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 associated 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 modes. A complex empirical orthogonal function analysis of the divergent circulation brings out two dominant modes with essentially the same vertical structures in all months of the year. The first mode, which depicts the global monsoon, has a simple vertical structure with a maximum in vertical motion at about 400 mb, divergence in the upper troposphere that is strongest at 150 mb and decays to zero amplitude above 70 mb, and convergence in the lower troposphere with a maximum at 925 mb (ECMWF) or 850 mb (NCEP). However, this mode has a rich three-dimensional spatial structure that evolves with the seasons. It accounts for 60% of the annual cycle variance of the divergent mass circulation and dominates the Hadley circulation as well as three overturning transverse cells. These include the Pacific Walker circulation; an Americas–Atlantic Walker circulation, both of which comprise rising motion 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 migrate and evolve considerably with the seasons and have about a third to half of the mass flux of the peak Hadley cell. The annual cycle of the two Hadley cells reveals peak strength in early February and early August in both reanalyses. A second monsoon mode, which accounts for 20% of the variance, features relatively shallow but vigorous overturning with the maximum vertical velocities near 800 mb, outflow from 750 to 350 mb, and inflow peaking at 925 mb. It is especially strong over Africa where the shallow, mostly meridional overturning migrates back and forth across the equator with the seasons. It influences the Middle East, has a signature over Australia, and is also an important component of the overturning in the tropical eastern Pacific and Atlantic, and thus of the convergence zones in these regions. The relationship of the global monsoon to the regional monsoons is described over six zonal sectors: Africa, Australia–Asia, North America, South America, and the Pacific and Atlantic Oceans. Only the two ocean areas do not undergo 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 cross sections highlight the importance of the shallow overturning cell in lower-troposphere monsoon activity. The steadiness of the overturning circulation is determined by comparing the signal of the seasonal mean vertical motions at 500 mb with the standard deviation of the transient daily variations. Locations where this signal exceeds 60% of the daily noise correspond closely with the regional centers of the monsoon.