Zonal circulations over the equatorial Indian Ocean

The NCEP-NCAR 1958-97 upper-air dataset and surface observations have been analyzed for evidence of zonal-vertical circulations along the Indian Ocean equator and their role in climatic variability. The long-term mean upper-tropospheric circulation is dominated in boreal winter by divergent outflow from the southern Indian Ocean northwestward into southern Asia, and in summer from southern Asia southwestward into the Southern Hemisphere. In boreal autumn only, divergent easterlies blow from Indonesia along the equator into an uppertropospheric convergence band over East Africa, and only then a closed zonal-vertical circulation cell materializes along the Indian Ocean equator, between the centers of ascending motion over Indonesia and of subsidence over equatorial East Africa, and featuring westerlies in the lower layers. The boreal autumn zonal-vertical circulation varies interannually. A regime of intense circulation features accelerated equatorial surface westerlies, enhanced subsidence, and deficient rainfall at the coast of East Africa. Tn the high phase of the Southern Oscillation [anomalously high (low) pressure at Tahiti (Darwin)] this regime is preferred. The regime of weak zonal circulation has the opposite departure characteristics.