Low-Frequency Variations in the Atmospheric Branch of the Global Hydrological Cycle

Abstract

According to the atmospheric water balance equation, the divergence of the water vapor flux is responsible for the exchange of water vapor between its source and sink regions. Because the water vapor flux divergence is primarily determined by the divergent circulation, time variations of the global hydrological cycle reflect the pronounced low-frequency modes of the global divergent circulation, that is, the annual and intraseasonal (30–60 day) modes. The annual variation of the hydrological cycle is illustrated in terms of hemispheric-mean hydrological variables for the Northern and Southern Hemispheres, while the intraseasonal variations of the global hydrological cycle are illustrated with mean values over two hemispheres that form an east-west partition of the globe. This partition is defined by the 60°E–120°W great circle and was chosen so that the mean precipitation difference and the divergent water vapor transport between the two hemispheres was maximized. Two years (1979–80) of daily precipitation estimates from the Goddard Laboratory for Atmospheres and 14 years (1979–92) of upper-air data generated by the Global Data Assimilation System at the National Meteorological Center are used in making quantitative estimates of the annual and intraseasonal variations in the global hydrological cycle. The annual variations in hemispheric-mean precipitation [P̂] and water vapor flux divergence [∇ · Q̂ ] for the Northern and Southern Hemispheres are comparable with amplitudes of about 0.5 ∼ 0.7 mm day−. Both [P̂] and [∇ · Q̂] vary annually in a coherent way in each hemisphere so that water vapor diverges from the winter hemisphere, where [P̂] reaches its minimum, to the summer hemisphere, where [P^] attains its maximum. In fact, the hemispheric-mean divergence of water vapor flux changes sign during the annual cycle. Intraseasonal variations of hemispheric-mean precipitation 〈P̃〉, evaporation 〈Ẽ〉, and water vapor flux divergence 〈∇ · Q̃〉 in the two hemispheres in the cast-west direction are comparable with amplitudes of about 0.1 ∼ 0.2 mm day−1, although amplitudes in some cases exceed 0.3 mm day−1. Hemispheric-mean precipitation 〈P̃〉 varies coherently in opposite phase for the two hemispheres, while 〈∇ · Q̃ 〉 varies so that water vapor diverges from the hemisphere of maximum 〈P̃〉 to the hemisphere of minimum 〈P〉. Intraseasonal variations of 〈P̃〉, 〈Ẽ〉 and 〈∇ · 〉 are in accord with the eastward propagation of the intra seasonal global divergent circulation.