Simulating the atmospheric response to the 11-year solar cycle forcing with the UM-UKCA model: the role of detection method and natural variability

Abstract

The 11-year solar cycle forcing is recognised as a potentially important atmospheric forcing; however, there remain uncertainties in characterising the effects of the solar variability on the atmosphere from observations and models. Here we 15 present the first detailed assessment of the atmospheric response to the 11-year solar cycle in the UM-UKCA chemistryclimate model using an ensemble of integrations over the recent past. Comparison of the model simulations is made with observations and reanalysis. Importantly, in contrast to the majority of previous studies of the solar cycle impacts, we pay particular attention to the role of detection method by comparing the results diagnosed using both a composite and a multiple linear regression method. We show that stratospheric solar responses diagnosed using both techniques largely agree with 20 each other within the associated uncertainties; however, the results show that apparently different signals can be identified by the methods in the troposphere and in the tropical lower stratosphere. Lastly, we focus on the role of internal atmospheric variability on the detection of the 11-year solar responses by comparing the results diagnosed from individual model ensemble members (as opposed to those diagnosed from the full ensemble). We show overall agreement between the ensemble members in the tropical and mid-latitude mid-stratosphere-to-lower-mesosphere, but larger apparent differences at 25 NH high latitudes during the dynamically active season. Our results highlight the need for long data sets for confident detection of solar cycle impacts in the atmosphere, as well as for more research on possible interdependence of the solar cycle forcing with other atmospheric forcings and processes (e.g. QBO, ENSO… etc.).Atmos. Chem. Phys. Discuss., https://doi

Improvements to stratospheric chemistry scheme in the UM-UKCA (v10.7) model: solar cycle and heterogeneous reactions

“…The results presented here are broadly similar to those shown by Bednarz et al (2018); they report a solar driven effect on total column ozone in the 60 • s -60 • N region of 4.1 (±1.9) and 5.6 (±1.4) DU/Wm −2 using two different methods, where the difference in total irradiance between solar maximum and minimum was 1.06Wm −2 . The difference between solar maximum and minimum shown in Figure 2 expressed as an absolute difference is around 4 − 5 DU.…”

“…It may be possible to extend our treatment of solar variability to the UM radiation scheme although it is unlikely to have a large effect given the smaller amplitude of the solar cycle at the relevant wavelengths. Bednarz et al (2018) find that the difference between solar maximum and minimum in the shortest UM wavelength band (covering 200-320 nm) is only 0.56%, and is less than 0.1% in the longer-wavelength bands.…”

“…The difference between solar maximum and minimum shown in Figure 2 expressed as an absolute difference is around 4 − 5 DU. Bednarz et al (2018) also find the ozone response to increase away from the equator but do not report values for the polar regions.…”

“…Angell (1989) also found that ozone leads the solar cycle but with a much smaller phase difference of around half a year. Bednarz et al (2018) also examine an implementation of a solar cycle in the UKCA model. They take a slightly different approach wherein the amplitude of the solar cycle is defined by the difference between the 1981 solar maximum and the 1986 solar minimum.…”

“…Also, our more comprehensive treatment of the look-up table component, which is used here for pressure levels above 20 Pa, is likely to produce better results in the upper model levels, for example, in the photolysis of oxygen shown inFigure 2(a). Our solar cycle implementation is solely in the UKCA module, unlikeBednarz et al (2018) who…”