619 Bulletin of the American Meteorological Society 1. Introduction El Niño–Southern Oscillation (ENSO) warm events are associated with climate extremes over many areas of the globe (Bradley et al. 1987; Ropelewski and Halpert 1987, 1989, 1996; Kiladis and Diaz 1989; Halpert and Ropelewski 1992). Because of the rela- tively high predictability of the ENSO phenomenon (Cane and Zebiak 1985; Cane et al. 1986; Latif et al. 1998; Neelin et al. 1998; Stockdale et al. 1998a,b), prior knowledge of the expected state of the equato- rial Pacific Ocean provides a significant source of pre- dictability of seasonal climate variability over much of the Tropics, and farther afield (Palmer and Ander- son 1994). Knowing what typically has occurred dur- ing previous ENSO events gives some indication of what is likely to happen during a present or upcom- ing event. For many parts of the world this knowledge provides a better estimate of the probable future cli- mate than the assumption that seasonal conditions will be the same as average. A composite of climate anomalies observed dur- ing past ENSO events provides some useful informa- tion about typical ENSO impacts (e.g., Bradley et al. Probabilistic Precipitation Anomalies Associated with ENSO Simon J. Mason* and Lisa Goddard + *Climate Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California. + International Research Institute for Climate Prediction, Lamont- Doherty Earth Observatory, Columbia University, Palisades, New York. Corresponding author address: Dr. Simon J. Mason, Scripps In- stitution of Oceanography, University of California, San Diego, Mail Code 0230, La Jolla, CA 92093-0230. E-mail: simon@lacosta.ucsd.edu In final form 6 October 2000.  2001 American Meteorological Society ABSTRACT Extreme phases of the El Niño–Southern Oscillation (ENSO) phenomenon have been blamed for precipitation anomalies in many areas of the world. In some areas the probability of above-normal precipitation may be increased during warm or cold events, while in others below-normal precipitation may be more likely. The percentages of times that seasonal precipitation over land areas was above, near, and below normal during the eight strongest El Niño and La Niña episodes are tabulated, and the significance levels of the posterior probabilities are calculated using the hy- pergeometric distribution. These frequencies may provide a useful starting point for probabilistic climate forecasts during strong ENSO events. Areas with significantly high or low frequencies or above- or below-normal precipitation are highlighted, and attempts are made to estimate the proportion of land areas with significant ENSO-related precipi- tation signals. There is a danger of overstating the global impact of ENSO events because only about 20%–30% of land areas experience significantly increased probabilities of above- or below-normal seasonal precipitation during at least some part of the year. Since different areas are affected at different times of the year, the fraction of global land affected in any particular season is only about 15%–25%. The danger of focusing on the impact of only warm-phase events is empha- sized also: the global impact of La Niña seems to be at least as widespread as that of El Niño. Furthermore, there are a number of notable asymmetries in precipitation responses to El Niño and La Niña events. For many areas it should not be assumed that the typical climate anomaly of one ENSO extreme is likely to be the opposite of the other extreme. A high frequency of above-normal precipitation during strong El Niño conditions, for example, does not guarantee a high frequency of below-normal precipitation during La Niña events, or vice versa. On a global basis El Niño events are predominantly associated with below-normal seasonal precipitation over land, whereas La Niña events result in a wider extent of above-normal precipitation.