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Interhemispheric Propagation of Stationary Rossby Waves in a Horizontally Nonuniform Background Flow

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  • 1 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • | 2 College of Global Change and Earth System Science, Beijing Normal University, and Joint Center for Global Change Studies, Beijing, China
  • | 3 School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawaii
  • | 4 State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China
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Abstract

Significant interhemispheric teleconnections are identified that span the tropical easterlies in the boreal summer 300-hPa streamfunction, such as the North Africa–Antarctic (NAA) and the North Pacific–South America (NPSA) patterns. These patterns are not supported by traditional wave theory, since stationary waves in a basic state without meridional wind would be trapped in the easterlies. To describe the interhemispheric responses more realistically, two-dimensional spherical Rossby wave theory in a horizontally nonuniform basic state is considered. Conditions sufficient for the existence of one propagating wave are obtained, and the meridional group velocity of the wave is shown to have the same direction as the meridional basic wind at the traditional critical latitude. It is concluded that stationary waves with a specific wavelength can propagate across the easterlies from south (north) to north (south) via southerly (northerly) flows. Hence, energy transport by stationary waves on a horizontally nonuniform basic state may produce interhemispheric responses that could pass through the tropical easterly belt.

The wave theory and a barotropic model are then applied to idealized and climatological flows. Model results agree well with the theory. In boreal winter and summer, cross-equatorial flows steer stationary waves propagating from one hemisphere to the other across the tropical easterlies, especially over the Australian–Asian monsoon region. It seems that the large-scale monsoonal background flows play a critical role in the interhemispheric teleconnection. Additionally, the wave ray trajectory and model results suggest that the NAA pattern may result from Rossby wave energy dispersion.

Denotes Open Access content.

Corresponding author address: Prof. Jianping Li, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China. E-mail: ljp@bnu.edu.cn

Abstract

Significant interhemispheric teleconnections are identified that span the tropical easterlies in the boreal summer 300-hPa streamfunction, such as the North Africa–Antarctic (NAA) and the North Pacific–South America (NPSA) patterns. These patterns are not supported by traditional wave theory, since stationary waves in a basic state without meridional wind would be trapped in the easterlies. To describe the interhemispheric responses more realistically, two-dimensional spherical Rossby wave theory in a horizontally nonuniform basic state is considered. Conditions sufficient for the existence of one propagating wave are obtained, and the meridional group velocity of the wave is shown to have the same direction as the meridional basic wind at the traditional critical latitude. It is concluded that stationary waves with a specific wavelength can propagate across the easterlies from south (north) to north (south) via southerly (northerly) flows. Hence, energy transport by stationary waves on a horizontally nonuniform basic state may produce interhemispheric responses that could pass through the tropical easterly belt.

The wave theory and a barotropic model are then applied to idealized and climatological flows. Model results agree well with the theory. In boreal winter and summer, cross-equatorial flows steer stationary waves propagating from one hemisphere to the other across the tropical easterlies, especially over the Australian–Asian monsoon region. It seems that the large-scale monsoonal background flows play a critical role in the interhemispheric teleconnection. Additionally, the wave ray trajectory and model results suggest that the NAA pattern may result from Rossby wave energy dispersion.

Denotes Open Access content.

Corresponding author address: Prof. Jianping Li, College of Global Change and Earth System Science, Beijing Normal University, Beijing 100875, China. E-mail: ljp@bnu.edu.cn
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