The global monsoon as seen through the divergent atmospheric circulation

A comprehensive description is given of the global monsoon as seen through the large-scale overturning in the atmosphere that changes with the seasons , and it provides a basis for delimiting the monsoon regions of the world. The analysis focuses on the mean annual cycle of the divergent winds and as sociated vertical motions, as given by the monthly mean fields for 1979-93 reanalyses from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and European Centre for Medium- Range Weather Forecasts (ECMWF), which are able to reproduce the dominant m odes. A complex empirical orthogonal function analysis of the divergent cir culation brings out two dominant modes with essentially the same vertical s tructures in all months of the year. The first mode, which depicts the glob al monsoon, has a simple Vertical structure with a maximum in vertical moti on at about 400 mb, divergence in the upper troposphere that is strongest a t 150 mb and decays to zero amplitude above 70 mb, and convergence in the l ower troposphere with a maximum at 925 mb (ECMWF) or 850 mb (NCEP). However , this mode has a rich three-dimensional spatial structure that evolves wit h the seasons. It accounts for 60% of the annual cycle variance of the dive rgent mass circulation and dominates the Hadley circulation as well as thre e overturning transverse cells. These include the Pacific Walker circulatio n; an Americas-Atlantic Walker circulation, both of which comprise rising m otion in the west and sinking in the east; and a transverse cell over Asia, the Middle East, North Africa, and the Indian Ocean that has rising motion in the east and sinking toward the west. These exist year-round but migrat e and evolve considerably with the seasons and have about a third to half o f the mass Aux of the peak Hadley cell. The annual cycle of the two Hadley cells reveals peak strength in early February and early August in both rean alyses. A second monsoon mode, which accounts for 20% of the variance, features rel atively shallow bur vigorous overturning with the maximum vertical velociti es near 800 mb, outflow from 750 to 350 mb, and inflow peaking at 925 mb. I t is especially strong over Africa where the shallow, mostly meridional ove rturning migrates back and forth across the equator with the seasons. It in fluences the Middle East, has a signature over Australia, and is also an im portant component of the overturning in the tropical eastern Pacific and At lantic, and thus of the convergence zones in these regions. The relationship of the global monsoon to the regional monsoons is describe d over six zonal sectors: Africa, Australia-Asia, North America, South Amer ica, and the Pacific and Atlantic Oceans. Only the two ocean areas do not u ndergo a seasonal reversal required for monsoons, although they have direct overturning cells and they nevertheless participate in the global monsoon through the changes in large-scale overturning. The regional meridional cro ss sections highlight the importance of the shallow overturning cell in low er-troposphere monsoon activity. The steadiness of the overturning circulat ion is determined by comparing the signal of the seasonal mean vertical mot ions at 500 mb with the standard deviation of the transient daily variation s. Locations where this signal exceeds 60% of the daily noise correspond cl osely with the regional centers of the monsoon.