HARMONIC ANALYSIS OF THE BIENNIAL ZONAL-WIND AND TEMPERATURE REGIMES

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  • 1 U.S. Weather Bureau, Washington, D.C.
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Abstract

Based upon 78 months of mean-monthly zonal-wind and temperature data, the phase angle and amplitude of the third (26-month) harmonic is determined as a function of latitude and height for stations within or bordering the North Pacific Ocean. In tropical latitudes the 26-month zonal-wind oscillation can always be traced down to the 200-mb. surface and frequently much lower. However, the 26-month temperature oscillation is usually not detectable below the 100-mb. level. In temperate latitudes, particularly at high levels, a mean-monthly zonal-wind oscillation of about 26-month period occurs which, with considerable justification, can be associated with the 26-month oscillation in the Tropics. Analysis of the 26-month temperature oscillation shows that this oscillation is even more easily traced into the temperate latitudes at high levels, but with a rather pronounced phase shift, so that north of the Tropic of Cancer the minimum temperature in the 26-month oscillation occurs at approximately the same time as the maximum temperature occurs south of this latitude. The thermal wind resulting from this temperature change with latitude is consistent with the observed 26-month zonal-wind oscillation.

A search for the cause of the heating and cooling with which the biennial wind oscillation apparently is associated is complicated by the fact that in tropical latitudes the downward progression of the warming with time appears most likely associated with small-scale eddy heat fluxes, while in polar and temperate latitudes the heating and cooling take place nearly instantaneously at all levels, suggesting the influence of vertical motions associated with a reversible meridional cell.

Abstract

Based upon 78 months of mean-monthly zonal-wind and temperature data, the phase angle and amplitude of the third (26-month) harmonic is determined as a function of latitude and height for stations within or bordering the North Pacific Ocean. In tropical latitudes the 26-month zonal-wind oscillation can always be traced down to the 200-mb. surface and frequently much lower. However, the 26-month temperature oscillation is usually not detectable below the 100-mb. level. In temperate latitudes, particularly at high levels, a mean-monthly zonal-wind oscillation of about 26-month period occurs which, with considerable justification, can be associated with the 26-month oscillation in the Tropics. Analysis of the 26-month temperature oscillation shows that this oscillation is even more easily traced into the temperate latitudes at high levels, but with a rather pronounced phase shift, so that north of the Tropic of Cancer the minimum temperature in the 26-month oscillation occurs at approximately the same time as the maximum temperature occurs south of this latitude. The thermal wind resulting from this temperature change with latitude is consistent with the observed 26-month zonal-wind oscillation.

A search for the cause of the heating and cooling with which the biennial wind oscillation apparently is associated is complicated by the fact that in tropical latitudes the downward progression of the warming with time appears most likely associated with small-scale eddy heat fluxes, while in polar and temperate latitudes the heating and cooling take place nearly instantaneously at all levels, suggesting the influence of vertical motions associated with a reversible meridional cell.

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