Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a "green Sahara" to the present hyperarid desert. Our continuous 6000-year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site-specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large-scale dust mobilization from 4300 calendar years before the present (cal yr B.P.). Today's desert ecosystem and regional wind regime were established around 2700 cal yr B.P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate.

Abstract. Equatorial planetary scale wave modes such as Kelvin waves or Rossby-gravity waves are excited by convective processes in the troposphere. In this paper an analysis for these and other equatorial wave modes is carried out with special focus on the stratosphere using temperature data from the SABER satellite instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different equatorial wave types and the contribution of gravity waves is determined from the spectral background of the space-time spectra.Both gravity waves and equatorial planetary scale wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different wave types are calculated for the period from

Abstract. Equatorial planetary scale wave modes such as Kelvin waves or Rossby-gravity waves are excited by convective processes in the troposphere. In this paper an analysis for these and other equatorial wave modes is carried out with special focus on the stratosphere using temperature data from the SABER instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different equatorial wave types and the contribution of gravity waves is determined from the spectral background of the space-time spectra. Both gravity waves and equatorial planetary scale wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different wave types are calculated for the period from February 2002 until March 2006 and compared to previous findings. A comparison between SABER and ECMWF wave analyses shows that in the lower stratosphere SABER and ECMWF spectra and temperature variances agree remarkably well while in the upper stratosphere ECMWF tends to overestimate Kelvin wave components. Gravity wave variances are partly reproduced by ECMWF but have a significant low-bias. A case study for the time period of the SCOUT-O3 tropical aircraft measurement campaign in Darwin/Australia (in November and December 2005) is performed and we find that in the lower stratosphere also the longitude-time distribution of the Kelvin waves is correctly reproduced by ECMWF.

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The Halogen Occultation Experiment (HALOE) experiment on Upper Atmosphere Research Satellite (UARS) performs solar occultation (sunrise and sunset) measurements to infer the composition and structure of the stratosphere and mesosphere. Two of the HALOE channels, centered at 5.26 gm and 6.25 gm, are designed to infer concentrations of nitric oxide and nitrogen dioxide respectively. The NO measurements extend from the lower stratosphere up to 130 km, while the NO2 results typically range from the lower stratosphere to 50 km and higher near the winter terminator. Comparison with results from various instruments are presented, including satellite-, balloon-, and ground-based measurements. Both NO and NO2 can show large percentage errors in the presence of heavy aerosol concentrations, confined to below 25 km and before 1993. The NO2 measurements show mean differences with correlative measurements of about 10 to 15% over the middle stratosphere. The NO2 precision is about 7.5x10 '13 arm, degrading to 2x10 -12 arm in the lower stratosphere. The NO differences are similar in the middle stratosphere but sometimes show a low bias (as much as 35%) between 30 and 60 km with some correlative measurements. NO precision when expressed in units of density is nearly constant at lx10 '12 atmospheres, or approximately 0.1 ppbv at 10.0 mb or, 1.0 ppbv at 1.0 mb, and so forth when expressed in mixing ratio. Above 65 km, agreement in the mean with Atmospheric Trace Molecule Spectroscopy (ATMOS) NO results is very good, typically + 15%. Model comparisons are also presented, showing good agreement with both expected morphology and diurnal behavior for both NO2 and NO. hydrogen chloride (HCI), hydrogen fluoride (HF), methane (CH4), water vapor (H20), nitric oxide (NO), nitrogen dioxide (NOy), and aerosol extinction. Retrieved profiles cover an altitude range from the upper troposphere, in some cases, to the lower thermosphere for nitric oxide. Fifteen spacecraft sunrises and sunsets are observed daily and usually in opposite hemispheres, although at certain times these measurements occur on the same day and almost overlap in space. Details of the HALOE experiment, including geographic coverage, discussion of the experiment and instrument techniques, instrument ground test results, error mechanisms, in-orbit performance, initial pressure versus latitude cross sections, and orthographic projections are included in the HALOE overview by Russell et. al. [1993]. The purpose of 'this paper is to describe steps taken and 'the status of efforts to validate data from the NO gas correlation channel and the NO2 radiometer channel. All results were inferred using the most current archived HALOE data, version 17, released in November 1994. Version numbers were liberally changed during the continuous validation and evolution of the HALOE processing system. Attainment of research quality results coincided with version 16 in late summer of 1994, which was the first version released to the general science community. However, a second general processing ...

