite and reanalysis data show the presence of a summertime “moist pool”, extending from ~150mb to 70mb, over the arid Asia Minor region (Iraq to Pakistan). The pool appears to form through the trapping of water vapour, which ascends in discreet pulses. The localised ascending regions are associated with intense low PV formations at ~40N. Diabatic heating along the westerly jet appears to be linked to the negative PV tendencies, however the source of the diabatic heating is unconfirmed. We suggest that perturbations to the jet may cause temporary dynamic instabilities, which could result in mixing and vertical diffusion of heat into the tropopause layer. 395K Potential Vorticity (PVU) Time Figure 5: Left: ERA-40 JJA 1994. Averaged 25-45N at 395K. Above: Expansion of Potential Vorticity at 395K, day 43, 12/07/94 Introduction • Figures 4 and 5 show that the moist pool is trapped within the region of low potential vorticity at the anticyclone core. • Various authors have shown the importance of the northern hemisphere monsoon systems in the hydration of the upper troposphere. However, the mechanisms through which the monsoons moisten the tropopause layer have remained unconfirmed. • This study uses data from the ECMWF ERA-40 reanalysis model and observational data from the HALOE instrument onboard UARS, to study water vapour distributions over the strongest monsoon system: the Asian Summer Monsoon (ASM). Longitude • Periodically, regions of intense low PV form on the northern side of the anticyclone, at ~40N. After formation, these anomalies are advected southwards and around the anticyclone core. The formation of low PV anomalies is associated with the pulses of water vapour seen in figure 4. HALOE Results Diabatic Heating (Q/cp) Diabatic Heating and PV Generation 126mb 86mb 100mb • Figure 6 shows a band of diabatic heating at ~40N. Periodic intensifications of this heating could incite the localised negative PV tendencies and water vapour pulses shown in figures 4 and 5. • Water vapour ascending within these localised regions would be immediately trapped within the PV anomaly. • No similar PV anomalies form over India or Myanmar. Figure 6: 395K ERA-40 July 1993-99 (Interval: 0.5Kday-1 red(+), blue (-)) Figure 1: HALOE Water Vapour (ppmv) JJA (1993-99) • At 126mb, HALOE shows two moist pools. The pool over the Bay of Bengal results from the strong convection of the monsoon beneath. The pool over Asia Minor is unexpected, as it overlies an arid region. • By 100mb, only the Asia Minor moist pool remains. This pool is still present at 86mb, but fades by 70mb. • How could a source of water vapour at ~40N, create the Asia Minor moist pool? • PV tendencies at ~40N are quickly removed by isentropic advection (Fig 7a). The newly formed PV and water vapour anomalies are advected around the anticyclone. • The region over Asia Minor experiences a continuous negative PV tendency (Fig 7). Water vapour is trapped in this region and gradually builds over the summer. ERA-40 Results (a) Isentropic Advection PV (b) Diabatic Advection PV (c) Diabatic Creation PV 150mb 100mb 70mb Figure 2: ERA-40 JJA 1993-99 Water Vapour(ppmv). 150mb Figure 7: Three dominant components of PV tendency at 395k. Calculated using ERA-40, averaged July 1993-99. (Interval: 0.2PVUday-1, red (+), blue(-)) Green indicates zero PV tendency 100mb 70mb Source of Diabatic Heating • Diabatic heating at ~40N can not be explained by latent heating, as relative humidities do not exceed 30% • We propose that heating may result from the vertical turbulent mixing of heat, within the westerly jet. • The Richardson Number is naturally low at ~40N, due to the strong vertical wind shear (Fig 8). • We suggest that synoptic disturbances may result in a strengthening of the jet or weakening of the local buoyancy stability. Small perturbations to this sensitive region have the potential to cause temporary dynamic instabilities, leading to vertical mixing and heat transfer. Figure 3: ERA-40 JJA 1993-99 Vertical Wind Velocity (Shaded: red=ascending, blue=descending) and Horizontal Wind Velocity (Vectors). [Contours: 150mb (2Pas-1), 100mb & 70mb (1Pas-1)] 395K Water Vapour (ppmv) • ERA-40 shows a similar moist pool to HALOE, but suggests a continuous westward motion with height. • At 150mb, the water vapour distribution reflects the strong convection of the monsoon over India (Fig. 3). • Above 150mb, moisture forms a pool to the south of the anticyclone core, overlying Asia Minor. • The moist pools at 100mb and 70mb appear to overlie a region of descent. • Figure 4 shows the variability of water vapour on an isentropic level. The pool moves longitudinally with time and appears to form through a series of discreet cross-isentropic pulses. Figure 8: ERA-40 Richardson Number, July 1993-99 Conclusions and Future Work • A tropopause moist pool forms over Asia Minor, overlying a region of mean descent. • The pool is trapped within the low PV anomaly of the monsoon anticyclone. • The pool forms through trapping of water vapour that ascends in discreet pulses from below. These pulses appear to be linked to local increases in diabatic heating and intense low PV generation. • The source of the diabatic heating is work in progress. We suggest that the source is vertical diffusion of heat resulting from temporary dynamic instabilities. We are currently investigating the potential of transient perturbations in horizontal advection heating to incite these instabilities. Figure 4: ERA-40 JJA 1994. Averaged 25-45N at 395K. Acknowledgements We would like to thank the UK Natural Environmental Research Council (NERC) for funding this work. ERA-40 data was obtained from the British Antarctic Data Centre (BADC), courtesy of ECMWF. HALOE data was obtained from the HALOE homepage at the NASA Langley Research Centre website.