Stationary Waves in the Winter Stratosphere: Seasonal and Interannual Variability

A. K. Smith Geophysics Program, University of Washington, Seattle, WA 98195

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Abstract

Monthly average satellite and conventional data are used to determine the zonal mean wind and stationary waves 1 and 2 in the northern winter stratosphere for January 1973 through February 1977. Wave propagation is diagnosed with the aid of the Eliassen-Palm flux and the refractive index squared. Interannual variability in zonal mean wind and stationary wave amplitudes is greatest during the second half of winter (January-March), partially as a consequence of the irregular occurrence of major sudden warmings during those months. Both the seasonal trend and the interannual variability of the amplitude of wave 1 reflect variations in zonal mean wind, and the corresponding refractive index. The wave 1 amplitude maximum in the upper stratosphere occurs on the cyclonic side of the zonal jet, and its variability during mid-winter months is controlled primarily by refractive index variations associated with variations in the meridional curvature of the wind profile. During each year studied, a mid-winter minimum in wave 1 amplitude occurs at and above 1 mb as a consequence of a mid-winter curvature maximum. Interannual variability in wave propagation characteristics during late winter is sensitive to the zonal wind strength. Dissipation is favored and the speed of wave propagation is slower when there are relatively weak zonal winds.

Four major sudden warmings of wave 1 type occurred during the study period (McInturff, 1978). In three of the cases, the month prior to the warming was characterized by abnormally weak winds near the climatological mean jet (above 1 mb and equatorward of 40°N), a detached secondary jet at low altitude and high latitude (near 65°N and 3 mb), and abnormally large wave 1 amplitude throughout the middle and upper stratosphere. Similar, features have been noticed by O'Neill and Taylor (1979) and Palmer (1981) in instantaneous wind fields prior to the warnings of 1977 (wave 1 warming) and 1979 (wave 2 warming).

Abstract

Monthly average satellite and conventional data are used to determine the zonal mean wind and stationary waves 1 and 2 in the northern winter stratosphere for January 1973 through February 1977. Wave propagation is diagnosed with the aid of the Eliassen-Palm flux and the refractive index squared. Interannual variability in zonal mean wind and stationary wave amplitudes is greatest during the second half of winter (January-March), partially as a consequence of the irregular occurrence of major sudden warmings during those months. Both the seasonal trend and the interannual variability of the amplitude of wave 1 reflect variations in zonal mean wind, and the corresponding refractive index. The wave 1 amplitude maximum in the upper stratosphere occurs on the cyclonic side of the zonal jet, and its variability during mid-winter months is controlled primarily by refractive index variations associated with variations in the meridional curvature of the wind profile. During each year studied, a mid-winter minimum in wave 1 amplitude occurs at and above 1 mb as a consequence of a mid-winter curvature maximum. Interannual variability in wave propagation characteristics during late winter is sensitive to the zonal wind strength. Dissipation is favored and the speed of wave propagation is slower when there are relatively weak zonal winds.

Four major sudden warmings of wave 1 type occurred during the study period (McInturff, 1978). In three of the cases, the month prior to the warming was characterized by abnormally weak winds near the climatological mean jet (above 1 mb and equatorward of 40°N), a detached secondary jet at low altitude and high latitude (near 65°N and 3 mb), and abnormally large wave 1 amplitude throughout the middle and upper stratosphere. Similar, features have been noticed by O'Neill and Taylor (1979) and Palmer (1981) in instantaneous wind fields prior to the warnings of 1977 (wave 1 warming) and 1979 (wave 2 warming).

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