Probabilistic Precipitation Anomalies Associated with ENSO

Extreme phases of the El Nino-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 Nino and La Nina episodes are tabulated, and the significance levels of the posterior probabilities are calculated using the hypergeometric 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 precipitation 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 emphasized also: the global impact of La Nina seems to be at least as widespread as that of El Nino. Furthermore, there are a number of notable asymmetries in precipitation responses to El Nino and La Nina 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 Nino conditions, for example, does not guarantee a high frequency of below-normal precipitation during La Nifia events, or vice versa. On a global basis El Nino events are predominantly associated with below-normal seasonal precipitation over land, whereas La Nina events result in a wider extent of above-normal precipitation.