GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 21, PAGES 2967-2970, OCTOBER 15, 1996 Baroclinic wave variations observed in MLS upper tropospheric water vapor Elizabeth M. Stone and William J. Randel National Center for Atmospheric Research, Boulder, Colorado John L. Stanford Department of Physics andAstronomy, Iowa State University, Ames William G. Read and Joe W. Waters JetPropulsion Laboratory, CaliforniaInstitute of Technology, Pasadena Abstract. Upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder are usedto investigate the structure and evolution of eastward traveling medium-scale wave features in Southern Hemisphere summertime. The extratropical SouthernHemispheresummer circulation pattern is frequently dominated by medium scale waves which exhibit life cycles of baroclinicgrowth and barotropic decay. The water vapor field during suchlife cycles is examinedhere and found to be well correlated with meteorological fields derived from European Centre for Medium Range Weather Forecasts global analyses. From mid January to mid February 1992 several episodes of growth and decay in the amplitude of eastward travelingwaves are foundin the water vapor and meteorological data at levels of the upper troposphere, with zonal waves four, five and six being predominant modes. The water vapor data are compared with derived potential vorticity(PV) fields,with strong anticorrelation observed in middle and high latitudes. The results are consistent with model paradigms for the structure andevolution ofbaroclinic disturbances, coupled with the known characteristics of high PV and low water vapor mixing ratios in lower stratospheric air parcels and thereverse for upper tropospheric air. Introduction scalewave. disturbances (zonal wave 5) in SH measurements of totalcolumn ozone from the Total Ozone Mapping Spectrometer (TOMS). Mote et al. [1991] find baroclinic wave signatures in TOMS ozone data along Northern Hemisphere oceanic stormtrack regions. Signatures of baroclinic waves have been studied MLS 215 hPa Water Vapor % ½4 The extratropical Southern Hemisphere (SH) summer circulation pattern is frequently dominated by medium scale waves(zonal wavenumbers 4-7), which propagate eastward with periods near 10 days [Salby, 1982;Hamilton, 1983;Randel and Stanford, 1985]. The medium scale waves often exhibit well defined life cycles of baroclinic growthandbarotropic decay, and appear very similar to the idealizedbaroclinic wavesstudied by Simmons and Hoskins [1978], Barnes and Young [1992], and Thorncroft et al. [1993]. NorthernHemisphere baroclinic waves are commonlylongitudinallylocalized along stormtrack regions [e.g., Blackrnon et al., 1984], whereas SH features often have wave maxima distributed more symmetrically arounda latitude circle. These systems provide a dominantmeans of heat and momentum transport in thetroposphere. Since baroclinic waves are fundamental to tropospheric dynamics, they canbe expected to impact atmospheric constituent distributions. Schoeberl and Krueger [ 1983] observed medium Copyright 1996 by the American Geophysical Union. Paper number 96GL02576. 0094-8534/96/96GL-02576505.00 250 hPa Geopotential Height Figure 1. Polar plots of the SH for 20 January 1990. (a) MLS water vapor field (ppmv) at 215 hPa and (b) ECMWF 250 hPa geopotential height (m). Lines of constant latitude andlongitude aredrawn every 20 degrees. Meridional coverage is from20øS to the pole. Geopotential height contours areincremented by 150m. 2967