Divergent Circulations Inferred from Nimbus-7 ERB: Application to the 1982-1983 ENSO Event

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Abstract
The Nimbus-7 Earth Radiation Budget (ERB) Experiment data set is used to identify and describe the planetary-scale, quasi-stationary irrotational flow anomalies which develop in the tropospheric wind field during the major El Niño/Southern Oscillation (ENSO) event of 1982-1983. A statistical correspondence was developed (Krishnamurti and Low-Nam, 1986) between the broad-band Outgoing Longwave Radiation (OLR) field and the divergent wind at 200mb during the FGGE year. The resultant regression relationship is here applied to a sequence of OLR fields that cover the period of the El Nibo episode to estimate the associated upper-level divergence. Further analysis then yields estimates of the velocity potential, divergent flow, and vertical motion. Vigorous thermally-direct Hadley and Walker circulation anomaly modes are demonstrated to exist in the tropics. The drought regions over Indonesia, northern Australia, Africa, and northern Brazil are found to lie in regions of subsidence resulting from the anomalous east-west circulations that form during the ENSO event. The drought regions over the northern Pacific Ocean from the Philippines to Hawaii and in the southern Pacific Ocean are demonstrated to lie in descending branches of the north-south Hadley circulation anomalies. Teleconnections into the mid-latitudes are clearly discernible in the fields. A wave train, with ascent regions over New Zealand, the equatorial Pacific Ocean, and the southern United States, and forced descent regions in the subtropical Pacific Ocean north and south of the equator, is evident as a dominant feature. At the height of the El Niño, the monthly-averaged mean vertical motion anomaly over the primary sea surface temperature maximum in the equatorial Pacific Ocean reaches 1cm/sec while the resultant divergent, east-west 200mb flow anomalies reach 4m/sec. The vertical motion anomalies display a high degree of correspondence to independently-derived estimates of the global cloud-cover distribution.