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Solar irradiance variability on hourly to decadal scale from SCIAMACHY and its impact on middle atmospheric ozone and ozone-climate interaction.

Principial investigator:

Dr. Mark Weber
Universität Bremen
Institut für Umweltphysik
Otto-Hahn Allee 1
28359 Bremen

Co investigator:


Solar irradiance variability on decadal and longer time scales play an important role in the energy budget of the upper atmosphere. Changing UV radiation levels alter stratospheric and mesospheric ozone and other trace gases (particularly oxygen and nitrogen) by photolysis. This in turn modifies atmospheric heating (by absorption) and the wave structure. Vertical dynamical coupling is believed to propagate wave perturbances into the lower atmosphere and therefore influences large scale atmospheric circulation over a large altitude range. Lower stratospheric ozone distribution and its inter-annual variability is mainly governed by the meridional residual circulation and modification of circulation patterns by solar activity have potentially an impact on stratospheric ozone well down to the tropopause region.

A more direct effect such as sea-surface temperature (SST) changes in the past have been linked to solar activity. The sharp increase in CO2 and other greenhouse gases due to anthropogenic releases are related to the global warming trend observed. The relative contribution of both forcings on the present and past global climate are still a matter of scientific debate. Long-term changes in surface and tropospheric temperatures modifies large-scale atmospheric circulation in both troposphere and stratosphere via tropospheric forcings of planetary scale waves that deposit angular momentum into the stratosphere driving the meridional residual circulation. The latter is also known as Brewer-Dobson circulation transporting stratospheric ozone into high latitudes from the equatorial source region. The large inter-annual variability of stratospheric ozone is to a large extent determined by the strength of planetary-scale wave activity during hemispheric winter. This process plays an important role in the ozone-climate interaction and has to be accounted for when long-term trends in stratospheric ozone are assessed. It is still uncertain if the upper atmosphere will cool (due to anthropogenic increases in IR emitting constituents: CO2, CH4, H2O) or warm (due to changes in atmospheric circulation that alters the temperature structure). A cooling stratosphere will lead to more ozone depletion by heterogeneous reactions on liquid aerosols and/or polar stratospheric clouds.

With recent advances in global observations of atmospheric composition and solar output from different satellite platforms, a better understanding of solar activity-ozone-climate interaction may be achieved. In this proposal several investigations using trace gas observations and direct measurements of the extraterrestrial solar spectra from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) in combination with other available satellite data are planned. SCIAMACHY provides measurements of various trace constituents (ozone, NO2, BrO, etc.) on a global scale at moderately high vertical resolution. In addition, it routinely measures solar radiation from the near UV (220 nm) up to the near infrared (2385 nm) on hourly time scales starting in the maximum of solar cycle 23 (2002) and the potential to continue these measurements towards the minimum of solar cycle 23 (2007). SCIAMACHY is one of the first satellite instruments providing routine solar measurements covering not only the UV spectral range but also the entire visible, near and short wave infrared range with moderately high spectral resolution (0.2-1.2 nm) and, therefore, permits the investigation of the spectral composition of the total solar irradiance (TSI) and its variation.

One of the main focii of the proposed work is to investigate the UV/vis irradiance variability in the spectral range 220-2385 nm from an hourly to daily scale and put it in perspective to decadal changes in solar activity. Solar activity proxy indicator derived from the Mg II emission are correlated with UV spectral irradiance changes and can be used to monitor and model solar activity in cases where no solar spectral measurements are available. The identification of suitable proxies for irradiance variability in the visible and NIR spectral range is another important issue.

The second focus in this study is on the connection between solar irradiance changes and mesospheric ozone on short time scales. Relation between short-term solar activity (outside charged particle precipitation events) and mesospheric and upper stratospheric ozone are studied with respect to 27 day cycles down to hourly time scales. Combining stratospheric ozone data (total columns and profiles) from several satellite platforms (Solar Backscatter UV SBUV/SBUV2, 1979-present) and SCIAMACHY (2002-present) a long-term trend assessment using multivariate regression including updated solar activity proxies from this study, Eliassen-Palm-flux diagnostics (describing tropospheric forcing) and other relevant atmospheric quantities (stratospheric aerosol loading, QBO: quasi-biennial oscillation) are carried out to investigate possible sign of ozone recovery, here defined as a moving away from a long-term linear decline, that is expected after effective stratospheric chlorine loading reached its peak at the end of the 90s. The important question remains, what are the relative contribution of long-term circulation changes, on one hand, and of changes in chemical conditions as expressed by the turnaround in the stratospheric halogen loading, on the other hand.

The work packages proposed for this study are divided as follows:

  • WP 100: UV/vis solar irradiance variability
  • WP 200: Long-term solar proxies for UV and vis/NIR irradiance variability
  • WP 300: Impact of short-term solar variability on high altitude ozone
  • WP 400: Stratospheric ozone trends and the solar cycle

This project covers several important themes of CAWSES: characterization of variability of solar radiation, atmospheric composition changes on various time scales, investigation of coupling mechanism from the troposphere up to mesosphere, and solar activity-climate interaction.