Seasonal Variation of Upper Tropospheric and Lower Stratospheric Equatorial Waves over the Tropical Pacific

Christopher K. Wikle Department of Geological and Atmospheric Sciences, Department of Statistics, Iowa State University, Ames, Iowa

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Roland A. Madden National Center for Atmospheric Research, Boulder, Colorado

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Tsing-Chang Chen Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa

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Abstract

Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).

* Current affiliation: NCAR/Geophysical Statistics Project, Boulder, Colorado.

Corresponding author address: Dr. Christopher K. Wikle, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: wikle@ucar.edu

Abstract

Upper tropospheric and lower stratospheric wind data spanning 31 years from 1964 to 1994 were analyzed at rawinsonde stations in the central/western Pacific. Traditional spectral and cross-spectral analysis led to the conclusion that there is a significant signal with periods between 3 and 4.5 days, which the authors link with the dominant antisymmetric waves predicted by theory to have these periods, mixed Rossby–gravity waves, and equatorial Rossby waves. Then the authors applied the seasonally varying spectral analysis method developed by Madden to study the average seasonal variation of these waves. The seasonally varying analysis suggested that there are significant twice-yearly maxima in equatorial wave activity throughout the upper troposphere and lower stratosphere, with peaks occurring in late winter–spring and in late summer–fall. The twice-yearly signal was most prominent at the 70-hPa and 100-hPa levels. Similar and consistent results were also shown by an autoregressive cyclic spectral analysis. The cyclic spectral analysis suggested that the frequency characteristics of the υ-wind wave power are different during the two maxima at some stations. In addition, the seasonally varying squared coherence between the u and υ winds and the associated phase implied that there is horizontal momentum flux associated with these waves and that the sign of the flux is different during the two maxima. The differences in wave characteristics during the maxima periods may be related to different wave modes, seasonal variation of the basic zonal state, or possibly to different equatorial wave forcing mechanisms (i.e., convective versus lateral excitations).

* Current affiliation: NCAR/Geophysical Statistics Project, Boulder, Colorado.

Corresponding author address: Dr. Christopher K. Wikle, NCAR, P.O. Box 3000, Boulder, CO 80307-3000.

Email: wikle@ucar.edu

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