ABSTRACT
A variety of impact-oriented indices have been developed to quantify various aspects of drought. The atmosphere exhibits preferred regional or even global patterns of variability. El Nino is an anomalous warming of the eastern equatorial Pacific. The Southern Oscillation is a global-scale seesaw in surface pressure, with centers of action near Indonesia-North Australia and in the southeast Pacific. The tendency for persistence of drought regimes invites speculation about the possibility of a natural feedback between anomalous, drought-related surface conditions and the drought-sustaining atmospheric circulation. Pronounced decadal-scale fluctuations occur in many regions, but the degree to which they exhibit a systematic behavior is hard to establish from the relatively short time series of precipitation measurements. Atmospheric regimes or oscillations that are large in amplitude and global in scale, have time scales comparable to or longer than the monthly or seasonal prediction period, and exhibit a systematic and relatively predictable evolution are of fundamental importance to climate prediction.
