The Geostrophic Poleward Flux of Angular Momentum in the Month of January 1949

From sea level, 700, 500, 300 and 200 mb daily synoptic charts over the whole of the northern hemisphere and 100 mb charts over three fourths of the hemisphere, the mean geostrophic poleward flux of angular momentum is computed for January 1949. A northward total flux is found in all latitudes, with the maximum at latitude 32 1/2° N of 5.4 × 10¹⁶ t m² s^–2. Four fifths of this flux occurs between the 500 and 100 mb levels. From the meridional convergence of the flux, the mean zonal surface stress is computed. The sign of this derived stress is found to be in good agreement with the distribution of the surface zonal wind: its average magnitude, in the region of the surface westerlies, is 1.2 dynes cm^–2.