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Measurements of the temporal and spatial variations in HNO3, particularly those from the Nimbus 7 limb infrared monitor of the stratosphere (LIMS) satellite experiment, are compared to both a twodimensional chemical/dynamical model and to chemistry/parcel trajectory analyses. Significant discrepancies are found between the observed and modeled variations in the winter season, especially in the polar night region. The study of the evolution of HNO3 suggests that an important source exists for this species in the high-latitude winter stratosphere that is not included in presently accepted photochemical schemes. Possible reactions to account for this discrepancy are explored.The time constant appropriate to meridional transport in the lower stratosphere is of the order of a few months. Thus the observed distribution of HNO3 in the lower stratosphere is dependent upon the effects of both dynamical and chemical processes, particularly in middle-and high-latitude winter, where its photochemical lifetime exceeds a month (see Figure 1). If the photochemistry of this species is well understood, then its observed evolution over short time intervals (of the order of a few days) should be simulated by a parcel trajectory model including chemical effects along the parcel path (see Austin and Tuck [1985-] for a description of the technique). Two-dimensional coupled chemical/dynamical models have also achieved at least qualitative success in simulating the longer-term seasonally and zonally averaged morphology and evolution of a number of chemical species such as N20 [`/ones and Pyle, 1984; Guthrie eta!., 1984; Ko eta!., 1985], CH4 [Solomon and Garcia, 1984], and ozone [Stordal et al., 1985]. Such models should therefore yield distributions of HNO 3 that are similar to appropriately averaged observations if the chemistry of HNO 3 is well characterized in the model. Note that the model used here includes the effect of wave-induced chemical eddy transport for HNO3, using an observed climatology for planetary wave amplitudes [see Garcia and Solomon, 1983]. 2. SPATIAL AND TEMPORAL VARIATIONS IN HNO3: MEASUREMENTS AND MODELS Nitric acid has been measured from stratospheric balloons using a variety of techniques, including infrared emission and absorption [Harries et al., 1975; Evans et al., 1978; Murcray et a!., 1975; Coffey eta!., 1981; Fischer eta!., 1985; etc.], filter collection [Lazrus and Gandrud, 1974], and inferences from ion composition [Arnold eta!., 1980]. Recently, satellite distributions of HNO 3 have been measured by the limb infrared monitor of the stratosphere (LIMS) experiment on board Nimbus 7. These data can provide altitude profiles from about 100 to 3 mbar, over the latitude range from 84øN to 64øS, yielding nearly global coverage for the stratosphere. Data were obtained continuously along 14 orbits per day from November 1978, through May 1979 (the duty cycle was 11 days on and 1 day off). Details of the experiment and validation of the HNO3 measurements are presented by Gi!!e et al. [1984]. Wofsy, S.C., Tempor...

Ozone from all instruments usually agrees to within 0.5 ppmv (•5%) in the upper stratosphere and •0.25 ppmv in the lower stratosphere; larger differences in the midstratosphere are primarily due to sampling differences. Individual profile comparisons, selected to match meteorological conditions, show remarkably good agreement between all instruments that sample similar latitudes, although some small differences do not appear to be related to sampling differences. In the Southern Hemisphere (SH) midstratosphere, the instruments (ATMOS, SAGE II, and POAM II) with observations confined to high latitudes measured low EqLs in air drawn up from low latitudes that had formed a "low-ozone pocket"; they measured much lower ozone at low EqL than those that sampled low latitudes. A low-ozone pocket had also formed in the Northern Hemisphere (NH) midstratosphere (a month earlier than this phenomenon has previously been reported), also resulting in differences between instruments based on their sampling patterns. POAM II sampled only high latitudes in the NH, where extravortex sampling did not include tropical, high-ozone air, and thus measured lower ozone at a given EqL than other instruments. Ozone laminae appear in coincident profiles from multiple instruments, confirming atmospheric origins for these features and agreement in some detail between ozone observed by several instruments; reverse trajectory calculations indicate such laminae arise from filamentation in and around the polar vortices. Both EqL/0 and profile comparisons indicate overall excellent agreement in ozone observed by all seven instruments in early November 1994. When care is taken to compare similar air masses and to understand sampling effects, much useful information can be obtained from comparisons between instruments with very different observing patterns.