The northern annular mode in summer and its relation to solar activity variations in the GISS ModelE |
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| Institution: | 1. Institute for Terrestrial and Planetary Atmospheres, Stony Brook University, Stony Brook, NY 11794-5000, USA;2. NASA Goddard Institute for Space Studies and Columbia University, New York, NY, USA;1. Shanxi Climate Center, Taiyuan 030006, China;2. Climate Change Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;3. Egyptian Meteorological Authority, Cairo 11784, Egypt;4. The Abdus Salam International Centre for Theoretical Physics, Trieste 34100, Italy;1. Instituto de Investigaciones en Fisicoquímica de Córdoba (I.N.F.I.Q.C.), CONICET, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina;2. Facultad de Ciencias Exactas y Naturales, Universidad Nacional de Asunción, Campus Universitario, San Lorenzo, Paraguay;3. Physikalische & Theoretische Chemie/FBC, Bergische Universitaet Wuppertal, Wuppertal, Germany;1. National Research Institute of Astronomy and Geophysics, Cairo, Egypt;2. National Data Center, Cairo, Egypt;3. Faculty of Science, Cairo University, Egypt;1. School of Instrumentation Science and Engineering, Harbin Institute of Technology, Harbin 150001, China;2. Aerospace System Engineering Shanghai, Shanghai 201109, China |
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| Abstract: | The northern annular mode (NAM) has been successfully used in several studies to understand the variability of the winter atmosphere and its modulation by solar activity. The variability of summer circulation can also be described by the leading empirical orthogonal function (EOF) of geopotential heights. We compare the annular modes of the summer geopotential heights in the northern hemisphere stratosphere and troposphere in the Goddard Institute for Space Studies (GISS) ModelE with those in the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. In the stratosphere, the summer NAM obtained from NCEP/NCAR reanalysis as well as from the ModelE simulations has the same sign throughout the northern hemisphere, but shows greater variability at low latitudes. The patterns in both analyses are consistent with the interpretation that low NAM conditions represent an enhancement of the seasonal difference between the summer and the annual averages of geopotential height, temperature and velocity distributions, while the reverse holds for high NAM conditions. Composite analysis of high and low NAM cases in both model and observation suggests that the summer stratosphere is more “summer-like” when the solar activity is near a maximum. This means that the zonal easterly wind flow is stronger and the temperature is higher than normal. Thus increased irradiance favors a low summer NAM. A quantitative comparison of the anti-correlation between the NAM and the solar forcing is presented in the model and in the observation, both of which show lower/higher NAM index in solar maximum/minimum conditions. The temperature fluctuations in simulated solar minimum conditions are greater than in solar maximum throughout the summer stratosphere.The summer NAM in the troposphere obtained from NCEP/NCAR reanalysis has a dipolar zonal structure with maximum variability over the Asian monsoon region. The corresponding EOF in ModelE has a qualitatively similar structure but with less variability in the Asian monsoon region which is displaced eastward of its observed position. In both the NCEP/NCAR reanalysis and the GCM the negative anomalies associated with the NAM in the Euro-Atlantic and Aleutian island regions are enhanced in the solar minimum conditions, though the results are not statistically significant. |
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