Upper-air circulation of the Southern Oscillation from the NCEP-NCAR Reanalysis

Summary ?The upper-air circulation characteristics of the Southern Oscillation (SO) are studied from the 1958–97 NCEP-NCAR Reanalysis. The low/high phase of the SO is defined by anomalously small/large values of the Tahiti minus Darwin surface pressure difference. For the fields of 200 mb topography, divergence and divergent flow, 500 mb vertical motion, and 1000 mb topography, during January, April, July and October, differences are computed between ensembles of years of extremely low versus high SO phase, and tested for statistical significance. The 1000 mb topography bears out SO characteristics consistent with earlier work on surface pressure, such as the seesaw between the western and eastern Pacific in all seasons, and in the low SO phase in the boreal winter a northward displaced near-equatorial trough over the Atlantic. In addition, the new dataset afforded documentation of functional upper-air processes. Thus, overall positive/negative 1000 mb topography anomalies tend to be associated with enhanced/reduced subsidence or reduced/enhanced ascending motion. Ensemble differences in mid-tropospheric vertical motion are coherent with those in upper-tropospheric divergence and divergent flow. Characteristic of the low SO phase are significant positive anomalies of upper-tropospheric topographies especially in the tropics and most prominently over the eastern Pacific, reflecting the anomalously warm atmospheric column, and during the boreal winter a train of centers with alternatingly positive and negative topography differences extending from the tropical Pacific across North America to the Atlantic. Received October 10, 2001; revised May 8, 2002; accepted May 31, 2002